Thrips are tiny, slender insects with fringed wings that feed on a large variety of plants, by puncturing and sucking up the contents. Common names for thrips include thunderflies, thunderbugs, storm flies, thunderblights, storm bugs and corn lice. There are many different species of thrips (over 5,000), most of which are pests (often in the family Thripidae), while certain species (black hunter thrips and sixspotted thrips) are beneficial predators that feed only on mites and other insects.
Thrips are extremely active, feed in large groups and leap or fly away when disturbed.
Lifecycle and Description
Adults are minute, slender insects with narrow fringed wings and measure about 0.5 mm to 1 mm long. Immatures (called larvae or nymphs) are similarly shaped with a long, narrow abdomen but lack wings. Thrips are not strong fliers but can readily travel long distances by floating with the wind or being transported on infested plants. Most thrips range in color from translucent white or yellowish to dark brown or blackish, depending on the species and life stage.
Adults and pupae overwinter in garden soil, sod, debris and cracks in barks. In spring, emerging females lay eggs in plant tissues including flowers, stems and leaves. Eggs hatch in 3-5 days in warm weather, or in weeks in colder weather. Nymphs feed for 1 to 3 weeks, then depending on the species either drops down to the soil to pupate or pupates on the plant itself. Pupal stage lasts 1 to 2 weeks before adults emerge. There can be 5 to 15 generations a year outdoors and year-round generations in greenhouses.
Thrips have been found on various plant orders showing their wide host plant range including various berries, field crops, forage crops, grass flowers, legumes, peonies, privet hedges, roses, trees, truck crops, vines and weeds. They are known to cause damage to avocado, beans, citrus and blueberry plants, figs, azalea, hypericum, photinia, rhododendron, garlic, onion, pepper, carrot, squash, gladioli, toyon, impatiens, petunia, cucurbits, grapes, strawberry, stone fruit among others. They seem to prefer grasses and yellow or light-colored blossoms.
Thrips, both adults and nymphs cause damage to the plant by piercing and sucking out cells on the leaf surface, leaving a silvery stippling or streaking on leaves. Severe thrip infestations can stunt and distort plants, damage flowers and scar fruit. Some species are responsible for spreading tomato spotted wilt virus (TSWV) and impatiens necrotic spot virus (INSV).
Organic Control and Prevention of Thrips
Producing thrips-free crops requires a multi-faceted approach, as with many other pests.
1. Eliminate sources: Remove weeds and grass from in and around your garden areas to eliminate alternate hosts for thrips. Remove and dispose of old, spent flowers after your harvest. Proper disposal of plant residues is also important.
2. Start and stay clean: An important part of keeping your crops void of thrips is to make sure the young plants are clean. Examine purchased plants or transplants for thrips before placing them in your greenhouse or garden. Covering openings to greenhouses has found to reduce pest problems by up to 70%. Vigorous healthy plants normally outgrow thrips damage, but excessive use of nitrogen fertilizers may promote higher populations of thrips.
3. Monitor: The most reliable way to detect thrips is with a yellow or blue sticky trap. Branch beating or shaking foliage or flowers onto a sheet of paper or a beating tray or sheet can help monitor and rid of a few thrips. Hosing down thrips from infested plants does help reduce pest numbers to a certain extent.
4. Row covers: Row covers, hot caps, and other types of cages can exclude thrips and other pests from vegetables and other young herbaceous plants. Basically, any type of covering that excludes insects but allows light and air penetration can be used.
5. Reflective mulch: Reflective mulch is said to confuse and repel flying insects that are searching for plants because the reflected ultraviolet light interferes with the insect’s ability to locate plants. This is more effective during early growth when plants are small and do not cover the mulch. Reflective mulch has been shown to delay or sometimes even prevent pests such as winged aphids, thrips, leafhoppers and whiteflies.
6. Pruning: Prune and destroy injured and thrips infested terminals of plants. Shearing is not recommended though as it stimulates new susceptible growth. Instead, prune by cutting plants just above branch crotches and nodes.
7. Beneficial insects: Release commercially available natural predators to prey on thrips such as minute pirate bugs, damsel bugs, ladybugs, lacewings, predatory mites (Amblyseius cucumeris) and parasitic wasps or attract them to your garden by planting alfalfa, goldenrod and yarrow.
8. Biological insecticides: Biological insecticides such as Beauvaria bassiana and Verticillium lecanii are an effective strategy against thrips. Both are fungi that grow naturally in soils and are proven to help rid of thrips and many other pests. Spinosad insecticide is based on a compound found in a bacterial species and it helps kill thrips among several others.
9. Soaps and oils: Spray a safe organic insecticidal soap or horticultural oil such as neem oil on afflicted plants to aid with thrips control.