Grain Sorghum: Nearly Perfect Storm in Lubbock and Lynn Counties

After writing in this newsletter last week that fall armyworm was not a significant threat this year, Katelyn Kesheimer, Lubbock and Crosby county IPM Agent, and I visited some fields in southern Lubbock county and south to the middle of Lynn County in the last four days. I take it back; fall armyworm is very numerous in sorghum in these places south of central Lubbock County where my traps are located. We encountered fields at panicle exertion or already booted that had as many as six worms per head, with an average of 2-3 mid-sized worms being the norm. For the most part these were fall armyworms in southern Lubbock County, but corn earworms seemed to increase in frequency as we went south. In a field 6 miles west of Tahoka we were seeing something like the 70% fall armyworm and 30% corn earworm. The age structure of the populations was approximately 45% small larvae, 45% medium larvae and 10% large larvae, but of course this will change quickly. Large larvae are by far the most destructive, and the goal is to treat the field before many of them are present. Kerry Siders, IPM Agent in Hockey, Cochran and Lamb counties, reported in his newsletter tonight that headworms were increasing in his counties, and the majority of these were corn earworms.

Fall armyworm larva feeding on a sorghum panicle in southern Lubbock County yesterday.

Prior to the arrival of sugarcane aphids, control options for caterpillars would have been a pyrethroid, Lannate or Carbaryl. Pyrethroids are not very effective on fall armyworms over 1/2 inch in size, so some area crop consultants are now adding a pint of Lorsban to act as a synergist with pyrethroids. HOWEVER, WE FOUND SUGARCANE APHIDS IN ALL OF THESE FIELDS. The use of a pyrethroid and/or Lorsban would eliminate the biological control agents in the field that are suppressing the sugarcane aphid population.

What we have now in these areas is a real problem. The best control practice would be to go after the worms with a soft insecticide that does not kill the biological control agents that keep sugarcane aphid in check. These insecticides would be Blackhawk or Prevathon. Blackhawk is approximately half the price of Prevathon, but DowAgroSciences has told us that there are no supplies of Blackhawk left in the warehouses because of brisk sales this year in the mid-South. So that leaves Prevathon, which is excellent on both caterpillar species. However, an application of 14 oz/acre of Prevathon, the lowest labeled rate, would cost on the order of $18 per acre + application costs. We cannot recommend less than labeled rates, but area Independent Crop Consultants tell us that 10 oz of Prevathon with 5 GPA by air provides good control of both caterpillar pests. This rate would cost approximately $12.80 per acre + application costs.

If one chooses to follow the pyrethroid + Lorsban path in a field with sugarcane aphids, then it is likely that a follow-up application will be needed for the aphids; at least 5 oz/acre of Sivanto or 1.25 oz/acre of Transform. But this is not a given; the aphids south of Lubbock County do not seem to be increasing as fast as they did in years past. Scouting will be essential.

At this point we do not know what to recommend with so many headworms in the system and aphids in the field; it comes down to economics. There are no inexpensive options here that do not elevate risk from sugarcane aphid, and we can't predict the future with respect to whether sugarcane aphids will require treatment later. (But note that some fields in southern Lubbock County are well over treatment thresholds for both pests.)

We have a sorghum headroom treatment threshold calculator here.  Our written thresholds and scouting procedures are here on page 23.

Another unknown is sorghum midge. The late planted crop is at risk, and with Blackhawk (which is effective on sorghum midge) out of the picture, we will have to resort to pyrethroids, which in turn will increase the risk of sugarcane aphid while not being much use on fall armyworm. Yes, there are no inexpensive answers to this emerging multi-pest situation.