Many orchid books mention thrips as an unwanted guest in an orchid greenhouse. In the northwest thrips don’t tend to be a problem as much as fungus gnats. Fungus gnats are very small (1/16-1/8 inch long) and fly at a slow and clumsy rate. They really aren’t that damaging to orchids in themselves, though they do bite into the leaves. The main problem that they bring to the orchid grower is that they spread disease as they bite into plant after plant. If you see an increase in fungus problems or viruses in your orchid collection, you probably have fungus gnats flying around too.
Fungus gnats are worth killing. They aren’t just a nuisance as they fly around your growing space, they are actively biting into plants, particularly soft parts of plants such as bacterially infected leaves, and spreading disease around. They are difficult to eradicate completely, but the spread of disease in your greenhouse will often be directly proportionate to the number of fungus gnats you have flying around. All the options below can (and perhaps should) be used in conjunction with each other.
|Flypaper||Many companies make sticky pads that are essentially flypaper. With fungus gnats the color is irrelevant, though thrips are attracted to blue. Since fungus gnats live and breed in the wet bark or moss that your orchids are growing in, and they don’t fly very well, the ideal placement for the flypaper is down near the base of the plant. Some sticky pads come with small stakes for this very purpose. The papers should remain sticky for quite a long time (even with waterings) but should be replaced if they lose their stickiness. There is nothing forcing the gnats to fly into the papers – it’s pure chance – so the more traps you have, the merrier (for you, not for the gnats).||Safe indoors.|
|Cleanliness||Reduce detritis by picking out rotting leaves. Gnats reproduce in rotting organic material. You should make a habit out of “cleaning up” your orchids.||Safe indoors.|
|“Mosquito Dunks”||Place some Bacillus thuringiensis israelensis (it must be the “israelensis” variety to be effective against fungus gnats) in your humidity trays or in dishes. You’ll find this bacteria in garden stores as a preventative for mosquitoes. The bacteria is harmless to humans and pets, so there are no worries there. That said, this inexpensive solution is probably only moderately effective against fungus gnats in a greenhouse or orchid room setting because the gnats have lots of other places to go than your humidity trays.||Safe indoors.|
|Wheat & Soil||Some people have experimented with pots of wheat and soil as a lure to the female fungus gnats. The females lay their eggs in the pots and then you take the pots out and discard them before the eggs hatch. Note that last, very important sentence: before the eggs hatch. If you don’t take the pot out, you’re just giving them a nice breeding ground.||Safe indoors.|
|Pyrethrin||Pyrethrin is an insecticide that kills (or deters) fungus gnats. While it is made from chrysanthemums and is used on clothing to prevent malaria-infested mosquito bites, it is certainly not entirely safe. It can cause a variety of symptoms (including death) in humans and is extremely toxic to fish. If you have fish in or near your growing area you should use pyrethrin carefully or not at all and take every precaution to avoid ingestion by humans or pets.||Toxic to fish and humans.|