2013 has been a challenging year for thrips; to date we have treated significantly more acres with foliar sprays than normal. Granted thrips infestations vary across the state, but as a whole this is a tough thrips year. So why is damage so bad? The images below illustrate excessive thrips damage on the left and a picture of two leaf cotton with little to no thrips injury (a rare site to date in 2013) on the right.
Cotton planted in late April and early May grew off slowly which exacerbated (made worse) plant injury symptoms associated from thrips feeding. Thrips numbers were only moderate during the first half of May but plant injury was severe in some areas. Thrips feed in the terminal bud and when cotton is not growing rapidly the thrips are feeding on the same unfurled leaf for an extended time resulting in increased damage symptoms. We have observed in research trials that a slow growing seedling incurs significantly greater damage than a rapidly growing seedling with similar thrips infestations. Slow growing seedlings have also extended the time which plants are susceptible to thrips. Seedlings are susceptible to thrips until they reach the 4-leaf stage and are growing rapidly. Recall that seed treatments are only active on thrips for about 3 weeks and much of our early planted cotton has taken 4-5 weeks to reach the 4-leaf stage.
Beginning late last week (about May 23rd), thrips numbers significantly increased in many areas. We normally expect the peak in thrips populations to occur earlier but the peak appears to be later than normal (maybe 2 weeks later) this year. Historically thrips numbers taper off on cotton planted after May 10, but this year the peak has been delayed and we can only hope numbers will taper off soon. Another factor which may explain the recent surge in thrips infestations is the dry conditions we are experiencing. Thrips reproduce and build populations on many different host plants. After a moderately wet spring, alternate host plants have begun to dry down and thrips are on the move looking for a green plant.
As I have looked at cotton this spring one of the most dramatic observations I have made is the impact of tillage on thrips populations. Thrips injury is significantly less in reduced tillage systems compared with conventionally tilled cotton; especially when there is a large amount of cover crop residue on the soil surface.
A common question I have received is whether the seed treatments are working. The answer is absolutely yes! The control which they offered is very evident in small plot trials where we commonly compare treatments to an untreated control. Granted foliar sprays have been needed, but the seed treatments did help. It would be near impossible to manage thrips with foliar sprays alone on a year like we have had. Years ago it was not uncommon for an in-furrow granule tube to stop up periodically and we were reminded at how bad thrips could be without an at-plant insecticide treatment.
In terms of management, take the time to scout and make foliar applications if thresholds are exceeded. Foliar sprays should be applied if thrips exceed 2-3 per plant and immatures are present. Immature thrips are crème colored and wingless where as adults will be brown or blackish and have wings. Seedlings remain susceptible to thrips injury until they reach the 4-leaf stage and are growing vigorously. Excessive injury during early seedling stages (1-2 leaf) has a greater impact on yield potential than injury on 3-4 leaf cotton. So if you have a problem on 1-2 leaf cotton, address it in a timely manner. We would expect thrips numbers to begin to decline soon but in the interim, scout, scout, scout.
Immature thrips bottom left and adult top right.
When foliar applications are made on cotton which has severe damage, it is likely that the next leaf to unfurl will also have damage (remember that thrips are feeding on unfurled leaves in the terminal bud). The second leaf to unfurl should look better. Retreatment of a field should be based on thrips counts and not the appearance of the next leaf to unfurl. There is no question that maturity has been delayed on some early planted fields, but cotton is resilient and will recover in most situations. In severely damaged fields some terminals may abort and the plant may “sucker out” and appear as “crazy cotton” with multiple terminals. I am amazed each year at how bad plots with no at-plant insecticide for thrips can look during the seedling stage and that they can recover and still make respectable yields.