The uzifly, Exorista sorbillans (Diptera: Tachinidae), a parasite of the silkworm, Bombyx mori L. (Lepidoptera: Bombycidae), causes heavy losses to the silk industry. This parasitoid harbours a Wolbachia endosymbiont, which controls the fly's reproduction. In the present study a method for curtailing this notorious pest by administering Wolbachia-targeted tetracycline via its silkworm host's diet is investigated. Tetracycline not only influenced the larval growth of the silkworms' by decreasing larval duration, increased silk production and fecundity, without affecting hatchability, it also decreased the reproductive fitness of the uzifly endoparasite by killing the Wolbachia. The antibiotic exerts a beneficial influence by affecting the intestinal flora of silkworm larvae. On the other hand the reproductive fitness of uzifly was greatly reduced in terms of different reproductive abnormalities. When male and female flies that emerged from treated host silkworms were crossed and males from untreated hosts and females from treated hosts were crossed, approximately 72% and 97% of the eggs failed to hatch, respectively. However, of the eggs from crosses between male and female flies that emerged from untreated hosts and between males from treated hosts with females from untreated hosts, an average of 30% failed to hatch and the Wolbachia infection enhanced the fecundity of uziflies. These results demonstrate that the Wolbachia may be essential for uzifly reproduction and that Wolbachia-targeted antibiotics have a beneficial influence on silkworm growth while decreasing the reproductive fitness of the uzifly, E. sorbillans.
Many gregarious insects aggregate in naturally occurring refuges on their host plants. However, when refuges are filled, they may be forced to aggregate on exposed areas of the plant. This study examines the effects of refuge saturation on group size and defence against parasitism in larvae of Ammalo helops Cramer (Lepidoptera: Arctiidae) that form day-resting groups on the trunks of weeping laurel, Ficus benjamina L., in El Salvador. Population densities, group sizes and parasitism were recorded on eight trees for each of four generations in 1995 and 1996. When population densities were low, all larvae were located in small groups in naturally occurring structural refuges (rotted out holes, spaces between crossing branches and under aerial roots) on the host plant. In contrast, when population densities were high and structural refuges were full, many larvae formed significantly larger groups (density refuges) on the open trunk. Between 20 and 24% of late-instar larvae were parasitized and this was inversely dependent on the size of within tree populations, in spite of populations being fragmented among structural refuges. Similarly, in a study carried out at a different location on young trees without structural refuges, parasitism of larvae was inversely related to group size. Although parasitism rates decreased with increasing group size, most larvae preferentially selected the small naturally occurring refuges, where groups were restricted to low densities. If this behaviour is an adaptive trait, I speculate that parasitism (or some other unmeasured mortality factor) is lower in naturally occurring refuges than in large open groups.