Contrasting scales of oviposition and parasitism in praying mantids

Abstract

We report on spatial patterns of parasitism of oothecae (egg cases) of praying mantises (Stagmomantis limbata) by torymid wasps, Podagrion spp. Using collections of mapped mantid oothecae from Riparian sites in the Sonoran desert and Grassland sites in the Chiricahua Mountains (both in Arizona, USA), we characterized the spatial distributions of oothecae and parasitism. The likelihood of an egg case suffering some parasitism was higher at Grassland sites, which had high oothecal densities, than at low-density Riparian sites. However, experimental isolation of Grassland oothecae to densities comparable to Riparian sites reduced parasitism rates. At Riparian sites, parasitized oothecae exhibited on average the same extent of parasitism as parasitized oothecae at high densities but with much greater variation. Indeed, large fractions of Riparian oothecae suffered both severe (Ͼ50%) and light (Ͻ20%) parasitism, whereas most parasitized Grassland oothecae suffered intermediate levels of parasitism. Analysis of first nearest neighbor distances indicated that the parasite load of an ootheca did not depend on its immediate isolation. However, extending the analysis to include subsequent nearest neighbors (using a technique from spatial statistics called the R(K) function), we found that even though oothecae of S. limbata were spatially clustered, some oothecae in a (statistically defined) cluster escaped parasitism when overall oothecal densities were low. This pattern suggests that when oothecae are sparsely distributed, Podagrion wasps exploit only a fraction of the oothecae available locally, even though the oothecae are strongly aggregated relative to their overall density. We suggest this lack of congruency in the scales of oothecal deposition and parasitism at low densities (which is absent when oothecae are at high densities) may be explained in part by behavioral aspects of the parasite's reproduction, including increased host fidelity by relatively sedentary female parasites.

Habitat edges as a potential ecological trap for an insect predator

“…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.…”

“…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.…”

“…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.…”

“…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.…”

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

“…Data for oothecae marked but left in the field until after emergence (b; see Fig. 1) were estimated from regression relationships (Fagan & Folarin 2001). winter-collected oothecae suffered unusually extensive bird predation.…”