Host use in relation to food availability and larval development in the specialist herbivore Epilachna admirabilis (Coleoptera : Coccinelidae)

We compared the effects of intrinsic quality and seasonal abundance of two host plants, Gynostemma pentaphyllum and Trichosanthes cucumeroides (Cucurbitaceae), on host use by a phytophagous lady beetle, Epilachna admirabilis. Although, only G. pentaphyllum exists as an available host in April, T. cucumeroides sprouts from late May and its leaf abundance surpasses that of G. pentaphyllum in August, the egg-laying period for E. admirabilis. In the laboratory, T. cucumeroides supported higher growth than did G. pentaphyllum and larvae grew faster as the dietary proportion of T. cucumeroides increased. We expected females to switch to T. cucumeroides for feeding and egg deposition. However, most adults remained and laid eggs on G. pentaphyllum. There is a poor correlation between larval performance and adult egg-laying site selection. On the other hand, first and second instar larvae showed a greater preference for T. cucumeroides in a host preference test. Additionally, 70-80% of first instar larvae were found on G. pentaphyllum in the field, and the number of larvae on T. cucumeroides increased with developmental stage. We suggest that larvae often switch food sources from G. pentaphyllum to T. cucumeroides depending on the intrinsic quality and food availability of these two host plants.