Agronomic Crop Insect Scouting

David Owens, Extension Entomologist, owensd@udel.edu

Alfalfa

Continue scouting for blister beetles in flowering alfalfa and as well as potato leafhopper. In the alfalfa recommendations page, there is an economic threshold table developed by Penn State for potato leafhopper. Remember that with potato leafhopper, early harvest is also an option. Harvest will destroy most of the nymphs and the adults will leave the field. If there are multiple ‘cuttings’ in a local farmscape or if there are snap beans nearby, scout those crops after an infested field is cut.

Pasture Grass

Begin monitoring for true armyworm. The threshold used in other states for armyworm is around 2-3 per square foot, depending on crop growth and value. During the day, it will be low in the canopy or hiding under debris but will come up in the evening and overnight hours. They are rather sporadic, but can quickly defoliate a field when present in large numbers. They are susceptible to pyrethroids.

Soybean

Full season soybean fields are at R2. At this point, our defoliation thresholds begin to decrease. The primary defoliators present are leaf feeding worms and grasshoppers. We have had variable efficacy with pyrethroids in 2019 and 2020 trials. The biggest curiosity active in some fields right now is green stink bug. We have had a large green stink bug flight this year, and near-threshold (5 per 15 sweeps) populations have been reported from R2 soybean. At this stage they are not an immediate threat to the crop, but may need to be controlled between R3 and R4. I suspect that lower thresholds may be justified in Plenish soybean, but do not have hard research to back that up. Remember that stink bugs tend to aggregate in fields, especially close to woods (where green stink bugs develop their first generations). Pay attention to wooded edges, especially small fields with a large wooded perimeter, and woods that have a lot of wild cherry, maple, and tree of heaven. Green stink bugs are susceptible to pyrethroids, brown stink bugs are more difficult to kill with pyrethroids, and the best pyrethroid for brown marmorated stink bugs is bifenthrin.

Together with UMD extension ag agent Alan Leslie and extension entomology specialist Kelly Hamby, we have updated a Dectes Stem Borer fact sheet. For right now, it can be found here: https://extension.umd.edu/resource/dectes-stem-borer-management-soybeans-fs-1196.

Field Corn

Many corn fields are at VT or R1. Scout these fields for brown stink bugs. You do not have to look at the entire plant, brown stink bugs are going to concentrate around the ear zone. Fields at V14 to VT with 10 or more stink bugs present on 100 plants are above threshold, once silking begins to about R2, the threshold rises to 28 bugs per 100 plants. More information and management considerations can be found here: https://corn.ces.ncsu.edu/stink-bug-management-in-corn/.

If you do not observe threshold level stink bugs, scout for grasshopper defoliation and silk feeding. If neither pest is present in concerning numbers, an insecticide is very unlikely to benefit the crop even if tank mixed with a fungicide.

Western corn rootworms are now active. Fields that were in corn in 2021, are in corn in 2022, and will be in corn in 2023 should be scouted between now and late July to assess for potential concern in 2023. Western corn rootworm lay overwintering eggs in corn and larvae feed on corn roots in the spring. Crop rotation breaks this lifecycle. Insecticide application to an infested field is unlikely to be of benefit for next year. Thresholds used out West are 1 beetle per plant.

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