Thrips are tiny, barely visible insects that are found on leaves, blossoms, buds, and leaf sheaths of plants. Adult and immature thrips damage plants by puncturing plant cells with their mouthparts, leaving behind white streaks or blotches flecked with specks of black fecal material. Adult females lay 25-200 eggs in plant tissue. The eggs are nearly impossible to detect. Nymphs are similar to adults, but are smaller, wingless, and often lighter in color. Nymphs feed from 7-10 days, and then pupate either on the ground or on the leaf, depending on the species. Ground pupation makes control more complex, since pupae are then protected from most biological or chemical controls (the soil-dwelling predaceous mite, Hypoaspis miles, may be used for control of thrips in growing media or greenhouse floors). Adults emerge from the pupation site after 4-14 days, depending on the temperature.
Identification and Monitoring
It is often easier to find thrips damage than to find the actual thrips. Damaged plants may have short, whitened lines where thrips have fed, often with tiny dark specks of fecal deposits. Several species of thrips are found in greenhouses, and they are difficult to distinguish. Two species of flower thrips, Frankliniella tritici and F. occidentalis (Western flower thrips), damage flowers and leaves, and pupate mostly on the ground. The onion thrips, Thrips tabaci, has several hundred host plants, including many vegetables and ornamentals, and pupates mostly on the ground. The banded greenhouse thrips, Hercinothrips femoralis, damages foliage, and pupates on the leaf.
Biological control programs should begin at the first sign of thrips. Sticky traps may help to detect thrips up to a month before they are seen on plants. Yellow sticky cards or ribbons should be hung just at the tops of plants and examined weekly. Blue sticky cards will sometimes, but not always, catch more thrips than yellow sticky cards. Thrips are the tiniest insects you will find in any numbers on sticky traps. Thrips are found with wings folded, so they appear like tiny brown flecks of peat moss. Some specimens will show hairs on edges of wings when viewed with a hand lens; often their stocky antennae stick out at the front in a V-shape.
Healthy, vigorous plants usually outgrow thrips damage. Thrips like dry conditions, so keep plants well-watered, and relative humidity high. Periodic flooding of greenhouse floors will drown ground-pupating thrips. Keep floors clean of debris, or cover with plastic or other mulch. Consider screening greenhouse vents. Control weeds in and around greenhouses to prevent a build-up of thrips in these areas. In the case of moderate to heavy thrips infestations, use a biorational pesticide to reduce their numbers before using biological controls. Consult with IPM Laboratories to choose a pesticide that can reduce thrips numbers without destroying your biological control program.
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