HPIPM:Thrips

Authors:Sue Blodgett, Frank B. Peairs[1] revision (Michael J. Brewer, Frank B. Peairs & Jay D. Donahue original)

Identification (and life cycle/seasonal history)

Onion thrips, Thrips tabaci Lindeman, and western flower thrips, Frankliniella occidentalis (Pergande), are known to damage alfalfa regionally. Both species have multiple generations each year, populations can build up quickly if conditions are favorable. Large populations are often associated with hot, dry conditions.

Thrips are small, slender-bodied insects with two pairs of narrow fringed wings. Female western flower thrips are about 1 mm long and are yellow to dark brown in color. The smaller male is pale yellow and has a narrower and more pointed abdomen. Adult female onion thrips are similar in size and are yellow with brownish blotches on the thorax and abdomen. Antennae are gray with the first segment lighter than other segments. Males are rare.

It is difficult to tell these two species apart without the aid of a good microscope. The shape of the antenna and the arrangement of the setae (hairs) behind the head are the most reliable characteristics to use to separate these two species (see http://txspace.tamu.edu/bitstream/handle/1969.1/87053/pdf_1476.pdf?sequence=1 and http://wci.colostate.edu/shtml/ot.vs.wft.shtml).

The life cycles of these two species are similar. The adults overwinter in protected areas such as under plant debris. They become active during warm days in spring and are present until cold weather in fall. Adult females insert their eggs into tissues of many different plant species. Upon egg hatch there are two active, feeding stages (Instar I and II larvae) that resemble the adult in general shape, but are smaller and lack wings. Thrips larvae are pale yellow and feed on developing leaf tissue. Two additional, non-feeding larval stages are found in the soil. A generation is completed in about three or four weeks during the summer.

Plant Response and Damage

Thrips feed by puncturing the epidermis with a stout mandible, puncturing deeper mesophyll cells with a fine pair of stylets (maxillae) and sucking out the cell contents. This feeding is sometimes described as "rasping-sucking", but is more accurately described perhaps as "puncture, poke and suck". As chlorophyll is removed from feeding, the damaged area appears puckered, silvery or whitened, and may have yellow flecking. Thrips also excrete tiny dark tar spots which can be useful when diagnosing onion thrips feeding injuries.

In alfalfa, thrips feed on developing leaf tissue. Scarring occurs at feeding sites, which, in turn, cause the leaves to distort as they emerge. Damaged leaves also may be curled or distorted, with a cupped or puckered appearance. Heavy infestations of western flower thrips 7-10 days before cutting have resulted in leaf drop and losses in quality in western Colorado.

Management Approaches

Chemical Control

Reliable scouting methods have not been developed for thrips in alfalfa. A combination of sweep net sampling and visual inspections for leaf damage is a good way to determine the presence of thrips in an alfalfa crop. Also, buds and flower can be tapped into a white cup or onto a piece of white paper or cardboard.

There is no consensus on the importance of onion thrips and western flower thrips in alfalfa, although onion thrips are considered to have more damage potential. Research based economic thresholds are not available.

Due to constantly changing labels, laws and regulations, the sponsors of this web site assume no liability for the suggested use of any chemicals noted in the management section. Contact your local pest management personnel for current information on labeled pesticides that can be used for the intended plant disease, insect pest or weed.