Brood reduction caused by sibling cannibalism in Isodontia harmandi (Hymenoptera: Sphecidae), a solitary wasp species building communal brood cells

Abstract

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.

Sibling Cannibalism in Group-Living Larvae of the Solitary Wasp,Isodontia harmandi(Hymenoptera: Sphecidae): Effects of Prey Availability and Size Discrepancy

“…In this study we first determined whether the larval cannibalism of I. harmandi is induced by prey shortage. Although our previous study showed that per capita prey provision was smaller in large broods and that cannibalistic events occurred more frequently in large broods (Imasaki and Endo 2022), the occurrence of cannibalism was not related directly to the amount of prey. Here, we experimentally examined the likelihood of cannibalism in pairs of wasp larvae under controlled conditions with different levels of prey availability in artificial nest chambers.…”

“…Female wasps of Isodontia harmandi build a nest in pre-existing holes, such as abandoned beetle tunnels and the cut ends of horizontally piled bamboo canes (Tsuneki 1963). They use several prey species from among the tettigoniid insects as food for their larvae (Iida 1934, Iwata 1936, Tsuneki 1963, Imasaki and Endo 2022). Females lay an egg on the first prey (pedestal prey) brought into a cavity closed with a loose temporary plug, and repeat the prey-hunting and egg-laying cycle a few to a dozen times within the same brood cell.…”

“…Our previous study showed that the total amount of prey per cell did not increase with the number of eggs (clutch size), and therefore for a larger clutch the amount of prey provided decreased significantly on a per capita basis (Imasaki and Endo 2022). In addition, it was concluded that brood reduction occurs routinely through larval cannibalism in I. harmandi nests, based on two lines of evidence: decline in the size of the average brood during the developmental stages of the brood in nests collected from the field, and frequent observations of larval cannibalism in broods reared under laboratory conditions (Imasaki and Endo 2022).…”

“…The solitary wasp, Isodontia harmandi (Pérez) (Sphecidae: Hymenoptera) has some features that are advantageous to the study of sibling rivalry in both cannibalistic and resource-based contexts. The female wasp of this species builds a large communal brood cell in a preexisting cavity, in which several to a dozen of her offspring grow together, relying on the resources she provided (Iida 1934, Iwata 1936, Tsuneki 1963, Imasaki and Endo 2022). This developmental fashion is similar to those of gregarious parasitoids rather than those of other solitary wasps that build a separate cell for a single offspring.…”

“…The second purpose of this study was to examine the factors other than prey availability influencing the occurrence of cannibalism: sex and siblinghood. As brood of I. harmandi is usually composed of either all females or all males (single sex-brood; Iida 1934, Iwata 1936, Tsuneki 1963), Female larvae may be more aggressive than male larvae under the same condition of resource availability, because female wasps are larger than male wasps (sexual size dimorphism; Tsuneki 1963, Imasaki and Endo 2022) and therefore females require larger amount of prey to complete development. Furthermore, if there is a sex difference in the likelihood of cannibalism, sex-biased cannibalism would occur in mixed-sex broods, which has rarely been reported in I. harmandi nests.…”