Tuesday, July 25, 2017

Sorghum Headworm Control: Consider Sugarcane Aphids

Sugarcane aphid is just beginning to build in fields in select counties on the Southern High Plains, and as of this writing I know of no fields that have required treatment. The sugarcane aphid distribution map can be found here. So far the aphids are building fairly slowly. This article is posted in the Sugarcane Aphid News site because sugarcane aphids should be considered if headworm control becomes necessary.

The less than good news is that fairly high numbers of headworms (corn earworm + fall armyworm) are being found in panicles. I was in a field in northeastern Crosby county last week that had 1-3 medium to large worms per head, and this field was later treated. Katelyn Kesheimer, IPM Agent in Lubbock and Crosby counties, just reported a field near Shallowater in Lubbock County that had a large number of worms. Stan Carroll, the Research Technician who runs the cotton bollworm/corn earworm traps at the Lubbock Center, told me this morning that he emptied the traps Tuesday night and had a high number of moths in them when he checked them Wednesday morning. We are therefore experiencing a big flight of cotton bollworms/corn earworms. The good news, if you can call it that, is that the fall armyworm trap captures are still well below average.

Insecticide selection for headworms is complicated now that we have sugarcane aphid or the threat of sugarcane aphid in the system. Most of our older insecticides like pyrethroids, Sevin, Lannate etc. will provide control, but they will also eliminate the beneficial insects from the field and leave it more open to damage by the sugarcane aphid. Newer insecticides like Blackhawk and Prevathon will preserve the beneficial insects, but they are more expensive than the older products. Besiege is a combination product; it has the same active ingredient as Prevathon but with a pyrethroid as well. Besiege will not preserve beneficial insects. If a headworm treatment is needed then the risk of sugarcane aphid will have to be factored into the choice of insecticides. As an additional complication, we think our corn earworm is still susceptible to pyrethroids in spite of some slippage downstate, but we know that fall armyworm is less susceptible to pyrethroids, especially the larger worms. One good thing is that headworms do not require the high gallons per acre of spray that sugarcane aphids do, so applications can be made with 3-5 GPA - but check the label for the specific product you intend to use.

Treatment thresholds are based on the size of the worms, number of worms per acre, heads per acre, control cost and value of the crop. For example, in the table below a treatment would be justified at 14,000 large worms (longer than 1/2 inch) per acre when the cost of control was $10/acre and the grain value was $7.00/cwt. To put this in perspective, if the field had 28,000 plants per acre, this would be one large worm per two plants. The online headworm threshold calculator is here.