Large numbers of shore flies were observed in greenhouses this week. Adult shore flies are small, black flies about ⅛ inch long (about the same size as fruit flies) with about 5 light colored spots on each wing, short antennae, and short legs. Shore flies should not be confused with fungus gnats which have long, skinny antennae, skinny legs, and look more like tiny mosquitoes than flies.
Shore flies’ larvae are about ¼ inch long when mature, and yellowish-brown with no head capsule. Female shore flies lay eggs in areas where algae accumulate such as soil surfaces, benches, and greenhouse floors. Larvae are found just under the top layer of soil, where they feed on algae. Adults move and fly rapidly but stay close to their breeding sites.
Both adults and larvae feed on algae (not plants), therefore shore flies are primarily a nuisance pest. Damage is caused primarily by excrement (fly specks) left on the foliage of bedding plants and other ornamental plants. If the numbers of shore flies in the greenhouse are too high this can become a deterrent to customers. Shore flies also have the potential to transmit fungal diseases such as Pythium root rot, Thielaviopsis and Fusarium.
Shore flies can be monitored using yellow sticky cards, which are especially useful for detecting adults. Place the sticky cards near the soil surface of potted plants and benches, and check cards weekly to determine whether numbers are increasing or decreasing. The first step in managing shore flies is to avoid overwatering or excessive use of water during irrigation and to apply only optimum levels of fertilizers for adequate plant growth. Excessive watering and fertilizer use encourages algal growth. Eliminate standing water and algae in pots, propagation areas, on benches and under benches.
Options for biological control:
Releases of rove beetle Atheta coriaria, soil-dwelling predatory mite Stratiolaelaps scimitus (formerly known as Hypoaspis miles), or Steinernema feltiae nematodes can help control shore flies. If using the soil dwelling predatory mite, make sure there is no standing water. Shore flies can survive in standing water but the predatory mites
cannot. Make releases soon after shore flies are detected.
Options for chemical control:
Distance (Pyriproxyfen), Citation (Cyromazine), and AzaGuard or Azatin (Azadirachtin) can be applied as a drench or sprench at labeled rates. Distance and Citation can also be applied as a media surface spray. Rotate between mode of action classes (IRAC numbers) to avoid resistance. Always read and follow label directions.
- Geoffrey Njue, UMass Extension Greenhouse Crops and Floriculture Program with Jim Mussoni, Private IPM Scout.