Greenhouse Insect and Mite Pest Management
Insect and Mite Pests Commonly Found in Greenhouses
Shore Flies
Shore flies, Scatella stagnalis, are a nuisance pest in the greenhouse (See Figure 27.8). Adult shore flies are black with reddish eyes and gray wings and appear more robust than the delicate-looking fungus gnats. The dark shore flies have bristle-like antennae shorter than their heads (fungus gnats have antennae much longer than their heads). Each wing of the shore fly has five pale spots, while there is a Y-shaped vein on the tip of the fungus gnat's wing. They are sometimes confused with dark-winged fungus gnats. Both insects thrive in a moist greenhouse environment, have similar biology, and feed on algae. Shore fly larvae are not known to feed on live plant tissue, while fungus gnat larvae can cause economic damage to plant roots. In high numbers, the adults of both pests can be a nuisance to employees and to customers in retail greenhouses.
Life Cycle
Their life cycle is like that of the fungus gnat. Female shore flies lay white eggs on algal scum or in very wet areas with decomposing organic matter (e.g., potting mix, pots, floors, near water spigots). Eggs hatch in 2 to 3 days into tiny maggots. Larvae reach a mature length of one inch (2.5 cm) in 3 to 6 days.
Types of Damage
The primary damage caused by shore flies is “fly specks” or excrement left on foliage of seedlings or mature plants. Adult flies have been implicated in spreading fungus spores. Adults can also be a nuisance problem.
Monitoring
Monitor weekly for shore fly development with yellow sticky cards, especially in propagation areas. Hang cards vertically for shore fly monitoring. Visual inspection for adults is usually adequate to determine if there is a problem. Inspect both plants and soil surface for adult shore flies.
Cultural Management Strategies
Water management and sanitation are essential in alleviating problems with shore flies. Avoid overwatering and fertilizer runoff; provide good drainage. Over watering, water leaks, and poor drainage may result in buildup of fungus gnats. Shore flies are typically more abundant in greenhouses with soil floors than those with concrete flooring. Allowing the soil to dry as much as possible, without injuring the plants, is an effective way to kill many maggots.
Biological Management Strategies
Predatory mites (Hypoaspis miles) are effective in controlling shore flies because they feed on the larvae of the insect. However, the predatory mite is only effective on soil and not standing water. Since shore flies reproduce in water just as they do in damp soil, the predatory mites may not offer a comprehensive control mechanism on their own.
Biorational Management Strategies
Microbials. Soil treatments can be used to control larval stages of shore flies. The following treatments are approved for greenhouse vegetables. Azadirachtin, a botanical, when ingested or absorbed by insect larvae, the molting process is interrupted.
Chemical Management Strategies
Aim insecticide applications for shore flies at either larvae or adults. The best long-term chemical control for shore flies targets the larvae stages. Insecticidal drenches or growing medium surface sprays (“sprenches”) of Distance or Adept are particularly effective against larvae. Certain insecticides may not affect eggs or pupae, and as such, repeated applications may be necessary.
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