The Curious Case of Parasitoids


The Curious Case of Parasitoids

weevil parasitoid

Although they differ from predators and parasites, parasitoids are important in pest suppression. Unlike predators, parasitoids do not kill their host immediately and unlike parasites, they eventually kill their host. Parasitoid wasps and flies have been keystone agents in biological control, in part, because of their specialization on certain pest families and species. Their life histories inspire science fiction. They are cryptic insects, but upon close observation, they can be found everywhere. Mass production of parasitoids has expanded their use against several pests.

Many sci-fi movies have utilized (either on purpose or by accident) the intriguing development of parasitoids as a

ichneumonid wasp
The ichneumon wasp is harmless, although the long ovipositor of the female is intimidating. Their larva live as parasites inside caterpillars and other larval insects.
proxy for alien development. Parasitoids lay eggs in or on their host and the parasitoid larva hatches and survives on or within the host. Eventually the host dies when the parasitoid has completed development. The temporary relationship between the parasitoid and its host can be quite dynamic and is geared toward the survival of the parasitoid. In an extreme case, Glyptapanteles parasitoid wasp pupae are protected from other predators by the actual host caterpillar from which they fed upon, and can be seen in this YouTube video.

You don’t have to look far to find parasitoids. Having a keen eye for a healthy versus parasitized insect is key. Identifying features include parasitoid exit holes or exposed parasitoid larvae and pupae. Aphid populations on many plants can be teeming with aphid parasitoids. Parasitized aphids (also called ‘mummies’) will have a

Nebo School District's School IPM Inspection Team
Parasitized aphids ("mummies") are easy to identify in that their bodies are bulbous, they have stopped eating or moving, and if the adult wasp has exited, there will be a hole in the abdomen.
bulbous appearance, straw-like or sometimes black color, and adult parasitoid exit hole. Of the several parasitoids released against alfalfa weevil in Utah, only one, Bathyplectes curculionis, has become well established. The pupal case of the weevil parasitoid is quite distinct with a white stripe (shown at right). One of the more noticeable parasitized hosts is the tomato hornworm that when parasitized by a Braconid wasp, has multiple white pupal cases on its outer body. Wheat growers that have to deal with cereal leaf beetle may be familiar with the resident larval parasitoid, Tetrastichus julis or Tj for short, which aids in extensive beetle suppression. It is important to conserve these beneficial parasitoids by avoiding broad-spectrum insecticides whenever possible, and planting nectar-producing plants for the adults.

In smaller settings or enclosed areas such as backyard gardens, greenhouses, or high tunnels, parasitoids may be purchased and released to aid in pest management. Note that each parasitoid is specific to a particular family of insects or to a single species. In addition to their host specificity, the parasitoid also deposits eggs in or on specific life stages (egg, larva, pupa, or adult) of the insect host. So matching up the parasitoid with the proper pest type and life stage is important for their success. Trichogramma wasps, for example, primarily attack the egg stage of various insects. Parasitoids are conveniently sold in ready-to-use dispensers such as a shaker bottle or a card that hangs on a plant stem. They are shipped with cool packs and can be immediately applied or briefly stored according to directions.

-  Ricardo Ramirez, Entomologist