Farmer’s Agronomy Update 8-19-21
Focus on Insects
-Other Insects in Soybeans
Armyworms are the hot topic the past 10 days. Check your forage crops, such as hay, and alfalfa fields as well as late planted soybeans. There are some hot spots around the Hwy 36 and I-70 corridor. These larva can feed in a hurry and should be dealt with swiftly with an adequate insecticide with good efficacy on larva. Thresholds vary depending on crop and source of info. For hay and alfalfa an average of 4 or more, non-parasitized larva per square foot would justify an application. In soybeans these pests tend to move off of grassy weeds after they are killed with a POST-applied herbicide application and move to the next available food source which is the soybeans. There are a couple ways to evaluate if you reach the threshold; by defoliation which is 15-20% defoliation in the reproductive stages, by a sweep net with 10-15 or more per 25 sweeps.
- Other insects of concern in soybeans that are requiring treatment are loopers, green cloverworms, pod worms (corn earworm), stink bugs. For loopers the concern is leaf feeding, with a threshold of 15% leaf defoliation during the reproductive growth stages. Loopers are more difficult to control than green cloverworms which also has a threshold of 15-20%. For podworms and stink bugs it’s easier to lay a white cloth or cardboard down below the plants and shake them to dislodge the pests, an average of 1 or more per linear foot would be a concern.
To differentiate between larva species in the field it’s usually easiest to count their prolegs (not true legs). Soybean loopers have 2 pair, green cloverworms have 3 pair, and podworms (corn earworm) have 4 pair.
-Corn Borer (only concerning to non-GMO/conventional corn growers) moths are still flying in places while several other areas already have 4th instar larva boring into the stalks. Once they are in the stalks they are protected and won’t be killed by an insecticide app. Scout fields, sampling 20 random plants in a row in 5 different parts of the field. If moths, eggs, and young larva (not feeding in the stalks yet) are found then chances for controlling them is at their highest potential which is still only 70% control at best. If you are only finding larva in the stalk then you’ll likely not see much return, if any, by applying an insecticide. Best bet is to evaluate the infestation level and target those fields with the highest infestations for harvest just as soon as possible to avoid any additional yield losses due to deteriorating stalk quality as a result of the stalk feeding. Each year I find scenarios where corn borer causes 60-100+ bu/ac yield reduction. They require intensive scouting.
Stay vigilant and let us know what we can do to help!
Jaime Farmer Pioneer Field Agronomist West Central Missouri