Extremely High Thrips Numbers in Mississippi Cotton
In the last 7 days there have been numerous reports of extremely high thrips numbers infesting cotton fields across the state. Although nearly all cotton has an insecticide seed treatment on the seed, high numbers of adults are overwhelming the seed treatments in many cases. I have had reports of 25-30 adult thrips per plant on cotyledon cotton, with the first true leaf already knotting up. With cool temperatures over the last 7 days cotton has not been growing off well which is exasperating the problem. The current threshold for thrips in MS cotton is an average of one thrips per plant with immature thrips present up until about the 4th true leaf. The reason we have always recommended that immatures should be present is because seed treatments are taken up systemically by the plant and thrips have to eat before they are controlled (which means that just because adults are there don’t mean they are not being controlled). The presence of immatures indicates that the seed treatments have broken down and reproduction is taking place. This threshold works fine under normal conditions. The situation we are in on a lot of this cotton is not normal with regards to thrips numbers present. In situations where extremely high numbers of adults are infesting cotton it will be necessary in some cases to go ahead and make foliar treatments to avoid injury. I assure you that you are getting some protection from the seed treatments but with that many adults having to feed to be controlled, we are taking much more injury than we are use to with seed treatments. I would not assume that every field across the state is at these high levels and requires treatments. We should scout each field and make decisions to treat based on pressure in those fields (areas). We also have examples of cotton that was treated behind a seed treatment at the one leaf stage that required an additional treatment 5 days later. Yesterday (5/19/11) I sent crews across the north and south delta making random collections from cotton fields to determine species composition. In LA they have had similar problems this year with 20-30% of the population being western flower thrips (which we will not control well). As soon as we get these broken out to species I will post that information here if western flower thrips are present at appreciable numbers. Products of choice for thrips control are: Acephate 0.2-0.25 lbs, under extremely high numbers I may would consider running this up to 1/3 lb, Bidrin at .2 lb or 1:40, Dimethoate at .2 lb or 1:20. Keep in mind that acephate is the most likely of these to flare mites but mites will not flare if they are not already present in the field. Also, we have been experimenting with a product from Dow called Radiant that has looked equally as well as the standards at 1.5 oz per acre.
