There are many acres of grain and forage sorghum planted late in North Florida. We usually experience the first freeze in late November and have plenty of time to make a crop after spring vegetables or corn silage harvest. We are finding that much of our later planted sorghum is under pressure from insect pests. We should plan to protect this grain until about mid dough stage. The caterpillars should not be much of a problem after that time because they are less attracted to the plant and they can also cause less yield reduction at that stage.
We are finding many Sorghum Webworms in this crop. They are tough to find hiding in the head when they are small, but are usually found in very large numbers when present. Farmers are reporting 15-20 per head. The economic threshold is actually 5 per head. The University of Georgia suggests these may not be controlled by pyrethroid herbicides, so you might choose another caterpillar-type product rather than a broad spectrum pyrethroid for control if this is your target pest. Other caterpillars such as earworm or armyworms have a lower threshold of 1-2 per head, and again not all of those will be controlled by pyrethroids. However, I am seeing only webworm at this time.
Much of this sorghum in our area is grown for silage for dairy cows. We have had some important questions about which products are safe to use and when can we harvest forage following treatment. The classic answer is “it depends.” I have attached the table from the 2013 Georgia Pest Management Handbook which shows the insecticides labeled for sorghum and the preharvest interval when harvested for forage/silage. I noticed that some of the more popular caterpillar materials are not labeled for sorghum and some have a 30 day preharvest restriction. After you scout your crops, please only use labeled products and respect preharvest intervals.