2016 University of Minnesota Cooperative Black Cutworm Trapping Network Report #4

Week of April 9-April 15, 2016

This newsletter is available in a print-friendly pdf format: 2016 Black Cutworm Network Issue 4

BCW 2016-04 MapThe warm weather the past week saw considerable progress in field work and corn planting - a good thing. 

Weather systems brought some black cutworms into the state - not a good thing. Ten trap locations reported moth captures from April 12-15.

We had a strong northerly flow most of last week. April 9-10 and April 13 looked particularly favorable for insect migration.

A trapper in Rock County found a significant capture of eight moths on the morning of April 12.

Lincoln, Nicollet, Waseca and Steele Counties captured more than 5 moths over a 2-night period from April 14- 15. This is less than a trigger for predicting the potential economic damage, but it shows that some moths were moving northward in most of southern Minnesota.

When might the larvae from these captures lead to cut corn plants? Like other insects, the rate at which black cutworm develop is related to temperature. Similar to corn development, degree days can be used to predict the developmental stage of the black cutworm. Fortunately for us, the 50ºF base for corn is similar enough to that of the black cutworm that we can use it to roughly estimate the development of both.

How do we know where to start accumulating degree days? That's where the pheromone traps come in! They provide the time of moth arrival, a general area of arrival and most importantly, a "biofix" from where to start counting degree days. Degree days can also estimate when the larvae will be done feeding. 

The eggs will hatch after 90 DD and the growing larvae will be large enough to cut small corn plants just after 300 DD. Smaller plants, sugarbeets for example, might be cut a bit earlier. To be on the safe side, be sure you are scouting corn before 300 degree-days after the moth flight have been accumulated, initially focusing on leaf feeding. Table 1 shows when cutting might start in counties with a significant moth capture of eight or more moths over two nights.

For more information on black cut development see: Black Cutworm Prediction

Date of significant trap capture Trap Location Projected cutting date
28 - Mar Swift Co. 19 - May
12 - Apr Rock Co. 21 - May

Table 1. Dates and locations of significant 2016 captures of black cutworm moths in 2015. Projected cutting date is based on accumulating 300 DD base 50ºF after a significant capture and uses 30-year historical data.

I have been using the Corn GDDDST calculator at the U2U climate website for calculating corn/black cutworm degree days. This site requires registration (free) but allows you to enter a black cutworm biofix date (or corn planting date) for your own county https://mygeohub.org/groups/u2u/gdd. The site uses the current year's weather data and also historical average data for projections. Several universities are involved in this website and there is plenty other neat stuff at this site to peruse during rainy days.

Fields that were worked before moth arrival are at lower risk for egg-laying. In the next issue we will list some guidelines for assessing a corn field's risk of a black cutworm problem.

- Bruce and Travis