Factors influencing parasitism of Trichogramma dendrolimi on eggs of the Asian corn borer, Ostrinia furnacalis

Laboratory studies were made to determine the capacity of Trichogramma dendrolimi to parasitize eggs of Ostrinia furnacalis, as affected by the rearing host species, substrate of host eggs, host age, original locality of host populations, and cold storage of host eggs. Wasps reared from eggs of Antheraea pernyi showed parasitic capacity on eggs of O. furnacalis on average twice as high as that of the wasps reared from eggs of Corcyra cephalonica. When the age of O. furnacalis eggs at 26 °C increased from 0–6 h to 18–24 h, the proportion of wasps that successfully parasitized host eggs, the number of host eggs parasitized, and the rate of parasitization all decreased by >50%. The number of O. furnacalis eggs parasitized per female T. dendrolimi increased with the number of host eggs available, and reached 22.9 in a 24 h period. However, the parasitic capacity of female T. dendrolimi on eggs of O. furnacalis laid on plant leaves was similar to that of O. furnacalis eggs laid on wax paper. Levels of parasitism of O. furnacalis eggs from two widely separated localities, i.e. Changchun (43.50° N, 125.20° E) and Hangzhou (30.18° N, 120.07° E), were similar. Cold storage of O. furnacalis eggs at 4 °C for 5 days did not affect parasitization. Results obtained in this study indicate that although O. furnacalis is a less preferred and less suitable host than many other hosts, such as Dendrolimus punctatus, Actias selene ningpoane, Philosamia cynthia, A. pernyi, C. cephalonica, within the host-species range of T. dendrolimi, the parasitoid has the potential to achieve 50–60% or even higher rates of parasitization of O. furnacalis eggs in corn fields under suitable conditions, and could be used in the biological control of the pest.     

To mark the host or the patch: Decisions of a parasitoid searching for concealed host larvae

We found evidence for patch marking in the parasitic wasp Halticoptera laevigata (Hymenoptera: Pteromalidae) foraging for concealed hosts. Wasps attack larvae of the fruit fly Myoleja lucida (Diptera: Tephritidae) in fruits of honeysuckle. A special feature of this host-parasitoid system is the limited food supply of a patch (i.e. a fruit of honeysuckle), which allows the successful development of only a single host fly larva. Females of the parasitoid H. laevigata were found to mark the host patch with a pheromone and to abandon the patch following oviposition into a single host larva. Field data revealed that eggs of the parasitoid were spread out evenly among infested patches, with several larvae of the host fly left unparasitized in those patches that contained more than one host. Since many parasitic insects mark the parasitized host after oviposition, we assumed host marking to be the ancestral character state and studied the patch-marking behaviour of H. laevigata as a derived character state as an alternative foraging strategy. We used stochastic dynamic modelling to investigate under what conditions mutant (patch) markers would be able to invade a population of normal (larval) markers. The models suggested that, under a variety of conditions, wasps marking the patch obtained higher fitness than wasps only marking the larva. Consequently, the results from our model predict the evolution of the patch-marking behaviour found in the empirical investigation. Finally, we discuss alternative pathways to the evolution of patch marking and point out under what circumstances the evolution of a patch-marking behaviour can generally be expected.     

Adult feeding and lifetime reproductive success in the parasitoid Aphytis melinus

The diet of adult females of the parasitoid Aphytis melinus DeBach (Hymenoptera: Aphelinidae) includes host insects and sugar-rich foods such as nectar and honeydew. We compared the contributions of host feeding to longevity and fecundity in A. melinus females in the presence and in the absence of honey meals. First, we assessed the longevity of females that were not allowed to oviposit. While the longevity of females fed honey was significantly increased by host feeding (median ages were 30.5 days for host-fed females and 17 days for females not allowed to host feed), the lifespan of parasitoids not fed honey did not exceed 3 days for any individual and there was no effect of host feeding on longevity in this group. In the second set of experiments, we assessed the fecundity and longevity of females allowed to oviposit. We conducted two experiments, one in which honey was continuously available, and one in which honey was not available. In both experiments, daily observations were made of females that were either allowed to host feed or manually prevented from host feeding. In the presence of honey, host feeding significantly increased both fecundity and longevity, and in the absence of honey, parasitoids died within 2 days and host feeding had no significant effect on either fecundity or longevity. The lifetime fecundity of females fed honey but not hosts exceeded the initial egg complement by 60% on average. Approximately one host per day was used for host feeding whether honey was supplied or not, and each host-feeding meal contributed approximately 3.9 eggs to the lifetime fecundity of honey-fed females. In the last experiment, we compared the rate of egg resorption over a 36-h period in A. melinus females that were deprived of hosts and either fed honey or starved. While no egg resorption was detected in honey-fed females over this time period, starved females resorbed approximately 9 eggs. Thus, the availability of a sugar-rich food interacts strongly with host feeding in influencing longevity and fecundity and has a strong direct effect on egg resorption.     

In vitro rearing ofExorista larvarum on tissue culture-based diets

Exorista (=Tachina) larvarum (L.) (Diptera, Tachinidae), a polyphagous parasitoid that attacksLymantria dispar L. andHyphantria cunea (Drury), was rearedin vitro from egg to adult on four tissue culture media-based diets (TMM-FH, SCHNEIDER'S, EX-CELL 400, and SF-900). The kind of tissue culture media in the diets did not influence the adult yield (34 to 55%) and puparium weight (26–27 mg). Adult yield and the puparium weight ofE. larvarum developed on TNM-FH and SCHNEIDER'S-based diets containing different amounts ofGalleria mellonella pupal extract (PE) (0, 1.25, 2.5 and 5%), were lower on diets without PE. In diets without PE development times from oviposition to adult emergence, were shorter on TNM-FH (19 days) than on SCHNEIDER'S-based diet (25–26 days). The adults that developed on artificial diets were able to parasitize the factitious hostG. mellonella and produce viable progeny. The results demonstrate thatE. larvarum is the most promising parasitoid ever studied forin vitro mass production.     

Dependence of the parasitoid Gonia cinerascens on the hormones of its lepidopterous hosts

Development of first instar larvae of Gonia cinerascens, which rest in the muscles of host caterpillars, is triggered by the release of the host's ecdysteroids when the juvenile hormone is absent. Ecdysteroids act on the parasitoid directly and at the same time induce physiological and biochemical changes in the host, which are indispensable for the parasitoid's development. These changes do not occur when metamorphosis of the host is suppressed with the juvenile hormone. Normally the parasitoids initiate development at the larval-pupal transformation of the host, but under experimental conditions, they do so whenever a high ecdysteroid titre is coupled with the proper internal environment in the host, that is in decapitated caterpillars, isolated host abdomens, and when implanted into host pupae. Activated parasitoids moult into the second instar and migrate to the exuvial space of the host; this migratory behaviour is also triggered by ecdysteroids and may be induced experimentally in the first instar parasitoids. Unknown clues direct the migrating parasitoids under the wings and appendages of the host pharate pupal stage. The second instar parasitoids, which anchor to the integument of the host pupae, apparently develop independently of the host's hormones: they can produce third instar larvae, pupae, and adult flies when cultured in vitro.     

The time and egg budget of Leptopilina clavipes, a parasitoid of larval Drosophila

Abstract.

• 1 For the understanding of the influence of natural selection on the persistence of host selection behaviour in populations of Drosophila parasitoids it is important to know whether parasitoids will become time- or egg-limited. We investigated whether the Drosophila parasitoid Leptopilina clavipes (Hartig) meets egg- or time-limited conditions in the field.
• 2 To this end the following aspects of the parasitoid's life were studied: egg load at emergence, travelling velocity between patches, patch residence times, oviposition rates and life expectancy. Together with the results from earlier studies on host and patch distributions, this formed the input of a ‘Monte Carlo’ simulation model, in which the life history of an individual parasitoid can be traced.
• 3 The simulations revealed that under the conditions found in the field 12.9% of the parasitoid population is egg-limited. The model was also run for a number of scenarios which reflect ‘good’ or ‘bad’ circumstances. In most cases a significant proportion of the parasitoid population proved to be egg-limited.
• 4 For the measurement of travelling velocities and patch residence times a marking method, especially applicable to small-sized parasitoids such as L.clavipes, is described. Marking did not affect survival, host habitat location or host detection rate. Parasitoids were found to be attracted to the odour of fruit-bodies of Phallus impudicus, the most important breeding substrate of their hosts.

     

Competition between two primary parasitoids,Gyranusoidea tebygi Noyes and Anagyrus mangicola Noyes,attacking the mealybug Rastrococcus invadens Williams and the influence of a hyperparasitoid Chartocerus hyalipennis Hayat

Two primary parasitoids of the mealybug Rastrococcus invadens Williams, (Hemiptera: Pseudococcidae), Gyranusoidea tebygi Noyes and Anagyrus mangicola Noyes (Hymenoptera: Encyrtidae), were studied. Both primary parasitoids were capable of eliminating the mealybug host but on occasions the parasitoids went extinct before the mealybugs. Three of four parasitoids known to attack G. tebygi also attacked A. mangicola. The latter species was more heavily parasitized than the former, especially by the gregarious species Chartocerus hyalipennis Hayat. In competition experiments the presence of hyperparasitoids slightly slowed the speed of extinction of either the mealybug or the primary parasitoid. A. mangicola was heavily parasitized by C. hyalipennis and the primary went extinct while many suitable mealybug hosts were still available. There were two situations where the primary parasitoids were in direct competition; in the first G. tebygi was more successful than A. mangicola while the reverse was true for the second. It is likely that the superiority of either parasitoid would depend on particular conditions but the introduction of A. mangicola is unlikely to lessen the control being exerted in West Africa by G. tebygi.     

Estimations of number of active loci,dominance and heritability in polygenic inheritance of gregarious behavior inMuscidifurax raptorellus [Hymenoptera: Pteromalidae]

Estimates of the minimum number of genetic loci governing expressions of gregarious oviposition inMuscidifurax raptorellus Kogan & Legner were enabled by the verification of recombinant males, which were secured from virgin hybrid females, formed by crossing cohorts from solitary and gregarious populations. Examinations of parasitization behavior in female progeny that had F₁ male fathers indicated the presence of hybrids among the males. Estimates of gene number, made on the basis of variances in P₁, F₁, F₂ and backcross progeny, and by observing behavior in second and third order backcrosses, ranged from two to 19, with most between two and five. However, backcrossing data suggested that at least eight loci were actively segregating for this characteristic. Semi-dominance of the solitary trait (D=0.63 to 0.84), and unequal gene effects probably caused these gene estimated to be lower than the actual number. Estimates of the coefficient of heritability, in the broad sense based on parental and F₁ and F₂ variances indicated that variability of gregarious behavior in the experimental environment was influenced >60% by genotypic factors, offspring-parent regression analyses gave estimates >38%.      

Structural damage to oak leaves alters natural enemy attack on a leafminer

A field experiment was conducted to test the role of structural changes in oak leaves caused by folivory on natural enemy attack of leafmining larvae and pupae of Cameraria sp. nov., while controlling for induced chemical responses. Damaged and intact leaves of Quercus emoryi were sealed with a fluorocarbon telomer in 1988–1989 to prevent release of long-range or contact chemical cues that might be perceived by searching parasitoids and predators. These leaves were attached to Q. emoryi leaves harboring first instars of Cameraria, but otherwise were undamaged. Rates of attack by natural enemies of leafminers in leaves with sealed, damaged leaves attached were significantly greater than those in control leaves, indicating that structural damage alone influences attack by natural enemies. Survival of leafminers in leaves with attached damaged leaves was significantly less than that of controls, suggesting that structural changes in leaves due to folivore feeding affect population dynamics of Cameraria via increased attack by the third trophic level.

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Résumé Des expériences ont été faites dans la nature pour évaluer l'influence des modifications structurales provoquées par la défoliation sur les attaques des ennemis naturels des chenilles mineuses et des chrysalides de Cameraria sp. nov. En 1988–89 des feuilles saines et endommagées de Quercus emoryi ont été enduites de télomère de fluorocarbone pour empêcher l'émission d'indicateurs chimiques à distance ou de contact pouvant être utilisés par les parasitoïdes et les prédateurs lors de leur prospection. Ces feuilles ont été attachées à des feuilles apparemment intactes de Q. emoryi mais hébergeant des Cameraria du premier stade. Les attaques par les ennemis naturels des mineuses étaient plus importantes sur feuilles associées à des feuilles endommagées mais enduites de fluorocarbone que sur feuilles témoins, ce qui montre que les dégâts structuraux seuls influent sur les attaques par les ennemis naturels. La survie des mineuses dans les feuilles associées à des feuilles endommagées était plus faible que chez les témoins, ce qui suggère des changements structuraux dans les feuilles provoqués par les phytophages affectant la dynamique de population de Cameraria via une attaque accrue par le troisième niveau trophique.