Fecundity of diamondback moth females when offered honey with pyridalyl, a selective insecticide

We previously reported that the presence of pyridalyl, a selective insecticide, in aqueous honey (50% v/v) negatively affects the survival of diamondback moth (Plutella xylostella: DBM) adults. However, it remains unclear whether toxicity of pyridalyl in aqueous honey affects the fecundity of adult female DBMs. We analysed the survival and fecundity of adult DBM under the water, honey or honey + pyridalyl conditions in glass tubes containing Japanese mustard spinach (Brassica rapa: Komatsuna) leaves for 10 days. The survival of adults under the honey (50% v/v) and honey + pyridalyl (100-fold dilution) conditions was significantly higher and lower, respectively, than under the water condition during the experimental period. The number of eggs laid by DBM females each day (fecundity) under the water condition was significantly lower than that under the honey condition throughout the experimental period, except on day 1. In contrast, the numbers under the water and honey + pyridalyl conditions were not significantly different, except on day 2 (the honey + pyridalyl condition was significantly lower). To study the effects of plants grown in pots on the survival and fecundity of DBM females, we conducted experiments using acrylic cages containing potted komatsuna plants. The survival trends under the honey, and honey + pyridalyl conditions in the cages were similar to those in the glass tubes. Fecundity was evaluated based on the total number of subsequent generations (DBM larvae and pupae) on day 10. The numbers were significantly higher in the honey condition than in the water condition. In contrast, the number in the honey + pyridalyl condition was not significantly different from that in the water condition. Given our previous findings that the longevity and fecundity of Cotesia vestalis, an effective natural enemy of DBM larvae, remained unaffected by the presence of pyridalyl in honey as a food supply, our research suggests that introducing devices providing honey infused with pyridalyl in greenhouses as a nutritional source for C. vestalis could be an effective biological control strategy for managing DBM populations within greenhouse environments.