Agronomic Crop Insect Scouting

Soybean
Continue scouting for spider mites and defoliators. Hot dry weather has slowed soybean growth, making plans much more susceptible to both types of pests. Further, our full season beans are in their reproductive stages when thresholds decrease. We need a canopy leaf area index value of 3.5 to 4 for full yield potential. This would mean that there are 3.5 to 4 acres of leaves for every acre of ground below. An excellent publication with photographs of what these LAI values look like can be found: https://www.pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/444/444-203/SPES-74.pdf.

Sorghum
Sorghum is beginning to head. There are three pests that need to be scouted for: sorghum midge, corn earworm, and white sugarcane aphid. Sorghum midge is a tiny orange-ish fly that infests flowering sorghum, and only flowering sorghum. Once pollen anthers turn orange, it is no longer susceptible to midge. Some folks use a clear Ziploc to cover the head and tap midges out. Thresholds are one midge per head with 25-30% heads blooming. Pyrethroids will do a good job on midges.

Corn earworm moths are highly attracted to flowering sorghum and will lay eggs in the head. Thresholds are 1-2 per head. One caterpillar/head can consume 5% of the yield potential. Sample 5 heads in 10 locations in the field by banging the head into a 5 gallon bucket and count any earworms, fall armyworm, or sorghum webworm. Thresholds for webworms are 5 larvae per head.

The third pest to be on the lookout for is sugarcane aphid. Last year we detected enough aphids to say that they do get up this far north. They form dense colonies on the underside of leaves, they are smooth, light yellow with black cornicles, antennae, and feet, and are small. The yellow sugarcane aphid is a seedling pest and is a larger, darker yellow aphid with long hairs. White sugarcane aphid reproduces quickly and is resilient to pyrethroids. I do not think that it will be abundant this year in our area, but parts of Virginia’s Eastern Shore have had yield limiting populations in recent years. Sugarcane aphid thresholds depend on plant growth stage; at boot to milk, thresholds are 50 aphids per leaf on 20 – 30% of plants. Look under upper and lower canopy leaves.

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