Manage Greenhouse Tomato Pests With Natural Enemies

Aphidoletes larvae

Aphidoletes larvae attack an aphid that is paralyzed by their venom. Aphidoletes attack many species of aphids. Photo courtesy of Applied Bionomics LTD.

Greenhouse biological control programs originated with greenhouse tomatoes with whitefly control with Encarsia in the 1930s in Canada and England. Later, in the early 1970s, spider mite control with predatory mites became important on cucumbers in both England and the Netherlands, where spider mites had become resistant to pesticides.

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Today, biological control programs for tomatoes control many pests, including fungus gnats, aphids, thrips, whiteflies, and spider mites. Absence of pesticides reduces plant damage from sprays and allows bumblebee pollination, resulting in higher tomato yields than ever before.

Biocontrols are most economical when pest populations are low, so start at or before the very first sign of the pest. One way to detect the very earliest traces of a pest is to grow indicator plants that are attractive to the pest in question. For example, snap beans are extremely attractive to spider mites and thrips and quickly show damage that is easy to see. Simply grow them in pots in the greenhouse, being sure to place some near entrances where pests are likely to enter. For early detection, snap bean plants are the best insurance policy you can buy.

Natural Enemies for Whitefly Control

Encarsia formosa was first used for control of whitefly in greenhouses in the late 1920s and is still the most effective biological control of greenhouse whitefly today. However, sweet potato whitefly, also known as Bemisia, is common wherever ornamentals are also grown. Encarsia is less effective on sweet potato whitefly, for which you need to obtain Eretmocerus. In all cases, excellent control can be achieved if the parasites are released weekly after the first sign of the pest. After four to six weekly releases, the parasites often will maintain control season-long unless more whitefly are brought into the greenhouse on another crop. Anyone who waits until several generations of whitefly occur before starting parasite releases will have great difficulty in regaining control, because there will be a hundred-fold to a thousand-fold more whitefly for the parasites to kill.

Delphastus catalinae is a tiny lady beetle that specializes in whitefly eggs and small scales, but it does not seem to establish in tomatoes well and is therefore not used much.

Natural Enemies for Aphid Control

This is another pest where identification is very important to control by parasites. There are three parasitic wasps used in tomato greenhouses. Aphidius colemani attacks melon aphid and green peach aphid, but not potato aphid, which is also a common pest in greenhouse tomatoes. Aphidius ervi and Aphelinus abdominalis attack potato aphid. It is important to repeat parasite releases about once a week after the first release to create constant pressure of adults.

Aphidius colemani can be maintained on banker plants of barley infested with cereal aphids. This aphid does not attack broadleaf plants and is no threat to them. The system does not work for the very large potato aphid parasites, Aphidius ervi, because the cereal aphids are too small to be hosts for them. In the case of potato aphid, IPM Laboratories has developed a new banker plant system that offers the larger pea aphids to Aphidius ervi.

Aphidius ervi adult

Aphidius ervi is a tiny wasp that parasitizes potato aphid, one of the two species of aphids that commonly attack tomatoes. Photo courtesy of Beneficial Insectary

One way to get around the aphid identification question is to release the predator Aphidoletes aphidimyza, which kills many species of aphids. This is a gnat-like fly that lays eggs in aphid colonies. The tiny orange larvae are voracious aphid predators. Aphidoletes must be released every week for two or more weeks to create constant pressure of adults. When daylength is short, they will stop reproducing unless given supplemental light to lengthen the days. A second strategy is to release very small numbers of aphid midges every two weeks when aphids are anticipated. They are excellent searchers and will find aphids that you have not detected.

Natural Enemies for Thrips Control

Neoseiulus cucumeris, commonly called Cucumeris, is a predatory mite used for thrips control in tomatoes. It is available in bran that can be shaken from a bottle, or it can be hung from the plants in little packets called mini-sachets. Since they do not reproduce well on tomato plants, they must be reapplied every two weeks for the loose material, or every four weeks for the mini-sachets. It is essential to maintain a continuous presence of Cucumeris for four or more weeks, because the mite only kills the first larval stage of thrips after the egg. The predator does not attack adult thrips or second larval stage of thrips.

Orius insidiosus is a predatory bug that does not survive well in tomatoes, although it is excellent for controlling thrips on peppers.

Natural Enemies for Spider Mite Control

The predatory mite Phytoseiulus persimilis works well for a quick knockdown of smaller two-spotted spider mite outbreaks. However, it does not persist well in tomatoes long term. The sticky hairs prevent it from traveling freely around the plants. However, a tiny midge called Feltiella acarisuga thrives in tomatoes with spider mites and can fly among hot spots. Release Feltiella when spider mites are first seen. Snap bean indicator plants are especially helpful to time the Feltiella release and serve as a comfortable habitat for them to colonize.

Natural Enemies for Fungus Gnat Control

Fungus gnats can be controlled with beneficial nematodes (Steinernema feltiae), predatory soil-dwelling mites (Stratiolelaps scimitus), and/or predatory soil-dwelling beetles (Dalotia coriaria). For best results, they should be applied when the crop is first started. The two predators, Stratios and Dalotia, are generalists that will reproduce themselves throughout the season.

One of the great benefits of biocontrols is that they take resistance pressure off pests, so that many pests revert to their genes that are susceptible to pesticides. There are myriad examples

where pesticides have regained their killing power after a year of biocontrols. But this only happens for a single generation because the resistant gene-pool bounces back in a single generation. With biocontrols, that single treatment may give the only correction that is needed.

To be successful with biological controls, you must plan to start even before you plant your crop. Prepare in advance to watch for the very first signs of pests and create a release schedule to meet the first generation of pests. Weekly scouting is vital to be constantly vigilant about upsurges in pest numbers or any changes in pest species.

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