Manage Onion Thrips

Manage Onion Thrips

Onion thrips

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Onion thrips is the most important insect pest of onions. This insect causes significant economic losses, by reducing yields up to 60%.

Thrips damage to young onions is more devastating than on larger plants late in the growing season; however, thrips feeding opens up the onion to secondary infections. Therefore it is important to protect the onions from thrips damage through the entire growing season.

Difficult To Manage
Onion thrips is difficult to control because the mobile stages of this insect are found mainly in the narrow spaces between the inner leaves where spray coverage is difficult to accomplish. In addition, the eggs are laid into the leaf tissues where they may escape control. Reinfestation of fields can occur from surrounding non-crop vegetation and immigration of thrips from nearby fields. Currently, the most important tool for commercial onion growers is the judicious use of insecticides.

Insecticides should be used as part of an integrated pest management strategy, keeping in mind the following points: (1) before making an application, determine the average number of thrips on your onions, and (2) check the weather forecast, since hot, dry spells will likely help the numbers of thrips rise quickly in the near future, while cool, wet weather will help slow population increases.

Choice Of Control
Most onion growers have to make multiple applications of insecticides in a season. Before choosing a product for onion thrips control, the following points should be taken into consideration:
(1) There are relatively few products registered on onion, so (2) maximum application rates are quickly exceeded if the same product is applied multiple times in a season. (3) Therefore, multiple products have to be used in rotation.

It’s important to use different classes of insecticides within a season because the more often a particular class is used, the higher the chances are of onion thrips becoming resistant to it. Insecticides are lethal to most, but not all individuals, and those that survive are able to pass on this ability to their offspring. The more resistant survivors left in a field, the more insecticide insensitivity will be a problem for growers (i.e. the product won’t kill the insects).

So we need to find out which insecticide rotations and combinations do the best job suppressing onion thrips numbers without exceeding maximum application limits. The insecticides registered for onion thrips control differ in their efficacy, and based on the time in the growing season some products may be more appropriate to use than others.

For example, at the beginning of the growing season, when onion thrips populations are low, less effective products may do an adequate job of suppressing thrips numbers. On the other hand, the most effective insecticides should be saved for the later part of the growing season, when thrips are abundant and difficult to control. Given these efficacy differences among insecticides, the use of treatment thresholds can further save time and money for growers.

Insecticides should only be applied when the amount of thrips per plant has climbed above threshold, so as to prevent unnecessary insecticide applications. In the case of less effective insecticides, a lower threshold is suggested for use (e.g. one thrips per leaf), and in the case of more effective insecticides, a higher threshold is recommended (e.g. three thrips per leaf).

Lastly, growers should consider using penetrating surfactants when applying insecticides for onion thrips management. Commonly used products like Radiant (Dow AgroSciences), Movento (Bayer CropScience), and Agri-Mek (Syngenta Crop Protection) must penetrate the onion leaf surface to maximize their efficacy against thrips. Caution also should be taken to avoid co-applications of these insecticides with chlorothalonil-based fungicides, which have been shown to reduce the level of thrips control. AVG

This information was presented at the 2011 Great Lakes Fruit, Vegetable and Farm Market Expo in Grand Rapids, MI. Szendrei and Byrne are in the Department of Entomology at Michigan State University.