Phytochemicals that Regulate Oviposition Mistakes of Eurema mandarina on Oxalis corniculata

Eurema mandarina is a pierid butterfly that primarily feeds on plants in the family Fabaceae. In mainland Japan, adult females preferentially lay eggs on Albizia julibrissin and Lespedeza cuneata. In the field, females may oviposit on non-fabaceous plants, although rarely. When maintained in our laboratory with their natural hosts removed, females showed moderate oviposition responses to Oxalis corniculata (Oxalidaceae), but newly hatched larvae failed to grow to the second instar on the leaves. This indicated that females made oviposition mistakes on O. corniculata because it is not suitable for larval development. We attempted to identify the phytochemicals that regulate oviposition of E. mandarina on O. corniculata. Females hardly responded to the methanolic leaf extract and its aqueous fraction but responded weakly to the most polar subfraction. Further fractionation of this subfraction by ion-exchange column chromatography revealed high oviposition-eliciting activity in the acidic subfraction and low activity in the acidic/neutral/amphoteric and neutral/amphoteric subfractions. Mass spectrometry and oviposition bioassays identified erythronic and threonic acids as stimulants in the acidic subfraction, and arabinose and arabitol as deterrents in the neutral/amphoteric subfraction. Leaf samples of O. corniculata varied greatly in the composition of the four components; however, none contained the concentration of stimulants necessary to induce oviposition without being counteracted by deterrents. These results suggest that oviposition mistakes on O. corniculata, though infrequently, occur when females discover non-hosts with sufficiently high concentrations of stimulants and sufficiently low concentrations of deterrents.