Thrips and Herbicide Co-Applications

There is no question that we are seeing increased tolerance to the seed treatments with tobacco thrips in cotton. We made an effort this year to encourage the use of acephate either in-furrow or as an overtreatment to the normal seed treatments to extend control and hopefully limit the number of foliar applications. To date, feedback has been extremely positive where this has been applied. Thrips pressure this year is a mixed bag. It ranges from extremely light to heavy depending on where you are in the state. It is important to evaluate the presence of immatures as an indicator that seed treatments need to be supplemented with foliar applications. There will always be adult thrips on cotton without exception. This does not necessarily translate to injury. However, the presence of immatures is the key to knowing if seed treatments are failing.

We have been getting a lot of questions the last 7 to 10 days about potential injury or problems associated with mixing thrips insecticides with POST herbicides over cotton. This will no doubt be controversial, but we are going to lay it out like we would if you called.

There are a few herbicides that are commonly applied to cotton at the time that thrips are also problematic. These include dimethenamid (Outlook), acetochlor (Warrant), glufosinate (Liberty 280), and metolachlor-based products (Dual Magnum, Sequence, etc.), and now dicamba and 2,4-D herbicides. Many of these are applied alone or sometimes in mixtures with glyphosate.  There has been numerous questions regarding Liberty 280 application alone and in combination with the aforementioned products on Glytol/LibertyLink. WideStrike, WideStrike 3, Xtend, or Enlist varieties. (It is extremely important to remember that NO INSECTICIDES are currently labeled to go out with the dicamba or 2,4-D herbicides.

We have evaluated numerous insecticide/herbicide combinations over the years across the Mid-South for compatibility and crop safety. While we cannot guarantee every adjuvant/herbicide/insecticide combination, we have certainly evaluated the most likely combinations.

Take Home: Metolachlor-based products, Outlook, and Warrant applied in combination with Liberty 280 on WideStrike and Xtend cotton in particular can and will often burn (injure) cotton without any insecticide in the tank. Our data indicates that Widestrike 3 and GlyTol/LibertyLink varieties are more tolerant to these combinations. The addition of Bidrin or Dimethoate to the aforementioned tankmixes CAN increase injury particularly on WideStrike and Xtend cotton but also on WideStrike 3 and GlyTol/LibertyLink cotton. Burn is nearly always cosmetic and poses no risk to yield in most cases. The addition of an insecticide can sometimes enhance the burn but rarely above the herbicide’s own injury potential; however, this depends on the product being added. There are four insecticides commonly recommended for thrips control, which include acephate, Radiant, Bidrin, and Dimethoate. Acephate and Radiant are the safest followed by Bidrin. Dimethoate is the least safe and is the only one in the group that consistently causes any additional burn when mixed with herbicides. We are perfectly fine with mixing the insecticide alone with either Liberty 280 or metolachlor/Outlook/Warrant, but not in combination.

The reason herbicide/insecticide mixtures are controversial is everyone has a story about a time when they burned cotton when an insecticide was in the tank. Chances are you would have burned the cotton anyway from the herbicide alone based on all of our testing.

There are pros and cons with each of the insecticides we use for thrips: Orthene and Radiant are the safest by far but Orthene can flare mites and aphids and is not as rainfast as the rest. Radiant is also very safe to mix with herbicides but is more expensive, Bidrin can increase burn somewhat over acephate and Radiant but is very rainfast, Dimethoate works very well on thrips and is rainfast quickly after application but has the greatest likelihood of burn, especially mixed with herbicides.

No one likes walking injured fields trying to determine the cause, so the simple answer you will hear most often is do not mix the insecticide with the herbicide. Fact is, I would not hesitate to mix one of these thrips insecticides from an injury standpoint and save the money on application fees, but be aware of the technology and the herbicide combinations and the condition of the crop before doing so.