Effects of nectar-producing plants on Diadegma insulare (Cresson), a biological control agent of diamondback moth, Plutella xylostella (L.)

Effects of nectar-producing plants (NPP) on the longevity, fecundity, oviposition and nectar-collecting behaviour of Diadegma insulare were studied. Longevity and fecundity of D. insulare were varied with morphological characters of NPP’s flowers. Brassica kaber (D.C) Wheeler, Barbarea vulgaris R. Br. and Daucus carota L. supplied nectar and resulted in D. insulare longevity and fecundity equal to when honey+water used as food sources. Others were not significantly better than no food at all. Chenopodium album L. and Thlaspi arvense L. did not provide available nectar, however, adults parasitoid fed on honeydew excreted by aphids feeding on the plants. An increase in longevity and fecundity was correlated with flower corolla opening diameter but not with corolla length. The oviposition behaviour within the first minute of exposure to diamondback moth larvae was highly correlated with longevity and fecundity of D. insulare, which we considered indices of food quality. We observed five nectar-collecting behaviours of D. insulare. The most striking behaviour, on B. vulgaris and B. napus L. flowers, involved chewing at the base of the corolla and creating holes that probably released the floral nectars. D. insulare visit more frequently and spent longer time particularly at the base of flower supporting longer life and high fecundity. The charactersitics of NPP’s flower and behavioural flexibility of D. insulare should be manipulated to increase its impact in integrated diamondback moth management.