Onion Thrips and Flower Thrips

Problem type: Insect

Name of problem: Onion thrips and flower thrips

Plant name(s): Onion (and relatives), tomato, bean, cucumber, cabbage, cauliflower, squash, asparagus, turnip, green pepper, carrot, pea, potato, corn, alfalfa, clover and many weed species; garden flowers and house plants

Symptoms / Characteristics:
The nymphs use their piercing mouthparts to suck the juice from plant tissue, creating whitish streaks or patches on leaves, buds and petals. Feeding injury is initially concentrated near the leaf veins. As feeding continues, these streaks expand into blotches or patches. Small, black fecal pellets may also be visible in and around feeding streaks. Heavily infested leaves become distorted, wilt and turn brown. Entire plants may appear scorched. Damage is most severe in hot and dry weather. Heavy rainfalls actually help to remove the insects from host plants and reduce feeding injury. In onions, bulb production is poor. In garden ornamentals and house plants, flowers may be streaked and deformed. Affected flower buds often fall from the plant. Thrips are also associated with virus transmission.

Onion thrips (Thrips tabaci) adults range in color from light yellow to dark brown and can reach a mature length of approximately 2 mm. The body is quite flattened and pointed at the posterior end. Females have four long, narrow wings with long hairs attached. Males are wingless. The wingless nymphs, responsible for the damage, are similar to the adults except that they are smaller. They are typically pale yellow in color. On the prairies, there may be several generations per year and feeding damage may occur throughout the growing season. There will be fewer generations during seasons in which cool and wet conditions persist.

Flower thrips, a similar looking insect, is also common on the prairies. They inhabit both the garden and the greenhouse and may even be found on house plants. Flower thrips prefer to feed on flower petals, especially roses. In the greenhouse they will feed on leaves and developing fruits. There are many different species of thrips, each causing similar feeding injury to their respective hosts. Species identification often requires microscopic examination.

Control / Preventions:
Thrips are not easily seen on a plant because of their small size. Shaking suspect or symptomatic leaves over a white piece of paper may help to confirm the presence of the insect. Yellow, white or blue sticky traps can also be used as a monitoring aid. Severely infected leaves and flowers should immediately be removed and destroyed. Spraying a strong stream of water over the top of the plants will wash the insects from the plants and reduce overall thrips populations. In the fall, remove and destroy crop residue and plant debris in order to reduce potential overwintering sites. Good weed control is essential in the spring and throughout the growing season as weeds can act as an alternative host for the thrips.

There are many natural predatory insects that will effectively reduce thrips populations in the garden. These include ladybugs, lacewings, minute pirate bugs and hover flies. Incorporating a wide variety of plant species into the garden design will provide food and habitat for many of these beneficial insects. For this reason, spraying insecticides in the garden is discouraged. Furthermore, foliar sprays may be ineffective as the tiny insects are often protected within flower buds and leaf sheaths. Insecticides used to control onion maggots will typically control onion thrips as long as the insects are exposed and thorough coverage is achieved. Predatory mites can be used to control flower thrips in the greenhouse. Inside the home, use an insecticide that is registered for controlling thrips in house plants. House plant insecticide strips and sticky traps can also be used.