Behavioral Ecology of Mycophagous Thysanoptera.

Abstract

Behavioral studies of Hoplothrips pedicularius (Haliday) and Hoplothrips karnyi (Hood) indicate that: (1) females oviposit communally under bark; (2) males fight each other with their enlarged, armed forelegs in defense of oviposition areas; (3) large males win fights; and (4) males unsuccessful in fights adopt an alternative mating tactic of attempting to 'sneak' matings. Males of H. karnyi exhibit a 'fighter-flier' polymorphism, determined during the second instar by nutritional conditions and population density. In rearing experiments, females of H. karnyi produced about 25% male offspring, close to the sex ratio predicted from the relative proportions of male and female offspring that develop wings. Elaphrothrips tuberculatus (Hood) is a bivoltine winged thrips that lives on clusters of hanging dead oak leaves. In this species males are produced by viviparity and females by oviparity, a mode of reproduction termed monogeny. Oviparous females guard their clutches of eggs and males fight each other with their enlarged forelegs in defense of individual egg-laying females. Field observations indicate that large males win fights and smaller males attempt to 'sneak' matings during ovipositions. Four years of collection data indicate that the intensity of sexual selection for large male body size covaries with the sex ratio. In a year when large oviparous females began breeding earlier than smaller ones in spring, males and females paired assortively for size. The proportion of oviparous and viviparous females varies seasonally. However, because the relative cost of each sex also varies, the sex allocation ratio remains near 1:1 in each generation. Females tend to be viviparous if their offspring will develop into relatively large adults and oviparous if their offspring will be of average size or small. These findings support the Trivers-Willard hypothesis of conditional sex ratio manipulation.