A question we are often asked is, "When should I start scouting for black cutworm?"  We can't say with absolute certainty when moths will migrate from the south, but after the moths get blown in on passing storm fronts, we can predict with very good accuracy when to expect larval feeding activity.  The reason is that the growth and development of insects is directly dependent upon accumulated heat units, known as pest degree days.

The development of any given insect species occurs only above a specific base temperature and stops or levels off above a cutoff temperature.  Precisely calculating degree days can be complicated, so we often use a simple average method expressed in the following formula:  [(daily high + daily low)/2]-base temperature.  The base temperature for black cutworm is 50° F.  If the daily high is 65° F and the low is 45° F on a given day, we would accumulate [(45+65)/2-50] = 5 degree days on that day.           

Degree days then accumulate daily for each insect species from a specific starting date on the calendar, referred to as the biofix date.  Insects that overwinter locally can benefit from any day that is warm enough to drive development.  Alfalfa weevil overwinters locally as eggs in the alfalfa stem and has a biofix date of January 1.  For migratory insects, like black cutworms and armyworms, the biofix date varies each year and is event-driven.  The biofix date for black cutworm is the first significant moth flight, defined as that day when 8-10 moths are captured over a 2-night period in a pheromone trap.  Universities typically maintain trap networks to monitor and publicize these event-driven start dates.

Why is this important?  Highly predictive relationships exist between accumulated degree days and the important life cycle stages of each insect species.  We can leverage this knowledge to anticipate when to time our scouting activity and control applications.  Black cutworm eggs hatch in 90 degree days, but we should start watching for corn cutting activity at 300 degree days from the moth flight.  We should start scouting alfalfa for pinhole feeding by weevils after 250 degree days.  Corn rootworm egg hatch reaches 50% at about 700 degree days. 

Because we have experienced a warmer than usual winter, we are accumulating degree days faster than normal for insects that overwinter locally.  As an example, we have already accumulated 382 degree days for alfalfa weevil at the Belleville station, versus 264 for the 11-year average.  Consequently, we have seen unexpectedly early feeding injury from alfalfa weevil.  We may see similar patterns as the spring wears on with other 'local' insects, like European corn borer and corn rootworm.

The Illinois State Water Survey hosts a powerful web-based degree day calculator tool at http://www.sws.uiuc.edu/warm/pestdata/.  The site offers data specific to your region and information on a wide range of pests.  Your FS Crop Specialist can help you monitor degree days for key pests, help you scout your fields for pest activity, and recommend and apply an appropriate insecticide as needed.

Tim Laatsch serves the FS System and is GROWMARK's insect and plant disease technical manager. He can be reached at [email protected]