Nesting strategies and disease risk in necrophagous beetles

Abstract

While the effects of carcass decomposition on microorganisms have been demonstrated in recent years, little is known of how this impacts necrophagous insects. A common assumption is that insects that exploit carcasses are exposed to a high density of potentially harmful microorganisms, but no field data have so far validated this. Necrophagous beetles such as the Scarabaeinae have complex nesting behaviors with elaborate parental care. So here, we begin to explore whether this conjunction of life history and nesting behavior represents an adaptive response to the threat posed by microbes in these environments, mainly by entomopathogens. We evaluated the density and distribution of fungi and bacteria from soil near the carcasses, and their ability to infect and kill insects that are in contact with this soil during the decomposition process. Our data showed an increase in the density and activity of opportunistic or facultative pathogens during the apex of decomposition, when there is a predominance of necrophagous insects. Meanwhile, the survivorship of bait insects decreased when in contact with soil from this period of decomposition, indicating a potential risk of infection. However, the density and activity of these microorganisms decreased with distance from the carcass, mainly with depth, which would benefit tunneller beetles in particular. We have thus provided the first field data to show that necrophagous insects are indeed exposed to high densities of potentially harmful microorganisms. Furthermore, we propose that some parental care strategies may have arisen not only as a response to competition, but also as adaptations that reduce the risks of disease. Although we have focused on carrion feeders, we suggest that the same occurs with coprophagous beetles, as both carrion and dung are nutrient‐rich resources.

The nesting preference of an invasive ant is associated with the cues produced by actinobacteria in soil

“…The risk of predators, pathogen infection and abiotic condition is a major criterion used by insects when choosing nesting sites [1,2]. In particular, for social insects that nest in soil, pathogens are a significant danger that varies greatly from place to place [3][4][5][6]. Though some…”

How to escape from insect egg parasitoids: a review of potential factors explaining parasitoid absence across the Insecta

“…B 287: 20200344 other insects bury their eggs in soil, such as necrophagous beetles of the Scarabaeinae. These beetles eat and breed in decaying organic matter (dung and/or carrion) and show complex nesting behaviour with elaborate parental care, which is seen as an adaptive response to the threats of microbes in these environments [39]. Generally, insects that bury eggs seem to escape from egg parasitism, with some exceptions (see below).…”

Defences against brood parasites from a social immunity perspective

“…Avoidance behaviours can be employed against classic parasites in a social immune context, for example by avoiding laying eggs or raising young in contaminated locations. Carrion-breeding dung beetles have been shown to roll the carrion balls that they use to provision their young a distance from the carcass, either horizontally, or by digging to depths of up to 1 m, at which the concentration of microbes, particularly those that cause infection, is greatly reduced [37]. The removal of corpses from a communal nest is a social defence found in ants [38], bees and termites [39], thus reducing the risk of infection to other colony mates.…”

“…Both benefit from avoiding parasitism in the first place, whether that is from classic or brood parasites. More direct defences employed prior to parasite encounter, such as the construction of defensible [56,69] or concealed nests [13,37,69,70], or the collection/production of anti-parasite substances [57 -64] have the potential to induce some conflict, because the donor is paying high energetic and time costs for the construction/protection of the nest/kin, and may have to do this on multiple occasions for future broods. The donor must balance the costs of investing now against its residual reproductive value, and therefore recipients will value greater levels of investment in protection than donors will be selected to provide [10,14,33].…”