Recent questions I have been asked: When/if should I put an insecticide out with my harvest aid for sugarcane aphids in sorghum? Or should I wait to do this until harvest?
First, I’ll refer you to a good article from Mississippi State University from last week (http://bitly.com/AphidHarvest). This article also addresses if, when, and what harvest aids can be applied.
The short answer is we do not have an established threshold for making these decisions. It requires some guessing. You should treat if aphid populations exceed the recommended treatment threshold and you are still two to three weeks away from applying a harvest aid. You are still at risk to some yield loss until physiological maturity, and aphid numbers may explode to unmanageable numbers before harvest aids are applied.
The harder question is what to do if you break threshold within seven to 10 days of applying a desiccant. Obviously, it depends how many aphids are present.
I think you can wait and make a decision at the time of applying the harvest aid if there are only scattered hot spots and generally low numbers of aphids (e.g., average of 50 to 100 on flag leaves).
If you are ready to make a harvest aid application, one possible threshold would be the presence of many hot spots within the field (not just the edges) or aphids are present at moderate or high numbers on most plants. Moderate might mean 50 to 100 aphids on the vast majority of flag leaves. I would definitely treat if averaging of 100 or more aphids on the flag leaves.
The whole point is to prevent large numbers of aphids from moving into the heads after applying a harvest aid, thus creating harvest problems. Aphid populations typically do not build much after harvest aids are applied (because the plant is dying), but they tend to crawl to the heads after harvest aids are applied.
I suspect a few scattered hot spots will not cause a substantial problems during harvest.