Heterorhabditis bacteriophora

The parasite being featured today is one of a handful of parasitic nematodes that you can purchase from your local gardening supplies store. There microscopic worms, called entomopathogenic nematodes (EPN), are commonly used by both farmers and gardeners as a weapon against a range of insect pests. Most people who use such worms probably don't give much thought to how these worms kill insects - as long as they work as intended, then it is out-of-sight and out-of-mind. But it is worth mentioning how these worms actually do their killing.

Image source: Peggy Greb, USDA Agricultural Research Service, Bugwood.org

Technically speaking, Heterorhabditis bacteriophora itself doesn't kill the insect, the real killer is a bacterial symbiont that it carries. Bacteria from the Photorhabdus genus have co-evolved into a mutually beneficial partnership with nematodes like H. bacteriophora. When the worm infiltrates into an insect, it unleashes its deadly bacterial payload. The released Photorhabdus then proceeds to multiply rapidly, flooding the insect's system with toxins and converting its innards into a nutritious goop for the nematodes to grow in.

It takes the worms about 20 days of bathing and drinking in the bug goop to complete incubation and produce the next generation of infective worms - in the mean time, they do not want to be disturbed. However, to most larger animals, an immobile insect is simply an easy meal - if a bird or another animal happens upon the worm-filled bug, they just gobble it up - and this kills both the nematodes and its bacteria.

A few years ago, it was discovered that the worm-bacteria duo also change the colour of the insect in order to deter birds. About 24 to 36 hours after the insect has been killed, it starts to glow, and over the course of a week its colour changes from a tangerine orange to a bright pink red. But what about for hungry animals that don't judge prey with their eyes?

In this study, researchers conducted a series of experiments to see if this parasite has some other tricks to ward off those predators. For their model predator, they used carnivorous ground beetles (carabids) and offered them frozen waxworms (caterpillar of wax moth), some of which were parasite-free, others had previously been infected with H. bacteriophora for a few days

They found that the beetles consistently preferred the parasite-free waxworms, in fact the beetles are more likely to stay away from waxworms which had been festering with H. bacteriophora for longer periods. To further examine if it was indeed the smell rather than colour that were deterring the hungry beetles, the researchers mashed up some of the infected waxworms, and the beetles still preferred the mashed-up parasite-free waxworms over those with nematodes and its bacterial symbiont.

It seems that the H. bacteriophora-infested cadaver not only change colour, but also emits a repellent smell. But wait, isn't that just what happens with a rotting corpse? However, the odour associated with insects killed by a H. bacteriophora infection is distinctively different from decaying insects that had died through other causes. The researchers notice this themselves while raising colonies of H. bacteriophora. This is probably because while most dead insects are colonised by a variety of different microbes, those killed by these nematodes are colonised almost exclusively by Photorhabdus and its host worms.

It should be pointed out that there might be an alternate explanation for why those carabid beetles avoided the infected waxworm. Heterorhabditis bacteriophora can infect a variety of insects, and depending on how far along the infection has developed, the beetle can potentially become the parasite's next victim if it starts chowing down on a H. bacterophora-killed insect. So they might be avoiding the infected waxworms for self-preservation rather than responding to some kind of special repellent emitted by the parasite colony. But if that is the case, this still achieves the intended effect, which is for the colony of growing parasites to be left alone. Furthermore, another study has shown insects killed by similar nematodes are also distasteful to other animals as well, such as fish which cannot be infected by H. bacteriophora

While killing the host is a good (and drastic) way of shutting down the immune system, thus leaving the parasite free to do what it wants, it also leaves it vulnerable to other predators and microbes that might make a meal of the now defenceless host. So H. bacteriophora and its symbiont have to provide it with a new type of protection.

This study also provides a nice tip for any prospective zombie fiction writers - if you want some kind of science-y explanation for why your walking dead do not succumb to the multitude of organisms that would gladly feast on a human cadaver, H. bacteriophora just handed you an idea, straight from wonderful mother nature.

Reference:
Jones, R. S., Fenton, A., & Speed, M. P. (2016). “Parasite-induced aposematism” protects entomopathogenic nematode parasites against invertebrate enemies. Behavioral Ecology 27: 645-651.