| Thrips are minute, slender-bodied insects less than
1/16 inch to about 3/16 inch in length. With
magnification, you can identify the adults that range
in color from yellow to black and have four long,
narrow, fringed wings. The young nymphs are
smaller, wingless, and range in color from yellow to
white. Many thrips are plant feeders attacking
flowers, leaves, fruit, twigs, or buds.
Symptoms and Diagnosis
Thrips congregate by the hundreds and leave black,
varnish-like fecal deposits on the underside of leaves.
To feed, they scrape and puncture the epidermal layer
of the host plant, then suck the cell sap as it flows into
the wound. As a result of the rasping, the foliage
appears dusty or silver and dull. Some species may
burrow between the upper and lower leaf surfaces to
feed. Blossoms can become streaked with brown and
can wither prematurely. Injured leaves are twisted or
discolored and scarred. The fruit of host plants is often
pitted. A few species are beneficial predators of mites
and small insects including other thrips, and some
feed on fungal spores. Several species may bite
people. Some species act as vectors of plant disease.
Thrips are poor fliers, so that damage may first appear
in one location and then slowly spread over the plant.
Thrips will be most apparent on flowers, especially
white, yellow, and other light-colored blooms.
Life Cycle
Eggs are inserted into leaves, fruit, stems, or the bark
of trees and shrubs. One to two weeks later, pale,
wingless nymphs emerge and feed actively until they
pupate. Some species enclose the pupa in a cocoon
on the ground or on the host plant. The winged
adult thrips emerge from the cocoon or pupal stage.
Generations may occur every two to three weeks,
but the largest populations are present from late
spring to midsummer. Thrips overwinter in the egg
stage.
Integrated Pest Management Strategies
1. Knock off thrips with a forceful spray of cold
water in small gardens, preferably early in the
morning. Use a nozzle that produces a fine spray
of water, and thoroughly wash off the undersides
of affected leaves. The control is most effective
when repeated at least 3 times, either daily or
every other day.
2. Keep plants well irrigated and adequately misted
since thrips prefer a dry environment.
3. Prune and destroy injured and infested plants.
Do not compost them.
4. Predacious spiders, mites, and pirate bugs help
control thrips. Conserving naturally occurring
populations of beneficial insects by avoiding
persistent pesticides is important to biologically
controlling thrips.
5. Aluminum foil and other disorienting mulches
have been used with some success to reduce
thrips infestations.
6. Two applications of pyrethrum applied 3 to 4
days apart will treat serious infestations.
7. Insecticidal soaps are effective for temporary
reduction of thrips populations. Spray every 3
days for 2 weeks. To control iris thrips, spray
insecticidal soap at the base of each plant every 5
to 7 days until the infestation has been controlled.
To prevent thrips problems, soak iris divisions and
gladiolus corms for 30 minutes in a hot water
bath (110–125 degrees F). Dig up gladiolus bulbs
early in the fall and let them cure in open trays in
a shady location with good air flow such as a
garage for a few weeks before soaking them. Then
allow the bulbs to dry thoroughly
before storing.
8. Spray infested plants with a systemic insecticide such as imidacloprid (Merit) since thrips frequently burrow inside the plant tissue.
9. If necessary, use chemical insecticides such as acephate (Orthene), bifenthrin, imidicloprid, or spinosad. Spray 2 or more times at intervals of 7-10 days to control thrips as they hatch. |
