Eastern Flower Thrips
Frankliniella tritici

Biology
The eastern flower thrips, I, is a tiny, yellow to brown insect less than 1/16 inch long. Adults are attracted to flowers of many different plants; their rasping feeding causes premature wilting of flower parts and blossom drop, as well as fruit deformities in some crops. Although Stannard (1968) reported that eastern flower thrips have never been found overwintering outdoors in Illinois (overwintering in greenhouses is presumed to occur), populations of this insect do develop each year throughout the state as a result of long-distance migrations from southern states on high-level winds associated with weather fronts. Stannard (1969) also reported that immigration of thrips may occur simultaneously with immigration of the potato leafhopper.

Eastern Flower Thrips. Actual length = 1/16 inch (1.5 mm).

Damage
Although Forbes (1892) and Slingerland & Crosby (1914) described buttoning or blighting of strawberry fruits in experiments in which eastern flower thrips were confined on plants during bloom and fruit set, thrips damage to strawberries has not been reported from the Midwest in the last few decades. As a result of controlled studies and field observations in California, Allen & Gaede (1963) reported that low populations of a related species, the western flower thrips, caused no appreciable damage to strawberries, but that very high populations caused golden brown discoloration of fruit that rendered berries unmarketable. A recent revision of California's Integrated Pest Management for Strawberries (Strand 1994) notes that western flower thrips can in fact be beneficial because they feed on mites, but control of this thrips is recommended if populations exceed 10 per blossom because fruit damage might then outweigh the benefits of predation on mites.

The 1994 strawberry crop failed to develop normally in many areas of the east central United States; berries failed to enlarge or ripen, remaining golden brown and leathery. Fruit symptoms matched those of thrips-damaged fruit described by Forbes (1892) and by Allen & Gaede (1963). In Illinois, this problem was greatest in the central portion of the state and less severe in the far south and the north. Whether or not eastern flower thrips caused these dramatic losses in 1994 cannot be determined, but several observations indicate that it did.

In 1994, eastern flower thrips were collected commonly on apple flowers in Urbana by mid-May, soon enough to cause damage to local strawberries. No monitoring was done in strawberries at that time. An earlier-than-normal immigration of potato leafhoppers was reported from many locations around the Midwest. If, as Stannard (1969) suggested, thrips and leafhoppers immigrated at the same time on the same weather system, it is very possible that an earlier-than-normal immigration of eastern flower thrips occurred throughout much of the east central U.S. in 1994, and that it coincided with strawberry bloom and fruit set in a large portion of Illinois and several other states. In "after-the-fact" evaluations, eastern flower thrips were numerous (sometimes > 40 per flower) on late blossoms in central Illinois fields where earlier fruits had failed to ripen normally. In more southern locations, fruit set may have preceded thrips immigration, allowing the crop to escape damage. In more northern locations, strawberries may not have begun to flower when thrips first arrived, causing immigrant thrips to seek other flowering plants instead of settling and building populations in strawberries.

Control
In Pennsylvania, Feland (personal communication) reported that producers found thrips to be numerous on blossoms; producers who used endosulfan (Thiodan), chlorpyrifos (Lorsban), or diazinon prebloom or at early bloom incurred less damage than producers who did not use an insecticide at that stage or used azinphosmethyl (Guthion). A similar trend of less damage where insecticides were used at prebloom or early bloom is supported by grower reports from Illinois.

Although conclusions that thrips caused much of the damage observed in Illinois in 1994 cannot be proven now, 1994's observations suggest practical steps for Midwest strawberry pest management in the future. To determine whether or not thrips control is warranted, strawberry growers should begin sampling for thrips by examining early flower clusters on early varieties and continue sampling all varieties as they begin to bloom. Tap flowers onto a white or very dark plate or saucer, and look for the slender yellow thrips. Alternatively, flower blossoms can be placed into a zip lock bag and shaken to dislodge thrips and allow counting. Although the relationship between thrips density and damage is not well understood, control is probably warranted only if populations exceed 2-10 thrips per blossom. This is a broad range of densities, but more a more precise threshold is not possible based on available research data. If insecticides are to be used for thrips control (or for tarnished plant bug control), make applications before bloom is well underway (by the time 10 percent of the plants have open blossoms) to avoid killing pollinators. Of the insecticides registered for use on strawberries in 1996, endosulfan (Thiodan) or chlorpyrifos (Lorsban) appear to be the most effective against thrips. Alternatively, newer insecticides containing azidarachtin (Align and Neemix) can be used; these insecticides are less toxic to honey bees than many other insecticides. Their effectiveness against thrips and tarnished plant bug has not yet been documented.

References Cited

1. Allen, W.W., & S.E. Gaede. 1963. "The Relationship of Lygus Bugs and Thrips to Fruit Deformity in Strawberries." Journal of Economic Entomology 56: 823-825.
2. Forbes, S.S. 1892. 'Notes on Thrips.' In: "Notes on Injurious Insects in Canada in 1892." Insect Life 5: 124-127.
3. Slingerland, M.V., & C.R. Crosby. 1914. Manual of Fruit Insects. MacMillan, New York.
4. Stannard, L.J. 1968. "The Thrips, or Thysanoptera, of Illinois." Illinois Natural History Survey Bulletin 29 (4): 215-552
5. Strand, L.L. 1994. Integrated Pest Management for Strawberries. Publication 3351, Regents of the University of California, Division of Agriculture and Natural Resources, Oakland.