DETERMINANTS OF MULTIPLE HOST USE BY A PHYTOPHAGOUS INSECT POPULATION

The number of host species used by an insect population is an important component of niche breadth. The use of several species may arise from preference differences among the insects. Some may prefer one species and some another, and they may all use the plants which they prefer. However, there are at least two ways in which oligophagy could be generated without differences among insects in preference. First, the insects may have "neutral" preference, and use the first plant they encounter within the range of acceptable host species. Second, all insects may prefer A over B and C, but some may fail to encounter A and may then use B or C. It follows that some insects may not use their most highly preferred hosts, and that the relationship between host preference and host use may be indirect. These terms are employed in the following way: two insects differ in host use if they actually feed on different plant categories, usually different species. On the other hand, insects which, in the same situation, have different likelihoods of using particular plant categories are described as differing in their host preference. This distinction is important, since natural selection acts directly on host use, not preference. One can make no assumptions about the nature of selection pressure on preference without knowing how variation in use is related to variation in preference. In consequence, a study of this relationship, as undertaken in this paper, is an essential step in any investigation of the evolution of either preference or use. The work reported here is part of a more general study concerning the behaviors by which butterflies select their host plants and the ways in which natural selection may act upon these behaviors. How and why do conspecific populations differ in the identities of the host plants they use or in the diversities of the hosts they attack? Such questions can be answered by examining the ways in which the behavior of individuals generates patterns of host exploitation by populations, and by investigating the consequences of host selection decisions that the insects make. The principal question posed in this paper is: how does the use of four host species result from the behavior of individual insects in a population of the nymphaline butterfly Euphydryas editha? I also ask in what ways the behavior of butterflies in this oligophagous population differs from that of the insects in monophagous populations of the same species. The answers to these questions lie principally in the oviposition behavior of E. editha rather than in larval host selection behavior. This is because newly-hatched larvae must find food within two or three inches of the oviposition site (Singer, 1971). E. editha was chosen as the study organism for three reasons. 1) It shows extensive ecotypic variation in host use that depends at least in part on heritable variation in oviposition preference (Singer, 1971; White and Singer, 1974, and unpubl.). 2) There is equally striking variation in the degree of host specialization. Some populations are strictly monophagous. Others are usually monophagous, but occasionally include a second host species in their diet. In a few populations, such as the one described in detail in this paper, eggs are regularly laid on as many as four host genera. This variation among populations could have a simple explanation: if there were five acceptable host species, the insects could be monophagous where only one of these plants grows and oligopha-