Genes and environment interact to determine the fitness costs of resistance to Bacillus thuringiensis

Increasing ecological heterogeneity can constrain biopesticide resistance evolution

Microbial biopesticides containing living parasites are valuable emerging crop protection technologies against insect pests, but they are vulnerable to resistance evolution. Fortunately, the fitness of alleles that provide resistance, including to parasites used in biopesticides, frequently depends on parasite identity and environmental conditions. This context-specificity suggests a sustainable approach to biopesticide resistance management through landscape diversification. To mitigate resistance risks, we advocate increasing the range of biopesticides available to farmers, whilst simultaneously encouraging other aspects of landscape-wide crop heterogeneity that can generate variable selection on resistance alleles. This approach requires agricultural stakeholders to prioritize diversity as well as efficiency, both within agricultural landscapes and the biocontrol marketplace.

Resistance of Spodoptera frugiperda to Cry1, Cry2, and Vip3Aa Proteins in Bt Corn and Cotton in the Americas: Implications for the Rest of the World

The fall armyworm, Spodoptera frugiperda, is an economically important pest of corn, cotton, and soybean, and a major target of transgenic crops expressing Bacillus thuringiensis (Bt) proteins. In recent years, this insect has invaded most countries in Africa, Southeastern Asia, and Oceania, posing a great threat to food security. Successful use of Bt crops in the U.S. indicates that Bt technology can be an effective tool for management of S. frugiperda in other countries. Evolution of insect resistance is the primary threat to the long-term efficacy of Bt technology. There are many factors that may affect the rate of evolution of insect resistance to Bt crops, which include initial resistance allele frequency, the dose of Bt protein in Bt crops, cross-resistance, complete/incomplete resistance, and fitness costs associated with resistance. Currently, the high dose/refuge and gene-pyramiding approaches are the two main IRM strategies used in the U.S. to combat evolution of insect resistance. In this paper, we review research on resistance of S. frugiperda to Cry1, Cry2, and Vip3Aa proteins. Specifically, we discuss the resistance allele frequencies of S. frugiperda to these three proteins in the field, the genetic basis of resistance, the patterns of cross-resistance, and the fitness costs associated with resistance. Experience and knowledge gained from these studies provide valuable information for the successful use of Bt crop technology for control of S. frugiperda worldwide.

Effects of cross-pollination among non-Bt and pyramided Bt corn expressing cry proteins in seed mixtures on resistance development of dual-gene resistant Helicoverpa zea

BACKGROUND

Seed mixture strategy can guarantee the compliance of planting non-Bt crops to host the susceptible insects for resistance management. However, pollen movement between Bt and non-Bt corn in the mixed plantings could reduce the efficacy of this strategy for ear-feeding insects. Few studies have evaluated the effects of cross-pollination among non-Bt and pyramided Bt corn in seed mixtures on the resistance development of insects possessing multiple resistances. Here, we provided the first study to investigate whether cross-pollination in mixed plantings of pyramided Bt corn producing Cry1A.105 and Cry2Ab2 would increase the dominance of resistance of dual-gene resistant populations of Helicoverpa zea, a target of pyramided Bt corn and cotton in the USA.

RESULTS

We compared the survival and development of susceptible, dual-gene resistant (resistance to both Cry1 and Cry2 proteins) and heterozygous genotypes of H. zea in the laboratory on non-Bt and pyramided Bt corn ears collected from mixed plantings and structured plantings in the field. We found higher fitness for F₁ heterozygous insects than for the susceptible insects of H. zea on both pyramided Bt corn and non-Bt corn in the mixed plantings.

CONCLUSION

These results suggest that cross-pollination in mixed plantings will significantly increase the dominance of resistance by supporting survival of heterozygous insects for dual-gene resistant populations of H. zea, and therefore accelerate evolution of resistance to pyramided Bt crops. © 2022 Society of Chemical Industry.

Rapid spread of a densovirus in a major crop pest following wide-scale adoption of Bt-cotton in China

Bacillus thuringiensis (Bt) crops have been widely planted and the effects of Bt-crops on populations of the target and non-target insect pests have been well studied. However, the effects of Bt-crops exposure on microorganisms that interact with crop pests have not previously been quantified. Here, we use laboratory and field data to show that infection of Helicoverpa armigera with a densovirus (HaDV2) is associated with its enhanced growth and tolerance to Bt-cotton. Moreover, field monitoring showed a much higher incidence of cotton bollworm infection with HaDV2 in regions cultivated with Bt-cotton than in regions without it, with the rate of densovirus infection increasing with increasing use of Bt-cotton. RNA-seq suggested tolerance to both baculovirus and Cry1Ac were enhanced via the immune-related pathways. These findings suggest that exposure to Bt-crops has selected for beneficial interactions between the target pest and a mutualistic microorganism that enhances its performance on Bt-crops under field conditions.

Effects of intraguild interactions on Anticarsia gemmatalis and Chrysodeixis includens larval fitness and behavior in soybean

BACKGROUND

Crop pest management requires an understanding of the complex interactions among species that potentially impact crop yield. In soybean, the velvetbean caterpillar, Anticarsia gemmatalis (Hübner), and the soybean looper, Chrysodeixis includens (Walker), are described as key pests, sharing the same feeding guild. We assessed the intraguild interactions of these species under laboratory conditions. Fitness cost study was conducted to examine the influence of competition on insect development. A video tracking system was used to evaluate behavioral parameters during larval interactions in scenarios with and without food availability.

RESULTS

In the fitness cost assay, pupal weight was not significantly affected, regardless of sex. However, larval and pupal survival were influenced by the competition, especially in third versus fifth instar scenarios. We detected 40.00% cannibalism and 46.67% predation when A. gemmatalis and C. includens third instars competed with A. gemmatalis fifth instar, respectively. Distance moved, distance between larvae, body contact (food available) and frequency in food of C. includens larvae were negatively affected by interactions. Anticarsia gemmatalis larvae showed highly active behavior, moving twice or more the distance compared to C. includens larvae, and A. gemmatalis spent more time in body contact with food.

CONCLUSION

Our results suggest that A. gemmatalis has a competitive advantage over C. includens. This study provides important information regarding lepidopteran behavior in soybean. We recommended that additional studies are necessary to understand the effects of interactions, especially in field conditions. © 2021 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Relative fitness and stability of resistance in a near-isogenic strain of indoxacarb resistant Helicoverpa armigera (Lepidoptera: Noctuidae)

BACKGROUND

A strain of Helicoverpa armigera with 171-fold resistance to indoxacarb was introgressed with a susceptible strain by serial backcrossing and reselection with indoxacarb resulting in the creation of the near-isogenic GY7-39BC4 strain. Fitness was compared on artificial diet under diapause and non-diapause conditions in resistant, susceptible and F₁ progeny from a reciprocal backcross of the two strains using life history trait analyses. Selection experiments were used to determine stability of resistance.

RESULTS

There were no significant differences between strains in survival, female fertility or realized fecundity. A comparison of the intrinsic rate of population increase showed similar relative fitness between strains. Lower male fertility and male longevity in the resistant strain and one of the F1 strains compared with the susceptible strain suggests small non-recessive costs may be associated with male reproductive capacity in individuals with indoxacarb resistance alleles. However, there was no significant decline in resistance in the GY7-39 strain when reared in the absence of insecticide for five generations. Following an artificially induced diapause, survival was reduced by 52% and pupal weights were significantly lower in the resistant strain compared with the susceptible strain.

CONCLUSIONS

These results suggest indoxacarb resistance does not confer a major fitness cost under standard laboratory conditions. However, a survival cost associated with overwintering highlights the imperative for adoption of management strategies in northern regions of Australia where a winter diapause does not occur. © 2020 Society of Chemical Industry.

Applying a Selection Experiment to Test for Fitness Costs of Bt Resistance in Western Corn Rootworm (Coleoptera: Chrysomelidae) and the Effect of Density on Fitness Costs

Western corn rootworm, Diabrotica virgifera virgifera LeConte, is a serious pest of corn and is often managed with transgenic corn producing insecticidal toxins from the bacterium Bacillus thuringiensis (Bt). This pest has developed field-evolved resistance to all commercially available Bt traits, beginning with Cry3Bb1 in 2009. Fitness costs may accompany Bt resistance, where individuals with alleles for Bt resistance have reduced fitness on non-Bt corn compared to Bt-susceptible individuals. In conjunction with non-Bt refuges, fitness costs can delay the evolution of Bt resistance. Importantly, ecological factors may affect the presence and magnitude of fitness costs. For western corn rootworm, available data suggest that fitness costs of Bt resistance may be present in some cases. Using two Cry3Bb1-resistant western corn rootworm strains (Hopkinton and Cresco), a fitness-cost experiment was performed by rearing rootworm in the absence of Bt for six generations to test for fitness costs of Cry3Bb1 resistance and the effect of larval rearing density on fitness costs. Fitness costs were detected for both strains; however, strains were still resistant to Cry3Bb1 corn at the end of the experiment. Cresco experienced a greater loss of resistance at low versus high density, but no effect of density was detected in Hopkinton. Our study shows that fitness costs can accompany Bt resistance in western corn rootworm and may be more pronounced under low larval density. Even though fitness costs were present, it appears that rootworm populations may remain resistant to Cry3Bb1 corn for years after resistance has evolved.

Drivers of pesticide resistance in freshwater amphipods

Aquatic invertebrates exposed to pesticides may develop pesticide resistance. Based on a meta-analysis we revealed environmental factors driving the magnitude of resistance in the freshwater amphipod Gammarus pulex in the field. We showed that (i) insecticide tolerance of G. pulex increased with pesticide contamination in agricultural streams generally by a factor of up to 4. Tolerance increased even at concentrations lower than what is considered safe in regulatory risk assessment (ii) The increase in insecticide tolerance was pronounced at high test concentrations; comparing the LC50 of populations therefore potentially underestimates the development of resistance. (iii) Insecticide resistance in agricultural streams diminished during the spraying season, suggesting that adverse effects of sublethal concentrations in the short term contrast long-term adaptation to insecticide exposure. (iv) We found that resistance was especially high in populations characterized not only by high pesticide exposure, but also by large distance (>3.3 km) from non-polluted stream sections and by low species diversity within the invertebrate community. We conclude that the test concentration, the timing of measurement, distance to refuge areas and species diversity mediate the observed response of aquatic communities to pesticide pollution and need to be considered for the sustainable management of agricultural practices.

First documentation of major Vip3Aa resistance alleles in field populations of Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae) in Texas, USA

The corn earworm, Helicoverpa zea, is a major target pest of the insecticidal Vip3Aa protein used in pyramided transgenic Bt corn and cotton with Cry1 and Cry2 proteins in the U.S. The widespread resistance to Cry1 and Cry2 proteins in H. zea will challenge the long-term efficacy of Vip3Aa technology. Determining the frequency of resistant alleles to Vip3Aa in field populations of H. zea is critically important for resistance management. Here, we provided the first F₂ screen study to estimate the resistance allele frequency for Vip3Aa in H. zea populations in Texas, U.S. In 2019, 128 H. zea neonates per isofamily for a total of 114 F₂ families were screened with a diagnostic concentration of 3.0 μg/cm² of Vip3Aa39 protein in diet-overlay bioassays. The F₂ screen detected two families carrying a major Vip3Aa resistance allele. The estimated frequency of major resistance alleles against Vip3Aa39 in H. zea in Texas from this study was 0.0065 with a 95% CI of 0.0014–0.0157. A Vip3Aa-resistant strain (RR) derived from the F₂ screen showed a high level of resistance to Vip3Aa39 protein, with a resistance ratio of >588.0-fold relative to a susceptible population (SS) based on diet-overlay bioassays. We provide the first documentation of a major resistance allele conferring high levels of Vip3Aa resistance in a field-derived strain of H. zea in the U.S. Data generated from this study contribute to development of management strategies for the sustainable use of the Vip3Aa technology to control H. zea in the U.S.

Fitness costs of Vip3A resistance in Spodoptera frugiperda on different hosts

BACKGROUND

Fall armyworm, Spodoptera frugiperda, is a major target pest of the Vip3A protein. The fitness of Bacillus thuringiensis (Bt)-resistant insects on refuges is an important factor in determining the speed of resistance development. Fitness costs associated with Bt resistance can vary among host plants. Here, we provide the first experimental evaluation of the interactions between non-Bt hosts and fitness costs of Vip3A resistance in S. frugiperda.

RESULTS

Laboratory bioassays showed that survivorship, pupal weight, sex ratio, developmental time, fecundity, net reproductive rate, and intrinsic rate were not different among Vip3A-resistant (RR), -susceptible (SS) and -heterozygous (R_♂ S_♀ and R_♀ S_♂ ) strains of S. frugiperda on non-Bt corn, non-Bt cotton, and non-Bt diet. R_♂ S_♀ and R_♀ S_♂ also showed no differences on sorghum relative to SS. However, compared to SS, RR on sorghum showed significant reduction in pupal weight, and took longer time to develop to pupa and adult.

CONCLUSION

These findings suggest that recessive fitness costs of reduced pupal weight and growth rate were detected at the individual level in the Vip3A resistant populations of S. frugiperda on sorghum. However, at the population level in terms of net reproductive rate and intrinsic rate, fitness costs of Vip3A resistance was not evident in S. frugiperda. © 2018 Society of Chemical Industry.

Larval density mediates knockdown resistance to pyrethroid insecticides in adult Aedes aegypti

BACKGROUND

Understanding mechanisms driving insecticide resistance in vector populations remains a public health priority. To date, most research has focused on the genetic mechanisms underpinning resistance, yet it is unclear what role environmental drivers may play in shaping phenotypic expression. One of the key environmental drivers of Aedes aegypti mosquito population dynamics is resource-driven intraspecific competition at the larval stage. We experimentally investigated the role of density-dependent larval competition in mediating resistance evolution in Ae. aegypti, using knockdown resistance (kdr) as a marker of genotypic resistance and CDC bottle bioassays to determine phenotype. We reared first-instar larvae from susceptible and pyrethroid-resistant field-derived populations of Ae. aegypti at high and low density and measured the resulting phenotypic resistance and population kdr allele frequencies.

RESULTS

At low density, only 48.2% of the resistant population was knocked down, yet at high density, the population was no longer phenotypically resistant - 93% were knocked down when exposed to permethrin, which is considered susceptible according to WHO guidelines. Furthermore, the frequency of the C1534 kdr allele in the resistant population at high density decreased from 0.98 ± 0.04 to 0.69 ± 0.04 in only one generation of selection.

CONCLUSIONS

Our results indicate that larval conditions, specifically density, can impact both phenotype and genotype of pyrethroid-resistant populations. Furthermore, phenotypic susceptibility to pyrethroids may be re-established in a resistant population through a gene x environment interaction, a finding that can lead to the development of novel resistance management strategies that capitalize on density-induced costs.

Fitness of Bt-resistant cabbage loopers on Bt cotton plants

Development of resistance to the insecticidal toxins from Bacillus thuringiensis (Bt) in insects is the major threat to the continued success of transgenic Bt crops in agriculture. The fitness of Bt-resistant insects on Bt and non-Bt plants is a key parameter that determines the development of Bt resistance in insect populations. In this study, a comprehensive analysis of the fitness of Bt-resistant Trichoplusia ni strains on Bt cotton leaves was conducted. The Bt-resistant T. ni strains carried two genetically independent mechanisms of resistance to Bt toxins Cry1Ac and Cry2Ab. The effects of the two resistance mechanisms, individually and in combination, on the fitness of the T. ni strains on conventional non-Bt cotton and on transgenic Bt cotton leaves expressing a single-toxin Cry1Ac (Bollgard I) or two Bt toxins Cry1Ac and Cry2Ab (Bollgard II) were examined. The presence of Bt toxins in plants reduced the fitness of resistant insects, indicated by decreased net reproductive rate (R₀ ) and intrinsic rate of increase (r). The reduction in fitness in resistant T. ni on Bollgard II leaves was greater than that on Bollgard I leaves. A 12.4-day asynchrony of adult emergence between the susceptible T. ni grown on non-Bt cotton leaves and the dual-toxin-resistant T. ni on Bollgard II leaves was observed. Therefore, multitoxin Bt plants not only reduce the probability for T. ni to develop resistance but also strongly reduce the fitness of resistant insects feeding on the plants.

Fitness costs and life table parameters of highly insecticide-resistant strains of Plutella xylostella (L.) (Lepidoptera: Plutellidae) at different temperatures

BACKGROUND

In many cases, resistance alleles have been associated with fitness costs and are often dependent on environmental factors such as temperature. Here, we studied the effects of temperature on the overall fitness, including development, survival and reproduction, of three insecticide-resistant and one susceptible strain of diamondback moth (DBM), Plutella xylostella (L.).

RESULTS

The broader cross-resistance profile of the resistant strains previously selected by diamide and benzoylurea insecticides was tested. Cohort studies were conducted in the laboratory at three different temperatures (20 ± 1 °C, 25 ± 1 °C and 30 ± 1 °C), and involved fitness costs were estimated. We observed significant differences in the development time, with the susceptible strain showing a shorter developmental period from egg stage to adult stage compared with the resistant strains. Moreover, the resistant strains differed significantly between one another. Additionally, the population growth parameters varied among the strains, with the benzoylurea-resistant strain showing the highest costs affecting the overall fitness of this strain. A temperature of 30 °C was unfavourable for DBM development, resulting in a reduced fitness in all strains.

CONCLUSION

Benzoylurea selection pressure on a diamide-resistant P. xylostella strain resulted in lowest reproduction parameters and the longest generation time, as well as doubling the time among all strains tested. This suggests significant effects on the overall fitness and population growth parameters for diamide-resistant populations pressured by benzoylureas under applied conditions. © 2017 Society of Chemical Industry.

Evolution of host resistance to insect pathogens

Insect pathogens are widely used as a tool for sustainable pest management. Their complex mode of action was thought to make them immune to the evolution of resistance; however, several examples of field-based resistance to the bacterium Bacillus thuringiensis and a granulovirus have been recorded. Here I review the scenarios where resistance has evolved and discuss the likelihood of it occurring in other entomopathogens. I highlight recent research on the factors which might influence the evolution of resistance to insect pathogens, including the role of pathogen diversity, host nutrition and transgenerational effects.

Assessment of Inheritance and Fitness Costs Associated with Field-Evolved Resistance to Cry3Bb1 Maize by Western Corn Rootworm

The western corn rootworm, Diabrotica virgifera virgifera LeConte, is among the most serious insect pests of maize in North America. One strategy used to manage this pest is transgenic maize that produces one or more crystalline (Cry) toxins derived from the bacterium Bacillus thuringiensis (Bt). To delay Bt resistance by insect pests, refuges of non-Bt maize are grown in conjunction with Bt maize. Two factors influencing the success of the refuge strategy to delay resistance are the inheritance of resistance and fitness costs, with greater delays in resistance expected when inheritance of resistance is recessive and fitness costs are present. We measured inheritance and fitness costs of resistance for two strains of western corn rootworm with field-evolved resistance to Cry3Bb1 maize. Plant-based and diet-based bioassays revealed that the inheritance of resistance was non-recessive. In a greenhouse experiment, in which larvae were reared on whole maize plants in field soil, no fitness costs of resistance were detected. In a laboratory experiment, in which larvae experienced intraspecific and interspecific competition for food, a fitness cost of delayed larval development was identified, however, no other fitness costs were found. These findings of non-recessive inheritance of resistance and minimal fitness costs, highlight the potential for the rapid evolution of resistance to Cry3Bb1 maize by western corn rootworm, and may help to improve resistance management strategies for this pest.

Effects of high temperature on life history traits and heat shock protein expression in chlorpyrifos-resistant Laodelphax striatella

The resistance of the small brown planthopper (SBPH), Laodelphax striatella, to insecticides has been widely found in China, and has posed serious problems to efforts to control the pest. To determine the costs and benefits of resistance, the life tables of chlorpyrifos-resistant and -susceptible strains were constructed at 24 and 30°C. The results showed the resistant SBPH (YN-CPF) had lower fitness at 24°C, but slightly higher fitness at 30°C compared to the susceptible SBPH. Transcriptomic analysis showed there are five heat shock protein genes changed their expression, and the up-regulated genes are LsHsc70-1 and LsHsc70-2. The deduced amino acid sequences of LsHsc70-1 and LsHsc70-2 include three heat shock protein 70 (HSP70) family signatures, but LsHSC70-1 has the conserved HSP70 carboxyl terminal region of the "EEVD" motif, while LsHSC70-2 has the endoplasmic reticulum (ER) retention signal of the "KDEL" motif. The phylogenetic tree further identified LsHsc70-1 has closer evolutionary distances to cytoplasmic/nuclear HSP70s from human and Drosophila melanogaster, while LsHsc70-2 has closer evolutionary distances to HSP70s localized to ER. After treatment at 30-44°C, the expression of LsHsc70-1 and LsHsc70-2 was slightly increased in YN-CPF. These results suggested that LsHsc70-1 and LsHsc70-2 are members of Hsc70 family, localized to the cytosol/nucleus and ER, respectively. The up-regulated expression of these genes may protect the chlorpyrifos-resistant pest against damage under high temperatures, increasing its relative fitness, but the lower relative fitness of this population under optimal temperature may be the trade-off.

Fitness costs and stability of Cry1Fa resistance in Brazilian populations of Spodoptera frugiperda

BACKGROUND

The presence of fitness costs of resistance to Bacillus thuringiensis (Bt) insecticidal proteins in insect populations may delay or even reverse the local selection of insect resistance to Bt transgenic crops, and deserves rigorous investigation. Here we assessed the fitness costs associated with Cry1Fa resistance in two strains of fall armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae), derived from field collections in different Brazilian regions and further selected in the laboratory for high levels of resistance to Cry1Fa using leaves of TC1507 corn.

RESULTS

Fitness components were compared using paired resistant and susceptible strains with similar genetic backgrounds and F₁ generations from reciprocal crosses, all of them reared on non-transgenic corn leaves. No apparent life history costs in the larval stage were observed in the Bt-resistant strains. Moreover, the resistance remained stable for seven generations in the absence of selection, with no decrease in the proportion of resistant individuals. Larval respiration rates were also similar between resistant and susceptible homozygotes, and heterozygotes displayed respiration rates and demographic performance equal or superior to those of susceptible homozygotes.

CONCLUSION

In combination, these results indicate the lack of strong fitness costs associated with resistance to Cry1Fa in the fall armyworm strains studied. These findings suggest that Cry1Fa resistance in S. frugiperda populations is unlikely to be counterselected in Cry1Fa-free environments. © 2016 Society of Chemical Industry.

Biotic interactions govern genetic adaptation to toxicants

The genetic recovery of resistant populations released from pesticide exposure is accelerated by the presence of environmental stressors. By contrast, the relevance of environmental stressors for the spread of resistance during pesticide exposure has not been studied. Moreover, the consequences of interactions between different stressors have not been considered. Here we show that stress through intraspecific competition accelerates microevolution, because it enhances fitness differences between adapted and non-adapted individuals. By contrast, stress through interspecific competition or predation reduces intraspecific competition and thereby delays microevolution. This was demonstrated in mosquito populations (Culex quinquefasciatus) that were exposed to the pesticide chlorpyrifos. Non-selective predation through harvesting and interspecific competition with Daphnia magna delayed the selection for individuals carrying the ace-1^R resistance allele. Under non-toxic conditions, susceptible individuals without ace-1^R prevailed. Likewise, predation delayed the reverse adaptation of the populations to a non-toxic environment, while the effect of interspecific competition was not significant. Applying a simulation model, we further identified how microevolution is generally determined by the type and degree of competition and predation. We infer that interactions with other species—especially strong in ecosystems with high biodiversity—can delay the development of pesticide resistance.

Near-Isogenic Cry1F-Resistant Strain of Spodoptera frugiperda (Lepidoptera: Noctuidae) to Investigate Fitness Cost Associated With Resistance in Brazil

Field-evolved resistance to Cry1F maize in Spodoptera frugiperda (J.E. Smith) populations in Brazil was reported in 2014. In this study, to investigate fitness costs, we constructed a near-isogenic S. frugiperda-resistant strain (R-Cry1F) using Cry1F-resistant and Cry1F-susceptible strains sharing a close genetic background. A near-isogenic R-Cry1F strain was obtained by eight repeated backcrossings, each followed by sib-mating and selection among resistant and susceptible strains. Fitness cost parameters were evaluated by comparing the biological performance of resistant, susceptible, and heterozygous strains on artificial diet. Fitness parameters monitored included development time and survival rates of egg, larval, pupal, and egg-to-adult periods; sex ratio; adult longevity; timing of preoviposition, oviposition, and postoviposition; fecundity; and fertility. A fertility life table was also calculated. The near-isogenic R-Cry1F strain showed lower survival rate of eggs (32%), when compared with Sus and reciprocal crosses (41 and 55%, respectively). The number of R-Cry1F insects that completed the life cycle was reduced to ∼25%, compared with the Sus strain with ∼32% reaching the adult stage. The mean generation time (T) of R-Cry1F strain was ∼2 d shorter than R-Cry1F♂×Sus♀ and Sus strains. The reproductive parameters of R-Cry1F strain were similar to the Sus strain. However, fewer females were produced by R-Cry1F strain than R-Cry1F♀×Sus♂ and more females than R-Cry1F♂×Sus♀. In summary, no relevant fitness costs are observed in a near-isogenic Cry1F-resistant strain of S. frugiperda, indicating stability of resistance to Cry1F protein in Brazilian populations of this species in the absence of selection pressure.

Effect of crop plants on fitness costs associated with resistance to Bacillus thuringiensis toxins Cry1Ac and Cry2Ab in cabbage loopers

Fitness costs associated with resistance to Bacillus thuringiensis (Bt) toxins critically impact the development of resistance in insect populations. In this study, the fitness costs in Trichoplusia ni strains associated with two genetically independent resistance mechanisms to Bt toxins Cry1Ac and Cry2Ab, individually and in combination, on four crop plants (cabbage, cotton, tobacco and tomato) were analyzed, in comparison with their near-isogenic susceptible strain. The net reproductive rate (R₀) and intrinsic rate of increase (r) of the T. ni strains, regardless of their resistance traits, were strongly affected by the host plants. The ABCC2 gene-linked mechanism of Cry1Ac resistance was associated with relatively low fitness costs, while the Cry2Ab resistance mechanism was associated with higher fitness costs. The fitness costs in the presence of both resistance mechanisms in T. ni appeared to be non-additive. The relative fitness of Bt-resistant T. ni depended on the specific resistance mechanisms as well as host plants. In addition to difference in survivorship and fecundity, an asynchrony of adult emergence was observed among T. ni with different resistance mechanisms and on different host plants. Therefore, mechanisms of resistance and host plants available in the field are both important factors affecting development of Bt resistance in insects.

Environmental influences on mosquito foraging and integrated vector management can delay the evolution of behavioral resistance

Along with the scaled‐up distribution of long‐lasting insecticidal nets for malaria control has become concern about insecticide resistance. A related concern regards the evolution of host‐seeking periodicity from the nocturnal to the crepuscular periods of the day. Why we observe such shifts in some areas but not others and which methods could prove useful in managing such behavioral resistance remain open questions. We developed a foraging model to explore whether environmental conditions affect the evolution of behavioral resistance. We looked at the role of the abundance of blood hosts and nectar sources and investigated the potential of attractive toxic sugar baits for integrated control. Higher encounter rates with hosts and nectar sources allowed behaviorally resistant populations to persist at higher levels of bed net coverage. Whereas higher encounter rates with nectar increased the threshold where resistance emerged, higher encounter rates of hosts lowered this threshold. Adding sugar baits lowered the coverage level of bed nets required to eliminate the vector population. In certain environments, using lower bed net coverage levels together with toxic sugar baits may delay or prevent the evolution of behavioral resistance. Designing sustainable control strategies will depend on an understanding of vector behavior expressed in local environmental conditions.

Long-Term Resistance of Drosophila melanogaster to the Mushroom Toxin Alpha-Amanitin

Insect resistance to toxins exerts not only a great impact on our economy, but also on the ecology of many species. Resistance to one toxin is often associated with cross-resistance to other, sometimes unrelated, chemicals. In this study, we investigated mushroom toxin resistance in the fruit fly Drosophila melanogaster (Meigen). This fruit fly species does not feed on mushrooms in nature and may thus have evolved cross-resistance to α-amanitin, the principal toxin of deadly poisonous mushrooms, due to previous pesticide exposure. The three Asian D. melanogaster stocks used in this study, Ama-KTT, Ama-MI, and Ama-KLM, acquired α-amanitin resistance at least five decades ago in their natural habitats in Taiwan, India, and Malaysia, respectively. Here we show that all three stocks have not lost the resistance phenotype despite the absence of selective pressure over the past half century. In response to α-amanitin in the larval food, several signs of developmental retardation become apparent in a concentration-dependent manner: higher pre-adult mortality, prolonged larva-to-adult developmental time, decreased adult body size, and reduced adult longevity. In contrast, female fecundity nearly doubles in response to higher α-amanitin concentrations. Our results suggest that α-amanitin resistance has no fitness cost, which could explain why the resistance has persisted in all three stocks over the past five decades. If pesticides caused α-amanitin resistance in D. melanogaster, their use may go far beyond their intended effects and have long-lasting effects on ecosystems.

Construction and characterisation of near-isogenic Plutella xylostella (Lepidoptera: Plutellidae) strains resistant to Cry1Ac toxin

BACKGROUND

Resistance to insecticidal Bacillus thuringiensis (Bt) toxins has arisen in multiple populations of the worldwide Brassica pest Plutella xylostella (L.). To help elucidate the mechanism of resistance to Bt Cry1Ac toxin in a population from Florida, two pairs of near-isogenic lines (NILs) were developed.

RESULTS

NILs were generated using either backcross or recombinant inbred line methodologies and evaluated for near-isogenicity with inter-simple-sequence-repeat (ISSR) markers. Backcross line BC6F4 maintained a similar level of Cry1Ac resistance to parental strain DBM1Ac-R (>5000-fold) yet showed 98.24% genetic similarity to the susceptible parental strain DBM1Ac-S. Single-pair backcrosses between DBM1Ac-S and BC6F4 revealed that Cry1Ac resistance was controlled by one recessive autosomal locus. BC6F4 exhibited high levels of cross-resistance to Cry1Ab and Cry1Ah but not to Cry1Ca or Cry1Ie.

CONCLUSION

Near-isogenic strains were constructed to provide a reliable biological system to investigate the mechanism of Cry1Ac resistance in P. xylostella. These data suggest that resistance to Cry1Ac, Cry1Ab and Cry1Ah is probably caused by the alteration of a common receptor not recognised by Cry1Ca or Cry1Ie. Understanding Bt toxin cross-resistance provides valuable information to consider when developing pest control strategies to delay resistance evolution. © 2014 Society of Chemical Industry.

Trade-off between thermal tolerance and insecticide resistance in Plutella xylostella

Fitness costs associated with resistance to insecticides have been well documented, usually at normal temperature conditions, in many insect species. In this study, using chlorpyrifos-resistant homozygote (RR) and chlorpyrifos-susceptible homozygote (SS) of resistance ace1 allele of Plutella xylostella (DBM), we confirmed firstly that high temperature experience in pupal stage influenced phenotype of wing venation in insecticide-resistant and insecticide-susceptible Plutella xylostella, and SS DBM showed significantly higher thermal tolerance and lower damages of wing veins under heat stress than RR DBM. As compared to SS DBM, RR DBM displayed significantly lower AChE sensitivity to chlorpyrifos, higher basal GSTs activity and P450 production at 25°C, but higher inhibitions on the enzyme activities and P450 production as well as reduced resistance to chlorpyrifos under heat stress. Furthermore, RR DBM displayed significantly higher basal expressions of hsp69s, hsp72s, hsp20,hsp90,Apaf-1, and caspase-7 at 25°C, but lower induced expressions of hsps and higher induced expressions of Apaf-1,caspase-9, and caspase-7 under heat stress. These results suggest that fitness costs of chlorpyrifos resistance in DBM may partly attribute to excess consumption of energy caused by over production of detoxification enzymes and hsps when the proteins are less demanded at conducive environments but reduced expressions when they are highly demanded by the insects to combat environmental stresses, or to excess expressions of apoptotic genes under heat stress, which results in higher apoptosis. The evolutionary and ecological implications of these findings at global warming are discussed.

Fitness costs and stability of Cry1Ab resistance in sugarcane borer, Diatraea saccharalis (F.)

The sugarcane borer, Diatraea saccharalis (F.), is a major target species of transgenic corn expressing Bacillus thuringiensis (Bt) proteins in South America and the U.S. mid-south region. In this study, the fitness of seven insect genotypes of D. saccharalis were assayed on non-toxic diet, which included a Cry1Ab-susceptible strain (SS-2009), two Cry1Ab-resistant strains (RR-43A(BC), RR-L5B(BC)), and four F1 hybrids (F1-R43A(m)S(f), F1-R43A(f)S(m), F1-R5B(m)S(f), and F1-R5B(f)S(m)). The F1 hybrids were generated by reciprocal crosses of SS-2009 with RR-43ABC and RR-L5BBC, respectively. Biological parameters measured were neonate-to-pupa survivorship, neonate-to-pupa development time, pupal mass, pupa-to-adult emergence rate, and progeny (neonates) production. The overall performance of the two resistant strains and the four F1 genotypes was either similar or better than SS-2009 for all biological parameters measured, suggesting a lack of fitness costs associated with the Cry1Ab resistance traits in both RR-43A(BC) and RR-L5B(BC). In addition, resistance stability was evaluated by measuring the Cry1Ab susceptibility of RR-43A(BC) and RR-L5B(BC) in the absence of selection pressure. Laboratory bioassays showed that larval mortality of the two resistant strains did not significantly increase after selection pressure was removed for 16 generations across all Cry1Ab concentrations assayed. The results provide valuable information on assessing resistance risk and developing effective management strategies for the sustainable use of Bt corn technology.

The impact of strain diversity and mixed infections on the evolution of resistance to Bacillus thuringiensis

Pesticide mixtures can reduce the rate at which insects evolve pesticide resistance. However, with live biopesticides such as the naturally abundant pathogen Bacillus thuringiensis (Bt), a range of additional biological considerations might affect the evolution of resistance. These can include ecological interactions in mixed infections, the different rates of transmission post-application and the impact of the native biodiversity on the frequency of mixed infections. Using multi-generation selection experiments, we tested how applications of single and mixed strains of Bt from diverse sources (natural isolates and biopesticides) affected the evolution of resistance in the diamondback moth, Plutella xylostella, to a focal strain. There was no significant difference in the rate of evolution of resistance between single and mixed-strain applications although the latter did result in lower insect populations. The relative survivorship of Bt-resistant genotypes was higher in the mixed-strain treatment, in part owing to elevated mortality of susceptible larvae in mixtures. Resistance evolved more quickly with treatments that contained natural isolates, and biological differences in transmission rate may have contributed to this. Our data indicate that the use of mixtures can have unexpected consequences on the fitness of resistant and susceptible insects.

Diamondback moth ecology and management: problems, progress, and prospects

Agricultural intensification and greater production of Brassica vegetable and oilseed crops over the past two decades have increased the pest status of the diamondback moth (DBM), Plutella xylostella L., and it is now estimated to cost the world economy US$4-5 billion annually. Our understanding of some fundamental aspects of DBM biology and ecology, particularly host plant relationships, tritrophic interactions, and migration, has improved considerably but knowledge of other aspects, e.g., its global distribution and relative abundance, remains surprisingly limited. Biological control still focuses almost exclusively on a few species of hymenopteran parasitoids. Although these can be remarkably effective, insecticides continue to form the basis of management; their inappropriate use disrupts parasitoids and has resulted in field resistance to all available products. Improved ecological understanding and the availability of a series of highly effective selective insecticides throughout the 1990s provided the basis for sustainable and economically viable integrated pest management (IPM) approaches. However, repeated reversion to scheduled insecticide applications has resulted in resistance to these and more recently introduced compounds and the breakdown of IPM programs. Proven technologies for the sustainable management of DBM currently exist, but overcoming the barriers to their sustained adoption remains an enormous challenge.

Ozone toxicity and walking response of populations of Sitophilus zeamais (Coleoptera: Curculionidae)

Ozone is a recognized alternative to the fumigants methyl bromide and phosphine for the control of stored product insects. However, as with fumigants in general, the potential sublethal effects of ozone on targeted insect species may compromise its efficacy and has yet to be investigated. Here, we determined ozone toxicity of 30 field-collected populations of the maize weevil, Sitophilus zeamais (Coleoptera: Curculionidae), and assessed the walking response of adult insects from these populations to sublethal ozone exposure. Time-mortality toxicity to ozone at 50 ppm concentration in a continuous 2 liter/min flow indicated uniform susceptibility among the populations studied without any indication of ozone resistance (toxicity ratios [at LT50] > two-fold). In contrast, there was significant variation in walking activity among the maize weevil populations, which was not correlated with ozone susceptibility. This was not surprising because of the relatively uniform susceptibility to ozone among the maize weevil populations. Respiration rate affected ozone toxicity but not walking activity, whereas body mass was negatively correlated with walking activity but was not correlated with ozone toxicity. Based on our data, lower respiration rates may potentially lead to reduced ozone uptake whereas larger body mass limits walking activity. Ozone seems a promising alternative fumigant with low short-term risk of resistance development because of the high susceptibility and low variability of response to this compound. Furthermore, ozone reduces walking activity of S. zeamais that implies it likely reduces the chances of insects escaping exposure at the early stages of fumigation.

Effectiveness of the high dose/refuge strategy for managing pest resistance to Bacillus thuringiensis (Bt) plants expressing one or two toxins

To delay resistance development to Bacillus thuringiensis (Bt) plants expressing their own insecticide, the application of the Insect Resistance Management strategy called "High Dose/Refuge Strategy" (HD/R) is recommended by the US Environmental Protection Agency (US EPA). This strategy was developed for Bt plants expressing one toxin. Presently, however, new Bt plants that simultaneously express two toxins are on the market. We used a mathematical model to evaluate the efficiency of the HD/R strategy for both these Bt toxins. As the current two-toxin Bt plants do not express two new Cry toxins but reuse one toxin already in use with a one-toxin plant, we estimated the spread of resistance when the resistance alleles are not rare. This study assesses: (i) whether the two toxins have to be present in high concentration, and (ii) the impact of the relative size of the refuge zone on the evolution of resistance and population density. We concluded that for Bt plants expressing one toxin, a high concentration is an essential condition for resistance management. For the pyramided Bt plants, one toxin could be expressed at a low titer if the two toxins are used for the first time, and a small refuge zone is acceptable.

Effects of entomopathogenic nematodes on evolution of pink bollworm resistance to Bacillus thuringiensis toxin Cry1Ac

The evolution of resistance by pests can reduce the efficacy of transgenic crops that produce insecticidal toxins from Bacillus thuringiensis (Bt). However, fitness costs may act to delay pest resistance to Bt toxins. Meta-analysis of results from four previous studies revealed that the entomopathogenic nematode Steinernema riobrave (Rhabditida: Steinernematidae) imposed a 20% fitness cost for larvae of pink bollworm, Pectinophora gossypiella (Saunders) (Lepidoptera: Gelechiidae), that were homozygous for resistance to Bt toxin Cry1Ac, but no significant fitness cost was detected for heterozygotes. We conducted greenhouse and laboratory selection experiments to determine whether S. riobrave would delay the evolution of pink bollworm resistance to Cry1Ac. We mimicked the high dose/refuge scenario in the greenhouse with Bt cotton (Gossypium hirsutum L.) plants and refuges of non-Bt cotton plants, and in the laboratory with diet containing Cry1Ac and refuges of untreated diet. In both experiments, half of the replicates were exposed to S. riobrave and half were not. In the greenhouse, S. riobrave did not delay resistance. In the laboratory, S. riobrave delayed resistance after two generations but not after four generations. Simulation modeling showed that an initial resistance allele frequency > 0.015 and population bottlenecks can diminish or eliminate the resistance-delaying effects of fitness costs. We hypothesize that these factors may have reduced the resistance-delaying effects of S. riobrave in the selection experiments. The experimental and modeling results suggest that entomopathogenic nematodes could slow the evolution of pest resistance to Bt crops, but only under some conditions.

Effect of temperature on the fitness of a Vip3A resistant population of Heliothis virescens (Lepidoptera: Noctuidae)

Microbial insecticides derived from the soil bacterium Bacillus thuringiensis (Bt) have become increasingly important for pest management. In addition to crystal (Cry) insecticidal protein toxins formed during sporulation, vegetative insecticidal protein (Vip) toxins can be produced during the vegetative phase. Resistance to Cry toxins has been reported in laboratory- and field-selected populations of various Lepidoptera species and several studies have identified fitness costs associated with Cry toxin resistance. Here, fitness costs are examined in the first insect population to be reported with resistance to a Vip toxin, a laboratory-selected Vip3A-resistant subpopulation of the tobacco budworm, Heliothis virescens (L.) (Vip-Sel). The Vip-Sel population showed reduced survival to adult eclosion compared with an unselected subpopulation at all test temperatures, including the culture temperature (25 degrees C). Vip3A resistance was also associated with reduced egg viability and mating success and a lower intrinsic rate of population increase (r(m)) at temperatures below (20 degrees C) and above (30 degrees C) the optimal laboratory culture temperature. The latter findings agree with previous studies, that fitness costs associated with resistance are usually greater under stressful conditions. Such data can help predict the impact of fitness costs on the rate of development of resistance in the field and in the development of resistance management strategies that more fully exploit fitness costs.

Blocking the evolution of insecticide-resistant malaria vectors with a microsporidian

Finding a way to block the evolution insecticide resistance would be a major breakthrough for the control of malaria. We suggest that this may be possible by introducing a stress into mosquito populations that restores the sensitivity of genetically resistant mosquitoes and that decreases their longevity when they are not exposed to insecticide. We use a mathematical model to show that, despite the intense selection pressure imposed by insecticides, moderate levels of stress might tip the evolutionary balance between costs and benefits of resistance toward maintaining sensitivity. Our experimental work with the microsporidian parasite Vavraia culicis infecting two lines of resistant mosquitoes and a sensitive line suggests that it may indeed be possible to stress the mosquitoes in the required way. The mortality of resistant mosquitoes 24 h after exposure to the insecticide was up to 8.8 times higher in infected than in uninfected ones; if mosquitoes were not exposed to the insecticide, resistant mosquitoes infected by the microsporidian lived about half as long as uninfected ones and insecticide-sensitive mosquitoes (with or without the parasite). Our results suggest that biopesticides or other insecticides that interfere with the expression of resistance may help to manage insecticide resistance in programs of malaria control.

Selection for Cry3Bb1 resistance in a genetically diverse population of nondiapausing western corn rootworm (Coleoptera: Chrysomelidae)

Five short-diapause laboratory lines of western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), were selected for resistance to MON863, a variety of corn genetically modified with the Bacillus thuringiensis Berliner (Bt) transgene that expresses the Cry3Bb1 delta-endotoxin. Three of the selected lines were developed by incremental increase in the duration of exposure to MON863 over 11 generations (moderate selected lines). Two selected lines were developed from a control group by constant exposure to MON863 for at least 14 d posthatch over seven generations (intense selected lines). At the end of the experiment, survivorship, as measured by adult emergence, was approximately 4 times higher in each of the selected lines reared on MON863 compared with control lines. Estimates of realized heritabilities (h2) were 0.16 and 0.15 for the moderate and intense selected lines, respectively, and are consistent with h2 estimates reported previously from a variety of pest insects. These lines provide data necessary for evaluating the potential for Bt resistance within diabroticite beetles and will be useful for developing improved insect resistance management strategies.

Successfully resisting a pathogen is rarely costly in Daphnia magna

BACKGROUND

A central hypothesis in the evolutionary ecology of parasitism is that trade-offs exist between resistance to parasites and other fitness components such as fecundity, growth, survival, and predator avoidance, or resistance to other parasites. These trade-offs are called costs of resistance. These costs fall into two broad categories: constitutive costs of resistance, which arise from a negative genetic covariance between immunity and other fitness-related traits, and inducible costs of resistance, which are the physiological costs incurred by hosts when mounting an immune response. We sought to study inducible costs in depth using the crustacean Daphnia magna and its bacterial parasite Pasteuria ramosa.

RESULTS

We designed specific experiments to study the costs induced by exposure to this parasite, and we re-analysed previously published data in an effort to determine the generality of such costs. However, despite the variety of genetic backgrounds of both hosts and parasites, and the different exposure protocols and environmental conditions used in these experiment, this work showed that costs of exposure can only rarely be detected in the D. magna-P. ramosa system.

CONCLUSIONS

We discuss possible reasons for this lack of detectable costs, including scenarios where costs of resistance to parasites might not play a major role in the co-evolution of hosts and parasites.

Adaptive responses and latent costs of multigeneration cadmium exposure in parasite resistant and susceptible strains of a freshwater snail

Population response to anthropogenic activities will be influenced by prior adaptation to environmental conditions. We tested how parasite-resistant and -susceptible strains of the freshwater snail, Biomphalaria glabrata, responded to cadmium and elevated temperature challenges after having been exposed to low-level cadmium continuously for multiple generations. Snails exposed to cadmium for three generations were removed for the fourth generation, and challenged in the fifth generation with (1) chronic cadmium exposure over the entire life cycle; (2) lethal cadmium exposure of adults; and (3) elevated temperature challenge of adults. The parasite susceptible NMRI strain is more cadmium tolerant than the parasite resistant BS90 strain and remained more tolerant than BS90 throughout this study. Additionally, NMRI exhibited greater adaptive capacity for cadmium than BS90 and became more tolerant of both chronic and lethal cadmium challenges, while BS90 became more tolerant of lethal cadmium challenge only. Fitness costs, reflected in population growth rate, were not apparent in fifth generation snails maintained in control conditions. However, costs were latent and expressed as decreased tolerance to a secondarily imposed temperature stress. Adaptation to prior selection pressures can influence subsequent adaptation to anthropogenic stresses and may have associated costs that reduce fitness in novel environments.

Effects of four nematode species on fitness costs of pink bollworm resistance to Bacillus thuringiensis toxin Cry1Ac

Evolution of resistance by pests can reduce the efficacy oftransgenic crops that produce insecticidal toxins from the bacterium Bacillus thuringiensis Berliner (Bt). In conjunction with refuges of non-Bt host plants, fitness costs can delay the evolution of resistance. Furthermore, fitness costs often vary with ecological conditions, suggesting that agricultural landscapes can be manipulated to magnify fitness costs and thereby prolong the efficacy of Bt crops. In the current study, we tested the effects of four species of entomopathogenic nematodes (Steinernematidae and Heterorhabditidae) on the magnitude and dominance of fitness costs of resistance to Bt toxin CrylAc in pink bollworm, Pectinophora gossypiella (Saunders) (Lepidoptera: Gelechiidae). For more than a decade, field populations of pink bollworm in the United States have remained susceptible to Bt cotton Gossypium hirsutum L. producing CrylAc; however, we used laboratory strains that had a mixture of susceptible and resistant individuals. In laboratory experiments, dominant fitness costs were imposed by the nematode Steinernema riobrave Cabanillas, Poinar, and Raulston but no fitness costs were imposed by Steinernema carpocapsae Weiser, Steinernema sp. (ML18 strain), or Heterorhabditis sonorensis Stock, Rivera-Orduño, and Flores-Lara. In computer simulations, evolution of resistance to Cry1Ac by pink bollworm was substantially delayed by treating some non-Bt cotton refuge fields with nematodes that imposed a dominant fitness cost, similar to the cost observed in laboratory experiments with S. riobrave. Based on the results here and in related studies, we conclude that entomopathogenic nematodes could bolster insect resistance management, but the success of this approach will depend on selecting the appropriate species of nematode and environment, as fitness costs were magnified by only two of five species evaluated and also depended on environmental factors.

Effects of host plant and genetic background on the fitness costs of resistance to Bacillus thuringiensis

Novel resistance to pathogens and pesticides is commonly associated with a fitness cost. However, measurements of the fitness costs of insecticide resistance have used diverse methods to control for genetic background and rarely assess the effects of environmental variation. Here, we explored how genetic background interacts with resource quality to affect the expression of the fitness costs associated with resistance. We used a serially backcrossed line of the diamondback moth, Plutella xylostella, resistant to the biopesticide Bacillus thuringiensis, to estimate the costs of resistance for insects feeding on two Brassica species. We found that fitness costs increased on the better-defended Brassica oleracea cultivars. These data were included in two meta-analyses of fitness cost experiments that used standardized protocols (and a common resistant insect stock) but which varied in the methodology used to control for the effects of genetic background. The meta-analysis confirmed that fitness costs were higher on the low-quality host (B. oleracea); and experimental methodology did not influence estimates of fitness costs on that plant species. In contrast, fitness costs were heterogeneous in the Brassica pekinensis studies: fitness costs in genetically homogenized lines were significantly higher than in studies using revertant insects. We hypothesize that fitness modifiers can moderate fitness costs on high-quality plants but may not affect fitness when resource quality is low.

The effect of three environmental conditions on the fitness of cytochrome P450 monooxygenase-mediated permethrin resistance in Culex pipiens quinquefasciatus

BACKGROUND

The evolution of insecticide resistance and persistence of resistance phenotypes are influenced by the fitness of resistance alleles in the absence of insecticide pressure. Experimental determination of fitness is difficult, but fitness can be inferred by measuring changes in allele frequencies in appropriate environments. We conducted allele competition experiments by crossing two highly related strains of Culex pipiens quinquefasciatus mosquitoes. One strain (ISOP450) was permethrin resistant (due to P450-mediated detoxification) and one was a susceptible strain. Allele and genotype frequencies were examined for 12 generations under three environmental conditions: standard laboratory, temephos exposure (an insecticide to which the P450 detoxification mechanism in ISOP450 confers no resistance and which is commonly used in mosquito control programs) and cold temperature stress (mimics the colder temperatures within the habitat of this mosquito).

RESULTS

A fitness cost was inferred for the P450 mechanism in the standard laboratory environment. A greater cost was associated with the temephos exposed environment, suggesting the temephos placed an additional stress on the P450 resistant mosquitoes. No observed cost was associated with the P450 resistance locus in the cold temperature environment, but there was a significant heterozygote advantage. In all environments the fitness of the resistant homozygotes was the lowest.

CONCLUSION

The cytochrome P450-mediated permethrin detoxification resistance in Cx. p. quinquefasciatus can have an associated fitness cost in the absence of permethrin, relative to a susceptible allele. The strength of the cost varies depending on the environmental conditions. P450-mediated resistance is expected to decrease over time if the permethrin application is relaxed and to decrease at an even faster rate if permethrin is replaced with temephos. Additionally, these results indicate that a P450 resistance allele can persist (especially in heterozygotes) in colder temperatures and could potentially be carried into the Culex pipiens hybrid zone.

Fitness costs of insect resistance to Bacillus thuringiensis

Evolution of resistance by insect pests threatens the continued effectiveness of Bacillus thuringiensis (Bt) toxins in sprays and transgenic crops. Fitness costs of Bt resistance occur when, in the absence of Bt toxins, fitness is lower for resistant insects than for susceptible insects. Modeling results show that fitness costs can delay resistance by selecting against Bt-resistant genotypes in refuges where insects are not exposed to Bt toxins. In 77 studies including 18 species, fitness costs were detected in 62% of experiments testing for declines in resistance and in 34% of fitness component comparisons. Mean fitness costs were 15.5% for survival, 7.4% for development time, and 2.5% for mass. Although most fitness costs were recessive, nonrecessive costs can select more strongly against resistance. Because fitness costs vary with ecological conditions, refuges designed to increase the dominance or magnitude of fitness costs could be especially useful for delaying pest resistance.

A primer for using transgenic insecticidal cotton in developing countries

Many developing countries face the decision of whether to approve the testing and commercial use of insecticidal transgenic cotton and the task of developing adequate regulations for its use. In this review, we outline concepts and provide information to assist farmers, regulators and scientists in making decisions concerning this technology. We address seven critical topics: 1) molecular and breeding techniques used for the development of transgenic cotton cultivars, 2) properties of transgenic cotton cultivars and their efficacy against major insect pests, 3) agronomic performance of transgenic cotton in developing countries, 4) factors affecting transgene expression, 5) impact of gene flow between transgenic and non-transgenic cotton, 6) non-target effects of transgenic cotton, and 7) management of pest resistance to transgenic cotton.

Cross-resistance between a Bacillus thuringiensis Cry toxin and non-Bt insecticides in the diamondback moth

BACKGROUND

Bacillus thuringiensis Berliner (Bt) crystal (Cry) toxins are expressed in various transgenic crops and are also used as sprays in integrated pest management and organic agricultural systems. The diamondback moth (Plutella xylostella L.) is a major worldwide pest of crucifer crops and one that has readily acquired field resistance to a broad range of insecticides.

RESULTS

Selection of a subpopulation of the P. xylostella SERD4 population with the pyrethroid deltamethrin increased resistance to both deltamethrin and Cry1Ac relative to an unselected subpopulation. Selection of a second subpopulation with the Bt toxin Cry1Ac also increased resistance to both Cry1Ac and deltamethrin. A complementation test between the Cry1Ac-selected and deltamethrin-selected subpopulations suggested the presence of a common genetic locus or loci that control resistance to both insecticides. A piperonyl butoxide analogue with potent inhibitory activity against insect esterases significantly increased the toxicity of Cry1Ac and deltamethrin against the respective resistant subpopulations, but showed no such synergism with the unselected subpopulation of SERD4.

CONCLUSION

Selection of one resistance phenotype resulted in the simultaneous selection of the other. This phenomenon could be due to a single mechanism acting against both classes of insecticide or to genetically linked, but separate, mechanisms.

Temperature-related fitness costs of resistance to spinosad in the diamondback moth, Plutella xylostella (Lepidoptera: Plutelidae)

Fitness costs associated with resistance genes expressed in the absence of insecticides affect the evolution of insecticide resistance and the outcome of resistance management programmes. However, measurements of fitness costs may not be straightforward as they vary with environmental conditions. The diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae), has developed resistance to spinosad, the first insecticide of the Naturalyte class, after only a few years of field application of this product. In this study, we compared the performance of two homogenous strains of P. xylostella, one susceptible (SS) and the other resistant (RR) to spinosad at an unfavourable, low natural temperature regime, a favourable median-fluctuating temperature regime and an unfavourable high-fluctuating temperature regime. The RR strain showed only marginal fitness cost at the median temperature regime. At the low temperature regime, the RR strain failed to produce any viable offspring, while the SS strain achieved positive population growth. At the high temperature regime, the RR strain showed a 33% decrease in intrinsic rate of increase compared to the SS strain. The results demonstrate that fitness costs of resistance to spinosad are temperature-dependent, increasing in scale at unfavourably low and high temperatures; costs were particularly high at low temperatures. Suggestions for designing effective management programmes are made to delay or avoid development of resistance to spinosad by P. xylostella under different temperature conditions.

Environment-dependent trade-offs between ectoparasite resistance and larval competitive ability in the Drosophila–Macrocheles system

Costs of resistance are expected to contribute to the maintenance of genetic variation for resistance in natural host populations. In the present study, we experimentally test for genetic trade-offs between parasite resistance and larval competitive ability expressed under varying levels of crowding and temperature. Artificial selection for increased behavioral resistance was applied against an ectoparasitic mite (Macrocheles subbadius) in replicate lines of the fruit fly Drosophila nigrospiracula. We then measured correlated responses to selection in larval competitive ability by contrasting replicate selected and control (unselected) lines in the absence of parasitism. Experiments were conducted under variable environmental conditions: two temperatures and three levels of larval density. Our results reveal a negative genetic correlation between resistance and larval-adult survival under conditions of moderate and severe intra-specific competition. At both low and high temperature, percent emergence was significantly higher among control lines than selected lines. This divergence in larval competitive ability was magnified under high levels of competition, but only at low temperature. Hence, the interaction between selection treatment and larval density was modified by temperature. As predicted, larvae experiencing medium and high levels of competition exhibited an overall reduction in female body size compared to larvae at low levels of competition. Female flies emerging from selected lines were significantly smaller than those females from control lines, but this effect was only significant under conditions of moderate to severe competition. These results provide evidence of environment-dependent trade-offs between ectoparasite resistance and larval competitive ability, a potential mechanism maintaining genetic polymorphism for resistance.