Thrips are small insects that are commonly found in garden plants, but occasionally turn up to infest houseplants during winter months. Their appearance can often be blamed on plants being brought indoors with insects, eggs or larvae on them, from which they can spread to other plants.
Thrips can be identified by their size; they are very small, less than a millimetre wide and only a couple millimetres long. There are several different types of thrips, but most of them are dark in colour. They are difficult to see because of their small size, but they will often appear in groups on buds and blossoms. They can run, fly weakly, and jump.
Thrips can also be identified by the damage they cause to plants. These insects feed by piercing and scraping leaves and flowers, causing damage that often looks silvery and spotted. Small, brownish specks are sometimes found, and are the bugs' droppings.
Thrips often don't cause severe damage, but they can stunt young plants, cause a lot of damage to flowers, and transmit diseases between plants. Unfortunately, they are difficult to control, since eggs are inserted into plant tissues, and young and adult insects are adept at hiding in plants and soil. Washing plants, removing infested flowers and keeping growing temperatures cool and humidity high can help control them, but if the infested plant is not valuable, throwing it out may be the best way to keep the infestation from spreading. Insecticidal soaps or oils can be effective against thrips, but they must contact the insects. Blue or yellow sticky cards can alert you to the presence of thrips and provide some control of adult populations. If pesticides are desired, use a spray containing bifenthrin, permethrin, resmethrin, pyrethrins or neem oil every one to two weeks.