Orchids include more than 800 genera, more than 25,000 species and thousands of named cultivars. While orchids are found all over the world, most cultivated plants are from tropical forests. Orchids are hardy in cultivation but must be monitored for pests to prevent long-term damage.
Aphids are small insects that reproduce quickly and can rapidly infest an orchid. They are green, black or yellow soft-bodied insects that only get an eighth of an inch long. Aphids feed on the sap from leaves and flower stems, transmit viruses and excrete a sugary liquid called honeydew that causes sooty black mold to grow on plant leaves. Treat aphid infestations a malathion- or pyrethrin-based insecticide or use a 2 percent solution of insecticidal soap.
Scale insects are a common pest on cultivated orchids and are difficult to eradicate because of their hard casing. They look like small domed bumps up to an eighth of an inch long and can be brown, green or yellow. Treat scale insects with a 70 percent solution of rubbing alcohol applied with a paintbrush or soft toothbrush every 10 days until all the scales have been removed. In larger orchid collections or with a persistent infestation, use a systemic insecticide that is carried in the plant sap and ingested by the insects. Sprays are not effective against scale insects.
Mealy bugs are small white insects up to a quarter of an inch long that are covered in woolly filaments that look like cotton wool. They can infest an orchid's root as well as its leaves. Treat mealy bugs in the same way as scale insects, but search the surroundings because adult mealy bugs sometime hide away from their host plant. Mealy bugs can also be treated with insecticide sprays formulated for houseplants.
Slugs and Snails
Orchids cultivated in a greenhouse or with other plants are vulnerable to slugs and snails. One slug or snail can eat away the tender new leaves or flower spike of an orchid plant in a single night. Search your orchid plant and its surroundings at night to catch the culprits, and remove clutter around the plant to eliminate hiding places. Use slug pellets around the plant to kill any pests before they can do any damage.
Microscopic spider mites suck the juices out of plant cells, producing yellowish patches on leaves, especially on the undersides. Fine webbing on orchid leaves is another sign of spider mites. Spider mites thrive in dry conditions and can often be controlled by spraying your orchid twice a day with rainwater or distilled water to increase the humidity. Wipe each leaf with a damp cloth at the same time to physically remove mites. Spider mites are not affected by most insecticides and severe infestations need to be treated with a miticide or acaricide formulated for houseplants.
These tiny flying insects can seriously weaken an orchid by feeding on its sap and transmitting viruses. Whiteflies look like tiny white moths. They are best treated with an insecticidal spray containing pyrethrin, applied at 10-day intervals for a month.