Linking the Morphology of Sternal Glands to Rubbing Behavior by Vespa soror (Hymenoptera: Vespidae) Workers During Recruitment for Group Predation

Biology of the southern giant hornet, Vespa soror: nest architecture, morphological differences among castes, and the genetic structure of colonies

Giant hornets in the genus Vespa are apex predators that are known throughout Asia for their exceptional size and devastating group attacks on social insect colonies. The giant hornets include Vespa mandarinia, a well-studied and widespread temperate species, and Vespa soror, a poorly known sister species that is limited to subtropical and tropical regions of Southeast Asia. Both species have been recently documented on the west coast of North America, raising urgent questions about their potential impact in novel ecosystems. To better understand the biology of V. soror, we describe the nest architecture, caste morphology, and genetic structure of colonies collected in Vietnam. Comparisons of colony metrics between the two giant hornet species suggest important differences that are likely a consequence of the relatively warmer climate in which V. soror occurs. Like V. mandarinia, V. soror constructs large, underground nests of partially enveloped horizontal combs. However, compared to temperate V. mandarinia colonies, the longer nesting period of subtropical V. soror colonies likely resulted in relatively larger colony sizes and nests by the end of their annual cycle. Vespa soror workers and gynes were larger than males, distinguishable based on wing shape and body size (total length and measures of six body parts), and equivalent in size to female castes of V. mandarinia. We genotyped colony members from three mature nests, which revealed that males and females were offspring of singly mated queens. Two colonies were monogynous, but one colony was comprised of two unrelated matrilines. Polygyny has not been observed for V. mandarinia, but is more common in tropical hornet species. Our study sheds light on essential details about the biology of an understudied species of giant hornet, whose populous colonies and long nesting period suggest the potential for substantial ecological impact wherever they occur.

Comparison of the External Morphology of the Sternal Glands for Hornets in the Genus Vespa

Simple Summary

Hornets are a group of 22 species of highly social wasps in the genus Vespa. These conspicuous insects construct large nests that are maintained by one or more queens and their offspring. Unlike other insects, many species of social wasps have glands on the underside of their abdomen called the van der Vecht and Richards glands. Documented uses for these glands by workers of some hornet species include producing pheromones for nestmate recruitment during group attacks on prey and chemical defense of nests against ants. We confirmed that these glands were present in nine hornet species. To examine differences among species in gland morphology, we used scanning electron microscopy to image the external surface of worker bodies around the glands. Accounting for differences in body size, we found that giant hornets and their close relatives had comparatively larger glands than the other species, some of which had glands that were approximately half as large. Differences among species are best explained by how hornets hunt and not by the likelihood that their nesting habitat increases exposure to ant predation. However, more information about how hornet workers use their sternal glands is necessary to permit further interpretation of their comparative gland morphology.

Abstract

Many social wasps in the speciose subfamilies Polistinae and Vespinae have two sternal glands—the van der Vecht gland and the Richards gland—that are not found in other insects. The presence of these glands has been confirmed in only 6 of 22 hornet species (genus Vespa) and images of their fine structure have not been produced. Here; we characterize the external morphology associated with both glands for workers of nine Vespa species using scanning electron microscopy. All hornets had similar gland configurations; although gland-associated external features differed among species. Scaled for size, glands were equivalently sized for the giant hornets (V. mandarinia and V. soror) and their closest phylogenetic relatives (V. tropica and V. ducalis). Relative size of gland-associated structures was reduced by half for V. simillima; V. velutina; and V. affinis workers. The remaining species (V. crabro and V. analis) had intermediately sized features. Differences among species in external gland structure were best explained by selective pressures related to predatory behavior, rather than defense of nests against ants. However, a lack of information about how Vespa workers use their van der Vecht and Richards glands limits a comparative interpretation of the function of their external gland morphology.