Black Cutworm Prediction

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Since insects are cold blooded, their activities, including how quickly they grow, depend on the temperature of their environment. The effect of temperature on growth is known as temperature dependant development. An organism grows and develops faster if it is exposed to cumulative heat. Similar to predicting corn growth with degree day accumulations (a.k.a. growing degree, heat units, growing degree days), we can use degree-days to predict what stage the cutworm eggs, larvae or pupae will be at.

There are several ways to calculate degree-days for insect development but for crops and black cutworm the simple model works fine. First, you need to know the maximum and minimum daily temperatures. Secondly, you also need to know the minimum temperature (threshold or base temperature) at which growth occurs. Below this temperature, little development occurs. Conveniently, we can use a 50° F lower developmental threshold for both corn and black cutworms.

Degree day = (Maximum temperature + Minimum temperature) / 2   - developmental threshold temperature

Technically, limited development can occur when part of the day but not the average temperature exceeds the developmental threshold, development ceases at an upper temperature threshold (e.g. 86°F for corn), individual life stages can have different threshold temperatures and temperature dependant development rates, and some black cutworms go through an extra larval stage (instar). Fortunately, for our purposes, these subtleties can be ignored.

For an example of calculating degree-day accumulations: The daily high was 70°F and the daily low was 48°F. The degree-day accumulation would be: ((70+48) / 2) – 50 = 9. Daily degree-day accumulations are summed over the time period of interest.

To know when to start the degree day accumulations we need a “biofix”. That biofix is a significant moth capture (8 or more moths in a 2 night period) and is where the black cutworm trapping network comes in.

Cumulative Degree Days

Black Cutworm Stage

Black Cutworm Activity

0 (biofix) Signifiacant moth capture Egg laying
90 Egg hatch
91 - 311 1st - 3rd instar Leaf feeding
312 - 364 4th instar Cutting begins
365 - 430 5th instar Cutting begins
431 - 640 6th instar Cutting slows
641 - 989 Pupa No feeding

The black cutworm life cycle, from egg to moth, takes 1 ½ months or more. The simple degree-day model for development predicts larvae large enough to cut plants after 300 hundred degree days have been accumulated.

Only larger 4th – 6th instar cutworm larvae can cut corn plants. We can use degree-days to predict when larvae will be large enough to cause visible damage, begin to cut corn and cease feeding.

Scouting corn crops for black cutworms should start before 300 degree days after a significant catch accumulate. This will, of course, happen sooner if warm and later if cool but is about three weeks in a normal Minnesota spring. We will be making cutting projections as the season progresses and now you are armed with the knowledge to fine tune things for your own location.

A sine model degree-day accumulation  for black cutworm can be a bit more accurate with cool temperatures. It can be run for single or multiple coordinates at: Degree day models for those looking for excitement on rainy days.