2020 UMN Cooperative Black Cutworm Trapping Network Report #6

 2020 University of Minnesota Cooperative Black Cutworm Trapping Network 

For more information: https://z.umn.edu/bcw-reporting

May 14, 2020

Greetings:

Moth capturesDuring the past week, cooperator’s pheromone traps picked up more immigrating black cutworm moths; particularly in the western part of the state where two traps had significant captures (Figure 1, Table 1). The largest influx occurred on the May 4-6 southerly system.  A Lac Qui Parle County trap captured 9 moths May 5th-6th and a Murray County trap captured 12 moths over the same period. Traps in other counties also captured moths throughout the week. Fortunately, when and where these larger flights finally arrived in Minnesota, most corn and sugarbeet, and many soybean fields, had already been worked and planted thereby reducing their attractiveness to moths laying eggs.

While the timing of 2020 black cutworm moth arrival into Minnesota is similar to 2019, the frequency, distribution and magnitude of the flights have been much less.  When these data on this year’s fewer and smaller flights are combined with the early planting in southern and Central Minnesota, the risk of economic damage appears lower than many years. This risk, however, is not zero.

Corn plants cut off above the growing point will survive but corn and broadleaf crops cut below the cotyledons will be killed. Black cutworms need to be 4th instar or larger to be capable of cutting corn and soybeans. While younger larvae may cut small weeds, leaf-feeding is typically the first sign of the presence of young cutworms. Late instar larvae can cut up to five leaf corn and may kill plants by tunneling into growing points above or below the soil surface. Although stand reduction is closely correlated with yield loss in corn and sugarbeets, black cutworms can also reduce the yield of soybeans and other crops.

Date

County with capture

May 02

Faribault, Lyon (2), Olmsted, Steele (2)

May 03

Brown (3), Ottertail, Renville (4), Steele, Swift

May 04

Cottonwood (3), Jackson I, Meeker (2), Mower (2), Murray II (2), Redwood, Renville (2), Sibley (1), Swift, Waseca I

May 05

Cottonwood, Dakota (2), Faribault, Jackson II (3), Lac Qui Parle), Lincoln (2), Murray II, (8), Meeker (2), Renville (2), Steele, Swift (2), Waseca I (2), Waseca II (2),

May 06

Dodge (2), Fillmore (2), Jackson II, Lac Qui Parle (8), Lincoln,   Meeker, Murray I (2), Murray II (4), Redwood (4), Sibley, Steele, Watonwan (2)

May 07

Faribault (3), Martin (4), Ottertail, Redwood

May 08

Martin, McLeod, Watonwan

 Table 1.  Positive captures in pheromone traps reported April 25- May 1. Numbers in parentheses are multiple captures for that date

The black cutworm life cycle, from egg to moth, takes 1½ months or more.  Like all insects, cutworms grow and develop as they are exposed to cumulative heat. Similar to predicting corn growth with degree-day accumulations, we can use degree-days to predict what stage the cutworm eggs, larvae or pupae will be at. A simple daily degree-day accumulation = [(Maximum temperature + Minimum temperature) / 2] - developmental threshold temperature. These daily degree-day accumulations are then summed over the time period of interest. Conveniently, we can use the same 50°F lower developmental threshold for both corn and black cutworms. 

Black cutworm eggs, and for the most part larvae, are subterranean. As result, air temperature-based degree-day models can sometimes over predict development for larvae kept cooler by soil type, moisture and crop residue or by lack of food. Additionally, different BCW life stages have slightly different lower and upper threshold stages and even if the daily average is below 50oF, some development can occur when temperatures exceed the lower developmental threshold. Although there are other ways to calculate temperature-dependent development, the simple degree-day accumulation works well enough for our purposes – timing scouting for BCW.

The date of moth flight is used as a “biofix” to start accumulating degree days. Degree-days can be used to predict when larvae will be large enough to cause visible leaf-feeding damage, begin to cut corn and cease feeding and pupate (Table 2). Scouting corn and other crops for BCW should start before 300 degree-days accumulate after a significant catch and larvae are large enough to cut off corn plants. This is about three weeks in a typical Minnesota spring but will, of course, happen sooner if warm and later if cool.

Some counties may have had threatening numbers of moths go undetected by this trapping network. Black cutworm moth arrivals and captures tend to be localized, following strong southerly flows and rain events. The network does give us a general idea where moths arrived and combined with weather and climate data, it provides a basis to time scouting. For a couple early immigration events and this week’s significant captures, predicted dates for the start of leaf-feeding and for the beginning and end of cutting are shown in Table 3.

Although we did not have any significant flights until May 5-6 into the SW part of the state, there may already be some leaf-feeding from larvae resulting from earlier migrants. These larvae might be large enough to cut corn as early as May 24th. Since both BCW and frost injury prefer low–lying areas, the recent 4-5 night freeze event could make scouting more difficult in some parts of Minnesota.

Next week, we will summarize the 2020 season’s trapping results and provide some advice on cutworm ID and economic thresholds.  If you want to read ahead see: https://extension.umn.edu/corn-pest-management/black-cutworm.

A PDF of the black cutworm management fact sheet along with previous 2020 (and earlier) issues of this newsletter can be found at: https://z.umn.edu/bcw-reporting.

Temperature chart
Table 2. Temperature dependent development and feeding damage of the black cutworm.

Cutworm development chart

Table 3. Estimated black cutworm development for significant moth captures as of May 22. 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/ using current year and NWS forecast temperatures.

Include awareness of BCW and other cutworms in your spring scouting efforts. If you notice cutworm damage in your fields we would appreciate hearing about it!

Bruce Potter and Travis Vollmer

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