Adaptive Traits of Riptortus pedestris Nymphs (Heteroptera: Alydidae) for Locating Host Plants

Abstract

Abstract Riptortus pedestris (F.) (Heteroptera: Alydidae) females oviposit not only on host plants but also on nonhosts, which may impose high costs on nymphs in terms of locomotion energy and time searching for host plants. Therefore, we hypothesized that second-instar nymphs have developed adaptive traits that help them access host plants, because the first feeding stage occurs during the second instar in this bug. We compared responses to the aggregation pheromone, relative leg lengths, and locomotion performance using a servosphere locomotion compensator as tests of physiological, morphometric, and behavioral traits, respectively, among the instars. We also investigated the effects of delayed feeding in the second instar on subsequent survival and development. Our results indicated that second-instar nymphs might have responded more sensitively to synthetic aggregation pheromone than other instars. Morphological measurements showed that second-instar nymphs had the longest relative leg length compared with other instars. The experiment using the servosphere revealed that second-instar nymphs had higher locomotion performance than did older nymphs, which may allow second-instar nymphs to walk a distance comparable to older nymphs, although their body size is much smaller. However, we did find that more than a 48-h delay in feeding after the first-instar molt decreased subsequent survival rates and that a later first-feeding led to a longer developmental period during the second instar. We concluded that R. pedestris nymphs have evolved various adaptive traits to enhance the probability of accessing host plants in response to the costly oviposition habit of adult females that lay eggs on nonhosts.