David Owens, Extension Entomologist, owensd@udel.edu
Soybean
A couple of calls came in earlier this week about spider mite activity in soybean. Rain sometimes, but does not always, lead to mite population declines. If the rain is from a ‘pop up’ storm in which a lot of water falls on the field but then quickly dries, it is less likely to affect mites. If a field has been recently treated with a fungicide, it is less likely that rain will lead to mite reductions. Prolonged cool, cloudy, wet weather such as we have experienced from Monday night to Thursday is more likely to lead to mite reductions. The most important natural enemies for mites are other mites and fungal pathogens. Fungal pathogens need weather like we have had this week to cause epidemics. It may take them a few days to kill mites, but I expect mites in many of our hot spots in fields to decline rapidly in the coming days. These mites will appear gray and fuzzy. Predatory mites will appear as pear shaped, a single color (light straw or red), generally flat, and move quickly (for a mite). If you are not seeing stippling on new growth, examine stippled leaves carefully for live mite activity or call your local friendly Extension entomologist to collect a sample under magnification. I was in a field that a week or two ago I would have made a recommendation to treat, but now most of the mites are dead, the canopy is still well developed, and little stippling is occurring on new growth. If high mite populations are still present, stippling is on more than a third of the leaves on half the plants or leaves are dropping from the plants, your best options are Agri-mek, etoxazole (ex Zeal), followed by Dimethoate and a still-labeled chlorpyrifos (please note Lorsban no longer has an active federal label), followed distantly by bifenthrin.
Check full season fields for stink bugs!
If you are near the Greenwood area, CHECK FOR STINK BUGS. Our blacklight trap west of Greenwood has been catching high numbers of green stink bugs all season. Stink bugs are most problematic at R4 and R5, but if they are present before then, they have no reason to leave. If most stink bugs are green stink bugs, any pyrethroid will do a good job. If brown or brown marmorated stink bugs makes up a significant proportion of the population, treat with a high rate of bifenthrin.
Sorghum
Once sorghum is approximately 1-2 weeks past heading, begin scouting for headworm. This can include corn earworm, fall armyworm, and sorghum webworm. Sorghum webworm thresholds are greater than those of the other two. As a general rule, one earworm per head can result in a 5% yield loss. While scouting sorghum, begin scouting for white sugarcane aphids (now also referred to as sorghum aphid). This aphid typically arrives in our area between late July and early August and can cause yield loss as late as soft dough. It will form dense colonies underneath of mid-canopy or lower leaves. Do not confuse it with yellow sugarcane aphid which is larger and yellow, and corn leaf aphid, which tends to be green and dark greenish blue and a rectangular shape.