Cryptic combat against competing microbes is a costly component of parental care in a digger wasp

Microorganisms are ubiquitous and key selective agents in parental care behaviour. Various animal species have thus evolved diverse means to prevent and combat detrimental effects of microbial competitors and pathogens on their offspring. The European beewolf, Philanthus triangulum, is a solitary crabronid wasp that stores paralysed honeybees as larval provisions in subterranean nests. In the soil the highly nutritious provisions can be attacked by a large number of microorganisms. We investigated the effect of fungal infestations of larval provisions on the reproductive success of beewolves. Although fungal infestations reduced larval survival, female beewolves largely eliminated this effect by embalming larval provisions with ample amounts of hydrocarbons. However, this came at the cost of a reduced ability to apply hydrocarbons to the provisions of future offspring. This reduced investment in embalming resulted in a higher risk of mould infestations. Embalming of the prey in P. triangulum thus meets the criterion of parental investment and females have to balance this investment of hydrocarbons in current and future offspring to maximize their lifetime reproductive success.