By Adam Sisson, Integrated Pest Management; Laura Jesse, Plant and Insect Diagnostic Clinic; and Erin Hodgson, Department of Entomology
The black cutworm is a sporadic pest that clips early vegetative-stage corn. Scouting for larvae helps to determine if an insecticide application will be a cost effective decision. Scouting dates are based on the peak flight of black cutworm moths and accumulating degree days after the peak flight. The adults migrate north every season, and therefore it can be difficult to determine when moths arrive each spring. After females arrive and lay eggs, degree days are estimated to determine when larvae are capable of cutting corn.
To find out when moths arrive in Iowa, a volunteer network established and monitored black cutworm traps. We asked volunteers to start checking traps at the end of March, and so far they have contributed more than 1,000 data reports in 2014. The first moths were recorded in Muscatine County on April 3.
The map (Figure 1) shows the predicted cutting dates for the nine Iowa climate divisions. Predictions are based on actual and historical degree day data. In the east central, south eastern and south central climate divisions, a peak flight occurred in mid-April, and estimated cutting dates are shown in orange. The dates in black are predicted cutting dates for peak flights in late April. Large numbers of moths were collected over several days in early May, so continue to scout fields past the predicted cutting dates.
Figure 1. Estimated black cutworm cutting dates for each Iowa climate division based on peak flights of moths occurring in April 2014. The dates in orange represent estimated cutting dates from an early peak flight. Scouting should begin, and continue from, these dates as moths continue to arrive in the state.
Scouting
Fields that may be at higher risk for black cutworm damage include those that are poorly drained and low lying, those next to natural vegetation, and those that are weedy or with reduced tillage. Late-planted corn can be smaller and more vulnerable to larval feeding. Some Bt hybrids provide suppression of black cutworm, but young plants can still be clipped by larvae.
Scouts are encouraged to start looking several days before the estimated cutting dates because local larvae development may be different with weather specific to that area. Fields should be scouted for larvae weekly until corn reaches V5. Examine 50 corn plants in five areas in each field. Look for plants with wilting, leaf discoloration and damage, or those that are missing or cut (Fig. 4). Note areas with suspected damage (with a flag) and return later to assess further damage. Larvae can be found by carefully excavating the soil around a damaged plant.
Identification
Black cutworm larvae are light grey to black. Their skin appears grainy and there are four pairs of fleshy prolegs on the hind end (Figure 2). On each body segment, the pair of tubercles closest to the head is about one-third to one-half the size of the pair nearest to the abdomen (Figure 3). Black cutworm larvae can be confused with armyworms and other cutworms. Some characteristics can be used to set species apart, which are outlined further in this article on cutworm identification.
Figure 2. Black cutworm larvae have light grey to black, grainy skin.
Figure 3. Black cutworms are best distinguished by the dark tubercles on the middle of the back. On each body segment, the pair of tubercles closest to the head is about one-third to one-half the size of the pair nearest to the abdomen.