This past week I’ve been out in the field a lot, looking at a lot of onions. And good news for the growers, I haven’t found very many thrips. In integrative pest management, counting and sampling has quite an important role. Courses of action are based on thresholds established for certain pests and certain crops. These are economic thresholds and economic injury levels.
The economic injury level (EIL) is defined as the lowest pest population density that will cause economic damage. If a pest populations reaches the EIL, economic damage will be sustained. To prevent such damage, action can be taken at the determined economic threshold (ET). The ET is the point at which action should be taken to prevent further damage, and prevent pest densities from reaching the EIL. The ET is also called the action threshold, which is more descriptive. More information about EILs and ETs can be found here.
In New York State, the ET for onion thrips is 3 thrips per green leaf. When sampling, it is important to examine multiple plants in different areas of the field. Some growers sample up to 50 plants to evaluate thrips populations The calculation to determine whether or not the ET has been reached is:
(# of thrips counted)/(# of leaves counted)
Of course, it is also important to assess damage, for thrips as well as other pest species. For onion thrips on onion, this is the characteristic damage:
Different growers have differing levels of acceptable damage, depending on whether it is direct or indirect damage, or may have different thresholds at different points in the season. To spray or not to spray, that may still be the question.