Gradient Analysis Using Plant Modular Structure: Pattern in Plant Architecture and Insect Herbivore Utilization

Abstract

Distribution of five species of tropical insect herbivores was recorded in relation to gradients of modular length on their host plants. Patterns in plant architecture and herbivore responses were compared with a well-studied temperate herbivore, Euura lasiolepis Smith (Hymenoptera : Tenthredinidae). The five tropical species were each from a different family, and host plants belonged to different families and vegetation types. Using modular gradients of shoot or leaf length we showed that plant architecture was comparable directly and was similar even when trees, shrubs, and grasses were compared. Also, the membracid, cicadellids, psyllid, and cecidomyiid in the tropics showed patterns of attack on modular gradients consistent with the patterns derived from high ovipositional preference linkage with larval performance, known to exist in the temperate species. Strong positive relationships between module length and percentage of modules attacked in each length class were observed, with variance accounted for ranging from 68 to 96%. One pest species on forage grasses in the vicinity of Brasilia, Deois flavopicta Stal (Homoptera : Cercopidae), showed a weak response to the shoot-length gradient of its host grass (r 2 = 0.41) and an insignificant response per unit length of shoot, suggesting a generalized capacity to use wide variation in module size. A method was developed for direct comparison of species employing a comparison between a hypothetical ideal response to modular gradients and the actual response. The actual slope of the attack versus module length regression, expressed as a percentage of the hypothetical slope, provided estimates of herbivore responses to modular plant structure consistent with hypotheses on plant vigor and phylogenetic constraints.