Know Your Friends

Aphidius Wasps

The wasp genus Aphidius is a large group containing numerous species, all of which attack aphids and provide natural control of aphids in backyard gardens, commercial fields, and urban landscapes. The species discussed here are commercially available, and are generally used in greenhouses.

Aphidius are small braconid wasps. Females lay eggs singly in aphid nymphs. The wasp larvae consume the aphids from inside. As the larvae mature and the aphids are killed, the aphids turn into mummies. After the larvae pupate, each adult wasp emerges through an exit hole cut in the mummy. In addition to killing aphids directly, mechanical disturbance of aphid colonies by the searching behavior of the adult wasps causes many aphids to fall off the plants and die.

Aphidius matricariae [photo by Max Badgley], a 1/8 inch long, black wasp from Europe, is one of the most common and effective parasites of the green peach aphid. It is not a good parasite of cotton aphid or potato aphid. Females lay 50-150 eggs in aphid nymphs of all sizes. The aphids are killed in about 7-10 days, the aphids turn into smooth, shiny, and light brown to silvery-gold mummies. This wasp does not diapause under winter greenhouse conditions as readily as some natural enemies (such as the aphid midge Aphidoletes aphidimyza) so it can be an important part of an aphid biological control program from fall through early spring. On chrysanthemum, green peach aphid has been controlled within 2 months of planting when the wasp was introduced at the rate of 50 wasps per box of 500 cuttings.

A. matricariae performs best when it is established in the greenhouse early in the growing season. Native A. matricariae entering the greenhouse from outside sometimes become established and provide effective control. However, in the spring and summer these wasps are frequently attacked by their own native hyperparasites, which reduce the natural or introduced A. matricariae population. Adult wasps are attracted to the color yellow, so yellow sticky cards should be removed before releases are made.

Aphidius colemani is a cosmopolitan species-that is, it occurs in most parts of the world. It reproduces well on cotton aphid, green peach aphid, and other species, but not on potato or foxglove aphid. It looks similar to A. matricariae, but females lay more eggs-an average of 388-over their 4-5 day life span.

A. colemani is a promising candidate for aphid biological control in greenhouses because of its high reproductive potential, short development time, and ability to parasitize several species of aphids, especially cotton aphid, melon aphid and green peach aphid. It is more efficient at parasitizing cotton aphid on cucumber than is A. matricariae.

Aphidius ervi is another cosmopolitan species that parasitizes numerous aphid species in many different crops. In greenhouses, the black adults parasitize particularly potato aphid and foxglove aphid, which turn into a grey or brown mummy. A. ervi is most effective when released before aphid populations build up. Weekly introductions should be made at a rate of about 1.5 adults/ft2. If aphids are already present, use 5.5/ft2 and make introductions every three days if aphid populations are high.

These wasps can also be used in combination with predators, such as green lacewings or aphid midges, for a complete aphid biological control program in greenhouses. However, they may not be compatible with entomopathogenic fungi (eg. Beauveria bassiana) which kill the parasite larvae inside the aphid.

- Susan Mahr, University of Wisconsin-Madison

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