Attracting parasitoids by plants as defences against herbivorous insects

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Females of Cotesia glomerata (L.) (=Apanteles glomeratus L.)., a specialist parasitoid wasp of Pieris butterfly larvae, are attracted to volatile chemicals from leaves of a cruciferous plant Rorippa indica (L) Hieron damaged by larvae of three Pieris species, P.rapae L., P.melete Menetries and P.napi L. The volatile chemicals are emitted neither from saliva of Pieris larvae nor from leaf juice of the plant. Their effect continued for more than five days. Mechanical damage or damage by other herbivorous insects had less effect on the parasitoid. Therefore, this plant likely produces volatile chemicals to help the parasitoid attack the Pieris larvae.However, what the plant gains by attracting the parasitoids is not immediately obvious. When the Pieris larvae are killed by the parasitoid, their feeding activities have completely finished. Thus, it is not directly beneficial for the plant to help the parsitoid attack the larvae by paying the cost of producing attractive chemicals.Nearby conspecific plants of R.indica seems to be kin plants each other. Therefore, if butterflies grown on those plants by eggs on the nearby plants, genes of the plants responsible for helping the parasitoid parasitized will be favoured by natural selection. Each of the three Pieris species has effective avoidance/defense mechanisms against C.glomerata. Such mechanisms may well be the result of co-operation between plant and parasitoid. We attempted to interpret that plant R.indica damaged by larvae of Pieris butterfly species has evolved to produce some volatile chemicals attracting the parasitoid C.cotesia.