Many species that overwinter in Pennsylvania apparently have done so with high survival rates. For example, we’ve seen very strong populations of several species of maggots, cucumber beetles, and asparagus beetle in April and May. However, many of our pests tend not to overwinter, but arrive as long distant migrants. We can now confirm that the long-distant migrants listed below have also been reported in May.
At our research farm in Centre County, we have found a few corn fields with substantial black cutworm damage. Black cutworms are primarily pests of corn, but they can also attack tomato, pepper, and eggplant. These caterpillars are active at night, but were easy to find with a trowel by digging up the cut seedling. Presence of the black, greasy looking caterpillars verifies the source of damage, which can be difficult to diagnose in some cases, particularly in slug infested fields.
To hedge against any surprises, growers and field personnel would be wise to scout fields for cutting damage, and apply rescue treatments if larvae are small and the economic threshold is exceeded. Females lay eggs in dead vegetation on the soil surface and in weeds, where moisture is high. Young larvae feed on the leaves of emerging corn, whereas the older larvae cut the plant off at the base or bore into the plant. In sweet corn, check each planting weekly during the spike through the five-leaf stage. Check for small irregular holes in the leaves, and missing or cut plants.
If cutworms are present, examine 10 sets of 20 plants throughout the field and record the percent of cut or damaged plants. Look under clods of dirt and vegetation and the bases of plants for the larvae; if you see the larvae, record the average size and the number per 100 plants. In sweet corn during the two-leaf stages, apply a treatment if more than 10% of the plants show fresh signs of feeding. At the three to four-leaf stages apply treatment at a 5% level.
Also, use your judgment based on stand count. If you are at the minimum stand count, you may need immediate treatment, whereas more feeding can be tolerated if the stand is heavier than needed. During drier conditions, treatments may be less effective because cutworms may be feeding below the soil surface. In these cases, rotary hoeing or cultivation, as well as using higher spray volumes, may help increase the chances of contacting the insects with the pesticide.
After four or five weeks, the larvae pupate in the soil. Two more generations may occur, but damage does not tend to occur from later generations, so no treatment is needed if the population is finishing its larval feeding stages and pupating. Full grown larvae are about 1.75 inches in length and although feeding damage increases with larval size, feeding will stop when they pupate. For more details on black cutworm see: www.ento.psu.edu/extension/factsheets/black_cutworm.htm.
True armyworms are also active. These caterpillars can cause damage to small grains and corn, but tend to be most problematic in corn planted before a small grain cover crop was completely dead. Be sure to scout for damage from this pest because, true to its name, populations can generate quickly and march through fields causing extensive damage — in corn characteristic defoliation leaves just the midrib remaining- For details, go to http://ento.psu.edu/extension/factsheets/armyworm.
This is a year where you need to be scouting your fields and be prepared for earlier-than-usual pest pressure.