What comes to mind when you think of beneficial insects in your yard? Observant gardeners will often name one of the voracious predators of other insects. These include generalist predators such as assassin bugs, dragonflies, and ground beetles. Others, such as lady beetles, can be more specialized, feeding exclusively on aphids or scales. Predatory insects can substantially reduce the populations of pests. Growers can employ measures to increase their presence as natural controls, or sometimes to introduce them intentionally as biological controls in agriculture. 

Equally important to biological pest control, however, are the parasitoids. Insect parasitoids differ from parasites in that they kill their hosts. The typical parasitoid life cycle involves the infection of a host with eggs. After hatching, parasitoid larvae consume the host as they mature into adults, killing it in the process. Apart from these basic principles, parasitoids vary dramatically in their life histories and their relationships with hosts, which are the result of millions of years of evolution. For example, there are different strategies for host infection. Parasitoid eggs may be injected directly into the host, deposited directly or indirectly on the body surface, or laid near a food source for the host to inadvertently swallow. 

Most parasitoids are tiny harmless wasps and flies. Virtually all insects and arachnids are susceptible to parasitoid infection, including parasitoids themselves. Up to five levels of parasitism have been observed in some systems! Parasitoids are typically host-specific, meaning they are restricted to certain hosts. When a parasitoid population spreads throughout a host population, they present an obstacle to the host’s population growth. When the host becomes more abundant, so does the parasitoid. Parasitoids therefore lower the maximum host abundance that a given environment can support. From a pest management perspective, predators offer rapid control, while parasitoids are more effective in the long-term. 

Parasitoid diversity is bewildering. Most are chalcidoid wasps (Chalcidoidea), ichneumonoid wasps (Ichneumonoidea), and tachinid flies (Tachinidae), though many other groups are known. More than 22,000 chalcidoid wasps alone have been described, but there may be as many as 20 times that number. Parasitoids range in size from the 0.25mm Trichogramma wasps, parasitoids of caterpillar eggs, to the 3-inch Megarhyssa wasps. The latter are parasitoids of wood wasp larvae, and their ovipositor may be an additional 4 inches!

Parasitoids are essential to integrated pest management, particularly of insects resistant to insecticides. Chemical controls are sometimes more effective against a pest’s natural enemies than against the pest itself. This is well-documented in scale insects (including ground pearls in turf), whose waxy armor presents an obstacle to spray insecticides. Pyrethroid sprays can thereby increase scale insect abundance. The Cooperative Extension Plant Clinic in New Hanover County can help you develop integrated pest control strategies for your garden. You can encourage parasitoid presence by including native flowering plants on your property. For more information, you can contact the Plant Clinic for a consultation or browse factsheets from NC State Extension.