Thrips

Thrips are tiny, slender insects, 1 to 2 millimeters long (about 1/25 inch), easy to miss until you see their damage. Adults are yellow, brown or black, with narrow fringed wings; the young are pale yellow or white and wingless. Shake a flower or leaf over a sheet of white paper and they show up as moving specks.

Damage

Thrips puncture plant cells and suck out the contents. The empty cells fill with air and leave:

• silvery or bronze streaks and stippling on leaves, usually dotted with tiny black specks of excrement
• distorted, streaked or browned flower petals, and buds that fail to open
• scarred or misshapen fruit, such as rough patches on peppers and cucumbers

The bigger problem is disease. Western flower thrips and a few other species spread tomato spotted wilt virus (TSWV), which infects tomatoes, peppers, lettuce and many ornamentals. Infected tomato plants develop bronzed, spotted young leaves and fruit with pale rings, and there is no cure; pull them out.

Life cycle

Females insert eggs into leaves, stems or flower tissue. The eggs hatch in a few days, the young feed for one to two weeks, then drop to the soil or leaf litter to pass through two non-feeding stages before emerging as adults. In warm weather the whole cycle can take as little as two weeks, so populations build fast in hot, dry summers.

Plants commonly attacked

• Vegetables: onions and garlic (onion thrips), tomatoes, peppers, cucumbers, beans, lettuce
• Fruit: strawberries, grapes
• Flowers: roses, gladiolus, dahlias, chrysanthemums and many others

Prevention and control

Monitor. Blue or yellow sticky cards hung just above the plants catch adults and show when numbers are rising.

Water sprays. A strong spray of water knocks thrips off leaves. Rain and overhead irrigation also reduce onion thrips, which is why they are worst in dry spells.

Control weeds in and around the garden. Many weeds host both thrips and tomato spotted wilt virus.

Encourage predators. Minute pirate bugs, predatory mites and lacewing larvae eat thrips. Predatory mites such as Neoseiulus cucumeris can be bought for greenhouses and high tunnels.

Reflective mulch. Silver or reflective plastic mulch under tomatoes and peppers has been shown to reduce thrips landing on the plants and the spread of tomato spotted wilt.

Low-toxicity sprays. Insecticidal soap and horticultural oils, including neem oil, kill thrips they contact. Coverage is the hard part, because thrips hide in buds, flowers and leaf folds; spray the undersides of leaves and repeat every five to seven days. Spinosad is one of the more effective products against thrips, but it is toxic to bees until it dries, so spray in the evening and avoid open flowers.

Broad-spectrum synthetic insecticides such as pyrethroids are a poor choice: they kill the predators that keep thrips in check, and western flower thrips have developed resistance to many of them.

See garden pests for other pests, and common tomato diseases for other problems that cause spotted or bronzed tomato leaves.