Spatial variations in egg density and the intensity of parasitism in a neotropical chrysomelid (Cephaloleia consanguinea)

Abstract

Abstract. The chrysomelid beetle, Cephaloleia consanguinea (Hispinae), is a monophagous herbivore of Heliconia imbricata (Zingiberales: Heliconiaceae) in wet lowland forests of eastern Costa Rica. Within the study area parasitism by eulophids and trichogrammatids (Hymenoptera: Chalcidoidea), the most common source of C. consanguinea egg mortality, varied in intensity from 35% to 50% during 1974–77. Spatial relationships between the probability of parasitism and egg density per leaf were the net result of two opposing trends. The probability of at least one egg on an egg‐bearing H. imbricata leaf being encountered and parasitized (here termed a ‘discovery’ of that leaf by parasitoids) increased significantly as egg density per leaf increased. The conditional probability of parasitism for other eggs present, given an attack on one egg, decreased with increasing egg density. The net result was a decrease in the overall probability of parasitism as egg density per leaf increased. Host aggregation appears beneficial in reducing the intensity of parasitoid attacks. Despite this inverse relationship between the overall probability of parasitism and egg density per leaf, a model based on the binomial distribution provides a good description of the spatial aspects of this host—parasitoid interaction.