Check Seedlings for Thrips Insecticide Efficacy
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Article by Reisig and Huseth
Cotton seedlings struggle to get established in North Carolina for many reasons. One reason is that if cotton is left untreated, thrips will exceed the economic threshold (two larval thrips per seedling) in nearly every field. Therefore, insecticides (seed treatment or in-furrow) at planting and/or post-emergent foliar are a must for successful cotton production. Thrips resistance to neonicotinoid insecticides has increased the importance of foliar sprays over the past several years. Timely application of effective insecticides is the best.
Here are some of the things to consider for thrips at the start of this 2023 season:
Use the Thrips Infestation Predictor tool to your advantage. Plug in different planting dates to see which fields were planted into high risk windows. Focus attention on these fields, especially those that were recently planted and at higher risk. These fields should have young cotton that might benefit from an intervention.
The weather has been cool. This means that seedlings are slower to grow and can take on more thrips injury relative to warmer seasons. Seed treatments and at-planting insecticides will diminish quickly and foliar insecticides may be needed.
Thrips species can vary from field-to-field and year-to-year. We did a study across the Southeast looking at thrips species across the cotton growing season. At Plymouth, 90% of the population was tobacco thrips during 2014. The following year at Plymouth, nearly 50% was tobacco thrips and nearly 50% was western flower thrips. This variation in species was seen across the Southeast, although tobacco thrips were most common. Western flower thrips tend to be more difficult to control than tobacco thrips and could be a reason insecticide sprays don’t work as well as expected.
As we mentioned in a late-April 2023 article, acephate (Orthene) resistance has been documented in the Midsouth. One population of thrips sprayed with Orthene during 2022 in North Carolina was tested and can be categorized as “reduced susceptibility”. Therefore, growers should use acephate with caution this year and check behind sprays.
Growers who are not satisfied with foliar insecticide control should consider several things. First, early sprays are the most effective. Plants had more biomass and more yield in studies where sprays were timed with the first true leaf emerging from the cotyledons versus the second leaf (see Figures 9 and 10). Sprays made after the second true leaf rarely pay off, and almost never do when timed at the third and fourth leaf. If cotton is growing rapidly and thrips numbers are low to moderate, it’s generally better not to spray.
Second, if growers determine that they need to make a second spray, spinetoram (Radiant) at 3 oz plus a surfactant is a good choice (see Table 2). In addition to being effective for tobacco thrips, this insecticide has the advantage of being very strong on western flower thrips. Spinetoram is also in a different insecticide class than acephate (Orthene). Other insecticides like dicrotophos (Bidrin) and dimethoate are in the same insecticidal class as acephate and are not good rotational partners.
Finally, do not spray ThryvOn cotton for thrips.