A multi-generation mass breeding colony of the cabbage moth, Plutella xylostella, was found to be infected with a microsporidium, Vairimorpha sp., which is passed transovarially between generations. The microsporidian infection had little impact on the fitness of this lepidopteran pest. However, when Trichogramma chilonis parasitized such infected host eggs, the offspring of this parasitoid species suffered from severe deficiencies. Microsporidian spores, ingested by parasitoid larvae together with the host egg nutrients, gave rise to stages which developed in various tissues of the parasitoid, such as the flight muscle and the nervous system. This infection resulted in a significantly increased rate of metamorphosis failure (related to host age) and reduced longevity and reproductive performance of the parasitoids. There are two main consequences arising from our findings if T. chilonis is to be used in an integrated control strategy against P. xylostella: (1) T. chilonis must be raised on Vairimorpha-free host eggs to receive viable and efficaceous parasitoids for release and (2) if natural populations of the cabbage moth in cruciferous crops are infected with Vairimorpha to a significant extent, the parasitoid must be released repeatedly within infested crop areas.

In orchards, imidacloprid, a new chloronicotinyl insecticide, is used chiefly to control aphid and leaf-mining pest species. In preliminary field studies, a strong reduction of hymenopteran parasitoids was recorded shortly after imidacloprid treatment. However, as soon as 7 days after treatment, abundance of parasitoids had returned to pre-treatment levels. The recovery of the parasitoid population appeared mainly attributed to emergence of freshly emerged parasitoids. This supports the view that preimaginal stages of parasitoids which are inside eggs are protected from spray impacts. Using the codling moth Cydia pomonella (Lepidoptera, Tortricidae) and the egg parasitoid Trichogramma cacoeciae (Hymenoptera, Trichogrammatidae) as a model, we investigated the impact of treatment with imidacloprid on preimaginal stages of the parasitoids.Results showed that preimaginal stages of T. cacoeciae (which develop within the host eggs) were not adversely affected by a spray treatment with imidacloprid. However, when treated host eggs were stored under greenhouse conditions, emergence of parasitoids was significantly affected. This was assumed to be caused by spray residues that had been deposited on host egg surfaces during treatment. In contrast, if treated host eggs were subjected to field exposure conditions before emergence of the parasitoids, no statistically significant reduction in the emergence rate of parasitoids was recorded. Apparently, spray deposits on host eggs are quickly degraded under natural exposure conditions and are no longer active at the time of parasitoid emergence. From these results, and from the observations made under practical conditions, we conclude that a spray treatment with imidacloprid will not significantly influence the ecological function of the egg parasitoid fauna in orchards.