Treating Earworms Defoliating Soybeans Prior to Bloom and Eating Flowers During Bloom
I have received a number of calls and reports of very high earworm numbers in soybeans that are not blooming or at full bloom. Our online threshold calculator for earworms in podding beans can be found by visiting soybeans.ces.ncsu.edu, clicking on “insect management”, clicking on “thresholds” and scrolling down to the online corn earworm in soybean threshold calculator link. Should you treat soybeans blooming or prior to bloom?
Prior to bloom: Our official defoliation threshold is 30% defoliation throughout the canopy until bloom or two weeks prior to bloom (depending on how the crop looks) and 15% defoliation after bloom. A good guide to estimating defoliation can be found here.
These thresholds were recently re-evaluated in the Midsouth. Yield loss started somewhere after 66% defoliation at V6 in these studies. Incremental defoliation did not have an effect if soybeans were defoliated multiple times at 17.5% and 33% defoliation, but had a small effect if soybeans were defoliated multiple times at 66% defoliation.
These studies prove to me that our threshold of 30% defoliation is very very conservative. They also prove to me that it is nearly impossible for defoliation to nickle and dime us throughout the season. Earworms, in North Carolina, rarely defoliate plants to high levels like those in the Midsouth defoliation studies (more likely for armyworms or loopers). Therefore, we should maintain yields by avoiding sprays prior to bloom, even under heavy pressure, until our defoliation thresholds are triggered.
Once flowering: Recent studies in NC, funded by the North Carolina Soybean Producers Association, proved that earworms will eat flowers, but that we can tolerate very high populations (up to 3x the podding thresholds) without experiencing yield loss (note these studies were done with a determinate variety). The interaction of earworm feeding on blooming soybeans and yield is complex. Flower number and number of flowers fed on by earworms is related to yield. Perhaps unsurprisingly, the more flowers injured by corn earworm and the fewer number of flowers on the plant, the lower the yield. However there is no direct relationship between earworm numbers during blooming and yield. So earworm numbers during blooming are not a good threshold to use when making a spray decision.
While we don’t understand this phenomenon fully, I suspect that it has a lot to do with the soybean plant’s ability to compensate. My recommendation is to only treat earworm in blooming soybeans if they are present at the podding threshold levels and if the plants are stressed. If the plants are not stressed, they will compensate for earworm feeding even at high populations, perhaps by retaining flowers, creating more seeds per pod, or filling out heavier pods. One caveat is that earworms present during blooming could potentially remain once pods are present. We know the yield loss potential at this point is great, so scout fields with earworms during blooming (R1-R2) closely to eliminate them once pods are formed (R3).