The roots of orchids are often infested with small white worms that are the larval stage of fungus gnats, small, black winged flies that grow in moist potting mediums. The worms are only a few millimetres in length and translucent with black heads, usually present in the upper 2 to 3 inches of potting soil. The pests feed on the roots of the plant and achieve maturity in about three weeks after which they pupate in the soil. Fungus gnats and their larvae are encouraged by an overly wet growing medium and shady conditions. It is best to control fungus gnat worms as soon as possible as damage can lead to plant death.
Confirm the presence of the worms in your orchids by inserting 1/4-inch wide wedges of potato into the growing medium.
Remove after a few days and inspect the potato slices for the worms. The larvae can be seen on the potato as they slowly migrate to the potato and feed on it.
Place yellow sticky traps horizontally all around the lip of the pot. Lay more traps around the pot base. This will help to capture the adult fungus gnats and reduce population.
Mix the systemic insecticide imidacloprid into the growing medium to kill the worms and fungus gnat eggs in the pot. The chemical is sold under various trade names and is available in the form of slow release spikes, granule and sprays.
You can also introduce the nematode Steinernema feltiae in the growing medium as a biological control option for the worms. The parasitic nematodes enter the bodies of the larvae and release a bacterium that feed on the inner bodies of the larvae, killing the pests within four days. Bacillus thuringiensis is also effective when used as a soil drench but this is not available in retail stores and is only used in commercial greenhouses, cites the Colorado State University Extension.