2019 UMN Cooperative Black Cutworm Trapping Network Report #4

Report #3 Week of April 27-May 3, 2019

For more information: Black Cutworm Reporting Network

May 9, 2019


MN map showing black cutworm moth captures by countyFigure 1. Maximum two consecutive night black cutworm captures reported by county (April 27-May 3).

We saw more black cutworm (BCW) moths arrive April 27 to May 3 (Figure 1). Some traps captured moths over an extended period April 29 – May 2.

For the most part, moth activity was less than the previous week. Traps in Nobles, Murray, Faribault, and Swift Counties captured eight or more moths over two nights with a high of 12 on May 2 in one of the two Swift County traps.

Some areas have seen considerable fieldwork and corn planting May 6-7 while wet fields limited tillage in others.

Fields where spring tillage removed soybean residue and early emerging weeds are typically less attractive to egg-laying moths.  As significant BCW moth flights continue to arrive in Minnesota, a reduced number of highly attractive fields will concentrate egg laying to later worked and planted fields. Tillage and planting date will be important pieces of information for assessing cutworm risk.

The appearance of BCW moths this spring is not a reason to change your corn planting plan. Scouting can find problem fields before yield loss and post-emerge rescue insecticides work well for black cutworm.

Degree-days for BCW larval development and risk for corn damage for the April 20 - 26 and April 29 - May 2 significant captures are shown in Table 1. Slower development predictions compared to last week is due to cool weather and the use of National Weather Service forecast temperatures rather than historical averages. These are only projections. Temperature accumulations vary greatly based on residue, soil moisture, aspect, and slope. At this time, the best window for scouting emerged corn is after May 15 to before 300 degree-days after moth arrival or June 1.

Black cutworm development estimates tableTable 1. Estimated black cutworm development for significant moth captures. Current and forecast degree-days are generated from the U2U Corn Growing Degree-Day application at the Midwest Regional Climate Center https://mrcc.illinois.edu/U2U/gdd/ and current year and NWS forecast temperatures.

What makes a field high risk for black cutworm damage?

Field topography, tillage and crop rotation

Black cutworm moths arriving in Minnesota seek out areas with crop debris, sheltered areas, and low spots in the field to lay eggs. Early season weed growth is very attractive to the moths. Areas with dense populations of winter annual (e.g. shepherds’ purse, Capsella bursa-pastoris L.) and early-spring emerging (e.g. lambsquarters, Chenopodium album L.) broadleaf weeds in fields are often infested. Similarly, overwintering cover crops might attract egg-laying moths. Black cutworm damage associated with winter rye has been observed in Minnesota corn.

Egg-laying black cutworm moths are less attracted to fields after spring tillage. Unworked fields, or fields with reduced tillage where more crop debris is on the surface, attract more egg laying moths. The higher risk of black cutworm attack associated crop residues and tillage can be seen in tillage plots at the Southern Research and Outreach Center in Waseca, MN during 1985 (Figure 2).

Influence of tillage and previous crop on the percentage of corn plants cut by black cutwormFigure 2. Influence of tillage and previous crop on the percentage of corn plants cut by black cutworm. Waseca, MN. K. Ostlie, 1985.

Fall tillage that buries crop residue and spring tillage that eliminates early spring weed growth before the flight arrives reduces the risk and severity of black cutworm attack. Historically, soybean residue is more attractive than corn, but this may be partially due to the amount of fall tillage or to species and numbers of broadleaf weeds in the seedbank between the two crops. 

Predicting black cutworm development and damage: Degree days

Degree-days and black cutworm growth and development

Since insects are cold blooded, their activities, including how quickly they grow, depend on the temperature of their environment. This effect of temperature on growth is known as temperature dependent development. An organism grows and develops faster as 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 the simple model works fine for crops and black cutworm. First, you need to know the maximum and minimum daily temperatures. Secondly, you also need to know the minimum temperature (lower development threshold or base temperature) at which cutworm growth occurs. Conveniently, we can use a 50°F lower developmental threshold for both corn and black cutworms.

The degree-day concept is not exact under field conditions. Technically, temperatures where the eggs and larvae are located are slightly different than air temperatures. A sine model for accumulating degree-days can best account for limited larval development can occur whenever temperature for part of the day exceeds the developmental threshold. Development ceases at an upper temperature threshold (e.g. 86°F for corn plants). Individual life stages can have different threshold temperatures and temperature dependent development rates. Finally, some black cutworms go through fewer or extra larval stages (instars). Fortunately, for our purposes, these subtleties can be ignored and the following simple degree-day model is accurate enough to time black cutworm scouting efforts.

A daily degree-day accumulation = [(Maximum temperature + Minimum temperature) / 2] - developmental threshold temperature 

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.

The 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 over a consecutive 2-night period) and is where the black cutworm pheromone trapping network comes in.

The black cutworm life cycle, from egg to moth, takes 1½ months or more. Only cutworm larvae 4th instar or larger can cut corn plants. The simple degree-day model for black cutworm development predicts when larvae will be large enough to cut plants (when more than 300 hundred degree-days from a moth flight have been accumulated).

Degree-days can be used to predict when larvae will be large enough to cause visible damage, begin to cut corn and cease feeding (Table 3).

table of degree days relating to black cutworm development

Next week we will discuss scouting and management. If you just can’t wait, see: Black Cutworms or UMN Extension - Black Cutworm

During the spring, results of the 2019 trap network captures and predictions of BCW cutting will be posted weekly at: https://z.umn.edu/bcw-reporting. There will be additional alerts for any BCW high-risk areas picked up by the network!

This project is supported, in part, by the farm families of Minnesota and their corn check-off investment.

Until next week,

Bruce Potter and Travis Vollmer