During greenhouse visits last week a potpourri of issues were observed including fungus gnats, shore flies and high pH iron deficiency.
Fungus gnats and shore flies thrive in moist environments with decaying organic material and algae. Both of these species usually occur in the greenhouse at the same time, but it is important to distinguish between them because they require different management strategies.
Fungus gnats are small mosquito-like insects with long legs and antennae. They have two delicate wings with a Y-shaped vein in the wing pattern. The larvae are small (1/4 inch) long, translucent to white in color, with a black head. Shore fly adults are flies that resemble small house flies. They have stout bodies, with shorter legs and shorter antennae than fungus gnats. Shore fly larvae are maggot-like, plump, opaque, and have no distinct head capsule. They range from whitish, to yellowish, to brown in color.
Fungus gnat larvae feed on decaying organic matter, but also feed on plant roots, and this feeding can be very damaging to seedlings and young plants. The root damage can lead to plant wilt, yellowing of leaves, and leaf drop. Adult fungus gnats do not feed on plant tissue, but they can be a nuisance, especially in larger numbers. Both adult and larval stages of shore flies cannot feed on plant tissue; they also feed primarily on algae and/or decaying matter. Shore flies are primarily a nuisance in greenhouses, but large populations can reduce the marketability of plants.
To manage fungus gnats and shore flies it is important to manage moisture, and build-up of organic material and algae. Avoid overwatering, accumulation of media or decaying plant material under benches, and eliminate standing water on and under benches and on walkways in the greenhouse.
Biological control of fungus gnats and shore flies can be achieved by releases of the predatory mite Stratiolaelaps scimitus (formerly Hypoaspis miles) early in the season, drenching with beneficial nematode Steinernema feltiae, or releases of predatory rove beetle Dalotia coriaria. Contact your biological control supplier representative to help you set up a biocontrol program that is suitable for your operation.
Chemical control of fungus gnats can be achieved by media drenches of Flagship (MOA 4A), Distance (MOA 7C), Adept (MOA 15), Citation (MOA 17) or a media surface spray of Distance (MOA 7C), Pylon (MOA 13) or Citation (MOA 17). For shore flies, apply media drenches of Distance, Adept , Citation, or media surface sprays of Distance or Citation.
For more management information consult the latest edition of the New York and New England Greenhouse Floriculture Management Guidelines (https://www.negreenhouse.org/pest-guides.html), and always be sure to read and follow the instructions on insecticide labels.
High pH induced iron (Fe) deficiency symptoms were also observed on pansy. Iron deficiency presents as interveinal chlorosis (yellowing between veins) of new foliage. Iron deficiency is usually associated with high substrate pH, but overwatering can also cause iron deficiency symptoms. Pansies perform best at a root zone pH of 5.5-6.2. Substrate pH values above 6.2 inhibit Fe availability and induce interveinal chlorosis. To correct iron deficiency, iron chelate can be applied to the substrate. Make efforts to lower pH by changing to a more acidic fertilizer and avoid overwatering.
- Geoffrey Njue, Extension Specialist, UMass Extension Greenhouse Crops and Floriculture Program