Agronomy News Insect Scouting Tips

Soybean

The usual defoliators are starting to arrive, including bean leaf beetle, grasshoppers, and caterpillars. Control may be needed if there is 30% defoliation during the seedling and vegetative stages and 15% defoliation from bloom-pod fill. Adult Dectes Stem borer will be emerging over the next few weeks. Chemical control is not recommended since it would require multiple applications to reduce larval infestations, which is not economical. If a high number of adults are found, harvesting that field as soon as it matures will reduce losses associated with lodged plants.

Fields that have an open canopy, drought-stressed, or have recently had an insecticide applied are at higher risk for corn earworm (CEW). CEW larvae can feed on flowers without impacting yields because soybeans overproduce flowers. However, feeding during pod development can affect yield. An economic threshold calculator is available to assist with management decisions: https://soybeans.ces.ncsu.edu/wp-content/uploads/2017/08/CEW-calculator-v0.006.html.

Field Corn

As corn ears begin to form, check for stink bugs. Stink bugs will gather around the edges of fields, so scouting should be done at least 15 rows in. Thresholds are 1 stink bug per 4 plants when the ear is forming, and 1 stink bug per 2 plants from pollen shed to blister stage. Treatment is not recommended past the blister stage.

Alfalfa

Continue scouting for leafhoppers, by the time hopperburn (yellowing of leaf margins) is visible, yield has already been impacted. Cutting alfalfa will kill nymphs and drive adults out of the field for a period of time. Leafhopper thresholds per 100 sweeps increase as plant size increases: plant > 3 inches = 20 adult potato leafhoppers per 100 sweeps, 8 to 8 inches = 50 adult potato leafhoppers per 100 sweeps, 8 to 12 inches = 100 adults or nymphs per 100 sweeps and plant which are over 12 inches = 200 adults or nymphs per 100 sweeps.

Sorghum

Sugarcane aphids were found on the Eastern Shore last year and typically show up in fields late July and August. Check underside of leaves for insects. Honeydew will turn leaves shiny and is an easy to see indicator that aphids are present. Sugarcane aphids are light yellow with black cornicles, antennae, and feet. Thresholds depend on plant growth stage; at boot to milk, thresholds are 50 aphids per leaf on 25 – 30% of plants. There is documented resistance to resistance to pyrethroids.

New Dectes Fact Sheet

Authors: Kelly Hamby, Alan Leslie, and David Owens

Dectes stem borer (DSB; Dectes texanus) is a native species of long-horned beetle that can be a sporadic pest of soybeans. Damage to soybean plants is caused by the larvae, which feed internally on soybean stems. As the plant matures, the larvae girdle the stem at the base of the plant, which can cause lodging before soybeans are harvested. Continuous soybean production favors Dectes stem borer populations. Few management tactics effectively reduce damage once populations build up, and insecticides provide inconsistent protection. Timely harvest of infested fields can help avoid lodging yield loss. Publication is available online or a PDF can be downloaded. Dectes Stem Borer Management in Soybeans (FS-1196)

Be sure to check all labels carefully before combining insecticides and herbicides. Thresholds are based on sampling 100 plants (10 plants x 10 locations).

Alfalfa

Begin scouting for potato leafhoppers (PLH). Stubble insecticide applications are rarely needed and seldom provide reasonable control since adult leafhoppers will move out of the field after cutting. A rough threshold estimate is 20 PLH per 100 sweeps on alfalfa 3 inches or less in height, 50 PLH per 100 sweeps in 4-6 inch tall alfalfa, and 100 PLH per 100 sweeps in 7-11 inch tall alfalfa. A more precise threshold chart can be found on Penn State Extension website https://extension.psu.edu/potato-leafhopper-on-alfalfa.

Field Corn

Scout for armyworms and cutworm if growing a hybrid that doesn’t control for them. Treatment threshold for armyworms in corn is 25% infested plants with larvae less than one inch long. Large larvae feeding deep in the whorls will be challenging to control. When scouting for cutworm, check/dig around the base of the plants to determine if cutworms are present and alive. Cutworms tend to be more common in late terminated cover crop, late planted corn fields. Cutworm thresholds are 5% cut plants at V2-V4 or 10% of plants with signs of fresh leaf feeding.
Once plants start to silk, scout for Japanese beetles and stink bugs. For Japanese beetles, the treatment threshold is when there is less than ½ inch of silk and less than 50% pollination, and an average of 2 or more beetles per ear. For stink bugs, the threshold is 1 bug per 10 plants (V1 to V6), 1 bug per 8 plants (V14 to VT), and 1 bug per 4 plants (R1 to R2). Cereal leaf beetle, stink bugs, and Japanese beetles are ‘edge’ pests, so treatment may only be needed around field edges and pivot tracks.