Identification of Thrips:
Thrips are tiny, 1/5"(5 mm) long, seen as animated lines running along veins on the undersides of plant leaves. Often, their signs of damage, (leaves infested with thrips will appear brown or a mottled silver and dried rather than wilted) are more evident than sight of the thrips themselves. Their fecal spots may appear as black sooty spots.
Detecting Thrips in a Greenhouse:
Blue sticky traps have been proven effective to detect thrips in greenhouse crops, hung so that the top of the trap lies in a zone of 2 feet (61 cm) above the plants, with the bottom within an inch (25 mm) of the highest leaves. Control is most effective when beneficial insects are released as soon as thrips are noted on the sticky trap.
Physical Control of Thrips - Moisture Control:
Studies have shown that dry plants, or plants lacking sufficient moisture are most likely to suffer attack by thrips, so adequate water placement by irrigation and mulching can be the first defense in controlling thrips.
Physical Control of Thrips - Let the Weed Border Grow:
Many thrip species thrive on weed vegetation so destroying or removing their natural habitat can cause them to move onto the cultivated plants.
Physical Control of Thrips - Mulch with Foil or Newspaper:
Shiny aluminum foil used as mulch around the base of plants disorients the egg-laying of thrips while any type of mulch that prevents weed growth does double duty in suppressing the emergence of thrip pupae from the soil.
Biological Control of Thrips:
Predatory mites and Lacewing larvae are commercially available and are suitable for release onto indoor plants. However, the lacewing larvae will need to be captured by hand or gently vacuumed up and released outdoors when they mature into flying adults. In addition, predatory nematodes control most thrip species and research is ongoing to develop airborne transmission methods for nematodes dispersal. Last but not least, Pirate and Damsel Bugs are natural predators of thrips as well as numerous other insects and can be encouraged to visit the garden by plantings of alfalfa, goldenrod and yarrow.
Chemical Control of Thrips Using Least-toxic Treatments:
Thrips have been shown to develop resistance to most insecticides used on a regular basis,yet least-toxic options like insecticidal soaps and horticultural oils can aid the gardener with thrip control until beneficial insect populations are thriving or supplemented.