The diminutive Trichogramma wasp parasitizes hawk moth eggs. The 28 shown above came from a single Manduca egg. (Photo credit: Kristen Potter)

By Josh Lancette

You may not be able to teach an old dog new tricks, but it turns out you can teach an old parasitoid wasp new tricks—and only the old ones.

A new study in the Journal of Insect Science reports that Trichogramma wasps are able to learn as adults the olfactory cues to find plants that host hawk moth eggs, which the wasps parasitize, but they don’t learn the cues while pupating, which other insects have been shown to do.

In other words, old Trichogramma wasps can learn, but young ones can’t. This result came as a surprise to the researchers.

Josh Lancette

“We expected the wasps to use olfactory cues given off by the plants—it’s been documented by other researchers for other species,” said Keaton Wilson of the University of Arizona, one of the researchers and an author on the study.  “We were a bit surprised, however, by the fact that wasps didn’t learn cues while pupating inside their host egg. There are other really cool examples of insects retaining memories through metamorphosis, and it seems like learning the smells associated with host plants while you’re developing on a plant might be a tremendous boon to finding other plants of the same species once you emerge.”

The findings of the study are important because the wasps are often used as biocontrol agents of pests.

“We grow and release Trichogramma and other related species to combat agricultural pests, just as people release ladybugs in their gardens as pest control, but on a much larger scale,” said Wilson. “There have been success stories from biocontrol but also problems, and I think it’s imperative that we grasp the full effect that biocontrol agents have on natural ecosystems. Insects, even tiny ones, aren’t static organisms; they can learn, they can learn at different life stages, and information can even be passed through generations through mechanisms like maternal effects. We need a complete understanding of their behavioral ecology and how they function in natural systems in order to make sound predictions about the impacts of their release in biocontrol contexts.”

The wasps need to learn olfactory cues of plants in order to find potential hosts. When a potential wasp host is eating a plant, the plant emits different chemicals than it does when it isn’t being eaten. The wasps then smell those chemicals, and it signals to them that they can find a host on the plant.

“Like a lot of insects, these little wasps are great at smelling things,” said Wilson. “But, instead of trying to smell out the eggs, they smell out the plants that the eggs are on, because plants are smellier than eggs. Not only can they find plants by the way they smell, but these little wasps can learn what smells are linked to eggs, just like we learn to associate certain smells (like popcorn) with certain places (like the movie theatre).”

Read more: “Innate and Learned Olfactory Responses in a Wild Population of the Egg Parasitoid Trichogramma (Hymenoptera: Trichogrammatidae)” (Journal of Insect Science)

Josh Lancette is Manager of Publications at the Entomological Society of America.