Can vertebrate predation alter aggregation of risk in an insect host–parasitoid system?

Abstract

Summary1. Insect host-parasite systems allow investigations of the trophodynamics of ecological communities within a well-formed theoretical context. A little explored feature of such systems involves the interplay between generalized consumers and host-parasitoid dynamics. I report a study investigating how the impacts of generalized consumers, viewed here as interaction modifications, may influence the stability of a particular interspecific interaction. 2. In a study involving overwintering oothecae of the praying mantis Stagmomantis limbata (Hahn), birds damaged 36% of oothecae, 85% of which had also been parasitized by winter-active, multivoltine torymid wasps of the genus Podagrion. Birds preferentially preyed upon oothecae oviposited high on trees, but such predation was often incomplete, leaving both viable and parasitized mantid eggs inside damaged oothecae. 3. Two factors allowed bird damage to influence the distribution of risk of parasitism among oothecae and among hosts. These were (1) that some parasites − but no mantids − emerged successfully prior to bird predation and (2) that extensive post-damage foraging by parasitoids occurred, but that the nature of this foraging was altered little by oothecal damage. 4. In all, bird damage engendered a fourfold increase (from 1·5 to 6·5) in the CV 2 of parasitism risk among hosts (a stability criterion that has been proposed for hostparasitoid interactions) and increased the relative importance of host-density-dependent parasitism. The role of timing of the two natural enemy impacts for stability of the host-parasitoid interaction is discussed.

Habitat edges as a potential ecological trap for an insect predator

“…In this study, increased bird predation on mantid oothecae near edges may be due to higher densities of birds, or simply be a density‐dependent response to increased availability of prey. Differences in oothecal height have previously been shown to be associated with differing rates of bird predation (Fagan, 2002), but this was not evident in the data. In contrast, Podagrion egg parasitoids appear to respond to habitat spatial structure quite differently than do the mantids whose oothecae they are searching for (Fagan & Folarin, 2001), so it is not surprising that a spatial effect of egg parasitism was not identified among the three distance classes.…”

“…Because the entire region was affected by a La Niña event that greatly reduced productivity of the desert grasslands, densities of oothecae were generally very low in comparison with prior studies in the area (Fagan & Folarin, 2001; Fagan, 2002). Consequently, oothecae were assigned to one of three broad classes based on distance to edge (0–10, 11–50, and 51–100 m) and pooled among years to increase sample sizes.…”

“…Collected oothecae were brought to the lab to determine parasitism and predation rates following the procedures detailed in Fagan and Folarin (2001) and Fagan (2002). Emergence potentials of oothecae were estimated volumetrically; parasitoid exit holes were counted to obtain an accurate index of parasitism rates; and oothecae were scored for evidence of physical damage due to bird predation.…”

“…In southern Arizona, S. limbata is univoltine, with nymphs hatching in spring and oothecae (egg cases) being deposited from August to November by adults that do not overwinter (Roberts, 1937). Mantid oothecae are preferentially deposited on thin woody branches of shrubs from 1 to 5 m off the ground (Fagan, 2002) and are relatively obvious when present. Oothecae are subject to bird predation as well as attacks by torymid parasitoids from the genus Podagrion (Breland, 1941; Grissell & Goodpasture, 1981; Fagan & Folarin, 2001; Fagan, 2002).…”

“…Mantid oothecae are preferentially deposited on thin woody branches of shrubs from 1 to 5 m off the ground (Fagan, 2002) and are relatively obvious when present. Oothecae are subject to bird predation as well as attacks by torymid parasitoids from the genus Podagrion (Breland, 1941; Grissell & Goodpasture, 1981; Fagan & Folarin, 2001; Fagan, 2002). Because oothecae remain where they were originally deposited, and show easily quantifiable signs of bird predation and wasp parasitism (Fig.…”