Controlling the Headworm Complex in Mississippi Grain Sorghum

There have been numerous reports lately of high numbers of corn earworm and sorghum webworm in grain sorghum. I have heard some reports of numbers as high as 10X threshold. It is critical to scout and treat as needed to control these pest. There are three species that make up the headworm complex in grain sorghum; the corn earworm (bollworm), fall armyworm, and the sorghum webworm. These caterpillar pests can occur alone or in combination with one another. The following gives a brief description of each.

Corn earworm/fall armyworm: These two species often occur together and are about equally damaging so we count them together in a threshold of 1 per head greater than 1/2 inch in size regardless of which species you are finding. Many people wonder why we add the 1/2 inch size criteria on the threshold. There are a couple reasons, the first is there is a lot of mortality on small larvae due to predators and parasites as well as natural mortality. The bad news about a midge spray is it kills beneficial insects which can actually make the headworm problem worse; however, it is very necessary if midge are present. The second is they do not cause any appreciable damage until they are in the latest instars. Remember, most caterpillar pests regardless of crop eat 90% of the food they eat in there entire lifetime in the last 3 days!  Generally we start finding headworms about 5-10 days after the head blooms out. You can find them earlier but the bulk of the numbers are generally present after bloom.

Sorghum Webworm:  Webworms are grain feeders as well that often occur simultaneously with corn earworm and fall armyworm.  This year we have found more webworms than normal on later planted grain sorghum.  Webworms are much smaller than the other headworms so not as damaging on a worm to worm comparison but a problem nevertheless when numbers are high.  Because they are not as damaging the threshold is higher for this species, 5-6/head.

Sampling the headworm complex: Sampling the headworm complex is actually very easy. Simply take an individual head and shake it vigorously into your sweep net and count the worms. Some folks use a bucket but I use a sweep net. Both work fine. I usually shake 10 heads before I start counting. Example: Shake 10 heads and find 20 worms, 20 /10 = 2, so you are averaging 2 worms per head. You can actually see worms on a head when carefully searched but I absolutely would not recommend this method of scouting for headworms. You will greatly underestimate the number of worms and thresholds were not designed to account for this. Use a beat bucket or something similar.

Control Options for the Headworm Complex: In the past pyrethroids at the higher rates have been good options form controlling the headworm complex, however, we have had numerous failures over the last few years on corn earworm in grain sorghum.  Also, webworms have documented resistance. I really think the risk is too high to try to control any of the headworm complex with pyrethroids alone.

Control options: Prevathon 14 oz., Besiege 7 oz. (Besiege also contains the pyrethroid Lambda cyhalothrin), Belt 2-4 oz., and Blackhawk at 2-3 oz. (Recently labeled for midge and headworms).

Benefits of choosing the right product:

We conducted the test below a few years ago demonstrating the superior control with the diamide chemistries when they were first being used in grain sorghum in MS. Superior headworm control also resulted in better yields (the trial below only averaged 1/2 bollworm per head or 50% of threshold). We only achieved about 50% control with pyrethroids and nearly 100% control with Besiege. This resulted in 4.2 bushels in a tight head variety, 6.7 bushels in a semi-tight variety, and 4.4 bushels in a loose head variety over the untreated checks. This also resulted in 5.9 bushels in a tight head variety, 3 bushels in a semitight variety, and 1.9 bushels in a loose head variety over the pyrethroid. This shows that even though these products are more expensive, the superior control far outweighs the price difference in most situations.

Summary: Manage your expectations. You still have to get good coverage to get good control. It would seem that with the heads exposed that coverage would be fantastic and you should zero out everything.  I have found this is not the case with all products and all application methods. It is not uncommon to leave some worms in the fields even with the best product choices available. As, indicated, we have also seen some differences with tight head varieties resulting in less control than loose head varieties on occasion.  It is not uncommon for some of the worms to stay toward the inside of the tight heads and not become exposed to the products in a timely manner. As always, let us know if you have any questions.