Sphaerularia vespae

Hornets can put fear into the minds of many people, but today meet the parasite that the hornets fear (if they are capable of fear). Sphaerularia vespae is a parasitic nematode that infects the Japanese yellow hornet (Vespa simillima) and as far as infection goes, this one is quite a doozy. It specifically invade and resides in the gaster (abdomen) of female hornets where it grows and develop. The nematode ends up sterilising the host (much like other parasitic castrators we have featured on this blog), turning her into a cozy nursery for baby worms. But a new study has shown that they are capable of doing more than just castrate the hornet.

Photo of a queen hornet (from Fig. 2 of the paper)

In a previous study, a group of scientists noticed that the majority of overwintered hornet queens caught in bait traps were infected with S. vespae, so there is something about these nematode-infected hornets which seems to make them more likely to end up in those traps. During autumn/fall, queen hornets fortunate enough not to be infected with S. vespae would visit and poke around various nooks and crannies (usually decayed logs) in the forest to find a spot to hibernate. When the hornet find a place she likes, she will start excavating a hibernacula ( a place to hibernate) and line it with plant fibres that serve as nesting material. But queens that are parasitised and sterilised by S. vespae start visiting decaying logs much earlier during early to mid-summer.

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)

They saw that unlike other hornets, the nematode-infected queens never dig nor gather nesting material. They simply crawl inside a decayed log, hang out for a while, then fly off. That is because they have become sterilised couriers that visited potential hibernation sites only to drop off a special package in the form of S. vespae juveniles. A quarter of the infect queens they saw landing on decayed logs offloaded some nematodes (there were some hornets that moved out of sight so the scientist couldn't see what they were up to). But in addition to those observations, the scientists also captured some hornet queens and brought them back to the laboratory for further examination. They kept them in vials and noticed that over two-third of the infected hornets ended up releasing juvenile worms.

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.

Reference:
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.

Paragordius varius

Photo of adult worm by Matthew Bolek

The nematomorphs, or horsehair worms, are well-known for their ability to persuade their insect host to jump into a pool of water, thus allowing the adult worm to escape and reproduce. After mating, the adult worm lays eggs which comes out in these long, white, spaghetti-like strings (see photo on the right). The eggs hatch into free-swimming larvae that then infect aquatic invertebrates such as freshwater snails, mosquito larvae, or other small aquatic critters. Once it infects this host, the larva takes between 5-14 days to develop into a cyst stage which is ready to infect a cricket where it can mature into an adult.

The trouble with studying a parasite like the horsehair worm is that because they have multiple hosts in their life cycle, in order to keep them in a laboratory you would have to also maintain colonies of all its host animals on stand-by to act as sacrificial hosts for the hairworm larvae to infect. Additionally, those little invertebrates are not always "in season" and they may not be available in sufficient number when the infective stages of the parasite are available for experimentation.

If scientists can somehow put the life cycle of these parasites on hold at each stage until suitable hosts become available for the parasites to infect, not only would it become less logistically challenging to maintain them in the laboratory, it would also allow scientists to carry out more detailed studies on their life cycles. Fortunately, there is an aspect of their biology that may allow scientists to do just that - the parasite we are featuring today - Paragordius varius - along with other hairworms that live in temperate regions are capable of surviving through winter either as a dormant larva or a cyst inside an aquatic invertebrate that waits until spring comes when there are cricket hosts around. During the winter months the larval or cyst stage of the parasite simply stay in a state of suspended animation as their surroundings freezes over.

This is also good news for scientists who wish to study them - these worms' ability to survive freezing means that the larval stages can be "put on hold" until suitable hosts become available. To explore the tolerance limit of these parasites, a team of scientists put some P. varius larvae and snails infected with P. varius cysts under a series of different conditions including freezing at -30°C or -70°C for 15-30 days or dried out at room temperature or -70°C for the same period of time.

Photo of P. varius larva from Nematomorpha.net

They found that larvae that has been frozen in water at both -30°C or -70°C still managed to infect snails once they have been thawed and they did it just as well as those that were not previously frozen. The only group that did not fare as well were the larvae that have been dried out at room temperature for a month. The cyst stage of the parasite were not as hardy as the larvae and experience a slight decrease in the number of cysts in snails that have been frozen compared to those that were not, and being dried out dramatically decrease the survival of P. varius cysts. Nevertheless those that did survive the freezing process were still able to infect crickets once they were thawed. And while P. varius seems to cope better with getting frozen rather than being dried out, the team who conducted this study also found that the cysts in the snail were better at surviving desiccation at -70°C than at higher temperatures.

So not only did this study reveal an interesting adaptation that allow these hairworms to complete their life cycle in temperate regions, it also discovered a way of making it easier for scientist to study them in the future. What had originally evolved in these parasites as a way for them to put their life on hold during those freezing winter may now also be the key for researchers to find out more about them.

Reference:
Bolek, M. G., Rogers, E., Szmygiel, C., Shannon, R. P., Doerfert-Schrader, W. E., Schmidt-Rhaesa, A., & Hanelt, B. (2013). Survival of larval and cyst stages of gordiids (Nematomorpha) after exposure to freezing. Journal of Parasitology 99: 397-402.