Treatments for fungus gnats are best directed against the larval stage. Many growers use beneficial nematodes Steinernema feltiae sold under the trade names of Nemashield, Nemasys, Entonem or Scanmask applied as a drench. Fungus gnat larvae are killed in one or two days by the symbiotic bacterium that dissolves their internal contents.
It has been an interesting winter. Given the warm temperatures this year, growers are advised to open up overwintering greenhouses on warm days to ventilate. Either roll up sides, open doors, use fans, depending on the type of overwintering greenhouse. Ventilating will lower the relative humidity. Greenhouses that are closed up will result in moisture buildup and encourage plant diseases to develop. Also, take time to inspect plants in overwintering houses, particularly for disease and injury caused by rodents.
Small greenhouses (< 4,000 sq.) can be scouted as one unit. Larger greenhouses should be divided into 2,000 to 3,000 sq. ft. sections for ease of scouting. Scout propagation areas at least every 3 to 4 days. Use your prior experience to determine how many plants and which plants to inspect (those that are most susceptible to pests or diseases in your greenhouse). The more plants or locations inspected, the more likely it is that a problem will be detected in a timely manner, when treatments are the easiest.
Teach employees to inspect incoming plants as they arrive and before plants are moved into production areas. Look for the presence of insects, mites, diseases, and cultural problems such as nutritional deficiencies. If feasible, quarantine infested or problematic plants in an isolated greenhouse or area so they can be treated before they are placed in production areas. Here are a few tips for inspecting incoming plants: