Effects of Sex Ratio on Milkweed Beetle Emigration from Host Plant Patches

The effect of conspecifics on milkweed beetle, Tetraopes tetraophthalmus (Forster) (Coleoptera: Cerambycidae), emigration from patches of their host plant Asclepias syriaca was tested. The density and sex ratio of beetles was experimentally manipulated by repeatedly releasing marked individuals into two small, Tetraopes—free host plant patches. Beetle emigration rates were then monitored. Sex ratio affected the emigration rate of red milkweed beetles. Male beetles emigrated at a higher rate from host plant patches with male—biased sex ratios than from patches with female—biased sex ratios. Sex ratio had no effect on the rate of female emigration, and beetle density had no effect on the emigration rate of either sex. Male emigration rate was correlated with male flight frequency: males flew more when the sex ratio was male—biased. Males that had disappeared from a patch were more likely to return if the sex ratio was female—biased. While male behavior in the field was correlated with sex ratio, the actual determinant of male behavior was shown to be the number of receptive females in a patch. Males remained in patches with receptive females, but since not all unmated females were receptive, sex ratio was a better measure of mate availability than female density. Since the probability of mating was sex ratio dependent, male emigration patterns resulted in males leaving areas where mating was unlikely. Thus, movement patterns were directly linked to male fitness.