We are going on week 6 of a very high and sustained flight of bollworms in cotton, soybeans, and grain sorghum. The same control problems with pyrethroids we have had in soybeans have spilled over to grain sorghum and some cotton fields.
Soybeans: Over the last two weeks people have been reporting very low numbers of soybean loopers showing up. Last week numbers starting increasing to around half threshold in some fields in the south delta. This week I have gotten numerous reports of above threshold numbers and some treatments are going out now. We looked at a couple fields this week around Tchula and Greenwood that were running 30-40/25 sweeps (threshold is 19/25 sweeps or 20% defoliation). Most of the fields throughout the delta region have been treated with pyrethroids for bollworms in the last couple of weeks so loopers will likely blow up quickly in those fields in the absence of beneficial insects. A lot of the loopers we are finding now are less than ½ inch in size but there were many hatch-outs present that we didnâ€™t bother to count. By next week I expect more widespread applications will be going out in the delta.
Many folks are also reporting very high numbers of bollworm eggs and small worms in soybeans that are R5.5-R6 with no flowers left in the field. This is a little unusual but nevertheless seems to be the case this year. Keep a close eye on bollworms in soybeans, seems they will be with us for a while longer.
Treatments Options: Intrepid, Belt, and Steward. Do not try and control soybean loopers with pyrethroids or acephate. This will be tempting since we are still dealing with bollworms across most of the delta but you will be disappointed if you target a soybean looper with either of these two products alone. Mixing acephate and pyrethroid together will only suppress light numbers of loopers, nothing more. I typically see 20-30 percent control when mixing pyrethroids and acephate on soybean loopers.
Out of the 3 options (Intrepid, Belt, Steward) Belt and Steward are the only ones that will control both soybean looper and bollworm without a tank-mix. If you are using Intrepid, you must tank-mix a pyrethroid (preferably with acephate) to control the bollworms. If you have applied Belt in the last 3 weeks for bollworms, I would not be surprised if you continue to see residual control of loopers for a while longer. We saw residual out to 28-30 days from Belt last year in our plots. Intrepid at 4 oz also provided in excess of 21 days in test last year. Many factors affect residual so don’t expect that long in every situation. If rain sets in, and numbers continue to increase it may be much shorter.
Terminating Looper Sprays: I have had a graduate student working on the effects of defoliation on soybean yield for the last several years. The most susceptible stages for yield loss are R3-R5.5. Generally we saw negligible yield loss until we exceed 40% defoliation (thatâ€™s why the threshold is set at 20%, to keep you from getting to 40%). However, if you are between R3 and R5 and exceed 75% defoliation, yield loss can be substantial. Once beans reach the R6 stage we typically only see 5-10% yield loss with 100% defoliation. At R7 you will very likely not see any yield loss from 100% defoliation, therefore, you can terminate treatments for all defoliating pest at R7.
Grain Sorghum: I have had about 10X more calls about bollworms in milo this year than the 8 years combined since I have been at MSU. The same control issues we have seen have seen in soybeans is showing up in milo as well with the straight pyrethroids. We conduct headworm test in milo every year and the pyrethroids have generally only provided 60-80% control in my test. Although it would seem that coverage should not be an issue in milo with the head exposed, this is not the case, particularly with aerial applications. Couple that with increased tolerance to the pyrethroids and high bollworm numbers, and we have unsatisfactory control. The standard has been Lannate at 24 oz, but I have had numerous reports of not being able to get this product. This year Belt also received a grain sorghum label and quite a bit has gone out, however, it has become even harder to find in recent days. That brings you back to the pyrethroids. If you cannot get Belt or Lannate and must use a pyrethroid, max the rate out and go by ground with at least 10 gallons if this is possible. I treated some milo the other day with 1:32 Mustang Max with 10 gallons by ground with hollow cone tips and did a very good job (This was in Starkville where bollworms are likely much more susceptible to pyrethroids than the delta). Some folks are also reporting seeing a definite difference in control with â€œtight headâ€ varieties versus â€œloose headâ€ varieties. If this is truly the case, then coverage is obviously lacking.