|Photo of a queen hornet (from Fig. 2 of the paper)|
A team of scientists in Japan decided to find out just what those infected queens are up to. For three months between May and August, they made regular weekly visits to a predesignated sites in a forest at the foot of Mount Moiwa and set up a video cameras to observe the decayed logs in the morning and afternoon.
|Photo of a hornet releasing |
some S. vespae juveniles
(from Fig. 2 of the paper)
When they dissected hornets to see how many of them were infected and to check the developmental stage of their parasites, they found a seasonal pattern to the infections. Queens caught during May and June were mostly infected with fully-mature female worms and their eggs, while queens caught between July and throughout August were filled with juvenile worms that were ready to disembark and infect a new host - which just so happen to be the period when parasitised queens start making regular visits to potential hibernation sites.
So that is S. vespae's game - use the hornet as a mobile incubator/nursery, fly her around during summer to scope out the best pieces of real estate around the forest, then drop off a bundle of worms that can lie in wait like a booby-trap for an uninfected hornet queen to come along and settle in for winter. To complete its life cycle, S. vespae simply take advantage of a preexisting behaviour (seeking out hibernation sites) from the host's repertoire, and "switch it on" at a different time of year to fit the developmental schedule of the parasite's own offspring. Parasite manipulation isn't necessarily about teaching an old host new tricks, but to get the host to perform the tricks that it already knows in a brand new context.
Sayama, K., Kosaka, H., & Makino, S. (2013) Release of juvenile nematodes at hibernation sites by overwintered queens of the hornet Vespa simillima. Insectes Sociaux 60: 383-388.