Studies on the Natural Enemy Complex of the Holly Leaf-Miner: The Effects of Scale on the Detection of Aggregative Responses and the Implications for Biological Control

Abstract

We measured holly leaf-miner (Phytomyza ilicis) densities, and mortalities imposed by a guild of specific parasitoids and birds, using nested quadrats varying in size from 0.03 to 1 m2. Mortalities imposed by three species of pupal parasitoids, one of which behaves as a facultative hyperparasitoid, were independent of host density at all spatial scales. The larval parasitoid, Chrysocharis gemma, in contrast, apparently aggregated strongly in regions of high host density, imposing spatially density-dependent mortality. The effect was most obvious at the smallest sampling-scale (0.03 m2) and became progressively weaker as the data were aggregated into larger sampling units. It could not be detected at all in samples of 1 m2. Within the smallest sampling units, aggregative responses (as revealed by host mortality imposed by C. gemma) were strongest in regions of low overall host abundance, and vice-versa. These results are in accord with predictions of parasitoid behaviour based on optimum foraging theory. Predation by birds, mainly blue tits Parus caeruleus, was inversely density-dependent. The possibility that birds may aggregate in response to very large patches of the holly leaf-miner (>1 m2) is discussed. Consistent with recent theoretical predictions, the species imposing spatially density dependent mortality (C. gemma) has been used as an effective biological control agent against the holly leaf-miner in Canada. Pupal parasitoids, which do not impose spatially density-dependent mortality, failed as biocontrol agents. Our results are discussed in relation to two problems, namely that of detecting an aggregative response by natural enemies in the field, and what constitutes a 'patch'. Prey grouped on the scale evoking an aggregative response by predators constitute 'patches'. Patches of the same species will be much larger for large active predators than for small, inactive ones. If the wrong spatial scale is chosen, it may be impossible to find evidence for aggregative responses in the field, even though they are present.