Mate choice and host discrimination behavior of the parasitoid Trichogramma chilonis

Trichogramma chilonis Ishii (Hymenoptera: Trichogrammatidae) is an important natural enemy of many species of lepidopterous pests and a widely used biological control agent. Detailed knowledge about its mate choice and host discrimination behavior is lacking. In this study, we studied the mate choice and host discrimination behavior of T. chilonis in experimental arenas through video tracking. Males’ mate recognition capacity was realized by perceiving the sex pheromone of females. When offered two females of different species, male could distinguish the conspecific female from Trichogrammatoidea bactrae Nagaraja (Hymenoptera: Trichogrammatidae), a species that has overlapping hosts with T. chilonis. When placed with two females of different mating status, male preferred mating with the virgin female to the mated female. T. chilonis females could distinguish unparasitized host eggs from parasitized ones (parasitized by conspecific females or heterospecific females). They preferred to stay on and lay eggs in unparasitized host eggs. When T. chilonis females were only provided with parasitized host eggs (parasitized by T. chilonis and T. bactrae females), conspecific superparasitism occurred more often than heterospecific superparasitism. Furthermore, the host egg discrimination ability of T. chilonis females was mainly achieved through antennal perception.

Effects of Condensed Tannins and Catalpol on Growth and Development of Compsilura concinnata (Diptera: Tachinidae) Reared in Gypsy Moth (Lepidoptera: Lymantriidae)

Condensed tannin is generally considered an example of a quantitative plant allelochemical defense, and catalpol an example of a qualitative chemical defense. The effects of these compounds on the growth and survival of a tachinid parasitoid, Compsilura concinnata (Meigen), reared in the gypsy moth, Lymantria dispar (L.), were compared. Each chemical was incorporated into synthetic diets in a range of ecologically relevant doses and fed to host larvae. Larvae were fed in each of two ways: immediately after parasitization (one day after fourth instar molt), and from egg hatch onward. Growth and survival of unparasitized gypsy moth larvae on test diets were also monitored. No significant effect of either catalpol or condensed tannin on C. concinnata growth or puparial survival was observed. Tannin did lengthen development time of unparasitized host larvae from fourth stadium onward, and lowered pupal weights of larvae fed tannin from egg hatch onward. Catalpol had no significant impact on overall gypsy moth larval development, indicating that this insect is able to compensate for the reduction in weight gain reported to be caused by catalpol in younger larvae. Mortality in all experiments was insignificant. It appears that these phytochemicals are more similar in their effects on the parasitoid than was predicted based on their roles as toxins and digestibility-reducers in herbivores. The data also suggest that generalist tachinid parasitoids such as C. concinnata may be more tolerant of allelochemicals in their host's diet, than their hymenopteran counterparts.

BEHAVIOR OF THE PARASITOID PLEOLOPHUS BASIZONUS (HYMENOPTERA: ICHNEUMONIDAE) IN RESPONSE TO CHANGES IN HOST AND PARASITOID DENSITY

A synthesis of observations on density-dependent interactions in parasitoid populations is attempted, working from the particular aspects of discrimination against parasitized hosts and detailed observations on parasitoid behavior, to more general aspects of parasitoid ecology, dispersal, distribution, and response to host populations. Females of Pleolophus basizonus (Gravenhorst) are able to discriminate against parasitized hosts of Neodiprion swainei Middleton between narrow limits (mean densities of 0.4 to 1.4 eggs per cocoon) set by the probability of a female finding an unparasitized host. They show mutual interference in egg laying at high parasitoid:host ratios, and a density-dependent escape reaction to adult parasitoid density. These behaviors all result from the recognition of repellent trail odors left by searching females. Response of individual females to high parasitoid densities may therefore lead to dispersal, the occupation by the population of more, but not so favorable habitats, and an apparent density-independent response to host populations.