MANAGING THRIPS IN WESTERN COLORADO ONIONS

Bob Hammon, Tri River Extension, Grand Junction CO

Onion thrips, Thrips tabaci, is the primary insect pest of onions in western Colorado. They have been controlled with pyrethroid insecticides since the early 1990's. Onion thrips have acquired resistance to pyrethroids, and they are no longer effective in controlling them. Insecticide trials have been conducted since 2000 to document pyrethroid failures and find effective alternatives. Only one currently labeled material, methomyl, has been consistently effective in controlling onion thrips. Several insecticides have provided thrips suppression, but pyrethroids have consistently had control ratings less than 50%. Higher spray volumes have given better control when compared to the traditional 20 gal/A. Pre-bulb insecticide sprays were responsible for increased bulb size n 2004 trials. There was good correlation between pre bulb onion thrips counts and incidence of Iris Yellow Spot Virus six weeks later. This suggests that virus transmission may occur some time before symptoms appear in the field. Many onion varieties were tested for resistance to thrips feeding, with the more vigorous Spanish storage onions showing relatively more tolerance. Several weed species were sampled for early season onion thrips presence, and they were found on the majority of collections from hoary cress (whitetop), Cardaria draba. Onion thrips were found on all volunteer onion plants inspected during April and May of each year research was conducted. A comprehensive IPM plan for onion thrips will take into account variety selection, insecticide resistance management and manipulation of surrounding habitat to eliminate sources of onion thrips.

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