Agronomic Crop Insect Scouting

David Owens, Extension Entomologist,

Early Season Moth Activity
Many thanks to David Armentrout at the Lower Eastern Shore REC and to Joanne Whalen extension entomologist emeritus extraordinaire for assistance with check traps. Moth counts were low this week, but enough black cutworm were captured in the Harrington area to begin a degree day model to predict when larvae will first be big enough to cause issues in corn. Currently, it is too early to make predictions on cutworm risk, but these counts are not unusual. Please keep in mind there are other cutworm species!

Location # of Nights Total Catch  
Salisbury, MD 7 2 1
Seaford, DE 6 0 0
Sudlersville, MD 7 3 26
Harrington, DE 6 56 71
Smyrna, DE 6 1 4
Middletown, DE 6 0 16

Early Seedling pests of corn include birds, slugs, cutworms, wireworms, and white grubs. White grubs are probably the most problematic of the bunch. White grubs and wireworms tend to be more abundant in fields that have previously been fallow or grass pasture or sod. To scout for white grubs and wireworms, sampling can be done on 12 inches x 12 inches x 6 inches or, as Virginia Tech recommends, a more compact sample of 8 inches x 8 inches x 6 inches. Spring thresholds are 1 white grub per sample Wireworm thresholds are uncertain, but probably a little bit less than white grubs given their greater mobility in the soil. If sampling a field that has corn stubble in it, do not focus solely on old plant stubble but select sample sites randomly. It is not uncommon to find a white grub under old stalks and roots.

If a high number of white grubs are present, a higher rate of seed treatment combined with a pyrethroid in furrow will aid control. Wireworms can be controlled with a mid-rate seed treatment.

Last year there were several reports of clay backed cutworm activity, and some reports of bird damage. Birds can be distinguished from cutworm by their habit of going down the row, partially pulling seedlings up to devour the seed. Holes left by their beaks are going to have straight edges. Cutworm holes are going to be scattered between rows and be perfectly rounded. Cutworms will clip plants off at the base and partially drag the plant material into a burrow. Also look for circular, transverse holes in the leaves. Last year, we marked plants with cutworm feeding signs to see how they responded to an insecticide treatment. Corn was 1-2 leaf stage, and cutworms were overwintered (not new black cutworm). A pyrethroid application only somewhat reduced new injury, and plants quickly outgrew cutworm feeding. Birds can be managed using Avipel seed treatment, but it must be applied by certain applicators and is only available after purchase.

Finally, scout corn seedlings for slug damage, particularly in no-till fields with heavy residue and fields in Kent and New Castle County that received rain this week. Heavy feeding can stunt seedlings and may warrant control measures if defoliation reaches or exceeds 50% and plants are only growing slowly. Starter fertilizer and row cleaners help. Be sure to close seed slots.