Sex Ratio in Umbonia crassicornis Amyot and Serville (Homoptera: Membracidae)

Abstract

Mean sex ratio of Umbonia crassicornis Amyot and Serville offspring is biased toward females. Clutch size, but not the proportion of female offspring in the clutch, is related to female size. Survival of nymphs is, however, related to deviation from an equal sex ratio. We argue that nutrient quality received by offspring from the plant, rather than manipulation by the mother, influences sex ratio and the condition of offspring. INTRODUCTION Sex ratios favoring females are considered a major factor in the development of eusociality in insects, particularly the haplo-diploid eusocial Hymenoptera (Hamilton, 1964; Wilson, 1975; Trivers and Hare, 1976; but see Alexander, 1974; Alexander and Sherman, 1977). However, the basis for skewed sex ratios in diploid animals is poorly understood. Trivers and Willard (1973) suggest that sex ratio bias in mammals with low male parental investment could be influenced by the nutritional status of parent females at the time of reproduction. Control of sex ratios should occur in species where the reproductive success of males is highly variable compared to that of females. Since females in good condition are likely to produce strong progeny, they should bias their offspring toward sons which may experience greater reproductive success than their sisters. Conversely, a female in less than optimum nutritional condition should bias her offspring toward daughters since number of females is usually limiting and even weak daughters will find mates. This model (Trivers and Willard, 1973) is based on the following assumptions: (1) the condition of offspring is correlated to that of the parent; (2) the condition of offspring is maintained throughout adulthood, and (3) slight advantages in the adult male condition enhance reproductive success more than in females. Although this hypothesis has been explored for some mammals, there is no supporting evidence from insect studies (Thornhill, 1979). We examine if sex ratios in the presocial insect Umbonia crassicornis Amyot and Serville (Homoptera: Membracidae) support this hypothesis. Unlike other homopterans, females of some membracid species care for offspring (Wood, 1974, 1976a and b, 1977, 1978). Parent females of Umbonia crassicornis remain on eggs, make slits in the bark of the host plant to facilitate feeding by nymphs, maintain offspring aggregations and ward off potential predators. Maternal care is strongly correlated with nymphal survival, as the absence of parent females during the first three instars results in almost certain mortality of offspring (Wood, 1974, 1976a). Females do not desert offspring nor do aggregations of offspring move from the branch where eggs were deposited. Males make no investment in offspring except for gametes. Females mate only once while males mate several times. Most females are semelparous, but a small minority may deposit a second clutch.