Ken Wise and Jaime Cummings – NYS IPM Program
The NYS IPM Field Corn Pheromone Trapping Network has started trapping black cutworm (BCW) Agrotis ipsilon and true armyworm (TAW) Mythimna unipuncta moth flights in NYS. While it seems like it might be early, we have caught BCW and TAW moths this week in Western, NY in pheromone bucket traps. These moths migrate north on weather fronts from the southern US every year. Both BCW and TAW prefer feeding on grasses, such as grassy weeds, hay fields, small grains and corn.
Even though the number of moths caught this week were low, it indicates that they have arrived. From this point forward, we can set the “Biofix Date”. The biofix date is the point where we start to calculate the number of BCW and TAM degree-days. We can predict when the eggs that were laid by moths will hatch. Degree-days are calculated by taking the high and low temperature each day and averaging them from the biofix date. Next, subtract the base temperature of 50 degrees Fahrenheit, and this will give you the daily degree-days. Each day, add the number of BCW degree days and this will give you a total. When this reaches 90 BCW degree-days and 113 TAW degree-days, the eggs will start to hatch.
High Temperature + Low Temperature/2 – 500 F = daily BCW degree days
The easy way to calculate this is to use the NEWA Degree Day Calculator. This will calculate the degree-days from a weather station near your farm. Below is the information on degree-days for the lifecycle of BCW and TAW.
Black Cutworm Degree Days (Base 500 F)
Degree Days Stage Feeding Activity
0 Moth Capture Egg Laying
90 Eggs Hatch
91-311 1st to 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
Source: University of Minnesota Insect Pest of Corn-Stand Reducers Black Cutworm
True Armyworm Degree Days (Base 500 F)
Degree Days Stage Feeding Activity
0 Moth Capture Egg Laying
113 Eggs Hatch
612 Larval stages Leaf Feeding
909 Pupa No feeding
A large number of moths in a trap does not necessarily mean there is going to be damage in your corn. It will depends on where the moths lay eggs. If a trap near your farm has a large number of moths, it would suggest it is time to scout for larvae and signs of feeding damage.
A good time to start scouting is when you take plant population counts. BCW damage is easy to identify. The larvae will cut the plant near the base at the soil surface, while TAW will feed from the edge of the leaf to the mid rib.
BCW and TAW larvae are primarily nocturnal or night feeders. Normally, you will not see them during the day. BCW larvae are ½ inch to 2 inches. They appear as greasy gray with darker raised spots on each segment. They normally hide in the soil near the base of the corn or under residue that might be on the surface.
TAW larvae range from ½ to 1.5 inches long. They have orange and white strips running along the side. They also have a white strip running down the back. TAW will hide under surface residue, in the whorl of the plant or in cracks in the soil.
If you are at threshold, and the larvae are still small, try to treat only the infected corn and a 20 to 40 foot border around the area. When the larvae are large (1.25 inches +) they are harder to kill with an insecticide, and they will pupate soon. When pupating, they will stop feeding.
One of the issues with BCW and TAW is that there can be multiple flights on different weather fronts throughout the spring. This can cause multiple infestations with different sizes of larvae in a field. Still follow the economic threshold, and manage if needed.
Our pheromone-trapping network has 25 traps of each BCW and TAW placed in 19 counties across the state. The counts and degree-days for many locations across NY will be published weekly starting later in April in the NYS IPM Field Crops Pest Report .
References:
University of Missouri-True Armyworm
University of Minnesota Insect Pest of Corn-Stand Reducers Black Cutworm
Cornell University Field Crop-Armyworm
Purdue University-Black Cutworm
Cornell University Guide for Integrated Field Crop Management
This work is funded by the NYS Corn and Soybean Growers Association.
