Chemical Ecology of the host searching behavior in an Egg Parasitoid: are Common Chemical Cues exploited to locate hosts in Taxonomically Distant Plant Species?

Parasitoids are known to exploit volatile cues emitted by plants after herbivore attack to locate their hosts. Feeding and oviposition of a polyphagous herbivore can induce the emission of odor blends that differ among distant plant species, and parasitoids have evolved an incredible ability to discriminate them and locate their hosts relying on olfactive cues. We evaluated the host searching behavior of the egg parasitoid Cosmocomoidea annulicornis (Ogloblin) (Hymenoptera: Mymaridae) in response to odors emitted by two taxonomically distant host plants, citrus and Johnson grass, after infestation by the sharpshooter Tapajosa rubromarginata (Signoret) (Hemiptera: Cicadellidae), vector of Citrus Variegated Chlorosis. Olfactory response of female parasitoids toward plants with no herbivore damage and plants with feeding damage, oviposition damage, and parasitized eggs was tested in a Y-tube olfactometer. In addition, volatiles released by the two host plant species constitutively and under herbivore attack were characterized. Females of C. annulicornis were able to detect and significantly preferred plants with host eggs, irrespectively of plant species. However, wasps were unable to discriminate between plants with healthy eggs and those with eggs previously parasitized by conspecifics. Analysis of plant volatiles induced after sharpshooter attack showed only two common volatiles between the two plant species, indole and β-caryophyllene. Our results suggest that this parasitoid wasp uses common chemical cues released by many different plants after herbivory at long range and, once on the plant, other more specific chemical cues could trigger the final decision to oviposit.

Natural Enemies of Fall Armyworm Spodoptera frugiperda (Lepidoptera: Noctuidae) in Different Agro-Ecologies

Fall armyworm (FAW) Spodoptera frugiperda (J.E. Smith) and southern armyworm (SAW) Spodoptera eridania (Stoll) have become major threats to crops in Africa since 2016. African governments adopted emergency actions around chemical insecticides, with limited efforts to assess the richness or roles of indigenous natural enemies. Field surveys and laboratory studies were conducted to identify and assess the performance of parasitoids associated with spodopterans in Cameroon. FAW was the most abundant spodopteran pest. Telenomus remus (Nixon), Trichogramma chilonis (Ishi), Charops sp. (Szépligeti), Coccygidium luteum (Cameron), Cotesia icipe (Fernandez & Fiaboe), and Cotesia sesamiae (Cameron) are the first records in the country on spodopterans. Telenomus remus, T. chilonis, C. icipe, and Charops sp. were obtained from both FAW and SAW; C. luteum and C. sesamiae from FAW. The distribution of spodopterans, their endoparasitoids, and parasitism rates varied with host, season and location. In the laboratory, T. remus showed significantly higher parasitism on FAW than SAW, and significant differences in the development parameters between the two host eggs, with shorter development time on FAW. It induced significant non-reproductive mortality on FAW but not on SAW. Developmental parameters showed that C. icipe has a shorter development time compared to other larval parasitoids. Implications for conservative and augmentative biocontrol are discussed.

Host Patch Use and Potential Competitive Interactions Between Two Egg Parasitoids From the Family Scelionidae, Candidate Biological Control Agents of Bagrada hilaris (Hemiptera: Pentatomidae)

Two egg parasitoids from Pakistan, Gryon sp. nr. gonikopalense Sharma (Hymenoptera: Scelionidae) and Trissolcus hyalinipennis Rajmohana & Narendran (Hymenoptera: Scelionidae), are currently being investigated as potential classical biocontrol agents for Bagrada hilaris Burmeister. The former is the most promising because of its ability to attack B. hilaris eggs in soil, but the latter was recently discovered in California. This study detailed the patch use and oviposition behavior of both species and assessed their relative foraging efficiency. We also investigated possible competitive interactions by assessing 1) the occurrence of intra- and interspecific host discrimination, 2) mutual interference between females (extrinsic competition), 3) the outcome of multiparasitism (intrinsic competition), and 4) the effect of competition on host suppression. Our results showed that T. hyalinipennis females tended to probe repeatedly in the same hosts leading to longer host patch time and lower foraging efficiency. Both species avoided conspecific superparasitism, but not multiparasitism. When the two species foraged simultaneously, G. sp. nr. gonikopalense seemed to be slightly superior in extrinsic competition, while neither species seemed to have an advantage in intrinsic competition. Also, neither species could develop as a facultative hyperparasitoid, but they inflicted non-reproductive mortality on eggs containing 4-d-old larvae of their competitor. Lastly, host mortality inflicted by G. sp. nr. gonikopalense and T. hyalinipennis when acting together appeared to be additive. These results suggest that the presence of T. hyalinipennis in California is unlikely to reduce G. sp. nr. gonikopalense efficiency, but will instead improve the biological control of B. hilaris.

Effects of Host Age and Density on the Performance of Apanteles hemara (Hymenoptera: Braconidae), a Larval Endoparasitoid of Spoladea recurvalis (Lepidoptera: Crambidae)

The amaranth leaf-webber, Spoladea recurvalis (Fabricius; Lepidoptera: Crambidae), is a serious pest of Amaranthus sp. in Africa and Asia. Apanteles hemara (Nixon; Hymenoptera: Braconidae) is by far the most important larval endoparasitoid of the amaranth leaf-webber. We examined the effects of host density and age on the biological characteristics of A. hemara. The regression model of the number of hosts supplied to A. hemara against the number of larvae parasitized resulted in a curve corresponding to type II functional response, with a significant increase in the number of hosts parasitized up to the density of 30 hosts before being constant up to 40 hosts. In contrast, the parasitism rate decreased linearly with increasing host densities. Development time, sex ratio, and adult longevity were not significantly affected by host density. The immature parasitoid mortality was significantly higher at higher host densities. Apanteles hemara did not parasitize 7-d-old larvae and beyond, while parasitism was significantly higher among 1- to 2-d-old compared with 3- to 4-d-old larvae. Immature parasitoid mortality was 2.6 times higher in 1- to 2-d-old larvae compared with 5- to 6-d-old larvae. The developmental period of the parasitoid from egg to adult was longest among 1- to 2-d-old larvae and least among 5- to 6-d-old larvae. Nonreproductive mortality was markedly higher among 1- to 2-d-old larvae compared with the older larvae. Adult female A. hemara were significantly larger on 3- to 4-d-old larvae compared with either 1- to 2-d-old or 5- to 6-d-old larvae. We discuss the implications of our results for the interpretation of functional response in parasitoids, mass rearing, conservation, and augmentative biological control of S. recurvalis.

Evidence for trans-generational effects on egg maturation schedules in a syn-ovigenic parasitoid

The lifetime reproductive success of a female parasitoid is limited by (1) host (or time) limitation - the number of hosts available for oviposition during its lifetime; and (2) egg limitation - its egg supply. Host limitation is expected to select for increased longevity and/or foraging efficiency, while increased fecundity is predicted to evolve under egg limitation. If the limiting factor varies, phenotypic plasticity in egg maturation schedules may be advantageous, i.e. adjusting investment in egg production to host availability. In the polyembryonic parasitoid Copidosoma koehleri, environmental conditions experienced during development indeed influence resource allocation to egg maturation. However, whether parasitoids' maternal environment also influences their daughters' egg production has hardly been studied. To address this knowledge gap, we tested whether exposure of C. koehleri females to previously parasitized hosts (signaling intraspecific exploitation competition and risk of host limitation) reduces their daughters' initial egg loads. We presented newly-emerged females with hosts that were either fresh or parasitized by conspecifics. The following day, we exposed both groups to additional fresh hosts, and reared out the daughters of these previously experienced, 24+ h old, individuals. The daughters' egg loads and body sizes were similar under both experimental conditions. Nevertheless, their egg loads were ~30% higher, and body sizes were ~10% lower, than in daughters of just-emerged parasitoids. We suggest that female experience or age, but not conditions associated with host exploitation, trigger maternal effects on the reproductive and developmental physiology of their daughters.

Nonreproductive Effects of Insect Parasitoids on Their Hosts

The main modes of action of insect parasitoids are considered to be killing their hosts with egg laying followed by offspring development (reproductive mortality), and adults feeding on hosts directly (host feeding). However, parasitoids can also negatively affect their hosts in ways that do not contribute to current or future parasitoid reproduction (nonreproductive effects). Outcomes of nonreproductive effects for hosts can include death, altered behavior, altered reproduction, and altered development. On the basis of these outcomes and the variety of associated mechanisms, we categorize nonreproductive effects into ( a) nonconsumptive effects, ( b) mutilation, ( c) pseudoparasitism, ( d) immune defense costs, and ( e) aborted parasitism. These effects are widespread and can cause greater impacts on host populations than successful parasitism or host feeding. Nonreproductive effects constitute a hidden dimension of host-parasitoid trophic networks, with theoretical implications for community ecology as well as applied importance for the evaluation of ecosystem services provided by parasitoid biological control agents.

Impact of differences in nutritional quality of wingless and winged aphids on parasitoid fitness

Winged aphids are described as hosts of lesser quality for parasitoids because a part of their resources is used to produce wings and associated muscles during their development. Host lipid content is particularly important for parasitoid larvae as they lack lipogenesis and therefore rely entirely on the host for this resource. The goal of this study was to determine to what extent winged and wingless aphids differ from a nutritional point of view and whether these differences impact parasitoid fitness, notably the lipid content. We analysed the energetic budget (proteins, lipids and carbohydrates) of aphids of different ages (third instars, fourth instars and adults) according to the morph (winged or wingless). We also compared fitness indicators for parasitoids emerging from winged and wingless aphids (third and fourth instars). We found that in third instars, parasitoids are able to inhibit wing development whereas this is not the case in fourth instars. Both winged instars allow the production of heavier and fattier parasitoids. The presence of wings in aphids seems to have little effect on the fitness of emerging parasitoids and did not modify female choice for oviposition. Finally, we demonstrate that Aphidius colemani, used as a biological control agent, is able to parasitize wingless as well as winged Myzus persicae, at least in the juvenile stages. If the parasitism occurs in third instars, the parasitoid will prevent the aphid from flying, which could in turn reduce virus transmission.

Acceptability and Suitability of Three Liriomyza Species as Host for the Endoparasitoid Halticoptera arduine (Hymenoptera: Pteromalidae)

In the scope of using Halticoptera arduine (Walker; Hymenoptera: Pteromalidae) in a classical biological control program in East Africa, laboratory bioassays were conducted to evaluate the acceptability and suitability of the three economically important Liriomyza leafminer species to the exotic parasitoid. Searching time, number of oviposition attempts, F1 parasitoid developmental period, parasitism rates, sex ratio, host mortality, and body size indices were assessed. H. arduine parasitized and developed successfully in the three Liriomyza species reported in East Africa. Female parasitoids took on average between 10.45 ± 0.83 to 15.80 ± 0.91 (means ± SE) seconds to encounter their first host and made significantly more oviposition attempts on Liriomyza huidobrensis (Blanchard; Diptera: Agromyzidae) than Liriomyza sativae (Blanchard; Diptera: Agromyzidae) and Liriomyza trifolii (Burgess; Diptera: Agromyzidae) (P = 0.0006). Parasitoid development period from egg to adult ranged between 19.32 ± 0.96 and 22.86 ± 0.27 d. Parasitism rate ranged from 27.96 ± 3.86 to 44.10 ± 4.56 in the three host species and was significantly higher in L. huidobrensis than in L. sativae (P = 0.0397). H. arduine did not induce significant nonreproductive host mortality in any of the three Liriomyza hosts. A female-biased parasitoid sex ratio was observed in L. huidobrensis, a balanced sex ratio in L. sativae and a male-biased in L. trifolii. Parasitoids progeny were significantly larger on L. huidobrensis for both tibia and wing length than L. sativae and L. trifolii (P = 0.0109 and P = 0.0192, respectively). The implication for the environmentally friendly management of Liriomyza leafminers in East Africa is discussed.

Acceptability and Suitability of Three Liriomyza Leafminer Species as Host for the Endoparasitoid Chrysocharis flacilla (Hymenoptera: Eulophidae)

Liriomyza leafminers represent important threats to the horticulture sector in East Africa. Parasitism rates of local parasitoids are reported to be low and the endoparasitoid, Chrysocharis flacilla (Walker) (Hymenoptera: Eulophidae), was introduced in Kenya for a classical biological control program. Acceptability and suitability bioassays were conducted on the three economically important Liriomyza species in Africa (Liriomyza huidobrensis (Blanchard) (Diptera: Agromyzidae), Liriomyza sativae (Blanchard) (Diptera: Agromyzidae), and Liriomyza trifolii (Burgess) (Diptera: Agromyzidae). Foraging behavior, developmental time, sex ratio, parasitism rates, host pupal mortality, and body indices of C. flacilla were assessed. Results showed that the three Liriomyza leafminer species were accepted and suitable to C. flacilla. Foraging time was significantly shorter on L. trifolii than on L. sativae and L. huidobrensis. Ninety-eight per cent of females successfully oviposited in the three-host species. Female parasitoids were significantly aggressive in attempting to oviposit on L. huidobrensis than on L. sativae and L. trifolii. High parasitism rates ranging between 73 and 78% were observed from the three Liriomyza hosts, but no significant difference among hosts. C. flacilla-induced significant nonreproductive pupal mortality ranging from 23 to 35%, an attribute rare among endoparasitoids. In all three Liriomyza hosts, the parasitoid progeny was female biased. Parasitoid development period ranged between 16 and 24 d. Female parasitoids reared on L. huidobrensis were significantly bigger than those reared on L. sativae and L. trifolii. The acceptance to local Liriomyza leafminers and high host suppression ability is potential for considerations of C. flacilla in the management of Liriomyza spp. in Africa.

Phenotypic plasticity of pre-adult egg maturation in a parasitoid: Effects of host-starvation and brood size

Larvae of parasitoid wasps develop on a single arthropod host, and often face resource limitation that induces a tradeoff between egg maturation and somatic growth. Part of the variation in the growth-reproduction allocation was shown to be heritable, but how the larval developmental environment affects this allocation is not well-known. Detection of life history tradeoffs is often facilitated under stress conditions. We therefore exposed developing female larvae of the polyembryonic parasitoid Copidosoma koehleri (Hymenoptera: Encyrtidae) to laboratory manipulations aimed to restrict host resources (either host-starvation or high larval density). We compared the females’ body sizes and egg loads shortly after adult emergence (<24 h) to those of closely related control females, which developed at a lower larval density within non-starved hosts. Host-starvation reduced the females’ body sizes but not their initial egg loads. Females that experienced high larval density produced more eggs but were similar in body size to the low-density controls. Thus, the relative allocation to reproduction increased in response to both manipulations of host condition. Developmental duration and longevity were similar in all treatments. The negative correlation between body size and reproductive allocation, observed in the host-starvation treatment, is compatible with previous evidence from other parasitoids. In the high larval density treatment, however, reproductive allocation increased while body size was maintained, suggesting that the higher density increased rather than limited host resources per developing parasitoid female. The additional host resources that were diverted into egg production possibly resulted from increased feeding and body mass gain by hosts parasitized by large broods of wasps. Our results demonstrate phenotypic plasticity in resource allocation between growth and reproduction in a developing parasitoid. This plasticity may contribute to an adaptive balance between longevity and mobility vs. fecundity during the adult stage.

Superparasitism, immune response and optimum progeny yield in the gregarious parasitoid Palmistichus elaeisis

BACKGROUND

The subsequent deposition of an egg clutch by a female parasitoid into a host already parasitised either by itself or a conspecific (i.e. superparasitism) is a counterintuitive adaptive strategy, particularly considering the female parasitoid's ability to recognise the parasitised hosts. Such a scenario suggests that the adaptive value of superparasitism depends on the number of clutches laid in the same host, with consequences for parasitoid progeny yield. Here, we tested whether such is the case for the gregarious parasitoid Palmistichus elaeisis and explored its underlying basis.

RESULTS

Allowing female parasitoids to lay multiple egg clutches in a single melonworm host pupa, parasitoid progeny and fitness exhibited a peak or optimum at three egg clutches laid per host pupa. In addition, haemocyte count, encapsulation and melanisation decreased with the number of egg clutches laid per host pupa.

DISCUSSION

An optimum number of three clutches laid per host pupa was detected for P. elaeisis. As immune response via haemocyte production, encapsulation and melanisation decreased with the number of clutches laid per host, the higher parasitoid yield and fitness observed is the likely consequence of a compromised immune response coupled with an accommodative (i.e. scramble) larval competitive strategy allowing enough resources for optimum balance of parasitoid number and quality produced. © 2017 Society of Chemical Industry.

Sex allocation in a polyembryonic parasitoid with female soldiers: an evolutionary simulation and an experimental test

Parasitoid wasps are convenient subjects for testing sex allocation theory. However, their intricate life histories are often insufficiently captured in simple analytical models. In the polyembryonic wasp Copidosoma koehleri, a clone of genetically identical offspring develops from each egg. Male clones contain fewer individuals than female clones. Some female larvae develop into soldiers that kill within-host competitors, while males do not form soldiers. These features complicate the prediction of Copidosoma's sex allocation. We developed an individual-based simulation model, where numerous random starting strategies compete and recombine until a single stable sex allocation evolves. Life-history parameter values (e.g., fecundity, clone-sizes, larval survival) are estimated from experimental data. The model predicts a male-biased sex allocation, which becomes more extreme as the probability of superparasitism (hosts parasitized more than once) increases. To test this prediction, we reared adult parasitoids at either low or high density, mated them, and presented them with unlimited hosts. As predicted, wasps produced more sons than daughters in all treatments. Males reared at high density (a potential cue for superparasitism) produced a higher male bias in their offspring than low-density males. Unexpectedly, female density did not affect offspring sex ratios. We discuss possible mechanisms for paternal control over offspring sex.