In a nutshell: Most damaging thrips species worldwide; generalist pest found on many different types of plants, especially cucumber; plant cell feeders—causes silvery patches on leaves and fruit; damages and deforms fruits, flowers and new leaves; important viral disease vector.
What are western flower thrips?
The western flower thrips (Frankliniella occidentalis) are native to the west coast of North America; this is the most damaging thrips species for greenhouse plants. They develop fastest on flowering plants; under optimal conditions a population can double in four days; and, if the thrips have access to pollen, the number of eggs laid increases. This species of thrips can also survive the winter and cause problems early in the new growing season.
What kinds of plants do they attack?
Western flower thrips can be found on a huge range of plants, even weeds, but they’re an especially serious pest of cucumbers, peppers, eggplant, and many ornamentals.
What do they look like?
Adult females are 1.3–1.4 mm long and vary widely in colour, from almost white to yellowish-orange to almost black. Winter populations are darker than summer populations. Males are pale and smaller than the females, as 0.9–1.1 mm long. The female’s abdomen narrows down to a pointed egg-laying organ, and the male’s abdomen has two orange patches.
Female thrips deposit their eggs inside the tissues of leaves, flower petals and the soft parts of stems. The eggs are about 0.2 mm long, kidney shaped and white-yellow in colour.
Larvae begin feeding immediately after hatching and are 0.4–0.3 mm long, translucent, and white-yellow to orange-yellow in colour. They have large heads with bright red eyes. The second larval stage is slightly larger (0.7–0.9 mm).
The larvae usually pupate in the ground, not on the plant, although pupae can be found on the leaves and flowers and other sheltered spots. They prefer damp places or natural crevices, up to 15 mm below the soil surface. The pre-pupae and pupae are distinguishable from the larvae by their developing wing buds. The pupae have more developed wing buds than the pre-pupae as well as longer antennae that curve back over the head. Thrips in the pupal stages do not feed and only move if disturbed.
How do I know if my plants are under attack?
Adults and larvae feed by piercing the leaf cells and sucking out their contents. This damage leads to silvery patches on the surface of the leaf. The loss of chlorophyll reduces the vigour of the plant, and when infestations become serious, the leaves can shrivel.
Small black dots of their feces (frass) on the plant can indicate the presence of thrips, as well.
Western flower thrips prefer to feed on developing plant tissues, such as growing tips and flower buds. As these parts of the plant grow, they can become severely deformed. Heavily infested flower buds may not open at all, and fruit can be damaged, even with low thrips populations. “Pig tail” deformities in cucumbers are caused this way.
This species of thrips also transmits viral disease among plants, even in very low numbers. In fact, the western flower thrips are the most important vectors of both tomato spotted wilt virus (TSWV) and impatiens necrotic spot virus (INSV). Both of these viruses affect a wide range of plants, and often a single host plant will be infected by both.