|Fig. 1. Corn earworm on Corn|
|Fig. 2. Sorghum webworm|
You should begin sampling any sorghum fields that have heads entering themilk stage. Using the bucket/shake method is the best way to determine the average number of worms per head. I recommend doing a series of 10-head samples in each field. Shake 10 randomly selected heads into a white 5-gallon bucket and count the worms/10=average per head. This is one sample. Do several 10-head samples in each field taking samples from areas with any obvious differences (different head maturities, areas next to corn, etc.) then calculate an overall worm/head field average.
Recommendations are pretty variable across states, but an average of 2 worms per head, or more, is a pretty common trigger for a spray.
If treatment is needed use a spray boom/nozzle/gpa/psi system that delivers as much product to the heads as possible. Remember the heads are not a flat surface, so this is much more like covering a wheat head with a fungicide as opposed to a replant herbicide application. The head has structure and shape and needs to be completely covered. Spraying leaves is a waste of time and product the more product hitting the heads, the better. Directing sprays to the heads is even more important in varieties with compact heads (-vs- loose heads). When heads are compact, worms tend to burrow to the center, are not easily seen, and are not as vulnerable to sprays. Remember, the insecticides used for worm control depend on direct contact.
What insecticides should you use? Sorghum has fewer labeled insecticides than many commodities, but there is a pretty good selection with different modes-of-action. Pyrethroids include Tombstone, Mustang Maxx, Warrior, and Asana XL, and others. Non-pyrethroids include Belt, Blackhawk (was Tracer), and Lannate. There are a few others that combine active ingredients like Stallion (Mustang + Lorsban) and Consero (Prolex + Tracer). Note that I have not evaluated these products so cannot make comments about control, but colleagues in other states have experienced lack of control with pyrethroids, alone, when worm populations were high or worms were a large size (harder to kill) when sprays were applied.
Fig. 1. Corn earworm on corn. Cam Kenimer.Fig. 2. Sorghum webworm on grain sorghum. Alton N. Sparks, Jr. Bugwood.org