I’m sure, even if you haven’t seen them, most of you are at least somewhat familiar with the movies in the Alien franchise and the premise behind them. You’re probably extremely grateful that there aren’t any terrifying creatures like xenomorphs stalking the earth, right? Well, that’s not entirely true. For tobacco hornworms (Manduca sexta), the caterpillars of a species of sphinx moth throughout the Americas, the horrifying scenario of Alien is a regular occurrence. This is thanks to a tiny parasitoid wasp called Cotesia congregata in the braconid family. Parasitoids are different from parasites in that they depend on the host for survival and keep it alive until they no longer have a use for it. At this point, they move on to a different stage of life, destroying the host in the process.
When it comes time to reproduce, the female wasp seeks out a larval tobacco hornworm and lays her eggs inside the body of the unfortunate caterpillar. With the eggs, she deposits a special symbiotic polyDNAvirus as well. This virus lives and multiplies exclusively inside the wasp’s reproductive system and is like a part of the wasp itself. The virus can’t reproduce on its own because it doesn’t possess the necessary genes to replicate. Instead, the genetic codes for the virus are present in the wasp’s genome. When this virus enters the caterpillar along with the eggs, it infects the caterpillar’s cells and dismantles its immune system. This protects the wasp eggs from being encapsulated and killed by the caterpillar’s hemocytes (insect blood cells). In addition to immune suppression, the virus arrests the development of the host and prevents metamorphosis into a moth. It also causes the accumulation of certain proteins in the host’s nervous system that allow it to survive for longer periods of time without food or water and alters its metabolism. This serves to keep the host alive to protect the developing wasps until they move onto their next life stage.
While inside the tobacco hornworm caterpillar, the C. congregata larvae hatch from their eggs and feed on the host while they grow larger and larger. By the time they are old enough to move on, the larvae have essentially eaten the host from the inside and chew their way out of its body to spin cocoons so they can pupate into adults. The larvae don’t travel away from the caterpillar at all to do this and create their cocoons right there on its body. After they pupate, the wasps emerge as fully formed adults and take to the air in search of mates in order to start the gruesome cycle all over again.
As terrible as it is for the hosts, C. congregata and many other species of parasitoid wasps are used as a means of biological control for a myriad of Lepidopterans (butterflies and moths) and other insects. There are many different species of parasitoid wasps and each of them primarily parasitizes a specific host insect. Many hosts are considered agricultural pests and wasps can be purchased and released to do their horrifying work without the destructive effects of pesticides or other artificial pest control techniques. Still, not all the insects that parasitoid wasps prey upon become hosts. The majority of them get to develop into their adult stage like our intriguing friend, the tobacco hornworm.
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2. Beckage, Nancy E., et al. “Characterization and biological effects of Cotesia congregata polydnavirus on host larvae of the tobacco hornworm, Manduca sexta.” Archives of Insect Biochemistry and Physiology 26.2‐3 (1994): 165-195.
3. Beckage, Nancy E., and Michael R. Kanost. “Effects of parasitism by the braconid wasp Cotesia congregata on host hemolymph proteins of the tobacco hornworm, Manduca sexta.” Insect biochemistry and molecular biology 23.5 (1993): 643-653.
4. Beckage, Nancy E., and Dale B. Gelman. “WASP PARASITOID DISRUPTION OF HOST DEVELOPMENT: Implications for New Biologically Based Strategies for Insect Control*.” Annual Reviews in Entomology 49.1 (2004): 299-330.