Sorghum harvest is well underway in the Valley and is just starting in the lower Gulf coast of Texas. Much of the sorghum in south Texas looks very good considering the long drought period from April through much of May. The information is this newsletter will be relevant to late-seeded sorghum in south Texas and much of the sorghum in the Upper Gulf Coast and Winter Garden regions. We hope you enjoy and please let us know if you have questions around information in this newsletter.
Stink Bugs in Sorghum
Stink bugs are an insect pest that can affect row crops including sorghum. They get their name because when agitated they can release an unpleasant odor. All stink bugs have sucking piercing mouth parts. Stink bugs can cause significant losses in sorghum because they feed directly on the developing seeds. After feeding, the damaged seed is more likely to be affected by fungi or other plant diseases.
Last time I went out scouting I was mostly seeing rice stink bugs in the heads of sorghum. Regardless of what kind you have in your field, when populations reach the threshold you will want to treat. Consider managing the stink bug when you find a stink bug nymph or adult for every 1 to 2 heads during the milk through soft dough stage. During the hard dough stage stink bugs rarely cause damage to sorghum so treatment is usually not recommended.The training video above discusses different types of stink bugs that can be found in sorghum and shows a technique to scout for them faster. I highly recommend you watch it if you have sorghum in the flowering, milk and dough stages. Certain pyrethroid insecticides can be used to manage stink bugs at threshold in your field. However, if sugarcane aphids are present consider an alternative insecticide or tank Transform or Sivanto with the pyrethroid to minimize the probability of flaring aphids.
Sorghum headworm is a complex of two worm species, Helicoverpa zea (more commonly referred to as the corn earworm or cotton bollworm) and Spodoptera frugiperda (the fall armyworm). It is important to identify the species in your field to make the right management decision. Bollworm will have a light head capsule and their bodies are covered with small bristles that can be observed with a hand lens. Fall armyworm have a dark head capsule and a ‘Y’ shaped suture on the front of its head. The fall armyworm also has dorsal lines lengthwise on the body.
Worms damage sorghum by feeding directly on flowering parts and the developing grain but it is the last two larval instars (growth stages) that cause greatest damage. Natural mortality of small worms (1/4″ or smaller) is quite high and should not be considered when making a management decision. If headworm are 1/4″ or smaller it is suggested that the field be re-sampled in 2 to 3 days for a more accurate assessment of headworm populations. If headworms are different sizes then count only those larger than a 1/4 inch. Although any sorghum field is vulnerable to sorghum headworm, late-planted fields are more prone to infestations and hybrids with tight or compact heads generally have greater populations of worms compared with hybrids with loose or open heads.
Scouting sorghum for headworm should start shortly after flowering and continue weekly until the hard dough stage. It is suggested that heads be shaken vigorously into a white 5-gallon bucket to dislodge larvae. You can see the technique in the video at the top of the screen. The technique is the same for stink bugs and headworms. Headworms can then be counted and identified in the bucket. Sample at least 30 grain heads selected randomly across the field. In fields larger than 30 acres sample one grain head per
acre. The economic threshold for sorghum headworm in Texas is based on the cost of control, the grain value and the number of heads per acre. In general, the threshold for medium sized worms (1/4″ to 1/2″) half (one per two heads) to one worm per head. The threshold for worms larger than 1/2″ will be less than that for medium sized worms.
Pyrethroid insecticides should provide satisfactory suppression of bollworm on sorghum but will likely not provide a desired level of suppression against fall armyworm. Consider an alternative class of insecticide (there are several) to manage fall armyworm or a mixed population of headworm.
Soldier Beetles: Sound the Alarm or Support the Troops?
These are soldier beetles and they are plentiful right now. They are actually beneficial because they help pollinate the flower heads so don’t raise the alarm just yet. They are also potentially helpful because their larvae are predaceous and feed on other insects. Just leave them be and they won’t hurt you. Once flowers mature into heads you won’t be seeing them much any more, so support the soldier beetles while they are around.
Did you notice anything curious in the video in the last 20 seconds? If you look real closely you will see a very small red insect that looks like a mosquito. They aren’t mosquitoes filled with a blood meal. These are sorghum midges. They inject their eggs into developing sorghum heads and can lower yield when populations are large. To scout for sorghum midges you can watch a flowering head for a bit to detect their presence or use a plastic bag. To use the bag place it on top of the head and shake bag to scare the midges. They will typically fly to the top for easy counting. In Texas, the economic threshold for sorghum midge is based on cost of control, the value of the grain, and the number of flowering heads per acre but, in general, for sorghum in south Texas the threshold will range from half to one midge per head after checking 20 flowering heads for every 20 acres. If adult midge are found 3 to 5 days after an insecticide application a second application is suggested. Pyrethroid insecticides labeled for sorghum are generally effective against sorghum midge. In these situations treatment time needs to occur during the flowering stage to prevent losses. Treating during others head growth stages is ineffective.
If you did’t catch them in my first video watch this video and look closely. I will point out the sorghum midges to you with arrows. They are hard to pin down and find, but a predaceous fly of some sort, probably a robber fly caught one for me and was eating it. You can watch this at the end of the video. As always, it is important to only treat with pesticides when necessary so your insect allies can help you fight off pests like this one.
Thanks to Jason Thomas for helping write and contribute ideas, video footage and content for this newsletter. All videos, graphics and images were created by him unless otherwise noted in the image caption.
Robert Bowling, Ph.D.
Assistant Professor and Agrilife Extension Entomology Specialist
Texas A&M Agrilife Research and Extension Center at Corpus Christi
10345 Hwy 44
Corpus Christi, TX 78406