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Onion Thrips in Onions: Dispersal, Flight Habits, and Insecticide Resistance 1997

Project Leaders: Mike Hoffman, Chuck Eckenrode and Jody Gangloff


Improve the basic knowledge and management of the onion thrips in onions by

a) examining hourly flight activity of onion thrips before, during and after population peaks associated with surrounded field and forage crop phenology.

b) determining how time-of-day affects adult onion thrips activity and correlating this with other variables for use in prediction.

c) documenting and quantifying insecticide resistance build up in onion thrips populations inhabiting onion fields.

d) using resistance characteristics of sub-populations of onion thrips in alfalfa and onions to determine the significance of local migrations.


Two peaks of thrips flight activity were observed over all four commercial fields. These peaks are most likely the result of phenology of other crops such as the senescence of winter wheat and cutting of alfalfa. Despite weekly or otherwise regular applications of insecticides in all fields, the numbers of thrips per plant steadily climbed as summer progressed. A time-consuming but fairly reliable laboratory test that could be used to look at resistance levels of thrips was devised by mid-summer. One significant aspect of experiments on resistance levels was the lack of homogeneity of the test subjects in terms of their tolerance levels. Thrips problems were lessened and thrips remained susceptible for growers who practiced rotation, scouting, and spraying only once the economic injury level is reached.