Insect Herbivore Abundance in Tropical Monocultures and Polycultures: An Experimental Test of Two Hypotheses

Abstract

The population dynamics of six beetle pests (Chrysomelidae: Galerucinae) were studied for three seasons in monocultures and polycultures of the corn—bean—squash agroecosystem in Costa Rica. The study was designed to determine if beetle abundance differed among these treatments, and if so, to determine whether the differences were caused primarily by predators and parasites (Enemies Hypothesis) or by movement patterns of adult beetles (Resource Concentration Hypothesis). In polycultures containing at least one nonhost plant, the numbers of beetles per unit host plant (squash or bean) were significantly lower relative to the numbers of beetles on host plants in monocultures. This pattern was generally observed for each of the beetle species studied between 40 and 60 d after planting and continued until the end of the season. When a given beetle species fed on both crop species in a diculture (i.e., the squash—bean diculture), the general polyculture effect was reversed: beetle numbers were significantly greater on one or both of the host plants in the diculture relative to the respective monocultures. These effects were apparently caused by changes in species richness of the plots per se rather than changes in host density or total plant density in the plot. There were no differences in rates of parasitism or predation of beetles between monocultures and polycultures. Neither were there significant differences in beetle emergence between treatments that could account for differences in adult beetle numbers between monocultures and polycultures. Instead, several lines of evidence show that different movement patterns of adult beetles were primarily responsible for the observed pattern of beetle abundance and distribution. Direct measurements of beetle movements in the field showed that beetles tended to emigrate more from polycultures that included a nonhost plant than from host monocultures. Field observations and experiments showed that this was due to several factors: (1) beetles avoided host plants shaded by corn; (2) cornstalks interfered in some way with flight movements of beetles; (3) as beetles moved through polycultures they remained on nonhost plants significantly less time than on host plants and their overall rate of movement in polycultures was higher. These data suggest that the Resource Concentration Hypothesis rather than the Enemies Hypothesis can account for differences in beetle abundance between monocultures and polycultures. The implication of this finding for predicting herbivore abundance in yet untried agreocosystems are discussed. It is suggested that it is not sufficient merely to recognize that herbivores are less common in particular polycultures. Practical issues of agroecosystem design and more theoretical questions of plant—herbivore coevolution depend on knowing exactly what ecological processes create reduced herbivore loads in polycultures.