Description
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.
Crop Management
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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