Watch out for fungus gnats

Dave Smitley and Dean Krauskopf, Entomology

February 27, 2008 -- Constantly moist media is ideal for fungus gnat development. Cut up a potato into half inch strips (like a French fry) and stick 2 inch-long pieces into pots and plugs throughout your range. The top of the strip should be just above the media. Take a look at the strips after 48 hours. The black head capsules of fungus gnat larvae are easy to see against the white potato. If you wait longer than 48 hours to look at the strips, they start to rot and turn brown, making it more difficult to see the larvae. The best way to control fungus gnats is to allow the media to dry out whenever possible.

If fungus gnats become so abundant that the larvae cause root pruning damage (usually only on cuttings or very small plants) or the adults become a nuisance, apply Azatin XL, Adept, Distance, Marathon or Mesurol as a soil drench. A drench will control larvae for several weeks, preventing new emergence of adults, while a spray only kills adults for a few days and has no effect on the larvae in the soil.

Greenhouse insect update

Thomas A. Dudek, MSU Extension Horticulture and Marketing Educator
Dave Smitley, Entomology

February 27, 2008 -- During visits to area greenhouses, we have noted that the insect pressure is increasing and insects are not waiting for bright sunny weather to show up.

The first case of Impatiens Necrotic Spot Virus was confirmed recently, and we all know that thrips play a major role in spreading the virus. Thrips numbers are continuing to build up on sticky cards in many greenhouses. If sticky card counts exceed 10 per card per week, you may want to consider 2-3 sprays of Avid, Mesural, Orthene 97, Safari, Sanmite, Tristar or Conserve five days apart. If the sprays are effective, the weekly counts of thrips should go down.

Aphids are showing up on both rooted and unrooted cuttings, especially Calibrachoa and rosemary plants. Check plants carefully for the white cast-skins shed by aphids and spot-spray aphid-infested plants with one of the insecticides listed for aphids in the December 21, 2007 article titled Managing greenhouse insects as soon as they are found. Aphid and spider mite populations can build rapidly, so it's best not to delay your spot-spraying.

Spider mites are showing up on Lamium, especially the ‘White Nancy’ variety. Check off-color or speckled plants for the mites on the undersides of the leaves. Spot-spray infested plants with Akari, Avid, Floramite, Hexygon, Judo, Ovation, ProMite, Pylon, SanMite, Shuttle or Tetrasan as soon as they are found.

Finally, broadmites are being observed in ‘Galleria’ geraniums. Look for the stunted twisted growing points, which may turn black if the infestation is serious. These tiny mites can only be seen with a high-powered microscope. Spray infested plants with Avid, Akari, Judo, Pylon or SanMite. Leave a few plants unsprayed for comparison, to confirm your diagnosis. Unsprayed plants will continue to decline while sprayed plants will begin to look better after spraying.