Adaptations of Plutella xylostella adult females and larvae to waxy host plants

The diamondback moth (DBM), Plutella xylostella, is an oligophagous pest of cruciferous crops worldwide. Host plants of the DBM possess epicuticular wax, but thick wax is unfavorable to the selection of oviposition sites for female DBMs. How the DBM responds to host plant wax regarding oviposition site selection is largely unknown. The glucosinolates and wax levels in the cotyledons and true leaves of Chinese kale are different, which makes this plant an ideal host for exploring the oviposition behavior of the DBM. We found that although the true leaves contained more glucosinolates and waxy powder than cotyledons of healthy Chinese kale, DBM females preferred to lay eggs on the cotyledons over true leaves. However, the number of eggs laid on true leaves increased significantly when the waxy powder was artificially removed. Furthermore, the hatched larvae greatly preferred to feed and performed better on true leaves. In light of the current results, we propose that DBM females are hindered from laying eggs on the true leaves of Chinese kale by the outer layer of leaf wax powder. However, DBM females and larvae can adapt to the waxy host plant through adaptations: i.e., DBM females first lay eggs on the cotyledons where less wax exists, the hatched larvae then crawl to and spin silk nets on true leaves; the silk nets help subsequent DBM females overcome the obstacle of the wax and oviposit successfully on true leaves. These findings highlight new insights into plant–insect interactions.