Managing Insect Pest in Mississippi Grain Sorghum

Mississippi traditionally does not plant a lot of grain sorghum compared to some states but the acres we plant (42K according to USDA, 2013) have to be scouted and managed for insect pest at least weekly. Grain sorghum in Mississippi can have significant insect issues in some years, especially on late plantings. Below are just a few of the pests commonly found in MS grain sorghum and management tips.

Chinch Bugs

Chinch Bugs: Typically more of a problem in grain sorghum than corn because of the later planting dates and mostly dryland conditions. Chinch bugs thrive in hot dry weather and periods of drought are conducive for buildups. Also, it is common for grain sorghum to be purchased and planted without and insecticide seed treatment which greatly increases the likelihood of having a problem.

Sampling: Visual, remove lower leaf sheaths at the base of the plant and count adults and immatures. Also, observe movement in cracks in the ground.

Threshold: Same as corn, when 20% of the plants have 5 or more chinch bugs per plant on plants 6” or less. Unfortunately, most problems occur on corn that is greater than 6”, so what do you do then? This is one of those areas where you have to inject your common sense and experience. Chinch bugs can severely damage big grain sorghum if numbers get high enough, but what is that number? In the absence of a decent threshold on bigger grain sorghum I always advise to look at the crop and try and determine if the numbers you are observing are physically hurting or holding back the crop. This is not as hard as it may seem. Chinch bugs have piercing sucking mouthparts and tend to suck the plant down and you will often see lower leaves dying and a general loss of vigor. Another important factor to consider is if the crop is stressed from another factor such as drought. If so, it will not be able to tolerate near the numbers that a healthy actively growing plant can take. I have witnessed hundreds per plant on actively growing grain sorghum that was 24” tall with no problems but also seen drought stressed grain sorghum that was 24” tall being negatively affected by less than 50 per plant.

Treatment Options: Tough to control but can be done if needed. In the past, we have some success post directing a high rate of a pyrethroid or Lorsban 2 times 5 days apart at 15-20 GPA toward the base of the plant. If post directing is not an option, broadcast at 20 GPA but manage expectations.

Whorl feeding caterpillars: The whorl complex consists of fall armyworms and corn earworms or any combination of the two. However, often time in grain sorghum it is mostly made up of fall armyworm.

Sampling: Visual destructive. Pull whorls and unroll to determine species and age of larva.

Threshold: Same as corn in MS, 100% infestation but I feel more comfortable with 75%. Keep in mind also on very late planted grain sorghum that heavy fall armyworm grazing will slow plants down which can delay head emergence to some degree which may indirectly cause more sprays down the road (see midge section). Be aware that as grain sorghum approaches the boot stage where the head can easily be found in the whorl, fall armyworms can and will often times feed in the whorl and chew the neck of the developing head. I have witnessed this on several occasions if the armyworm infestations line up closely with boot. We have no threshold for this but I would likely use a number around 10-20% if you see this starting to happen. Don’t assume that all whorl infestations will cause this but be aware it can happen.

Treatment Options: Believe it or not a pyrethroid insecticide at 15-20 GPA will actually do a decent job on fall armyworms that are down in the whorl. Try to make applications as late in the afternoon as possible so hopefully our heavy dews in MS will move some down in whorl overnight. I have also seen where a light rain behind an application appeared to “wash” the insecticide down in the whorl and achieved 90% + control. Generally expect about 50% control but often this is more than enough to get you out of a jam. Use highest labeled rates.

Stem borers: Southwestern corn borers and sugarcane borers can be a severe problem in grain sorghum and have caused many a headache in Louisiana, however, in MS, I have searched hard and never been able to find a single infestation. It is likely that sugarcane borers are the most common species causing the damage which fortunately have not yet become established in any appreciable numbers yet in MS. For now just be aware it can happen.

Sorghum Midge

Sorghum Midge: Midge can be extremely damaging in grain sorghum if they are present in high numbers. Often time our normal planted grain sorghum in MS will have very few midge but as planting dates become later chances of midge being a problem increases. I have seen as many as 50 per head on very late planted grain sorghum. Most folks are applying an automatic application at 25-35% flowering. Midge is not hard to scout for and I feel comfortable making the decision to spray on threshold rather than automatic. Be aware that midge numbers are usually much worse around areas of Johnson grass.

Sampling: Visual or Bag Method. To visually sample for midge simple sit still for a minute and carefully watch the flowering head for midge movement. This is my preferred method and it actually works very well. It works better in the morning or evening when the wind is down but I have been successful at all times of day. This works better for young eyes because they are small……The bag method also works very well. Simply take a very light weight bag, something the consistency that your newspaper comes in works very well if you can find one bigger and gently place it over the head and tap the head easily and the midge will fly up to the top and you can count them. Keep in mind midge are very small and red in color. You may catch several “midge like” gnats but they won’t be red in color.

Treatment Options: Midge are easy to control. Mid to low rates of pyrethroids work well. Also Lannate and Dimethoate are labeled.

Headworm Complex:  The headworm complex is made up of fall armyworm, corn earworm, and sorghum webworm. In MS we mostly deal with fall armyworm and corn earworm. Sorghum webworm can commonly be found in MS grain sorghum but are seldom sprayed alone here. Headworms generally show up 5-10 days after flowering and more commonly a problem in fields that had midge sprays that killed out beneficial insects.

Bloom progressing down the head

Scouting: Bucket or Sweep net Shake. Scouting for headworms is very easy. Simply take a white bucket or your sweep net and bend the head over in it and vigorously shake it then count the worms. Divide the number of worms you counted by the number heads you shook and that gives you the worms per head you are averaging. Do this in several places in the field. I generally shake 10 heads before counting and I prefer the sweep net but either work equally well.

Threshold: One worm per head for corn earworm and fall armyworm published but new threshold calculator from Texas A&M, https://insects.tamu.edu/extension/apps/sorghumheadwormcalculator/ , suggest that based on current prices and our expected yield that it may actually be closer to ½ worm per head. I have some data that would agree with that from last year.

For sorghum webworm it is recommended to treat when numbers reach 5 to 6 per head.

Treatment Options: This one can be a little tricky in MS because the level of control we have seen with pyrethroids has fell off dramatically over the last few years with corn earworm. That basically leaves Belt and Lannate. Lannate is an old standard and Belt is a newly labeled diamide. It would seem we would get very good coverage on grain sorghum and control of headworms would be very good but we seldom get “great” control with any compounds on corn earworm in grain sorghum under high pressure for some reason. I have seen fields require multiple applications for corn earworms but only during periods of very high pressure.

Final thoughts There are many other compounds labeled for these pests mentioned above that I did not mention because I have not thoroughly evaluated. As always, read the label. If you have questions about other options besides those mentioned give me a call.