Sibling rivalry or brood reduction prevailing within bird nests is effectively avoided in solitary aculeate nests because the larvae of wasps and bees usually develop in each brood cell. However, a solitary wasp species, Isodontia harmandi, allows us to study brood reduction in a communal brood cell, where up to a dozen larvae develop in a group relying on prey provisioned by a female wasp. To demonstrate brood reduction in this species, we collected nests at various developmental brood stages from fields for five years (2010–2015). There was a significant decrease in the brood size between the nests sampled at the egg or hatchling stages and those at later stages when analyzing only data excluding nests that were parasitized, attacked by predators, or containing deteriorated prey. In whole brood-rearing experiments, we also confirmed that brood reduction occurred in 30 of 39 nests during larval stages and in 23 nests after cocoon stage. Larval survival was affected positively by total prey weight and negatively by brood size, though cocoon survival was not affected. A third-quarter (76%) of larval death was identified as sibling cannibalism through observation by time-lapse recording on multi-larvae rearing experiments. Therefore, we conclude that brood reduction routinely occurs as a result of sibling cannibalism in I. harmandi. Additionally, as we could not detect any positive effects of clutch size on the amount of provision, female wasps might overproduce offspring due to the unpredictability of available prey resources. Differences in brood size and reduction among sex categories were undetected, except for parental provisions. Thus, sibling cannibalism may efficiently regulate brood size in communal brood cells under prey shortage.
Scoliid wasps are ectoparasitoids that attack soil-dwelling scarabaeid larvae, and little is known about their host-searching behavior. In this study we investigated the cues used in host location by Campsomeriella annulata (Fabricius) and examined whether or not these wasps can detect hosts in the soil from the surface. In a dual-choice test with a Y-tube, female wasps were attracted to sand with host odor, sand with host feces, and sand used for rearing the host, the larvae of Anomala rufocuprea Motschulsky (Coleoptera: Scarabaeidae). In a dual-choice test for cues presented at a distance, the wasps did not discriminate between the Y-tube arms with and without cues. In an experimental arena in which host products and a host grub were buried 0.5 cm below the surface the wasps did not respond to the cues from the surface in terms of the burrowing frequencies and antennal tapping rates. Our results indicate that C. annulata searches for the host grubs by using kairomones, residual cuticular substances, or feces deposited in the soil as the grubs move through it, and that wasps cannot perceive the host in the soil from the surface. We discuss how scoliid wasps search for soil-dwelling hosts using cues that are reliable but not highly detectable.
Prey selection by a spider wasp, Batozonellus lacerticida, was investigated at the riverside of the Toyohira River in Misumai, Sapporo, Hokkaido over 7 years from 1981 to 1987. Seventeen species of araneid spiders were found in the study area and six of them were hunted by the wasps. Most (97.3%) of the 223 prey records obtained were of three species, Araneus macacus, A. marmoreus and A. pinguis. Nesting activity of the wasps continued until mid‐August in 1981, 1982 and 1986, but stopped in July in 1984, 1985 and 1987. The length of nesting period was related to whether the wasps successfully switched prey species from A. macacus to A. marmoreus and A. pinguis and A. pinguis or not Switching seemed to occur at the phase when density of A. macacus remained high, depending on the density of larger A. marmorus and A. pinguis. This switching had large effects on species composition and size distribution of actual prey. Chesson's index α calculated at every 10 day period revealed that the wasps preferred A. macacus to the two other species and large prey to medium and small ones. The effect of prey density on preference depended on whether A. macacus was present or absent. In the presence of it there were only slight differences in preference among various conditions of prey density. However, when A. macacus was absent, densities of both A. marmoreus and A. pinguis had some complementary effects on preference. The importance of prey size selection by pompilid wasps, and implications of density effects on preference are discussed.
The spatial and temporal distributions of scoliid wasps in the coastal sand dunes at Hakoishi, Kyoto Prefecture, Japan, were investigated using three different sampling methods in 2002 and 2003. Of eight scoliid species collected in the present study, five species, Scolia historionica, Campsomeriella annulata, Scolia decorata, Scolia oculata, and Megacampsomeris schulthessi, were dominant. The flying insects caught by Malaise traps and flower-visiting insects caught by insect nets were mostly males, and this biased pattern was due to the active mate-searching behavior of male wasps and their frequent visits to flowers to supplement energy consumed by such behavior. Given that the ground traps caught females exclusively, female wasps seemed to actively engage in host-searching behavior on and below the ground. Of the wasps caught by Malaise traps and flower-visit sampling, five dominant species showed spatially different habitat use: S. historionica and C. annulata mainly occupied the grassland zone on the plain (Gp), S. decorata occupied the grassland zone on the terrace (Gt) and the forest zone (Fp), S. oculata occupied the small scrub zone on the plain (Sp), and M. schulthessi occupied the small scrub zone on the terrace (St). Ground trap samples also indicated that S. historionica and C. annulata shared habitats. On the basis of the observed seasonal changes in wasp abundance and the degree of wing wear as an index of wasp age, S. historionica and C. annulata are thought to be bivoltine species, whereas S. decorata, S. oculata, and M. schulthessi are thought to be univoltine species. These scoliid wasp species may play an important role in pollinating coastal plants in the grassland zone.
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