Sex ratio and brood size in a monophagous outcrossing gall aphid, Tamalia coweni (Homoptera : Aphididae)

Sex allocation theory has been applied successfully in the case of spatiall y structured aphid populations, in which local mate competition can account for biased sex ratios. Likewise, a demographic effects model can explain s ex ratio bias when maternal investment in sons and daughters is asynchronou s, owing to developmental constraints. In one of the first studies to exami ne patterns of sex allocation in a phytophagous insect in which outbreeding is likely and both sexes are produced concurrently we measure sex ratio an d sex allocation for the gall-forming aphid Tamalia coweni (Cockerell). Whi le the sex ratio at a low elevation (800 m) site did not differ significant ly from 1:1, the sex ratio was slightly but significantly female-biased at a higher elevation (1350 m). At both sites, the variance in sex ratio among broods was significantly greater than binomial, suggesting active manipula tion of the sex ratio by aphid foundresses. Ten percent of the broods with more than four individuals contained exclusively males or exclusively femal es, a percentage that could not have resulted from random variation around an average sex ratio. Among non-unisex broods, the sex ratio became increas ingly more female-biased with increasing brood size. Local mate competition and non-adaptive demographic effects on the sex ratio could not account fo r the overall bias towards females. Apart from the possibility of cytoplasm ic or other genetic sex ratio distortion elements, the Trivers-Willard hypo thesis of condition-dependent sex allocation may best explain the observed sex ratio patterns in T. coweni.