The places where thrips first feed on onion foliage will have a silvery, faded look. - Whitney Cranshaw, Colorado State University, www.insectimages.org

Almost impossible to see because they are so small (1/6 of an inch long), thrips are thin, torpedo-shaped insects that feed on plant tissue. They vary in color from light yellow to brown. Thrips are common on onions and a host of other vegetables, including squash, cabbage, beans, tomato, and peas. On onions the tiny thrips hide in the crevices between leaves, making them even harder to find. A telltale sign of their presence is the silvery spotting of onion leaves where they destroy the green tissue as they feed. Later leaves may turn brown, and onion bulbs won’t develop properly. On squash and cucumbers, the leaves curl, crinkle, and look dwarfed. Cabbage leaves show bronzy, rough areas on and in the head.

It is not easy to control onion thrips because they hide in the folds of onion plants and in tight places on other plants. To spray, use an insecticidal soap and apply with strong pressure to help the spray get down between the leaves. To prevent recurring problems, clean up the garden at the end of the season, because onion thrips overwinter in plant debris.