David Owens, Extension Entomologist, owensd@udel.edu
Sorghum
Scout late sorghum for corn earworm and sugarcane aphid. Last week, a field of sorghum we had been looking at that just finished pollinating was over threshold for corn earworm, and our variety trial at Carvel was in various stages of head emergence through soft dough. The best way to scout for earworm is to use a ‘beat bucket’ – traditionally a 2-5 gallon bucket, although a sweep net can serve in a pinch. Gently bend the head to fit inside the bucket and shake it against the sides of the bucket several times to dislodge worms. Do this for 10 heads in a location, and 5 locations per field. Keep notes on how many larvae are small (1/4 inch or less), mid-sized (1/4 – ½ inch) and large (>1/2 inch). Texas A&M has a useful sorghum threshold calculator that takes into account control costs, grain value, and heads per acre and calculates a threshold based on the number of medium and large larvae. If your field is over threshold, we have several good options for earworm. In a 2019 spray trial, the greatest worm reductions came from Carbaryl (1.5 qts), Lannate (1.1 pints), Besiege (8 fl oz), Prevathon (now Vantacor), and Baythroid XL (2.8 fl oz). Before treating sorghum for corn earworm or the next pest of interest, remember to read labels carefully; the label is the law.
While scouting for earworm, take a few moments to flip some leaves back to look for white sugarcane aphids. When it is present in a field, it is obvious. Aphids produce copious honeydew which attracts flies, bees, and butterflies. It can cause yield loss as late as soft dough, particularly in drought stressed fields. So far this year in Delaware, its populations have been spotty, slow to build, and our soils have good moisture. There are two thresholds: 40-150 aphids per leaf or 30% of plants with aphids and scattered areas of honeydew slicks present on the upper surface of a leaf just below the aphid colony. In 2019, we found our first sugarcane aphid on August 8. By September 11, the field averaged between 227 and 644 aphids per leaf! If you deem a field needs treating, the only good options are Sivanto, Transform, and Sefina. Sivanto has a 2ee recommendation to use at 4 fl oz, although even lower rates will work very well. Lorsban and dimethoate annoy aphids for about a week before their population increases. Pyrethroids are completely ineffective. If sorghum is going to continue serving in your rotation, reach out to your agronomist – several varieties have some level of resistance or tolerance to aphids, and this far north, that may be all we need for late sorghum.
Hemp
Corn earworm continues to increase in hemp. Remember, our best options are microbial insecticides B.t. and polyhedrosis viruses (ex Gemstar), and these work best on small larvae. Aphids are also showing up on flowers. These can be controlled insecticidal soaps. If you are unsure what is allowable on hemp, please visit https://www.epa.gov/pesticide-registration/pesticide-products-registered-use-hemp. Useful biological control agents in enclosed spaces include lady beetles and lacewings.
Soybean
Corn earworm populations in double crop are spotty, some fields are at or near threshold, but many are clean. Other insects of interest that are active include stink bugs, soybean loopers, and bean leaf beetle. Bean leaf beetle populations typically peak in early September. Soybean looper populations are low, but continue scouting double crop soybean for defoliation through the R6 stage. We were recently called to a field with stink bugs. Stink bugs were concentrated along a woodline that contained walnut, maple, mulberry and wild cherry. A mixture of greens, browns, and brown marmorateds were present. Our threshold is 5 bugs per 15 sweeps until the beans reach R6, and then thresholds can start to increase. Nymphs should be included in that count.