Effects of Pesticides on Red Rot of Planted Sugarcane

Red rot, caused by Colletotrichum falcatum, is an important constraint to sugarcane production. In Louisiana, red rot primarily affects planted seed-cane and is more severe when billets (stalk sections) are planted rather than whole stalks. At planting, application of seed-treatment pesticides, particularly a combination of a fungicide and the insecticide thiamethoxam, has improved stand establishment and increased yields in billet plantings in Louisiana. However, information on the effect of chemicals on disease development is lacking. Greenhouse experiments were conducted to evaluate stalk rot symptom severity and initial plant growth for billets dip-treated with a combination of the fungicides azoxystrobin and propiconazole, thiamethoxam, a combination of both fungicides and the insecticide, and, as a control, untreated billets. Reductions in disease severity recorded for different treatments were similar for billets inoculated with the fungus or exposed to natural inoculum. Disease severity was consistently reduced by the combination treatment, while reductions resulting from treatment with fungicides and insecticide alone were variable. Reductions occurred for both internode and node rot severity. The effects of pesticide treatments on plant growth after 6 weeks were minor; however, there was evidence of disease adversely affecting germination, particularly for nontreated billets exposed to natural inoculum, where germination was reduced by one third. The treatments that reduced disease severity prevented this reduction. The results provide evidence that reduction in disease severity is an important contributor to the stand establishment and yield improvements observed for treated billets in field experiments.

Systemic insecticides for control of stem gall wasp in highbush blueberry

The gall wasp, Hemadas nubilipennis Ashmead, is a pest of highbush and lowbush blueberry and can pose a challenge to control with foliar sprays due to adult activity being during bloom and because larval development is within plant tissues. We hypothesized that systemic insecticides that move within the blueberry vascular system would reach areas where H. nubilipennis eggs are laid, causing larval mortality. Three application methods, crown injection, soil drench, and foliar spray were applied to potted 'Jersey' blueberry bushes at 50% and 100% rates to quantify systemic residue concentrations in shoots and leaves. Additionally, systemic insecticides were evaluated for control of gall wasps using single-shoot bioassays and measuring larval mortality at 0.01%, 0.1%, 1%, and 10% of field rate provided within a floral pick. Systemic insecticides tested in both studies included imidacloprid, flupyradifurone, and spirotetramat. The potted bush residue study determined that insecticides moved from three tested sites of entry: the roots, crown cavity, and foliage. Results from the shoot bioassays found that the mean percent larval survival of H. nubilipennis was negatively correlated with the concentration of AI detected in galls. Imidacloprid and spirotetramat were found to have the greatest potential for control of H. nubilipennis due to mortality in the shoot bioassays and similar residue concentrations in the potted bush studies to shoot bioassays. Future research should evaluate systemic insecticides applied in highbush blueberry plantings for control of H. nubilipennis using the bioassay mortality assessment method developed in this study.

Effectiveness of Systemic Insecticide Dog Treatment for the Control of Chagas Disease in the Tropics

Chagas disease, caused by Trypanosoma cruzi and transmitted by triatomines, can lead to severe cardiac issues and mortality in many mammals. Recent studies have shown that systemic insecticide treatment of dogs is highly effective in killing triatomines. Here, we assessed the impact of dog treatment on T. cruzi transmission. We developed a mathematical model of T. cruzi transmission among triatomines, dogs, humans, and rodents. We used the model to evaluate the impact of dog treatment regimens on T. cruzi transmission dynamics to determine their effectiveness in reducing T. cruzi infection among hosts. We show that a 3-month treatment regimen may reduce T. cruzi incidence among humans by 59-80% in a high transmission setting, and 26-82% in a low transmission setting. An annual treatment may reduce incidence among humans by 49-74% in a high transmission setting, and by 11-76% in a low transmission setting. However, dog treatment may substantially increase T. cruzi prevalence among dogs if dog consumption of dead triatomines increases. Our model indicates that dog treatment may reduce T. cruzi infections among humans, but it may increase infections in dogs. Therefore, a holistic approach targeting different hosts is necessary for Chagas elimination.

The Impact of Systematic Insecticides Against Emerald Ash Borer on Phenology of Urban Ash Trees

The continued threat of emerald ash borer (Agrilus planipennis; EAB) to North American ash trees (Fraxinus spp.) has necessitated the use of systemic insecticide treatments as a primary control strategy against EAB in urban centers. Altered tree phenology due to systemic insecticides could mediate nontarget effects on other insect species, such as seed weevils or leaf-feeders, but whether such injections alter phenological events has not been studied. This study assessed the effects of systemic injections of emamectin benzoate or azadirachtin relative to untreated controls on the spring and fall phenology of mature green ash trees in Saint Paul, MN, USA from fall 2017 to spring 2019. EAB was first detected in this area in 2009. Trees showed minor, visible signs of EAB infestation at study initiation, but not mortality. We examined six phenological events: bud swelling, budburst, flowering, leaf out, leaf color change, and leaf abscission using a visual survey protocol. The timing of phenological events was similar across the different treatments for all but two of events; budburst and flowering. Budburst and flowering occurred 7 d and 5 d earlier, respectively, in treated trees than untreated trees. Given symptoms observed, we posit that delays in these events in untreated trees were due to infestations of EAB and the treatments of emamectin benzoate or azadiractin simply preserved the original phenology. The results from this study suggest that systemic insecticides may mitigate changes in ash tree phenology such as delayed leaf out that may be early symptoms of emerald ash borer.

Effects of emamectin benzoate trunk injections on protection of neighboring ash trees against emerald ash borer (Coleoptera: Buprestidae) and on established biological control agents

The emerald ash borer (EAB), Agrilus planipennis Fairmaire, is the most destructive invasive pest on ash (Fraxinus spp.) in the United States. We determined whether ash trees injected with emamectin benzoate (EB) could protect untreated neighboring ash trees. We also determined whether the selective treatment of ash trees with EB injections had adverse effects on the establishment of introduced larval parasitoids Tetrastichus planipennis Yang and Spathius galinae Belokobylskij & Strazenac. In experiment one, trees were treated with EB and then retreated 3 years later. Five years post initial treatment, we found that 90% of treated ash trees retained healthy crowns, significantly higher than those of untreated control ash trees (16%). For experiment two, trees only received one treatment of EB and after 2 years 100% of treated ash trees retained healthy crowns, significantly higher than those of untreated ash trees (50%). In both experiments, we found that distance from the central EB-treated tree was not a significant predictor for tree health or presence of EAB exit holes. Although distance from the EB-treated trees appeared to have a significant positive relationship with woodpecker feeding signs on neighboring trees, such relationships did not result in significant differences in the proportion of neighboring ash trees retaining healthy crowns between EB treatment and control plots. The introduced EAB parasitoids appeared to have established equally well between treatment and control plots. Findings are discussed in the context of integration of EB trunk injection with biological control for protection of North American ash against EAB.

Seed treatment for managing fall armyworm as a defoliator and cutworm on maize: plant protection, residuality, and the insect life history

BACKGROUND

The highly polyphagous and invasive fall armyworm (FAW, Spodoptera frugiperda) can feed on different plant parts of host crops, damaging whorls and stalks in early maize growth stages. Systemic insecticide seed treatment (IST) could minimize this damage, although the residual efficacy may vary with the plant tissue damaged. Using damage rating scales and artificial infestation in controlled conditions, we determined the potential of IST against FAW attacking maize whorl leaves or the stalk base.

RESULTS

Chlorantraniliprole, cyantraniliprole, or thiodicarb + imidacloprid IST similarly killed > 80% FAWs for 1 or 2 weeks after plant emergence depending on the plant tissue attacked. The residual efficacy (i.e. time after plant emergence sustaining > 80% larval mortality) lasted from the first to the eleventh day (VE-V3 maize growth stages), while for cutworm on the maize stalk base, it lasted 3-7 days after plant emergence (V1-V2 stages). In terms of damage, the ISTs lasted 15 days after emergence (V4 stage) for FAW on whorl leaves and 10 days (V3 stage) for FAW feeding on the stalk base. The larvae surviving on the seed-treated plants underwent sublethal effects in growth and development, reducing insect fitness.

CONCLUSION

Diamide or carbamate + neonicotinoid seed treatments kill FAW larvae on maize whorls or stalks in favorable edaphoclimatic and insecticide-susceptibility conditions. The cumulative impacts of systemic IST on aboveground insect pests go beyond mortality. The ISTs studied can be valuable against FAW in maize, for instance, to help protect varieties that may not express sufficient insect resistance in maize early growth stages.

Impact of Imidacloprid Soil Drenching on Survival, Longevity, and Reproduction of the Zoophytophagous Predator Podisus maculiventris (Hemiptera: Pentatomidae: Asopinae)

Systemic insecticides when applied as seed treatments or soil drenches are often more toxicologically selective for natural enemies than target pests. This may not be the case, however, for omnivorous predators, which are at risk of extended exposure to systemically applied pesticides through ingestion while feeding on treated plants for nutrients or water. Such exposure may kill or have sublethal consequences for these natural enemies, compromising their role as biocontrol agents of agricultural pest species. The spined soldier bug, Podisus maculiventris (Say) (Hemiptera: Pentatomidae: Asopinae), is an important zoophytophagous biocontrol agent (i.e., able to substitute zoophagy by phytophagy for survival) that may be exposed to systemic insecticides in many agricultural systems. We, therefore, examined effects on P. maculiventris following exposure to cabbage plants subject to soil-drench treatments with imidacloprid, a systemic neonicotinoid insecticide. Predator survival, development, body weight, and reproduction were recorded. Imidacloprid significantly affected nymph survival and adult emergence, but not duration of the nymphal period or adult body weight. At one-twentieth the recommended field rate for whitefly and aphid management, imidacloprid treatments reduced longevity, fecundity, and fertility of female predators. These findings demonstrate that soil treatments with systemic insecticide can negatively impact zoophytophagous natural enemies.

Optimising systemic insecticide use to improve malaria control

Background

Long-lasting insecticidal nets and indoor residual sprays have significantly reduced the burden of malaria. However, several hurdles remain before elimination can be achieved: mosquito vectors have developed resistance to public health insecticides, including pyrethroids, and have altered their biting behaviour to avoid these indoor control tools. Systemic insecticides, drugs applied directly to blood hosts to kill mosquitoes that take a blood meal, offer a promising vector control option. To date, most studies focus on repurposing ivermectin, a drug used extensively to treat river blindness. There is concern that overdependence on a single drug will inevitably repeat past experiences with the rapid spread of pyrethroid resistance in malaria vectors. Diversifying the arsenal of systemic insecticides used for mass drug administration would improve this strategy’s sustainability.

Methods

Here, a review was conducted to identify systemic insecticide candidates and consolidate their pharmacokinetic/pharmacodynamic properties. The impact of alternative integrated vector control options and different dosing regimens on malaria transmission reduction are illustrated through mathematical model simulation.

Results

The review identified drugs from four classes commonly used in livestock and companion animals: avermectins, milbemycins, isoxazolines and spinosyns. Simulations predicted that isoxazolines and spinosyns are promising candidates for mass drug administration, as they were predicted to need less frequent application than avermectins and milbemycins to maintain mosquitocidal blood concentrations.

Conclusions

These findings will provide a guide for investigating and applying different systemic insecticides to achieve more effective and sustainable control of malaria transmission.

Sublethal and transgenerational effects of thiamethoxam applied to cotton seed on Chrysoperla externa and Harmonia axyridis

BACKGROUND

Thiamethoxam, when applied as a seed treatment, can contaminate plant products, such as extrafloral nectar, and have non-target effects on beneficial arthropods. This study assessed the non-target effects of thiamethoxam applied to cotton seed on the life history parameters of the predators Chrysoperla externa (Neuroptera: Chrysopidae) and Harmonia axyridis (Coleoptera: Coccinellidae).

RESULTS

Exposure of C. externa larvae to plants grown from thiamethoxam-treated seeds caused sublethal and transgenerational effects. Thiamethoxam treatment doubled the proportion of pharate adults and reduced egg fertility in C. externa F0 and F1 generations. In addition, the insecticide prolonged pupal developmental time in the C. externa F1 generation. Thiamethoxam treatment also had a transgenerational effect on exposed H. axyridis larvae, reducing pupal survival in the F1 generation. In the adult bioassay, thiamethoxam treatment reduced egg fertility of C. externa, prolonged the larval period, and reduced both fecundity and egg fertility of the F1 generation. Thiamethoxam also caused transgenerational effect on H. axyridis adults, reducing larval survival of the F1 generation.

CONCLUSION

Thiamethoxam seed treatment was harmful for both predators, but C. externa was more affected by the insecticide than H. axyridis. © 2018 Society of Chemical Industry.

Relative Toxicity of Spirotetramat to Riptortus pedestris (Hemiptera: Alydidae) and its Egg Parasitoids

Spirotetramat, a lipid biosynthesis inhibitor, is effective against sucking insect pests but harmless to insect natural enemies. As spirotetramat can be registered for the management of sucking insect pests such as aphids and bugs in soybeans, we evaluated the insecticide against Riptortus pedestris (Fabricius) (Hemiptera: Alydidae), one of the most important soybean pests in Korea, as well as its effect on two of its important egg parasitoids, Ooencyrtus nezarae Ishii (Hymenoptera: Encyrtidae) and Gryon japonicum (Ashmead; Hymenoptera: Platygastridae). Oral toxicities of five concentrations of spirotetramat (1.00, 0.50, 0.25, 0.13, and 0.06 ml/liter) were tested by feeding these test solutions to insects for 24 h after 12 h of starvation. The lethal median concentration (LC50) for second instars of the bean bug was 0.3 ml/liter after 48 h while values for fourth instars and adults were 9.2 and 19.0 ml/liter, respectively. The median lethal time (LT50) for bean bugs when exposed to a concentration of 0.50 ml/liter was 1.2-1.5 times less than that of the control, while in G. japonicum and O. nezarae it was 1.1-1.2 times less than the control. These results show that spirotetramat is less toxic to the egg parasitoids of bean bug than to bean bug itself and would thus be useful in an integrated management program for this pest.

Effectiveness of Fipronil as a Systemic Control Agent Against Xenopsylla cheopis (Siphonaptera: Pulicidae) in Madagascar

Fipronil was evaluated as a systemic control agent for the rat flea Xenopsylla cheopis (Rothschild), the main vector of Yersinia pestis (Yersin), the causative agent of plague, in Madagascar. The effectiveness of fipronil as a systemic control agent against X. cheopis was assessed by determining the toxicity values of the "Lethal Dose 50" (LD50). Two techniques were used to evaluate the systemic action of the insecticide on the vector: 1) an artificial feeding device filled with blood-fipronil mixture from which X. cheopis was fed and 2) rodent hosts, Rattus norvegicus (Berkenhout) and Rattus rattus (L.), which fed on fipronil-treated bait. As a standardized control method, the susceptibility of X. cheopis to fipronil was evaluated by exposure to impregnated paper within World Health Organization (WHO) insecticide test protocol to compare its effect to the systemic activity of the studied insecticide. Results showed that when administered in a systemic way, fipronil appears to be more effective: the toxicity level was evaluated to be ninefold higher compared with the WHO test. Compared with other methods, which require indiscriminate dusting of rodent burrows and human dwellings, fipronil applied in a systemic way enables the direct targeting of the plague vector. Thus, this method appears to be a superior alternative to fipronil-dusting for the control of the main plague vector in Madagascar. However, subsequent tests in the field are necessary to confirm the suitability of fipronil administration in a systemic way on large scales.

Does Drought Increase the Risk of Insects Developing Behavioral Resistance to Systemic Insecticides?

Increases in severity and frequency of drought periods, average global temperatures, and more erratic fluctuations in rainfall patterns due to climate change are predicted to have a dramatic impact on agricultural production systems. Insect pest populations in agricultural and horticultural systems are also expected to be impacted, both in terms of their spatial and temporal distributions and in their status as pest species. In this opinion-based article, we discuss how indirect effects of drought may adversely affect the performance of systemic insecticides and also lead to increased risk of insect pests developing behavioral insecticide resistance. We hypothesize that more pronounced drought will decrease uptake and increase the magnitude of nonuniform translocation of systemic insecticides within treated crop plants, and that may have two concurrent consequences: 1) reduced pesticide performance, and 2) increased likelihood of insect pests evolving behavioral insecticide resistance. Under this scenario, pests that can sense and avoid acquisition of lethal dosages of systemic insecticides within crop plants will have a selective advantage. This may lead to selection for insect behavioral avoidance, so that insects predominantly feed and oviposit on portions of crop plants with low concentration of systemic insecticide. Limited research has been published on the effect of environmental variables, including drought, on pesticide performance, but we present and discuss studies that support the hypothesis described above. In addition, we wish to highlight the importance of studying the many ways environmental factors can affect, directly and indirectly, both the performance of insecticides and the risk of target insect pests developing resistance.

Sex allocation theory reveals a hidden cost of neonicotinoid exposure in a parasitoid wasp

Sex allocation theory has proved to be one the most successful theories in evolutionary ecology. However, its role in more applied aspects of ecology has been limited. Here we show how sex allocation theory helps uncover an otherwise hidden cost of neonicotinoid exposure in the parasitoid wasp Nasonia vitripennis. Female N. vitripennis allocate the sex of their offspring in line with Local Mate Competition (LMC) theory. Neonicotinoids are an economically important class of insecticides, but their deployment remains controversial, with evidence linking them to the decline of beneficial species. We demonstrate for the first time to our knowledge, that neonicotinoids disrupt the crucial reproductive behaviour of facultative sex allocation at sub-lethal, field-relevant doses in N. vitripennis. The quantitative predictions we can make from LMC theory show that females exposed to neonicotinoids are less able to allocate sex optimally and that this failure imposes a significant fitness cost. Our work highlights that understanding the ecological consequences of neonicotinoid deployment requires not just measures of mortality or even fecundity reduction among non-target species, but also measures that capture broader fitness costs, in this case offspring sex allocation. Our work also highlights new avenues for exploring how females obtain information when allocating sex under LMC.