Black Cutworm Monitoring and Forecasting Program

Integrated Pest Management Program
Department of Entomology
Commercial Agriculture Program 

Introduction  Description and Life Cycle  Damage to the Corn Crop  What are Degree-Days?  Monitoring the Black Cutworm with Pheromone Traps  Forecasting Pest Development with the Degree-Day Model  Pest Advisories Help Time Scouting Visits  Benefits of the Monitoring Program  Black Cutworm Trap Counts

Use BCW Forecasting Program

Introduction 

The goal of the black cutworm monitoring and forecasting program is to decrease corn growers' reliance on at-planting insecticide applications for this sporadic pest while protecting yields from economic injury. Preventive insecticide treatments are rarely advisable for this early-season pest because of the difficulty predicting which fields will have economic infestations. 

The University of Missouri's recommended management plan advises timely scouting followed by treatment with a postemergence "rescue" insecticide if the economic threshold is exceeded. The timing of scouting visits can be forecasted and "fine tuned" by a two-step program:

• Monitoring a statewide network of pheromone traps.
• Predicting injurious stages of the pest based on its degree-day model of development (Table 1).

Description and Life Cycle

The black cutworm is by far the most destructive species of the cutworm complex in the Corn Belt as it cuts corn seedlings near the soil line. Black cutworms do not overwinter in the Midwest, except occasionally in the "Bootheel" region of southeastern Missouri. Moths overwinter in coastal areas of the Gulf of Mexico and migrate northward on strong southerly winds in the spring. Although migration into Missouri typically begins in mid-March, flights that result in economic injury to corn peak in April through mid-May.

Depending on diet and temperature, the black cutworm progresses through six or seven larval stages, or instars. However, larvae must reach the fourth instar before they are large enough to cut seedling corn. Although there are several generations of black cutworm in the Midwest each year, only the first generation injures corn.

Damage to the Corn Crop

The major symptoms of black cutworm injury in corn are:

• Leaf feeding, often consisting of small "pinholes" and light margin feeding by third-instar larvae.
• Seedlings cut at or just above the soil surface.
• Purplish, lodged plants partially cut or notched below ground.
• Wilting ("dead heart") due to tunneling within the stalks of larger plants by older larvae.

Black cutworm larvae can cut corn from the fourth-instar stage until pupation; consequently, the injurious cutting stage may last 2½ to 3 weeks, depending on temperature (See Table 1 above). Corn is vulnerable to cutting from emergence through the five-leaf growth stage. On average, each larva is capable of cutting four one-leaf stage corn seedlings or a single four-leaf stage plant before pupating. Damage to seedling corn may occur from early May through mid-June (in central Missouri) due to overlapping waves of migration. 

What are Degree-Days? 

The growth rates of many insects (and plants) are dependent on ambient temperature and to varying degrees photoperiod, nutrition, and/or environmentally-induced stress conditions. Temperatures must exceed a base threshold, or minimum developmental temperature, in order for an insect to grow. The base threshold, which varies by species, is 50°F for the black cutworm. 

Degree-days are simply defined as the number of degrees above the minimum developmental temperature multiplied by time (days). Ten degrees above the minimum for 5 days represents 50 degree-days (10° X 5 days) just as does 2° above the minimum for 25 days (2° X 25 days).

There are several methods, varying in complexity, which compute the accumulation of degree-days. The method used at the University of Missouri (See Figure below) accounts for temperature fluctuations throughout the day by averaging hourly readings: (Sum of hourly temperatures ÷ 24) - Minimum developmental temperature = Degree-day.

Sample calculation of daily degree-days (base developmental threshold of 50°F) over a two day period.

Monitoring the Black Cutworm with Pheromone Traps

The black cutworm monitoring and forecasting program is coordinated by the MU-IPM Program, in cooperation with the MU Commercial Agriculture Program's climatologists and programmers with the MU AgEBB (Agriculture Electronic Bulletin Board). The MU IPM program supports volunteer trappers in approximately 25 Missouri counties (view map). The traps are baited with a pheromone lure that attracts only the male black cutworm moth.

The first intensive pheromone trap captures of black cutworm moths mark the arrival of the first significant migratory flight. Intensive captures are indicated when at least eight black cutworm moths are captured over two nights in a sticky wing trap, or at least 17 moths are caught in one night in the larger Texas-style metal cone trap.

Forecasting Pest Development with the Degree-Day Model 

Results of the trapping network are complemented by a degree-day model that forecasts the growth and development of the black cutworm (See Table 1 above). The date of each first intensive capture serves as the "biofix," or designated starting point for accumulating degree-days based on the minimum developmental threshold of 50° F. From the date of each intensive capture, an average of 300 degree-days is required for the black cutworm to develop from the egg to plant-cutting fourth instar larval stage.

Degree-days are calculated from "real time" temperatures recorded on an hourly basis at a statewide network of 21 Campbell Scientific automated weather stations. The stations, established by the MU Commercial Agriculture Program and MU Outreach and Extension, are primarily located in those counties with the highest crop production (i.e., excluding those within the Ozark Plateau). It is necessary to use 30-year averages of temperature observations from each station to derive degree-days from the current to the future predicted stage of pest development. IPM specialists use an internet web browser to query the temperature database by the date of each intensive trap capture and the location of the weather station closest to that trap. 

Pest Advisories Help Time Scouting Visits 

Pheromone trap captures in three counties during two seasons reflect the irregular distribution of this sporadic pest by trap location and by year during the peak migration. Such records reinforce the need for a forecasting system that is more accurate than pest advisories based exclusively on calendar dates or crop development.

The MU Integrated Pest and Crop Management newsletter and the MU AgEBB provide forecasts of when to begin scouting cornfields for the damaging stages of the black cutworm. Although pest advisories indicate when to begin or intensify scouting efforts, the level of infestation cannot be predicted based on the number of black cutworm moths caught in the pheromone traps.

Black cutworm density is indirectly estimated by sampling for crop injury, specifically the percentage of plants damaged above and below ground. The corn growth stage, larval stage, and to a lesser extent plant population and soil moisture, also affect the decision to apply a "rescue" insecticide treatment to a heavily-infested field. 

Benefits of the Monitoring Program 

The likelihood of an unnecessary treatment of the noneconomic dingy cutworm is reduced by predicting the first appearance of the fourth-instar stage of the black cutworm. The dingy cutworm is an overwintering cutworm that is often found in cornfields one to two weeks before the black cutworm reaches its plant-cutting stage. The dingy cutworm is primarily a leaf feeder that rarely cuts corn plants, but it is very similar in appearance and is sometimes mistaken for the more injurious black cutworm.

The monitoring program also forewarns scouts of "secondary waves" of migratory black cutworms that may cause significant injury, especially in late-planted cornfields. Late-planted corn is more likely to emerge just as the pest population has developed to the fourth-instar "cutting" stage; whereas, in early-planted fields, young seedlings are large enough to escape the onset of the damaging pest stage. The potential for a cutworm outbreak exists only if eggs or larvae are already present in the field when the corn is planted. Consequently, high numbers of moth captures in late April (See Figure below) are potentially far more damaging when planting is delayed until early to mid-May. 

Late planting may "predispose" corn seedlings to black cutworm injury because the damaging pest stage more fully coincides with the susceptible crop stage.

In summary, the IPM program for black cutworm reduces reliance on at-planting insecticide applications by:

• Fine tuning scouting so that the most vulnerable cornfields can be initially targeted.
• Forecasting early cutting and other deviations from the normal infestation pattern.