Fitness consequences of body-size-dependent host species selection in a generalist ectoparasitoid

Sublethal effects of nine insecticides on Drosophila suzukii and its major pupal parasitoid Trichopria drosophilae

BACKGROUND

Although the pupal parasitoid Trichopria drosophilae is used in conservative and augmentative biocontrol of Drosophila suzukii infestations, current pest management strategies mostly rely on multiple insecticide applications. In this context, the aim of the study was to investigate the baseline toxicity of nine insecticides on D. suzukii larvae and their multiple sublethal effects (LC10 ) on immature stages of the pest feeding on contaminated diet and T. drosophilae developing within the intoxicated host.

RESULTS

Chlorpyriphos and azadirachtin showed the lowest and the highest LC10 , the values of which were 9.78 × 1013 and 1.46 × 103 times lower than their recommended label field rate, respectively. Among tested insecticides, imidacloprid, malathion and dimethoate were the only treatments that did not affect the juvenile development time of D. suzukii, while spinosad and the organophosphates chlorpyriphos and dimethoate did not influence fly pupal size. No sublethal effects were recorded on T. drosophilae degree of infestation (DI) and juvenile development time. On the contrary, cyazypyr and dimethoate negatively affected the success of parasitism (SP) and the number of progeny of the pupal parasitoid, in association with malathion for the first parameter and spinosad for the fertility. Compared to the untreated control, more female progeny emerged following azadirachtin exposure, while dimethoate caused the opposite effect. Imidacloprid, lambda-cyhalothrin and spinetoram decreased hind tibia length of emerged parasitoids.

CONCLUSION

This study provides new insights on the (eco)toxicological profile of nine insecticides and new information needed to support the deployment of T. drosophilae in the field within the sustainable management techniques against D. suzukii. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Performance of Chouioia cunea reared from a coleopteran alternative host as a biocontrol agent against the invasive lepidopteran pest, Hyphantria cunea

BACKGROUND

Chouioia cunea is a generalist pupal endoparasitoid. Native to Eurasia, the parasitoid has been mass-reared on an alternative lepidopteran host (Antheraea pernyi) to successfully control the exotic invasive lepidopteran pest Hyphantria cunea in China. To solicit more accessible hosts, this study evaluated the potential of an easily cultured coleopteran beetle (Tenebrio molitor) as a substitute for rearing C. cunea through comparing the relative performance of the parasitoids reared from both alternative hosts on H. cunea.

RESULTS

Compared with those reared from A. pernyi, the parasitoids reared from T. molitor (i.e., T. molitor vs. A. pernyi groups) performed equally well in terms of parasitism rate (94.4 vs. 88.9%), number of offspring produced per parasitized host (278 vs. 286), and female body length (1.334 vs. 1.351 mm), hind-tibia length (0.322 vs. 0.324 mm) and number of mature oocytes in the ovarioles (171 vs. 187), or even better based on offspring pre-emergence time (16.0 vs 16.9 days) and percentages of emerged offspring (99.8 vs. 99.1%) and female offspring (97.1 vs. 91.3%). Flight performance testing indicated that young C. cunea adults emerged from T. molitor had a similar percentage of actively flying wasps (76.9 vs. 72.9%) and a lower percentage of inactive wasps (2.3 vs. 10.6%) when compared to those reared from A. pernyi.

CONCLUSION

Given the remarkable adaptability of C. cunea and the vast availability of T. molitor as a common resource insect worldwide, this indigenous parasitoid could be mass-reared on T. molitor to improve the prospect of biological control of H. cunea in its invaded regions. © 2022 Society of Chemical Industry.

Potential effects of RNA interference of Asian longhorned beetle on its parasitoid

BACKGROUND

It is important to understand how non-target insects such as parasitoids may be impacted directly or indirectly by RNA interference with double-stranded RNA (dsRNA) that has emerged as a novel pest control tool. We examined the potential effects of a dsRNA targeting an inhibitor of apoptosis (IAP) of the Asian longhorned beetle Anoplophora glabripennis on its gregarious larval ectoparasitoid Ontsira mellipes, directly on adult wasp's survival via injection of 4 μg of dsIAP per wasp, and indirectly on the detectability and suitability of host larvae injected with 2, 4 or 8 μg of dsIAP per larva.

RESULTS

Compared with no injection or injection with a control dsGFP targeting a region of gene coding for a green fluorescence protein (GFP), dsIAP did not affect adult wasp's survival. Ontsira mellipes locates hosts in the wood by sensing their movement. Host larvae did not completely cease movement after the injection of dsIAP and were still detected and parasitized. Clutch size was reduced and only 3.8% of the parasitoid offspring developed into adults on host larvae treated at the highest dose. However, clutch size was not affected and 25.5% of the parasitoid offspring developed into adults on host larvae treated at the lowest dose. The fitness of developed wasps (development time, sex ratio, body size, and fecundity) was not affected when compared to the control treatments. No dsIAP was detected in parasitoid larvae.

CONCLUSION

The results show no direct effect of the dsRNA on its parasitoid, but the potential indirect effect of dsRNA-affected host on the parasitoid, which may be minimized through optimizing dsRNA dosage to promote compatible applications of both management options for this invasive forest pest. © 2022 Society of Chemical Industry. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.

Strong effects of food quality on host life history do not scale to impact parasitoid efficacy or life history

Parasitoids are small insects, (e.g., small wasps or flies) that reproduce by laying eggs on or within host arthropods. Parasitoids make up a large proportion of the world's biodiversity and are popular agents of biological control. Idiobiont parasitoids paralyze their hosts upon attack and thus are expected to only target hosts large enough to support offspring development. Host resources generally impact host attributes and life histories including size, development, and life span. Some argue slow host development in response to resource quality increases parasitoid efficacy (i.e., a parasitoid's ability to successfully reproduce on or within a host) due to longer host exposure to parasitoids. However, this hypothesis is not always supported and does not consider variation in other host traits in response to resources that may be important for parasitoids (e.g., variation in host size is known to impact parasitoid efficacy). In this study we test whether trait variation within host developmental stages in response to host resources is more important for parasitoid efficacy and life histories than trait variation across host developmental stages. We exposed seed beetle hosts raised on a food quality gradient to mated female parasitoids and measured the number of hosts parasitized and parasitoid life history traits at the scale of host stage- and age-structure. Our results suggest host food quality does not cascade to impact idiobiont parasitoid life histories despite large food quality effects on host life history. Instead, variation in host life histories across host developmental stages better predicts parasitoid efficacy and life histories, suggesting finding a host in a specific instar is more important for idiobiont parasitoids than finding hosts on or within higher quality resources.

Friend or Foe: Symbiotic Bacteria in Bactrocera dorsalis-Parasitoid Associations

Parasitoids are promising biocontrol agents of the devastating fruit fly, Bactrocera dorsalis. However, parasitoid performance is a function of several factors, including host-associated symbiotic bacteria. Providencia alcalifaciens, Citrobacter freundii, and Lactococcus lactis are among the symbiotic bacteria commonly associated with B. dorsalis, and they influence the eco-physiological functioning of this pest. However, whether these bacteria influence the interaction between this pest and its parasitoids is unknown. This study sought to elucidate the nature of the interaction of the parasitoids, Fopius arisanus, Diachasmimorpha longicaudata, and Psyttlia cosyrae with B. dorsalis as mediated by symbiotic bacteria. Three types of fly lines were used: axenic, symbiotic, and bacteria-mono-associated (Lactococcus lactis, Providencia alcalifaciens, and Citrobacter freundii). The suitable stages of each fly line were exposed to the respective parasitoid species and reared until the emergence of adult flies/parasitoids. Thereafter, data on the emergence and parasitoid fitness traits were recorded. No wasps emerged from the fly lines exposed to P. cosyrae. The highest emergence of F. arisanus and D. longicaudata was recorded in the L. lactis fly lines. The parasitoid progeny from the L. lactis and P. alcalifaciens fly lines had the longest developmental time and the largest body size. Conversely, parasitoid fecundity was significantly lower in the L. lactis lines, whereas the P. alcalifaciens lines significantly improved fecundity. These results elucidate some effects of bacterial symbionts on host-parasitoid interactions and their potential in enhancing parasitoid-oriented management strategies against B. dorsalis.

Searching of Underground Host Patches by a Pupal Parasitoid

When hosts are distributed in discrete patches, ways in which parasitoids search and move between patches affect variability in parasitism risk among hosts and host-parasitoid population dynamics. This study examined the patch searching behavior of the solitary pupal parasitoid Dirhinus giffardii (Silvestri) (Hymenoptera: Chalcididae) on its host Bactrocera dorsalis (Hendel) (Diptera: Tephritidae) which pupates underground. In a series of two laboratory experiments, host patches were created by burying pupae in peat moss, and the foraging behavior of the parasitoid was recorded. If D. giffardii can detect underground patches, the parasitoid would preferentially exploit high quality patches where the quality of a patch is represented by the number of unparasitized hosts in the patch. The first experiment investigated the effect of patch size (i.e., number of hosts) and host status (whether hosts are parasitized or unparasitized) on patch searching behavior. Results showed parasitoids were more likely to exploit a large patch than a small patch regardless of host status. The second experiment examined the effect of relative locations of patches by establishing three patches (one large patch and two small patches with unequal inter-patch distances from the large patch). The probability of parasitism was lower for the small patch close to the large patch than the small patch far from the large patch. The parasitism patterns described in the experiments have important implications on the distribution of parasitism risk among hosts and population dynamics.

Chinese oak silkworm Antherae pernyi egg, a suitable factitious host for rearing eupelmid egg parasitoids

BACKGROUND

Eupelmid egg parasitoids in the genera Anastatus and Mesocomys are important biological control agents for lepidopterous and hemipterous pests worldwide. The egg of Chinese oak silkworm Antheraea pernyi has been widely used for mass rearing of Trichogramma parasitoids. This study evaluated the suitability and optimal use methods of A. pernyi egg as a factitious host for the rearing of six eupelmid egg parasitoids (Anastatus fulloi, Anastatus gansuensis, Anastatus japonicus, Anastatus meilingensis, Mesocomys albitarsis and Mesocomys trabalae). Each parasitoid was tested for its oviposition preference and offspring performance on various differently treated host eggs (extracted from virgin moths or laid naturally by virgin or mated moths, and washed or unwashed prior to the use) in both no-choice and choice tests.

RESULTS

All treated A. pernyi eggs were readily parasitized by the six parasitoids. In general, A. gansuensis and M. trabalae preferred washed over unwashed eggs regardless of the fertilization status of host eggs, A. fulloi and A. meilingensis parasitized more unfertilized than fertilized host eggs, and A. japonicus and M. albitarsis did not show a preference among differently treated host eggs. Host egg treatment did not significantly affect offspring fitness (development time, survival, sex ratio and body size) nor reproductive potential of developed adult females for each parasitoid species, except for M. albitarsis (whose females contained more eggs when reared from unfertilized than fertilized host eggs).

CONCLUSION

Results suggest that manually extracted, unfertilized and washed A. pernyi eggs are most suitable for mass rearing of these eupelmid egg parasitoids in biological control programs.

Courtship behavior and identification of a sex pheromone in Ibalia leucospoides (Hymenoptera: Ibaliidae), a larval parasitoid of Sirex noctilio (Hymenoptera: Siricidae)

BACKGROUND

Ibalia leucospoides (Hymenoptera: Ibaliidae) is a larval parasitoid that has been widely introduced as a biological control agent for the invasive woodwasp,Sirex noctilio (Hymenoptera: Siricidae) in the Southern Hemisphere. In this study, the courtship behavior and identificaion of sex pheromones are described for I. leucospoides under laboratory conditions.

METHODS

For courtship behavior, both sexes were observed in a wire mesh observation cylinder (75 cm length ×10 cm diameter) for 15 minutes. The female body washes were analyzed using Gas Chromatography- Electroantennographic Detection (GC-EAD). Then the EAD-active compounds were tentatively identified using GC-Mass Spectrometry (GC-MS) and examined in olfactometer assays.

RESULTS

The courtship behavior included rhythmic lateral movements, mounting, head-nodding cycles in males, and wing-fanning in females. GC-EAD analysis of female body washes with male antennae revealed seven compounds which elicited antennal responses, four of which are straight-chain alkanes (C23, C25, C26, and C27). The identities of these alkanes were confirmed by matching the retention times, mass spectra, and male antennal activity to those of commercially obtained chemicals. In olfactometer assays, a blend of the four straight-chain alkanes was attractive to I. leucospoides males, and there was no response to blends that lacked any of these four compounds. Female body wash was no more attractive than the four-component blend. The ratios of EAD-active components differ between hydrocarbon profiles from males and females.

CONCLUSION

This study is the first investigation of cuticular hydrocarbons in the family Ibaliidae. It provides evidence that the ubiquitous alkanes (C23, C25, C26, and C27) in sex-specific ratios attract I. leucospoides males.

Optimal Conditions for Diapause Survival of Aprostocetus fukutai, an Egg Parasitoid for Biological Control of Anoplophora chinensis

Simple Summary

Diapause is a critical state of an insect’s life cycle when it undergoes arrestment of growth and/or reproduction to survive adverse environmental conditions and/or food shortage. Aprostocetus fukutai is a specialist egg parasitoid of the citrus longhorned beetle, Anoplophora chinensis, a high-risk invasive woodboring pest. The parasitoid overwinters as diapausing mature larva in the host egg and emerges in early summer in synchrony with the egg-laying of A. chinensis. Here, we determined the optimal conditions for diapause survival of this parasitoid. We showed that the parasitoid had a low (36.7%) diapause survival rate inside host eggs laid on potted plants due to desiccation or tree wound defense response under semi-natural conditions. Under laboratory rearing conditions, when parasitized host eggs were extracted from wood, the parasitoid did not survive at low humidity (44% RH). Survival rate increased with humidity, reaching the highest at 100% RH. Survival rate also increased with increasing chilling period temperature from 2 to 12.5 °C. Post-diapause developmental time decreased with increased humidity or temperature, but the reproductive fitness of the parasitoid was not significantly affected by the temperature regimes. Overall, high humidity (100% RH) and mild temperatures (12.5 °C) are the most suitable survival conditions for the diapausing parasitoid and thus should be used in laboratory rearing.

Abstract

Aprostocetus fukutai is a specialist egg parasitoid of the citrus longhorned beetle Anoplophora chinensis, a high-risk invasive pest of hardwood trees. The parasitoid overwinters as diapausing mature larvae within the host egg and emerges in early summer in synchrony with the egg-laying peak of A. chinensis. This study investigated the parasitoid’s diapause survival in parasitized host eggs that either remained in potted trees under semi-natural conditions in southern France or were removed from the wood and held at four different humidities (44, 75, 85–93 and 100% RH) at 11 °C or four different temperature regimes (2, 5, 10 and 12.5 °C) at 100% RH in the laboratory. The temperature regimes reflect overwintering temperatures across the parasitoid’s geographical distribution in its native range. Results show that the parasitoid resumed its development to the adult stage at normal rearing conditions (22 °C, 100% RH, 14L:10D) after 6- or 7-months cold chilling at both the semi-natural and laboratory conditions. It had a low survival rate (36.7%) on potted plants due to desiccation or tree wound defense response. No parasitoids survived at 44% RH, but survival rate increased with humidity, reaching the highest (93.7%) at 100% RH. Survival rate also increased from 21.0% at 2 °C to 82.8% at 12.5 °C. Post-diapause developmental time decreased with increased humidity or temperature. There was no difference in the lifetime fecundity of emerged females from 2 and 12.5 °C. These results suggest that 100% RH and 12.5 °C are the most suitable diapause conditions for laboratory rearing of this parasitoid.

A Comparison of Fitness-Related Traits in the Coleopteran Parasitoid Dastarcus helophoroides (Coleoptera: Bothrideridae) Reared on Two Factitious Hosts

The coleopteran parasitoid Dastarcus helophoroides (Fairmaire) (Coleoptera: Bothrideridae) is considered an efficient preventative method against the damage caused by a large number of cerambycid pests in China and other East Asian countries. Mass rearing of this parasitoid depends on screening appropriate factitious hosts. In this paper, the fitness-related traits of this parasitic beetle were explored using pupae of the two tenebrionid beetles, Tenebrio molitor Linnaeus and Zophobas morio Fabricius (Coleoptera: Tenebrionidae) as factitious hosts. The results showed that in dual-choice trials, D. helophoroides larvae preferred to attack Z. morio pupae. In no-choice trials, D. helophoroides reared on Z. morio pupae displayed significantly higher pupation and emergence rate, longer developmental duration of larval and larval-pupal stages, larger adult body size, shorter preoviposition period, more egg masses and egg production, and higher relative expected reproduction than those reared on T. molitor pupae. These results demonstrate that Z. morio is a more suitable factitious host for mass rearing of D. helophoroides.

Optimizing Parasitoid and Host Densities for Efficient Rearing of Ontsira mellipes (Hymenoptera: Braconidae) on Asian Longhorned Beetle (Coleoptera: Cerambycidae)

Ontsira mellipes Ashmead is a gregarious larval ectoparasitoid of woodboring cerambycids. It is native to North America but can readily attack the exotic Asian longhorned beetle, Anoplophora glabripennis (Motschulsky). This study aimed to develop an efficient rearing system for this parasitoid, as a potential novel association biocontrol agent for the beetle, by investigating the effects of different densities of host (two, three, or four larvae) and parasitoid (one, two, three, four, five, six, seven, and eight female wasps) on Ontsira's parasitization efficiency and reproductive outcomes. Results showed that overall parasitism and total numbers of parasitized hosts or progeny produced increased with host and/or parasitoid densities, but the number of parasitized hosts or progeny produced per female parasitoid decreased with parasitoid density at each given host density. Nonlinear regression indicated a consistent pattern of mutual interference as parasitoid density increased. Additional experiments showed that superparasitism (indirect interference) did not occur probably because the parasitoid detects hosts through vibration cues from host feeding and attacked (thus paralyzed) hosts are no longer detectable. Thus, the interference probably results from direct or exploitative competition. Interestingly, female parasitoids responded to increased parasitoid density with a significant increase in clutch size. Overall, per capita parasitization efficiency or reproductive outcomes were optimized at a low parasitoid-host ratio but with large group size of hosts and parasitoids. Therefore, an optimal combination of exposing three or four parasitoids to four hosts is proposed for efficient mass-rearing of this parasitoid.

Reproductive Traits of Ontsira mellipes (Hymenoptera: Braconidae), a North American Parasitoid, as a Novel Biological Control Agent for Exotic Anoplophora glabripennis (Coleoptera: Cerambycidae)

Ontsira mellipes Ashmead is a gregarious larval ectoparasitoid of woodboring cerambycids that is native to North America but can readily attack the exotic Asian longhorned beetle, Anoplophora glabripennis (Motschulsky). To evaluate the potential of the parasitoid as a novel association control agent for the pest beetle, this study investigated some key reproductive traits of the parasitoid, including egg maturation dynamics, and host size preference and suitability in association with the beetle. Results showed that female wasps emerged with a substantial portion (38%) of their lifetime complement of mature eggs and matured eggs rapidly, reaching a peak 4-6 d post-eclosion. The number of mature eggs was positively related to the female wasp's body size. Oviposition prompted production of more mature eggs by young female wasps. The parasitoid did not show a significant preference for large over small hosts in a choice test. Host size did not affect the parasitoid's offspring survival, developmental time, or sex ratio. However, clutch size increased with increasing host size. Female wasps that developed from large hosts had larger body size and consequently a higher mature egg load than those reared from small hosts. Neither longevity nor the total number of parasitized hosts over a female's lifetime was affected by the female's size, but the total number of offspring produced per female increased with the female's size. These results have important implications for improving rearing and field-release strategies as well as understanding the ecological mechanisms underlying host size selection in gregarious parasitoids.

Asymmetries affecting aggressive contests between solitary parasitoids: the effect of host species

Conflicts in animals are usually resolved based on asymmetries, where contest winners are often those that value a resource the most and/or those who have the greatest potential to retain it. In parasitoid wasps, contests between females determine which individual exploits hosts for offspring production. Previous studies on solitary parasitoids rarely considered the role of biotic factors generating phenotypic variation that could influence the strength of asymmetries. Some parasitoid species parasitize host species of various sizes, producing offspring that vary considerably in size and potentially fighting ability. In this study, we reared the egg endoparasitoid Telenomus podisi (Hymenoptera: Scelionidae) on two host species to measure the effect of body size on contest resolution and how it interacts with ownership and resource value (RV) asymmetries. Our results showed that ownership status best predicts the final contest outcome when similar-sized wasps fight over hosts. The frequency and outcomes of individual fights structuring contests were better explained by the difference in the number of eggs laid in the hosts by each female at a given time. When contestants varied in body size, larger intruders frequently dislodged small owners regardless of ownership and RV asymmetries. These results imply that body size is an important factor to consider in solitary parasitoid contests and that it can overshadow the effects of other asymmetries. Our study suggests that host community diversity could have a direct effect on parasitoid contests and that biotic communities, through their effects on animal phenotypes, may play an underrated role in contest resolution.

Peppermint essential oil inhibits Drosophila suzukii emergence but reduces Pachycrepoideus vindemmiae parasitism rates

Spotted Wing Drosophila (Drosophila suzukii; Matsumura) is an invasive fruit fly with the ability to oviposit in a broad range of agriculturally valuable fruits. Volatile organic compounds (VOCs) produced by botanical oils may reduce D. suzukii’s attraction to hosts and decrease survival, but it is unknown whether their efficacy varies across D. suzukii life stages or affects the survival and success of higher trophic levels. Through a series of laboratory bioassays, we evaluated the effects of peppermint (Mentha arvensis L.) oil produced VOCs on D. suzukii survival and the survival of and parasitism rates by a pupal parasitoid wasp, Pachycrepoideus vindemmiae (Rondani). First, we determined whether fumigation with peppermint oil VOCs at the pupal stage reduced adult emergence, and whether this depended on environmental conditions (i.e. soil moisture). Second, we evaluated whether fumigation with peppermint oil VOCs reduced or enhanced parasitism by the pupal parasitoid and whether this depended on the timing of peppermint oil VOC exposure (i.e. before, during, or after parasitoid access). Fumigation with VOCs of 4.5 mg of peppermint oil reduced D. suzukii emergence under moist soil conditions but dry soil had a similar effect on reducing adult emergence as peppermint oil presence. Peppermint oil VOC fumigation was toxic to adult P. vindemmiae, but developing P. vindemmiae were unaffected by peppermint oil VOC fumigation. Using peppermint essential oil as a fumigant may reduce D. suzukii emergence from the pupal stage. However, this could negatively impact P. vindemmiae dependent on the timing of application.

Life History and Host Preference of Trichopria drosophilae from Southern China, One of the Effective Pupal Parasitoids on the Drosophila Species

This study aims to evaluate several life-history traits of a T. drosophilae population from southern China and its parasitic preference of three Drosophila species. For mated T. drosophilae females, the mean oviposition and parasitization period were 27.20 and 37.80 d, respectively. The daily mean parasitization rate was 59.24% per female and the lifetime number of emerged progeny was 134.30 per female. Trichopria drosophilae females survived 37.90 and 71.61 d under host-provided and host-deprived conditions, respectively. To assess the potential for unmated reproduction in T. drosophilae, the mean oviposition and parasitization period of unmated females was 22.90 and 47.70 d, respectively. They had a daily mean parasitization rate of 64.68%, produced a total of 114.80 offspring over their lifetime, and survived 52 d. Moreover, T. drosophilae showed a preference towards D. suzukii based on the total number of emerged offspring under a choice test. Our findings indicate that T. drosophilae from southern China appears to be suitable for the control of D. suzukii in invaded areas, due to its reproductive potential.

Density-dependent distribution of parasitism risk among underground hosts

Variation in parasitism risk among hosts can arise from between-patch and within-patch factors, but considerably less information is known about the latter. This study investigated how distributions of the oriental fruit fly Bactrocera dorsalis influenced its parasitism by the pupal parasitoid Dirhinus giffardii in the laboratory. Because B. dorsalis larvae pupate underground, pupation depth was considered as an important factor that affects the risk of parasitism. When the density of B. dorsalis larvae was varied (1, 10, and 100 larvae per arena), average pupation depth increased with the density. When the depth of pupae was manipulated, the rate of parasitism differed by depths. Parasitism at 0 cm differed from the random parasitoid model expectation, but parasitism at 1 cm was not different from the model expectation. Few pupae at 2 cm were parasitized. In another experiment, when pupae were simultaneously presented at 0 cm and 1 cm depths, parasitism at 1 cm was weakened by the presence of puape at 0 cm. These results imply that the density of the host influences pupation depth as well as the distribution of parasitism and plays an important role in host-parasitoid dynamics.

Influence of the Rearing Host on Biological Parameters of Trichopria drosophilae, a Potential Biological Control Agent of Drosophila suzukii

Trichopria drosophilae is a pupal parasitoid that can develop in a large number of drosophilid host species including the invasive pest Drosophila suzukii, and is considered a biological control agent. We investigated the influence of the rearing host on the preference and performance of the parasitoid, using two different strains of T. drosophilae, reared on D. melanogaster or D. suzukii for approximately 30 generations. Host switching was employed to assess the impact of host adaptation on T. drosophilae performance. In a no-choice experimental setup, T. drosophilae produced more and larger offspring on the D. suzukii host. When given a choice, T. drosophilae showed a preference towards D. suzukii, and an increased female ratio on this host compared to D. melanogaster and D. immigrans. The preference was independent from the rearing host and was confirmed in behavioral assays. However, the preference towards D. suzukii increased further after a host switch from D. melanogaster to D. suzukii in just one generation. Our data indicate that rearing T. drosophilae for several years on D. melanogaster does not compromise its performance on D. suzukii in the laboratory. However, producing a final generation on D. suzukii prior to release could increase its efficacy towards the pest.

A Game of Russian Roulette for a Generalist Dinoflagellate Parasitoid: Host Susceptibility Is the Key to Success

Marine microbial interactions involving eukaryotes and their parasites play an important role in shaping the structure of phytoplankton communities. These interactions may alter population densities of the main host, which in turn may have consequences for the other concurrent species. The effect generalist parasitoids exert on a community is strongly dependent on the degree of host specificity. Parvilucifera sinerae is a generalist parasitoid able to infect a wide range of dinoflagellates, including toxic-bloom-forming species. A density-dependent chemical cue has been identified as the trigger for the activation of the infective stage. Together these traits make Parvilucifera-dinoflagellate hosts a good model to investigate the degree of specificity of a generalist parasitoid, and the potential effects that it could have at the community level. Here, we present for the first time, the strategy by which a generalist dinoflagellate parasitoid seeks out its host and determine whether it exhibits host preferences, highlighting key factors in determining infection. Our results demonstrate that in its infective stage, P. sinerae is able to sense potential hosts, but does not actively select among them. Instead, the parasitoids contact the host at random, governed by the encounter probability rate and once encountered, the chance to penetrate inside the host cell and develop the infection strongly depends on the degree of host susceptibility. As such, their strategy for persistence is more of a game of Russian roulette, where the chance of survival is dependent on the susceptibility of the host. Our study identifies P. sinerae as a potential key player in community ecology, where in mixed dinoflagellate communities consisting of hosts that are highly susceptible to infection, parasitoid preferences may mediate coexistence between host species, reducing the dominance of the superior competitor. Alternatively, it may increase competition, leading to species exclusion. If, however, highly susceptible hosts are absent from the community, the parasitoid population could suffer a dilution effect maintaining a lower parasitoid density. Therefore, both host community structure and host susceptibility will determine infectivity in the field.

Soil microarthropods and their relationship to higher trophic levels in the Pedregal de San Angel Ecological Reserve, Mexico

Soil fauna is essential for ecosystem dynamics as it is involved in biogeochemical processes, promotes nutrient availability, and affects the animal communities associated with plants. In this study, we examine the possible relationship between the soil microarthropod community on foliage production and quality of the shrub Pittocaulon praecox. We also examine the arthropods associated to its foliage, particularly the size of the main herbivores and of their natural enemies, at two sites with contrasting vegetation cover and productivity. The diversity of soil microarthropods was assessed from soil samples collected monthly under P. praecox individuals over 13 mo. Specimens collected were identified to species or morphospecies. Shrub foliage productivity was evaluated through the amount of litter produced. Resource quality was assessed by the mean content (percentage by weight) of N, C, S, and P of 30 leaves from each shrub. The mean size of herbivores and their natural enemies were determined by measuring 20 adult specimens of each of the most abundant species. We found a higher species richness of soil microarthropods and foliar arthropods in the open site, although the diversity of foliage arthropods was lower in the closed site. Shrubs growing in the closed site tend to produce more, larger, and nutritionally poorer (lower nitrogen content) leaves than open site. Herbivores and their natural enemies were also larger in the closed site. We found a significant positive relationship between the diversity and species richness of foliar arthropods and the nitrogen content of leaves. In general, species richness and diversity of both the foliar and soil fauna, as well as the size of organisms belonging to higher trophic levels, were affected by vegetation cover and primary productivity at each site. These findings highlight the need to simultaneously consider at least four trophic levels (soil organisms, plants, herbivores, and natural enemies) to better understand the functioning of these systems and their responses to environmental changes.

Hyperparasitism in a Generalist Ectoparasitic Pupal Parasitoid, Pachycrepoideus vindemmiae (Hymenoptera: Pteromalidae), on Its Own Conspecifics: When the Lack of Resource Lead to Cannibalism

Hyperparasitism is a normal behavior of parasitoids, which often happens among species. Conspecific hyperparasitism, such as some kinds of heteronomous hyperparasitic behaviors, has been only reported in some species belonging to Aphelinidae. In this article, the conspecific hyperparasitism of Pachycrepoideus vindemmiae (Pteromalidae) is reported, with Drosophila puparia as hosts. Hosts were exposed to P. vindemmiae females twice to parasitism with nine, twelve, and fifteen day intervals between the two exposures. None of the infested hosts emerged more than one offspring, and emergence of parasitoid offspring occurred in two obvious events, synchronously with the exposure time intervals, which suggested that offspring emerging during the first and second events would come from the primary and secondary parasitoids, respectively, and the inference with the developmental duration of offspring also indicated this. With two P. vindemmiae strains that could be identified by a simple sequence repeat marker, the above speculation of the origin of those offspring emerging during the two events was confirmed. Dissection of hosts exposed twice revealed a cannibalism behavior of larvae from the secondary foundresses on the primary conspecific pupae. Our results suggested a conspecific hyperparasitism behavior of the secondary parasitoids on the primary conspecifics. Measures showed a reduced body size for the adults from the conspecific hyperparasitism. Foundresses from the conspecific hyperparasitism had less fitness variables than those from primary parasitism, with shorter longevity, less life time fecundity, lower values of infestation degree, and lower success rate of parasitism. However, when the parasitoids from the conspecific hyperparasitism met healthy Drosophila puparia, their offspring would recover to normal size. Frequency of the conspecific hyperparasitism behavior enhanced with the decreasing of proportion of healthy hosts in the oviposition patch. The conspecific hyperparasitism of P. vindemmiae on the primary conspecifics would be helpful to last the population when healthy hosts are absent in the oviposition patch.

Tri-trophic movement of carotenoid pigments from host plant to the parasitoid of a caterpillar

Insect parasitoids normally produce white colored eggs. Habrobracon gelechiae (Hymenoptera: Braconidae) is a gregarious ectoparasitoid of various caterpillars. We found that adult female H. gelechiae lays yellow colored eggs when its larvae developed from host larvae of Choristoneura rosaceana and Epiphyas postvittana (both Lepidoptera: Tortricidae) that were fed green plant leaves, but white colored eggs when these same host larvae species were fed non-plant diets. This study investigated the causes of egg color in H. gelechiae and the possible consequences in terms of parasitoid fitness resulting from differential egg color. Using high-performance liquid chromatography we demonstrated that the yellow coloration resulted from the uptake of carotenoid plant pigments (mainly lutein and β-carotene) that were initially ingested by the caterpillar larvae from plant leaves, later absorbed by the parasitoid larvae (F0) feeding on the host and carried over to the adult parasitoids, and finally translocated to the eggs (F1) of the parasitoids. The amount of plant pigments consumed by the parasitoid larvae (F0) affected the intensity of the yellow color of the parasitoid's eggs (F1). Similarly, egg color was affected by the adult female parasitoids lifetime egg production and deposition rate. Further tests suggest that the observed differences in egg color did not have a genetic basis and did not affect egg viability or fitness. To our best knowledge, this is the first report of a tri-trophic and multi-stage translocation of carotenoid plant pigments in parasitoids. We discuss possible evolutionary significance and putative functions of the absorption of plant pigments by parasitoid species.

Efficiency of host utilisation by coleopteran parasitoid

In insect larvae, optimising food utilisation with respect to available meals and time is essential for achieving maximum adult body size, which is a relevant proxy of fitness. We studied the efficiency of food conversion, body size, mortality, and development time in a solitary idiobiont ectoparasitoid, Brachinus explodens (Coleoptera: Carabidae), reared in the laboratory on the pupae of another carabid genus, Amara. The efficiency of conversion index (ECI - ratio of ingested to assimilated food) was, on average, 54.1±1.1% (n=76), with a minimum of 26.9% and a maximum of 81.6%. The rate of increase in biomass gained (W(gained)) with biomass of the host was constant in females, but it decreased in males over the range of host body mass. Females, therefore, grew heavier from hosts of the same mass compared to males. Body length increased with the host mass and was correlated with W(gained) identically for both sexes. Mortality was unaffected by the host mass, but it significantly increased below 20°C. In contrast, the development time of the feeding phase of the larva increased with the host mass at 20.3 and 23.7°C, but it remained unaffected at 26.9°C and in all three temperatures considering pupal development. W(gained) increased with development time up to ca. 8 days of larval feeding at 23.7°C. To our knowledge, our data are the first on food utilisation in solitary idiobiont coleopteran ectoparasitoids, and they present the highest values of ECI in insects.

Effect of larval host food substrate on egg load dynamics, egg size and adult female size in four species of braconid fruit fly (Diptera: Tephritidae) parasitoids

Life history theory predicts that individuals will allocate resources to different traits so as to maximize overall fitness. Because conditions experienced during early development can have strong downstream effects on adult phenotype and fitness, we investigated how four species of synovigenic, larval-pupal parasitoids that vary sharply in their degree of specialization (niche breadth) and life history (Diachasmimorpha longicaudata, Doryctobracon crawfordi, Opius hirtus and Utetes anastrephae), allocate resources acquired during the larval stage towards adult reproduction. Parasitoid larvae developed in a single host species reared on four different substrates that differed in quality. We measured parasitoid egg load at the moment of emergence and at 24 h, egg numbers over time, egg size, and also adult size. We predicted that across species the most specialized would have a lower capacity to respond to changes in host substrate quality than wasps with a broad host range, and that within species, females that emerged from hosts that developed in better quality substrates would have the most resources to invest in reproduction. Consistent with our predictions, the more specialized parasitoids were less plastic in some responses to host diet than the more generalist. However, patterns of egg load and size were variable across species. In general, there was a remarkable degree of reproductive effort-allocation constancy within parasitoid species. This may reflect more "time-limited" rather than "egg-limited" foraging strategies where the most expensive component of reproductive success is to locate and handle patchily-distributed and fruit-sequestered hosts. If so, egg costs, independent of degree of specialization, are relatively trivial and sufficient resources are available in fly larvae stemming from all of the substrates tested.

Size-mediated adaptive foraging: a host-selection strategy for insect parasitoids

Foraging models are useful tools for generating predictions on predator-prey interactions, such as habitat or diet choice. However, the majority of studies attempting to explain adaptive behaviour using optimality criteria have assumed that there is no trait (e.g. size) variation among individual consumers or their prey. Hymenopteran parasitoids that attack the free-living stages of their host are an ideal system for studying the influence of body size on host selection because of the wide range of adult parasitoid sizes coupled with the defensive capabilities of their hosts. We report here our application of an experimentally parameterized host selection model to investigate the influence of parasitoid body size on the range of acceptable host instar classes. Using a demographic model, we compared the efficiency of parasitoids using an optimal host selection strategy against parasitoids using an indiscriminate host selection strategy over a range of different parasitoid body sizes. Net fitness accrual of parasitoids and the impact of host instar selection on aphid recruitment were assessed on different stage-structured aphid populations. Our results demonstrate that optimal host selection allows larger parasitoids to utilize a wider range of hosts. However, smaller parasitoids receive the greatest benefits from selecting hosts optimally by utilizing a restricted range of small, poorly defended hosts when they are abundant. We argue that the correlation between flexible host selection behaviour and adult body size may be a general phenomenon that applies to the majority of hymenopteran parasitoids that attack free-living, well-defended hosts. The potential of within-generation behavioural interactions to impact between-generation dynamics in host-parasitoid populations are discussed.

The impact of host aggressiveness on sex allocation by the gregarious parasitoid wasp Cotesia glomerata (L.)

Parasitoid wasps lay female eggs or a high proportion of female eggs in favourable host insects because female wasps require many more resources during their development. Many studies have tested the effects of host physiological status on the sex allocation of parasitoids, but few have attempted to test the effects of host behavioural traits. Cotesia glomerata is a gregarious parasitoid wasp that lays eggs in caterpillars of pierid butterflies. The brood sex ratio in C. glomerata females that attacked aggressive host caterpillars was compared with that in females that attacked less aggressive hosts. The male ratio was higher when C. glomerata attacked aggressive Pieris brassicae caterpillars than when it attacked less aggressive Pieris rapae crucivora caterpillars. However, when C. glomerata females were induced to oviposit in anaesthetized P. brassicae caterpillars, the male ratio in their offspring was significantly lower than when they attacked unanaesthetized caterpillars. C. glomerata was attacked by aggressive host caterpillars during oviposition bouts. It is likely that this aggressive host behaviour disturbed the fertilization process in ovipositing C. glomerata females. These results suggest that a behavioural defence by host caterpillars affects sex allocation in the parasitoid wasp C. glomerata.

Resource acquisition, allocation, and utilization in parasitoid reproductive strategies

Parasitoids display remarkable inter- and intraspecific variation in their reproductive and associated traits. Adaptive explanations have been proposed for many of the between-trait relationships. We present an overview of the current knowledge of parasitoid reproductive biology, focusing on egg production strategies in females, by placing parasitoid reproduction within physiological and ecological contexts. Thus, we relate parasitoid reproduction both to inter- and intraspecific patterns of nutrient allocation, utilization, and acquisition, and to key aspects of host ecology, specifically abundance and dispersion pattern. We review the evidence that resource trade-offs underlie several key intertrait correlations and that reproductive and feeding strategies are closely integrated at both the physiological and the behavioral levels. The idea that parasitoids can be divided into capital-breeders or income-breeders is no longer tenable; such terminology is best restricted to the females' utilization of particular nutrients.

Tri-trophic consequences of UV-B exposure: plants, herbivores and parasitoids

In this paper we demonstrate a UV-B-mediated link between host plants, herbivores and their parasitoids, using a model system consisting of a host plant Brassica oleracea, a herbivore Plutella xylostella and its parasitoid Cotesia plutellae. Ultraviolet-B radiation (UV-B) is a potent elicitor of a variety of changes in the chemistry, morphology and physiology of plants and animals. Recent studies have demonstrated that common signals, such as jasmonic acid (JA), play important roles in the mechanisms by which plants respond to UV-B and to damage by herbivores. Plant responses elicited by UV-B radiation can affect the choices of ovipositing female insects and the fitness of their offspring. This leads to the prediction that, in plants, the changes induced as a consequence of UV damage will be similar to those elicited in response to insect damage, including knock-on effects upon the next trophic level, predators. In our trials female P. xylostella oviposited preferentially on host plants grown in depleted UV-B conditions, while their larvae preferred to feed on tissues from UV-depleted regimes over those from UV-supplemented ones. Larval feeding patterns on UV-supplemented tissues met the predictions of models which propose that induced defences in plants should disperse herbivory; feeding scars were significantly smaller and more numerous--though not significantly so--than those on host plant leaves grown in UV-depleted conditions. Most importantly, female parasitoids also showed a clear pattern of preference when given the choice between host plants and attendant larvae from the different UV regimes; however, in the case of the female parasitoids, the choice was in favour of potential hosts foraging on UV-supplemented tissues. This study demonstrates the potential for UV-B to elicit a variety of interactions between trophic levels, most likely mediated through effects upon host plant chemistry.

Covariance of phenotypically plastic traits induces an adaptive shift in host selection behaviour

Flexibility in adult body size allows generalist parasitoids to use many host species at a cost of producing a range of adult sizes. Consequently, host selection behaviour must also maintain a level of flexibility as adult size is related to capture efficiency. In the present study, we investigated covariance of two plastic traits--size at pupation and host size selection behaviour-using Aphidius ervi reared on either Acyrthosiphon pisum or Aulacorthum solani, generating females of disparate sizes. Natal host was shown to change the ranking of perceived host quality with relation to host size. Parasitoids preferentially attacked hosts that corresponded to the size of the second instar of their natal host species. This resulted in optimal host selection behaviour when parasitoids were exposed to the same host species from which they emerged. Parasitoid size was positively correlated with host size preference, indicating that females use relative measurements when selecting suitable hosts. These coadapted gene complexes allow generalist parasitoids to effectively use multiple host species over several generations. However, the fixed nature of the behavioural response, within a parasitoid's lifetime, suggests that these traits may have evolved in a patchy host species environment.

Two different life-history strategies determine the competitive outcome between Dirhinus giffardii (Chalcididae) and Pachycrepoideus vindemmiae (Pteromalidae), ectoparasitoids of cyclorrhaphous Diptera

Dirhinus giffardii Silvestri and Pachycrepoideus vindemmiae Rondani are solitary parasitoids attacking puparia of many cyclorrhaphous flies. They are not typical ectoparasitoids, as they feed on host pupae within puparia that develop from the exoskeleton of host larvae. Dirhinus giffardii did not kill its host until the parasitoid egg developed into a larva, while P. vindemmiae permanently paralysed its host at the time of oviposition. As a result, ovipositing into a young host puparium (< 1 day old) in which the host pupa has not yet fully formed resulted in complete death of offspring in P. vindemmiae, but D. giffardii, although suffering higher mortality than in older host puparia, still showed a level of successful development. In a choice experiment, both parasitoids preferred to attack 2- to 3-day-old puparia in which the host pupae had fully formed, rather than 1-day-old host puparia. Pachycrepoideus vindemmiae always prevailed in competition because it injected venom that not only paralysed the host, but also caused the death of D. giffardii larvae in multi-parasitized hosts. Dirhinus giffardii preferred to attack unparasitized hosts rather than hosts previously parasitized by P. vindemmiae, while P. vindemmiae did not show a preference between unparasitized hosts and hosts previously parasitized by D. giffardii.