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

Abstract

1. The chrysomelid beetle, Cephableia consanguinea (Hispinae), is a monophagous herbivore of ffeliconia imbricata (Zingiberales: Heliconiaceae) in wet lowland forests of eastern Costa Rica.2. Within the study area parasitism by eulophids and trichogrammatids (Hymenoptera: Chalcidoidea), the most common source of Cconsanguinea egg mortality, varied in intensity from 35% to 50% during 1974-77.3. 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.imbria?a 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.4. 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. 5. 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.

Relative Impact of Interactions within and between Trophic Levels During an Insect Outbreak

“…It is doubtful that failure to detect spatial density dependence here is an artifact of sampling over the wrong spatial scale, since I examined parasitism over four scales (per leaf, twig, branch, and tree) that are likely to span the relevant patch size for searching parasitoids. In addition, the few significant positive (or negative) regressions were not associated with particularly lowdensity (or high-density) leaves or twigs, as might be expected if high host density per se was responsible for the lack of spatially density-dependent parasitism(Morrison and Strong 1981, Lessels 1985). This does not seem to be simply a consequence of the extremely high host densities resulting in egg depletion or foraging time restriction (Lessels 1985).…”

Aggregation in field parasitoid populations: foraging time allocation by a population of Diadegma (Hymenoptera, Ichneumonidae)

“…Our present understanding of parasitoid foraging in nature is based largely on inference from field patterns of parasitism (Morrison & Strong, 1980, 1981Hassell, 1980;Heads & Lawton, 1983), which has some limitations: absence of parasitism in a patch does not necessarily indicate absence of foraging parasitoids (Chesson, 1982), and parasitoids allocating foraging time in different ways may generate identical patterns of parasitism (Hassell, 1982). Our present understanding of parasitoid foraging in nature is based largely on inference from field patterns of parasitism (Morrison & Strong, 1980, 1981Hassell, 1980;Heads & Lawton, 1983), which has some limitations: absence of parasitism in a patch does not necessarily indicate absence of foraging parasitoids (Chesson, 1982), and parasitoids allocating foraging time in different ways may generate identical patterns of parasitism (Hassell, 1982).…”

Patterns of parasitism by insect parasitoids in patchy environments

“…Indeed, it is quite likely t o find a positive aggregative response leading to an inverse density dependent relationship between per cent parasitism and host density per patch. This range of patterns for the spatial distribution of parasitism has already been noted by Morrison et al (1 980) and Morrison & Strong (1980, 1981. They stress that in analysing relationships between parasitism and host density per patch, it is necessary to account for both the proportion of patches discovered and, within these, the proportions of host parasitized.…”