Although the growing season is winding down, we are still receiving some reports of insect activity and damage in soybeans. At this point, the risk is mainly to late-planted or double-cropped beans that are still maturing and still green when other fields are yellowing. Late-season soybean aphids, which feed on sap, do not cause yield drag after seed fill is complete. Stink bugs, grasshoppers, Mexican bean beetles, and bean leaf beetles can feed on pods later in the season.
Stink bugs feed directly on seeds by using their straw-like mouthparts to punch through the pod wall and suck seed contents. Earlier in seed development, this results in shriveled or aborted seeds. Later, feeding on more developed seeds causes only minor shriveling but can cause seed discoloration and open the pod up to disease. Heavy late-season feeding by stink bugs is more likely to affect seed quality than yield and is more of a concern for seed-beans. Feeding can also cause a “stay green” effect where affected plants mature later. This is most likely to be observed on field edges where stink bug feeding is usually heavier. Stink bugs do not affect beans after full seed maturity. Grasshoppers and beetles can chew on pods, either damaging the seed directly or encouraging disease until plant yellowing and full seed maturity.
Treatment is justified if the percent pod injury (counted on the whole plant, on the majority of plants examined) is approaching 10 to 15% (or 5% for seed-grade beans), insects are still present and active, and the field is not close to maturity. As harvest approaches, the pre-harvest interval (PHI) of insecticides is an important consideration. Most products labeled for these pests have a PHI ranging from 14-30 days, with the majority falling between 21-30 days – a consideration for harvest schedules.
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