What is in this article?:
• Since the spray was a full rate of a pyrethroid, it is likely these insects are resistant to this chemical.
• We simply cannot predict the areas in which pyrethroids will fail.
• Weknow we have resistant worms in our system. The safe bet is to spray a chemical other than a pyrethroid.
I received several calls earlier this week concerning the corn earworm in our North Carolina soybean crop.
These reports were unusual for at least one reason. Earworms seem widespread geographically, but are spotty from field to field.
Generally moths prefer to lay eggs in flowering soybeans, but two consultants have found earworms defoliating fields with young beans at V2-V3 (in addition to the usual).
Our threshold for defoliation prior to reproductive stages is 30 percent throughout the canopy. However, our crop is very late this year and we need all the foliage we can get to set a decent yield. It might be a good idea to temporarily lower the threshold to 15 percent to avoid losing too much on these young plants.
If the worms are already large, they will likely cycle out soon, so you might want to hold out on these situations.
I have also received several calls on spray failures in both soybeans and peanuts. I identified worms from Pamlico County, comparing sprayed to unsprayed fields in this case. The 14 worms I identified from the unsprayed field were all corn earworm.
However, in the sprayed field, 25 percent of the worms were tobacco budworm. This insect is tolerant to pyrethroids so the spray likely selected for these.