Wasps, hornets, and other Hymenoptera may live nearly solitary lives, live in huge colonies, or something in between. The European hornet, Vespa crabro, lives in a colony consisting of one queen mated to a single male. In Hymenoptera, females are typically diploid (having genes from both parents) while males are typically haploid (having genes only from the female parent). If you draw a diagram of this and stare at it for a long time, you may come to the same conclusions that Bill “Buzz Off” Hamilton came to several years ago. A female would benefit genetically from helping her mother raise more sisters to a greater extent than she would benefit from having her own offspring, because she will be related to her sisters by 75% but to her offspring by 50%. Depending on other conditions, of course.
Males, on the other hand, are related to their siblings by the usual 50%, but the females are related to their brothers by a low 25%. This causes an asymmetry whereby a sister may happily feed her brother to another sister, and the brother may mind less than you think.
So, with that backtground in mind, let’s look at the interesting phenomenon of Zombies among Vespa crabro.
Scientists looking at this species made predictions based both on theory and on comparisons to a second, similar, species of hornet, Dolichovespula arenaria. In Dolichovespula arenaria, workers (females) produce males (from their unfertilized eggs, as the queen does), but in Vespa crabro, there seems to be no males produced by workers.
Why do the former produce males at all? Well, if you are a worker in a eusocial insect colony, you can’t mate with a male, and therefore it would seem that you can’t have offspring. But since males develop from unfertilized eggs in these critters, you could produce males who might, if you are lucky, mate with the present queen, giving you nieces. In the species where this happens, we see the queen in conflict with these males, demonstrating that there is some strategic behavior going on here.
The lack of males produced by workers was a bit of a surprise:
In contrast to our prediction, the data show that hornet males are queens’ sons, that workers never attempt to lay eggs, rarely have activated ovaries, and that there is no direct aggression between the queen and the workers. This contrasts with other data for vespine wasps, which support relatedness predictions. Dolichovespula arenaria has the same kin structure as V. crabro and workers produce males in many colonies. The similarity between these two species makes it difﬁcult to explain why workers do not reproduce in V. crabro. Self-restraint is expected if worker reproduction signiﬁcantly reduces colony productivity but there is no obvious reason why this should be important to V. crabro but not to D. arenaria. Alternatively, queen control may be important … [but the] absence of expressed queen-worker conﬂict rules out physical control.
So what the heck is going on here? The scientists suggest that the wasps with the males manage this because the female converts the males into zombies that are under her control.
Indirect pheromonal control is a possibility and is supported by the occurrence of royal courts and queen pheromone in Vespa but not Dolichovespula. Pheromonal queen control is considered evolutionarily unstable, but could result from a queen-worker arms race over reproductive control in which the queen is ahead. The genetic data also revealed diploid males in one colony, the ﬁrst example in the vespine wasps, and two colonies with double matrilines, suggesting that occasional usurpation by spring queens occurs.
Foster KR, Ratnieks FL, & Raybould AF (2000). Do hornets have zombie workers? Molecular ecology, 9 (6), 735-42 PMID: 10849289