In most years it is not uncommon to overspray 20-30% of the cotton acres for thrips in MS. Last year (2011) was an anomaly in that we treated around 70% of the cotton acres for thrips due to extremely high pressure. It appears that this year is shaping up the same way. This week we have had numerous calls on young cotton with very high numbers of thrips. There have also been reports of Western flower thrips on cotton. WFTâ€™s generally make up less than 10% of the total thrips species we find on seedling cotton in MS, however, in some years they can make up a much larger percentage of the species present. WFTâ€™s are a problem, mainly because they are very difficult to control with our normal thrips materials.
When to Treat: Threshold for thrips in MS is when you find and average of one thrips per plant and immatures are present. The reason we say andâ€ immatures presentâ€ is because, if you are finding immatures it is a good sign your seed treatment is no longer holding. In years with extremely high adult numbers (like this year) you may have to treat on adults in some situations. I would let the plant growth response be your guide. If you are finding numerous adults per plant, but the plants are healthy and growing off well, I would not treat. However, if you have numerous adult thrips per plant and new leaves are showing damage, I would consider treatment on adults alone. The key is to base sprays on the youngest leaves so that you are not spraying on old damage. Remember, for seed treatments to work thrips have to feed. If there are a lot of thrips feeding, you will get damage even if the seed treatments are working and sometimes benefit from foliar applications. Donâ€™t do this unless necessary because the potential to flare spider mites exists, particularly with acephate.
What to treat with: Basically we have 4 options for thrips control, Orthene, Bidrin, Dimethoate, and Radiant. All of these have different strengths and weaknesses.
Strengths: Highly effective, relatively inexpensive, more flexibility, least likely to cause crop injury when mixed with Dual or Liberty
Weaknesses: Likely to flare spider mites and aphids if present at low levels
Strengths: Highly effective, less likely to flare spider mites and aphids
Weaknesses: A little more expensive, less flexibility
Strengths: Effective, relatively inexpensive, more flexibility, less likely to flare spider mites and aphids.
Weaknesses: A little less effective than Orthene or Bidrin at the lowest rate and most likely to cause injury when mixed with Dual or Liberty.
Strengths: Effective with an adjuvant, more effective on Western flower thrips, least likely to flare spider mites and aphids
Weaknesses: Expensive, must use an adjuvant
In general, all of these insecticides are effective and about equal in terms of thrips control. Selection of one of these insecticides should be based on cost, spectrum of species controlled, and impact on other pests. Orthene and Bidrin are very similar in terms of their level of control. However, Bidrin is a little more expensive than Orthene. On the other hand, Orthene is also the most likely insecticide to flare spider mites and aphids.
Dimethoate is slightly less effective than Orthene or Bidrin, but it is still a very good option because it does provide adequate control of thrips, is inexpensive, and less likely to flare other pests than Orthene.
Radiant is considerably more expensive than the other insecticides and is generally used as a last resort. The major strength of Radiant is its activity against Western flower thrips. Some Radiant has been used in cotton this year where an application of one of the other insecticides has provided less than adequate control and Western flower thrips were the primary species present. In these situations, tobacco thrips are usually the predominant species before the first application. This application kills most of the tobacco thrips, releasing the Western flower thrips.
When Radiant is used in these situations, it is important to use an adjuvant. In research that we conducted a few years ago, Radiant provided thrips control that was as good as any of the other insecticides when an adjuvant was used. In contrast, when Radiant was applied without the use of an adjuvant, thrips control was poor.
Another factor that can influence thrips on cotton is rainfall. A heavy rain, even for a short period of time, can kill a lot of thrips and let cotton plants get ahead of them. However, do not rely only on rainfall because thrips can buildup fast after rain. The key is to not spray when rain is expected. In some situations, a good rain can eliminate an insecticide application, but in most situations will only delay insecticide sprays.
In conclusion, watch for thrips reproduction on cotton that has an insecticide seed treatment, pay close attention to new leaves as they come out, and spray only when absolutely necessary to keep from flaring mites and aphids.