Saturday, March 22, 2008

Greenhouse - Thrips Resistance to Insecticides

Insecticide resistance is a very common problem for thrips and thrips management in greenhouses. The following is an article on the subject from the New England Greenhouse Update.

Thrips populations are increasing as the weather is getting warmer. Some growers have reported that Conserve (spinosad) is not controlling thrips as well as in past years. This may be due to pesticide resistance, inadequate pesticide coverage or other factors.

Pesticide resistance is genetic. An insect cannot become or acquire resistance during its life (within one generation). Resistance tends to occur when there is widespread application of an insecticide with some individuals surviving and passing on genetic factors. The surviving pests can transfer the resistance factors throughout the population allowing the population to become resistant over time. Repeat applications with the same type of pesticide will eventually leave only those with the resistant gene. One way to delay resistance is to rotate pesticides with different modes of action (MoA) every two to three weeks or after up to three successive applications of insecticides with the same mode of activity. Mode of action is how a pesticide affects the insect. Most pesticide labels now have a number on the label that correlates with their mode of action group to make it easier to recognize the MoA for rotation. Or you can look up the information on a chart in the Greenhouse Recommendation Guide.

To manage thrips, shorten spray intervals to 4-5 days and rotate pesticides with different modes of action using suggestions above for delaying resistance. A rotation program for thrips might include Conserve (spinosad-group 5), Avid (abamectin-group 6), Mesurol (methiocarb-group 1A), Pedestal (novaluron-group 15), Azatin (azadirachtin-group 18B). Note that Mesurol has a 48 hour REI and Pedestal is for immature stages. Other pesticides labeled for thrips that growers are using include Pylon (chlorfenapyr-group 13), Tristar (acetamiprid-group 4A) and BotaniGard (Beauveria bassiana). A couple of growers reported good control with Pylon. For a complete list of insecticides labeled for use against thrips, see the New England Greenhouse Floriculture Guide.

In addition to injury caused by their feeding, western flower thrips vectors impatiens necrotic spot virus (INSV) a serious disease of greenhouse crops. To confirm that plants are infected with INSV, plants can be submitted to a diagnostic laboratory or growers can purchase easy to use test kits that are available from Agdia. If plants are infected with INSV, there is no cure. Infected plants should be removed from the greenhouse. Some growers keep a few test kits on-hand in their refrigerator to test suspicious plants. It only takes a few minutes to do the test and it can provide peace of mind and early detection.

Reprinted from "Thrips and Pesticide Resistance" in the March 20 edition of the New England Greenhouse Update

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