SEX-RATIO PATTERNS AND POPULATION-DYNAMICS OF WESTERN FLOWER THRIPS (THYSANOPTERA, THRIPIDAE)

Frankliniella occidentalis (Pergande), the western flower thrips, is a common pest of greenhouse floral and vegetable crops. Studies in four vegetable greenhouses revealed a significant correlation between density and sex ratio of adults on sticky traps. At low densities, 80-100% of western flower thrips adults on traps were males. As density of thrips within a greenhouse increased, the proportion of females on traps also increased to 60-90%. Predictions about future population dynamics of western flower thrips can be made based on the sex ratio of adults caught on traps. This shift in adult sex ratio can have important consequences with respect to outbreak potential of thrips and damage to greenhouse crops. Laboratory experiments suggest that maternal age and sperm availability influence sex ratio and size of broods produced by individual mated females. Once mated, all females produced female-biased sex ratios (xBAR, 67% daughters). When virgin females were exposed to 1, 5, or 10 males for 1 wk, only those with 10 males produced daughters throughout their lifetime (up to 4 wk). These results are discussed with respect to sex ratio theory.