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Parasitic wasps may sound creepy, but they can actually improve your garden

A parasitic wasp is surrounded by host aphids.
A parasitic wasp is surrounded by host aphids.

Q. What are parasitic wasps?

Sally S., San Luis Obispo

A. A parasitic wasp is smaller than the more common yellow jacket and hornet. And it may seem creepy, but it’s used increasingly in controlling pests that attack crops and plants, stunting their growth and causing their leaves to curl, wilt or yellow.

Such pests include aphids, mealybugs, whiteflies, scale and tobacco hornworms.

One type of parasitic wasp specializes in attacking aphids. Here’s how:

A female parasitic wasp lays one egg inside the aphid. After the egg hatches, the larva feeds on internal tissues and secretes chemicals to transform the soft-bodied aphid into an aphid mummy — a bloated tan- to dark brown-colored shell. The larva completes its development safely inside the hardened aphid mummy and chews through the shell.

To put it another way: The host is eaten alive.

The parasitic wasp then emerges as an adult, mates, and looks for new hosts, all within its two- to three-week lifespan. Aphid mummies are easily spotted on plant foliage and are a tell-tale sign that parasitic wasps are present and at work in your garden.

You can encourage parasitic wasps by providing alternate food sources — such as plants and shrubs with dense clusters of small flowers, like Ceanothus and buckwheat or members of the carrot (Apiaceae) family, such as Queen Anne’s Lace, dill, cilantro and fennel.

Select a variety of plants with different bloom periods to keep parasitic wasps year-round. Be sure you eliminate ants around your plants to enable the parasitic wasps to do their job. If insecticides are necessary, look for those that pose the least risk to beneficial insects and follow label instructions.

For photos and more information on parasitic wasps and biological control, visit www.biologicalcontrol.info

Polly Nelson is a UCCE Master Gardener.

Got a gardening question?

In San Luis Obispo call 805-781-5939, Arroyo Grande, 805-473-7190 and Templeton, 805-434-4105. Visit us at http://ucanr.org/sites/mgslo/ or email us at anrmgslo@ucanr.edu. Follow us on Instagram at slo_mgs and like us on Facebook. Informative garden workshops are held the third Saturday of every month, 10 a.m. to noon at 2156 Sierra Way, San Luis Obispo. Garden docents are available after the workshop until 1 p.m. To request a tour of the garden, call 805-781-5939.