Greenhouse Insect and Mite Pest Management
Insect and Mite Pests of Greenhouses
Fungus Gnats
Fungus gnats (e.g., Orfelia spp., Bradysia spp.), common greenhouse pests, are dark, delicate-looking flies similar in appearance to mosquitoes (See Figure 25.3). Adult fungus gnats have slender legs with segmented antennae that are longer than their head. Wings are light gray to clear. Their long antennae distinguish them from the more robust shore flies. The Y-shaped vein on the tip of the wing separates them from shore flies, which have five clear spots on their wings and shorter antennae than those of fungus gnats. Fungus gnats are recognized as major insect pests in greenhouses and are one of the few insect pests in which the damaging life stage—the larva in this case—is located within the growing medium. They are especially a problem under excessively moist conditions during propagation, when plant cuttings or plugs are initiating root systems.
Life Cycle
Adult fungus gnats represent the visible “tip of the iceberg.” The rest of the life cycle takes place in the media, where adults lay eggs in the cracks and crevices. Fungus gnats develop through four stages—egg, larva (with four larval stages or instars), pupa, and adult. The tiny eggs and oblong pupae occur in damp organic media where females lay eggs and larvae feed.
Types of Damage
Adults cause minimal plant damage, but females lay eggs that hatch into larvae that damage plants by root feeding. The larvae prefer growing media with “high” moisture content and require fungi as a supplemental food source to complete development. Fungus gnat larvae feed on a wide-range of ornamental plants grown greenhouses. Young plants and/or seedlings are especially susceptible to injury from larval feeding, more so than mature plants.
Monitoring
Begin monitoring with yellow sticky cards in the greenhouse to determine if fungus gnats or other pests are present before starting seedlings. Adult fungus gnats are usually found on the soil surface. Horizontal placement of cards just above the soil surface is more effective than vertical placement. Since vertical placement is more effective for monitoring shore flies, a combination of horizontal and vertical placement is recommended.
Cultural Management Strategies
Water management and sanitation are essential in alleviating problems with fungus gnats. Avoid overwatering and fertilizer runoff; provide good drainage. Over watering, water leaks, and poor drainage may result in buildup of fungus gnats. Fungus gnats are typically more abundant in greenhouses with soil floors than those with concrete flooring. Allowing the soil to dry as much as possible is an effective way to kill many larvae.
Biological Management Strategies
Options in controlling fungus gnats include predacious mites (Hypoaspis miles), entomopathogenic nematodes (Steinernema feltiae), and rove beetle (Atheta coriaria). All are effective if applied before fungus gnat populations are abundant.
Parasitiods. The soil-dwelling predatory mite, Hypoaspis miles, is commercially available from most biological control distributors and is used in greenhouses to manage larval populations. It is important to make releases early in the growing season before fungus gnat populations are high. A single release has provided 6 to 8 weeks of control in some cases. The predators can establish and spreading throughout a greenhouse if the environment is favorable. Optimum development and reproduction of H. miles occurs at temperatures between 59- and 86-degrees F (15 and 30°C).
Predators. Atheta coriaria (rove beetle) is a generalist predator that feeds on on fungus gnats (eggs and larvae), shoreflies (eggs and larvae), and thrips (pupae). They disperse throughout a greenhouse by flying. Both larvae and adults feed on pests, but also may consume natural enemies such as H. miles. Temperatures of 65 to 80 degrees F (18 to 27°C) to and a relative humidity of 50 to 85 percent are optimal for survival.
Biorational Management Strategies
Microbials. The biological insecticide Bacillus thuringiensis var. israelensis tends to be more effective on first and second instars than the older (third and fourth) instars. Apply weekly until fungus gnat populations decline.
Chemical Management Strategies
Insecticides are typically used to deal with fungus gnats in greenhouses, but control of fungus gnats has become more difficult due to pesticide resistance. Since the larvae are the life stage that directly causes plant damage, most insecticides are applied as a drench to the growing medium. Make sure material is applied to a depth of 1 inch or more. Appendix B, Insecticides and Miticides Labeled for Greenhouse Use, lists insecticides and miticides labeled for control of insect and mite pests in greenhouses.
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