Parasitic Wasps

Many species in the subfamily Aphidiinae

By Jessica Walliser

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Once aphids have served as the unwitting hosts to parasitic wasp larvae, little remains of them but hollow shells.Parasitic wasps are a diverse and complex group of insects. Within the insect order Hymenoptera (which includes bees, wasps, ants, and sawflies) are some 40-plus families of parasitic wasps that include thousands of species. Technically, these insects are parasitoids rather than true parasites; parasites allow their hosts to live, while parasitoids eventually bring death. Though there is great diversity in the physical appearance and life cycles of these fascinating and important insects, they have one thing in common: They use other insects to house and feed their developing young. Many of the insects these wasps parasitize are considered garden pests, making them an incredibly important ally for gardeners who wish to keep pest populations in check.

Among the parasitic wasp clan are groups that utilize caterpillars, beetles, flies, scales, and other insects as hosts, as well as the specialized subfamily Aphidiinae, consisting of aphid parasites. There are approximately 114 Aphidiinae wasp species across North America, most of them measuring a mere 0.1 inch (2.5 millimeters)—certainly small enough to maneuver their way around a colony of tiny aphids.

Aphidiinae wasps are surprisingly common. Evidence of their work is easy to spot if you look closely at an aphid colony. An adult wasp attacks an aphid host by puncturing its “skin” and laying a single egg inside. The wasp larva hatches soon after and proceeds to excavate and consume the aphid from within, leaving only a swollen, brown exoskeleton behind. These bulbous parasitized aphid remains are commonly called aphid mummies, and their presence is a clear indication of a healthy population of Aphidiinae wasps. Once the wasp larva has matured within its aphid host, the adult wasp chews a circular hole in the mummy’s back, crawls out, and flies off.

Adult parasitic wasps of all species require not only host insects for their young but also nectar and pollen for energy. They lack mouthparts capable of extracting nectar from tubular flowers and so require plants with shallow, exposed nectaries to feed. Members of the carrot family (Apiaceae), such as angelica, chervil, fennel, dill, and cilantro, bear umbrella-shaped clusters (umbels) of minuscule flowers that are known to attract beneficial wasps. Other plant families that are attractive to parasitic wasps include the mint family (Lamiaceae) and the aster family (Asteraceae).

The vast majority of Aphidiinae wasps are incapable of stinging humans, and since they are so very small, most gardeners aren’t even aware of their presence. Within a few weeks of noticing a new aphid colony, you’ll begin to see the parasitized mummies—unless, of course, you’ve made the mistake of wiping out the wasps with an insecticide.

Photo: Jessica Walliser

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