Sorghum maturity ranges from late bloom to mature and harvested. Sorghum that is at or beyond hard dough is considered safe from insect pests. We are watching fields for the sugarcane aphid, stink bugs and headworms.
We have not been finding many stink bugs or headworms in sorghum fields but continue to scout until hard dough.
We have found sugarcane aphids under the sheath on the flag leaf on some plants.
Sugarcane aphids are appearing in some fields that may be harvested in the next 2-4 weeks. This poses a few tough questions.
1. Will the aphids reach high enough numbers to cause harvest issues with honeydew or will the beneficial insects and environment control them?
There is no way to know what the aphid population will do over the next few weeks. Beneficial insects such as lady beetles, syrphid fly larvae and green lacewing larvae can eat a lot of aphids but I do not know how many beneficial per leaf/plant will be enough to maintain control of the aphids. I have been using an Action Threshold of 100 sugarcane aphids per leaf (rechecking the field in 48 hours) when deciding on when to treat a field with an insecticide. I still want to see this threshold met before I pull the trigger and spray a field.
2. How many aphids in the head will cause enough sticky honeydew to create a problem?
Aphid thresholds in cotton drop from 50 to 15 per leaf once cotton bolls open to prevent sticky cotton (which is rarely an issue). The sticky problem is also solved with a 1⁄2 inch rain. I would stick with the above threshold with a slight modification. If your field has more than 100 aphids per leaf, or colonies of aphids above the flag leaf in more than 30-40% of the plants, treat the field.
3. Will a harvest aid appliction of glyphosate solve the aphid problem by drying the sorghum plant?
In the Rio Grande Valley, glyphosate application did not necessarily cure aphids in the head. Once the aphid gets in the head, they tend to stay there, even after glyphosate application kills the plant. As the plant dries down the aphids tend to move up the plant.
