Insect Management in Late Planted Grain Sorghum

Mississippi producers planted 65K acres of grain sorghum this year, up from 52K acres in 2011, and 12K acres in 2010. Grain sorghum traditionally in MS has been considered a “step child” crop that went on the worst dry land spots we had.  With recent prices we have started intensely managing grain sorghum acres in MS and are taking advantage of some of the benefits offered such as drought tolerance and rotational benefits. However, we have a large portion of grain sorghum that is planted late and requires intensive manage for insect pests.

There are numerous pests that attack grain sorghum in MS but the most consistent are chinch bugs, sorghum midge, the headworm complex, and to a lesser degree fall armyworm during boot stage. In this article I will focus on sorghum midge and the headworm complex since this is what we are dealing with on the late planted grain sorghum right now.

Sorghum Midge

Sorghum Midge:  Midge can be a devastating pest to grain sorghum if present in high numbers. In MS, we typically have low midge numbers (often never reaching threshold) on early planted grain sorghum, but as planting dates become later midge numbers can increase tremendously, particularly if there is johnsongrass in the area. Midge are only damaging during the bloom stage. Female midge can lay up to 130 eggs inside individual glumes during flowering and it’s the immatures that cause the damage as they develop in the seed causing blanks. Flowering starts from the top of the head and moves down the head until it reaches the bottom. The flowering period can last from 4-9 days depending on variety and other factors but I have typically observed about 4-5 day flowering period in MS with the varieties we are planting. The good news is there is a very short management period for midge.  The bad news is there is a very short management interval for midge, you can’t be late.  Typical management scenarios for sorghum midge for normal planting dates have been a low to mid-rate of a pyrethroid at 25-30% bloom on fields with fairly uniform head emergence.  On late planted fields, when midge numbers are high, it usually takes 2 applications, and 3 are not unheard of in fields with uneven head emergence.

Scouting for Midge:  there are a couple methods of scouting for sorghum midge.  The visual and the bag method are the most common.  The bag method consists of placing a light plastic bag carefully over the head, lightly tap the

30, 60, 100% Bloom

head, then quickly remove the bag and count midge. The visual method consists of simply observing the head for midge when winds are low. I personally prefer the visual method. Also, I check spider webs that are often easy to find in sorghum fields attached to the heads. Treat when midge numbers average 1/head.

Headworm Complex:  The headworm complex in MS generally refers to the complex of corn earworm, fall armyworm, and sorghum webworm.  They may occur individually or as a complex.  Over the last several years, the predominate headworm found has been corn earworm but this year we are finding more fall armyworms and sorghum webworms than years past on later planted grain sorghum.  When the headworm complex is present in very high numbers damage can be substantial as well.  In fact, we had at least one field that was not harvested last year due complete loss from corn earworm.

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 no matter which species you are finding. Generally I started finding headworms about 4-7 days after bloom. You will find them earlier but the bulk of the numbers are generally present shortly after. The bad news about the midge spray is it kills beneficial insects which can actually make the headworm problem worse; however, it is very necessary if midge are present.

Sorghum Webworm:  Webworms are grain feeders as well that often occur simultaneously with corn earworm and fall armyworm.  This year I 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.

Sorghum Webworm

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 5 heads before I start counting.

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 had numerous failures last year on corn earworm in grain sorghum.  Also, webworms have documented resistance in Louisiana and we have only seen marginal control here on that species.  I really think the risk is too high to try to control any of the headworm complex with pyrethroids alone.  I would recommend Belt at 2-4 oz, or Lannate at 24 oz. and would not rule out either of those two products also mixed with a mid-rate of a pyrethroid.

Summary: Manage your expectations.  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, particularly with an airplane.  It is not uncommon to leave some worms in the fields even with the best product choices available.  We have also seen definite differences with tight head varieties resulting in less control than loose head varieties.  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.