Garden Help: Problem spots? Put chilli thrips on suspect list

Insects are numerous on area landscape plants after a long, productive summer.

To check for problem spots walk around weekly and look for insect activity. Check both old and new growth, plant stems and turn leaves over to check underneath.

Specific insects to look for include caterpillars, grasshoppers, aphids, scale insects and thrips. Look for eaten, distorted or discolored leaves.

If damage is noticed, hold a white piece of paper under affected plants and gently tap the plants and look carefully for small crawling insects. Use a 10X lens to identify pests or place yellow sticky cards among plants to attract thrips.

Most of us are familiar with injuries caused by caterpillars, grasshoppers, aphids and scale insects, but recognizing thrips damage is more challenging.

A thrip's diet and life cycle

There are different types of thrips, and most live in plant flowers and leaves. Flower thrips and others have been around for years but one that is relatively new to Florida is Chilli thrips, Scirtothrips dorsalis Hood. It has been found on more than 112 host plants including many fruits, vegetables and ornamentals (banana, beans, chrysanthemum, citrus, corn, eggplant, grape, grasses, holly, impatiens, Indian hawthorn, ivy, jasmine, ligustrum, onion, peach, peanut, pepper, pittosporum, plumbago, rose, strawberry, tomato, viburnum, zinnia, etc.).

They are hard to see with the naked eye, but the damage is obvious. Adult thrips are about 0.5 to 1.2 mm long and are pale yellow to grayish-white in color. When the wings are held over the back, a dark line is visible down the center. The immature or larvae are off-white.

Chilli thrips prefer to feed on new leaves, buds and fruits, and depending on the plant, they may feed on the upper (cucumbers and strawberries) or lower surface (roses and peppers) of the leaf. Thrips have rasping-sucking mouth parts, so they inject their mouthparts and suck out plant fluids, thereby causing distinct symptoms. New growth is small and distorted and turns from bronze to black. Upward puckering of the leaves occurs on some plants like peppers. Plants may defoliate and eventually will die due to loss of leaves.

Roses are a favorite host plant and affected leaves have discolored purplish streaks and are distorted. Flower and vegetative buds turn brown and do not develop normally. Once an infestation occurs, plants deteriorate rapidly.

Thrips are worse during warm, dry weather so we had ideal conditions during the last month. When rainfall increases, chilli thrips numbers will decline.

Chilli thrips are very prolific and complete their life cycle in 14 to 20 days. Eggs are inserted inside plant tissue and develop into larvae. They then pupate and develop into adults with wings that allow them to move around easily in search of new plant material. An adult female lays 60 to 200 eggs in her lifetime.

Controlling thrips

Chilli thrips are not that difficult to control, but numbers build up quickly.

To control chilli thrips, prune affected plant material removing new growth and flower buds; bag it and get it out of the landscape.

Spray options include oils, soaps, spinosad, imidacloprid, etc. Rotate materials to avoid resistant pests and do not use broad spectrum insecticides that kill beneficial insects. Foliar sprays are more effective than soil applications and thorough coverage of plant material is critical.

Researchers are testing various biological controls and the most promising is a predatory mite that feeds on both chilli thrips and whitefly. The swirski-mite (Amblyseius swirskii) is very effective at controlling these pests in greenhouse environments.

If synthetic pyrethroids have been applied to the plant material over the last 60 days, the mites will not survive so wait until this material breaks down. Once the mites become established, avoid using broad-spectrum pesticides to protect your investment.

Other beneficial insects that feed on thrips include the minute pirate beetle, lacewings, syrphid flies, mirid bugs and predatory thrips.

Temperatures are dropping, so now is a good time to treat for many traditional pest problems. Before using any pesticides, check for beneficial insects. If lady beetles, lacewings or other beneficial insects are present, let them do their job and hold off on using pesticides. Insecticidal soaps and oils are softer on beneficial insects so if a pesticide is needed, make these your first choice.

Terry DelValle is a horticulture extension agent with the Duval County Extension Service and the University of Florida/IFAS.