Parasitic wasps turn caterpillars into mind-controlled bodyguards
The parasitic wasp Cotesia glomerata injects its eggs inside live caterpillars. The larvae feed on the host's body fat, eventually chewing their way out to spin cocoons. Surprisingly, the caterpillar survives this exit and is chemically manipulated to defend the wasp cocoons from predators, starving to death once the wasps hatch.
The Life Cycle of a Specialized Parasitoid
Cotesia glomerata is a small braconid wasp whose life cycle relies entirely on the exploitation of other insects. Unlike predators that immediately kill and consume their prey, parasitoids develop inside or on a living host, eventually causing its death only when their own development is complete. Cotesia glomerata specializes primarily in the larvae of pierid butterflies, particularly the large white butterfly Pieris brassicae and the small white butterfly Pieris rapae, both common agricultural and garden pests known as cabbage worms.
The interaction begins when an adult female wasp detects chemical cues released by plants that are actively being eaten by caterpillar hosts. Following these volatile compounds, the female locates a young caterpillar and uses her sharp ovipositor to pierce its cuticle. In a matter of seconds, she deposits a clutch of dozens of microscopic eggs directly into the host's body cavity, setting off a complex internal chain of events that seals the caterpillar's fate.
Subverting the Host Immune Defense
In nature, a caterpillar's immune system would normally recognize foreign bodies such as insect eggs and destroy them through encapsulation, a cellular process where hemocytes surround the invader and suffocate it with melanin. To prevent this defensive reaction, the female wasp injects specialized venom and symbiotic polydnaviruses alongside her eggs during oviposition. These viral particles infect the caterpillar's cells, disrupting its immune system and altering its internal physiology.
Once hatched, the wasp larvae remain inside the caterpillar's hemocoel, bathed in the host's hemolymph. They feed selectively on the caterpillar's non-vital tissues, such as the fat body, while carefully leaving vital organs intact. This precise feeding strategy keeps the caterpillar alive and feeding for several days, allowing it to continue gathering nutrients from host plants. The caterpillar effectively serves as a living incubator and nutrient supply, growing in size even as dozens of parasites multiply within its body.