Insect Strip

Insect Strip

Tuesday, March 31, 2015

Pale Western Cutworms and Brown Wheat Mites in Wheat

In the last newsletter, March 18, the article primarily covered greenbugs and Russian wheat aphids, with a little about army cutworms. And, there has been a lot of greenbug and Russian wheat aphid since the update. But, this week two other pests may be of concern for some wheat fields.

Pale Western Cutworms 
I just got in from looking at a Uniform Variety Trial just NW of Groom. While checking for greenbugs, I noticed there were spots in the field and at the east edge of the field that were drying down. I decided to dig around for cutworms because Jourdan Bell, Extension Agronomist-Amarillo, and I have seen a few cutworms in wheat at the USDA facilities in Bushland. What I found was Pale western cutworm (PWC) in relatively high numbers. It was easy to find 1 or more PWC larvae in a linear foot of drill row. The recommended guideline for treating PWC is when there are > 1 larvae per linear foot and the yield potential is good. If the yield potential is low the treatment level is when there are >2 larvae per linear foot (Kansas State University). These larvae are subsurface dwellers and feed on the root crown below the soil surface. They generally concentrate in dryer areas of the field. Populations that we see now are probably associated with dry conditions that occurred last spring. Several locations per fields should be scouted by digging and sieving a linear foot of soil x 3 inches deep betweentwo drill rows. If the numbers are above the treatment level, a pyrethroid insecticide application should be effective. For larvae > 1 inch and dry field conditions consider using the higher recommended rates and use higher spray volumes. Spot-treating may provide protection and prevent damage from spreading.
Pale western cutworm larvae, Photo 4/26/2012
Pale western cutworm damage from stand losses (right side of field. Photo 4/26/2012

Brown Wheat Mite 
The hot, dry conditions have been ideal for Brown wheat mites. I have received reports of infestations being sprayed. The fields I have seen have not had heavy enough numbers to warrant an insecticide treatment. The mites are primarily a problem in dryland fields, except under some limited-irrigation conditions.
Brown wheat mite, note the long front legs, Photo C. Patrick
The mite is about the size of a period. Under a hand lens the forelegs are distinctly longer than the other three pair. The life cycle from egg to adult is completed in just 10 to 14 days. All Brown wheat mite life stages are females (no males). They feed by piercing plant cells in the leaf that causes “stippling” leaf discoloration. Under heavy infestations plants become yellow, then dry out and die. The mites feed during the day and the best time to scout is mid–afternoon. 
Since these mites are associated with moisture stressed field conditions, it is difficult to determine whether or not there are any benefits from treating with insecticides. However, we know that their feeding adds to the yield losses. Unfortunately a good economic threshold is not currently available for determining when to treat. A rule of thumb for treating in early spring is when there are several hundred mites per foot of row. The wheat response to the treatment may not be evident until receiving a good rainfall. Hard driving rains in and by themselves can significantly reduce mite populations. 
Brown wheat mite damage to under irrigated field
Insecticide products typically used for control of the Brown wheat mites are dimethoate and chlorpyrifos. The following table also lists other insecticides that can be applied for control of the Brown wheat mite. 
Insecticides for Brown Wheat Mites
Chlorpyrifos (Many products)
0.5- 1 pt./acre
Cobalt (chlorpyrifos plus gamma-cyhalothrin)
7-13 fl. oz./ cre
Dimethoate 2.67
Dimethoate 4E
Dimate 4EC
0.75-1 pt./acre
0.33-0.5 pt./acre
0.33-0.5 pt./acre
Proaxis (gamma-cyhalothrin)
Declare (gamma-cyhalothrin)
3.84 fl. oz./acre

1.54 fl. oz./acre
Karate with Zeon technology (lambda-cyhalothrin)
Warrior with Zeon technology (lambda-cyhalothrin)
1.92 fl. oz./acre

3.84 fl. oz./acre