HOST PLANT-MEDIATED VARIATION IN OVERWINTERING SITE QUALITY - IMPLICATIONS FOR THE SIZE AND COMPOSITION OF POPULATIONS OF ACANTHOSCELIDES ALBOSCUTELLATUS (COLEOPTERA, BRUCHIDAE)

This study provides an example of how variation in the quality of over wintering sites provided by the host plant of an insect seed predator can influence both the probability of overwintering survival and the s ize and composition of postwintering populations. Thus, the concept of host plant quality is extended to include variation in the suitabilit y of the overwintering site of temperate region insects that overwinte r within, or in habitats created by, their host plant. Adult Acanthosc elides alboscutellatus (Horn) (Coleoptera: Bruchidae) overwinter insid e the fruit of Ludwigia alternifolia (L.) (Onagraceae). In early winte r, however, fruits begin to dehisce, i.e., one or more of the fruit's four sides and/or top are shed. Variation in the onset and extent of d ehiscence creates a range of overwintering habitats that vary in expos ure to ambient conditions. In this study the frequency of possible ove rwintering sites in natural populations of L. alternifolia was determi ned by monitoring the phenology of fruit dehiscence from October throu gh May in two populations for four years and for a third population fo r three years. Winter survivorship of adult A. alboscutellatus was ass essed experimentally in eight environments representative of the condi tions created by variation in dehiscence. These environments were prod uced by crossing four levels of exposure (degree of dehiscence) with t wo locations of the overwintering site, i.e., above or on the ground s urface. The onset, phenology, and overall frequency of fruit dehiscenc e varied markedly among populations and years. Exposure, location, and their interaction had strong effects on survival and accounted for 80 % of the observed variation in winter survival. Survivorship was highe r on than above the ground, and in both locations decreased with incre asing exposure. Thus, variation in fruit dehiscence among L. alternifo lia populations will influence the size of postwintering A. alboscutel latus populations by dictating the quality of overwintering sites. Adu lt beetles that overwinter inside indehiscent fruit experience selecti on for small body size, associated with high mortality, when they atte mpt to exit the fruit at eclosion. As a consequence, the frequency of fruit dehiscence at eclosion coupled with the relative survival rates of adults within indehiscent fruit will determine the body size compos ition of postwintering populations and hence the response to selection for small body size in this species.