7/2/2013 - By Matthew Shultz

This month I wanted to continue on with a beneficial insect theme and talk about a group of insects referred to as the parasitic wasps. There are nine families of garden wasps that make up this group collectively named the Parasitic Hymenoptera. Eggs and larvae of these insects are obligate parasites, which mean they have to feed or siphon energy off of a host insect. This is also what makes them beneficial in nature! Most of these wasps use pest insects as their feeding hosts, and therefore they can be used in IPM programs to attack common pest species found in the greenhouse or nursery. 

Most species of parasitic wasps are very small in size which makes them practically unnoticeable in the production area. Some growers know that they have populations of these beneficial wasps present, not because they see the adults flying around but because they see the egg or larval stage present. Parasitic wasps get their name because the adult female wasp stings its host and deposits an egg on or just inside the insect’s skin. The egg hatches and the larvae begins to feed on the host insect, siphoning off its energy, eventually immobilizing and killing the host pest insect. This immobile parasitized or mummified state is usually what most growers key in on. The pest insect at this point is either very unhappy or dead and it makes for easy scouting. Once the larva is fully developed it will pupate and will hatch out of the host insect.

Aphid parasitized by Aphidiidae wasp, often called an Aphid mummy, example of an internal wasp larva.
Photo by Matthew Shultz.

There are two major developmental differences within the parasitic wasp grouping, internal parasites and external parasites. Internal parasitic wasps deposit their eggs inside or on the host insect, the egg hatches and it continues to develop inside its host, hence the name. The larvae literally eat their way out of the host! Newly hatched larva might not kill the host immediately but as it develops the host will eventually die.

External parasitic wasps deposit their eggs inside or on the host in the same way internal wasps do. The difference being once the egg hatches, the larvae attach themselves to the outside of the host insect. The larva then spins a cocoon for protection. It feeds on the host insect by sucking or siphoning nutrients from it. The wasp starves and dehydrates the host insect in this developmental process.

Braconid wasp larvae on Tomato Hornworm, example of external parasitic wasp larvae.
Photo by Matthew Shultz.

Most species of parasitic wasps are native to our regions or at least naturalized, and should be conserved when present. Some notable families include the Ichneumonids and Braconids which parasitize caterpillars, Tiphidae which parasitize many white grub species, Encyrtids which parasitize mealybugs, soft scales, cutworms and loopers, and finally Eulophids which parasitize psyllids, scales, and flies.

Some families like the Aphid wasps (Aphidiidae family) are commercially available for purchase and can be released to help with control of aphid colonies. Early spring wasp releases can help growers combat aphid build ups, especially when houses are still covered and spraying is difficult, and will ensure a wasp population onsite all year long. When attempting to conserve these beneficial insects, growers should be highly selective when using insecticides. Certain chemistries will have detrimental effects to these beneficial insects. There are however many compatible chemistries available too. For more information on insecticide selection contact your Harrell’s tech services rep!