Effects of natural products on parasitoids of corn pests.

The physiological and ecological effects of plant-derived insecticidal compounds were investigated using the European corn borer Ostrinia nubilalis and the ichneumonid endoparasitoid Diadegma terebrans. Berberine, an isoquilonine alkaloids did not significantly affect duration of the immature stages or pupal and adult weights of D. terebrans. Survival to pupation and adult emergence declined with all concentrations of berberine, but the mortality levels did not correspond to the concentrations administered in the diet. In dual and no choice experiments berberine conferred an advantage to the developing parasitoids specially when present in the diet at frg/g. Percent parasitization was significanty increased in the 100 frg/g berberine treatment compared to the controls in dual choice tests. Berberine was not detected by HPLC analysis of the host and the parasitoid in any of the samples analyzed. The thiophene terthienyl did not significantly affect growth parameters of D. terebrans except in the 100 frg/g treatment. No females were produced at this concentration and male pupal and adult weights were significantly reduced. Survival to pupation and adult emergence were severely reduced with this treatment. A concentration of 100 frg/g alpha-t [alpha-terthienyl] in the diet somewhat improved the survivorship of parasitoids compared to the control diet. Dual choice tests with treated and control larvae of the same chronological age revealed that alpha-t was not beneficial at any of the concentration tested. Percent parasitization was significantly increased in 10 frg/g alpha-t treatment but significantly reduced at 100 frg-g alpha-t in the diet of the host. Exposure of D. terebrans to O. nubilalis larva fed azadirachtin in their diet at a concentration of 1 frg/g resulted in significant differences in pupal and adult weights. The number of pupae reaching the adult stage was also significantly reduced.