Egg Recognition : Its Advantage to a Butterfly

Ovipositing females of the pipevine swallowtail butterfly, Battus philenor, detect the presence of eggs laid by other females on their host plants . The presence of eggs on a plant inhibits oviposition by a female that discovers it . The selection pressure responsible for the evolution and maintenance of discrimination against plants with eggs appears to be lower survival from egg to adult of eggs laid on plants already containing eggs than on plants without eggs . Several authors have proposed that a preference in an ovipositing insect for a particular host plant or group of host plants is the result of natural selection for behaviour that discriminates against plants on which egg and larval survival is poor (Wiklund 1975 ; Chew 1977 ; Rausher 1978a, b ; Smiley 1978) . Although most empirical tests of this proposal have examined preferences for plants of one species over plants of another species, there is reason to suspect that ovipositing insects may also discriminate among plants within one species. Different plants of the same species may differ greatly in suitability for larval growth and survival (Singer 1972 ; Dixon 1976 ; Edmunds & Alstad 1978). If such differences are consistent and detectable by ovipositing insects, it is likely that those insects will evolve behaviours that lead to preferential oviposition on plants on which juvenile survival is greatest . The presence of other herbivores on a plant may render that plant less suitable for juvenile growth and survival than a conspecific plant without herbivores (Dixon 1973 ; McClure & Price 1975 ; Mitchell 1975) . When the amount of edible foliage per plant is small relative to the requirements of one larva, or when larvae are cannibalistic, it is likely that survivorship from egg to adult of an egg laid on a plant already containing other eggs or larvae will be lower than that of an egg placed on a plant free of herbivores. It may thus greatly benefit an ovipositing female to discriminate among host plants with respect to the presence or absence of other herbivores . The host plants of the pipevine swallowtail butterfly, Battus philenor, in southeast Texas are erect perennial herbs in the genus Aristolochia (Aristolochiaceae). Because one plant rarely has enough edible foliage to support the growth of a larva to maturity, each larva must abandon its *Present address : Department of Zoology, Duke University, Durham, North Carolina 27706. 1034 first host plant and wander until it discovers another host. Early instar larvae cannot move as far as larger larvae, and it is likely that the probability of discovering another host plant increases with larval size . Females that lay eggs only on plants that do not contain other eggs or larvae may thus maximize the amount of food available to the earliest instars. Their larvae would be larger upon leaving the first host plant than larvae produced by females that oviposit on plants already containing eggs or larvae. The evolution of the ability to discriminate against plants with eggs might therefore be expected in B. philenor . In this study I examined this expectation . In particular, I asked the following two questions : (1) is juvenile survival lower for eggs laid on a plant already containing eggs than for eggs laid on a plant not containing eggs? and (2) do females in nature discriminate against plants with eggs and oviposit on egg-free plants with greater frequency than on plants already containing eggs?