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
Insecticide
Rate
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

Wednesday, March 18, 2015

Texas High Plains Wheat Pest Update

Wheat Aphids
If it has been awhile since you have checked wheat fields for aphids, you may want to consider rechecking, even if you have already sprayed for greenbugs recently.  Last Friday, I stopped to look at a field. Greenbugs, Russian wheat aphids, and bird cherry-oat aphids were easily found. Winged forms (alates) of each of the aphids were active and feeding along with the non-winged aphids. These winged aphids will be flying and can be carried by our winds across the field and to other fields. For this reason, fields that have been previously treated could be re-infested.  And, with the warmer temperatures aphid infestations could develop to damaging levels before lady beetles and parasitic wasps, predominately Lysiphlebus testaceipes, can control the infestations.

Both the greenbug, Schizaphis graminum (Rondani), and the bird cherry-oat aphid, Rhopalosiphum padi (L.), are good vectors of the viruses that cause barley yellow dwarf/cereal yellow dwarf. The spread of the disease depend on the presence of the viruses in the aphids and the movement of the winged aphids in a field or across fields.
Greenbug colony with alate aphids, Photo Philip Sloderbeck, Bugwood.com
Bird cherry-oat aphids, Photo Frank Peairs, Bugwood.com
The salivary secretions of both the greenbug and the Russian wheat aphid, Diuraphis noxia (Mordw.), are toxic to the wheat plants. Greenbugs feed down in the wheat plant and symptoms of feeding is evident by young leaves turning yellow and orange-red spots on older leaves. Greenbug nfestations initially occur in concentrated patches within a field and as populations increase damage symptoms of yellow or irregular shaped patches can spread out across the field. 

Russian wheat aphid colony, Photo E. Bynum
Russian wheat aphid generally feed in the whorl of new shoots and tillers. Feeding cause the leaf to curl around the aphid colony and protects them from environmental conditions, natural enemies, and insecticides. The toxic saliva from the aphids cause damage to the leaves that are characterized by white, yellow, or purple longitudinal streaks down the leaf and stem. Also, feeding damage will cause prostrate growth of the plant.


Since lady beetles and parasitic wasps are also active now and could control aphids naturally, fields should be inspected and decisions to treat should be based on threshold levels. The “Glance ’n Go” sampling system (http://entoweb.okstate.edu/gbweb/index3.htm), as developed by Oklahoma State University and USDA-ARS, is a good tool for determining if greenbug or Russian wheat aphid infestations are at treatable levels. This sampling method takes into account the presence of parasitic wasp mummies when recommending treatment decisions. Our standard action threshold table for greenbugs can still be used for determining the need for a treatment, but it does not account for treatment costs and value of the crop. When using this action threshold take into account that when there are one to two lady beetles (adults and larvae) per foot of row, or 15 to 20 percent of the greenbugs have been parasitized, control measures should be delayed until you can determine whether the greenbug population is continuing to increase or declining. 

Action Threshold Table for Greenbugs
Plant Height (inches)
Number of greenbugs per linear foot
3 - 6
100 - 300
4 - 8
200 - 400
6 - 16
300 - 800

For the Russian wheat aphid, Texas A&M AgriLife research scientists developed economic thresholds based on infestation, damage, control costs and market value of the wheat. The calculated economic threshold represents the minimum number of tillers infest to justify an insecticide application. Individuals can predetermine the Economic Threshold level before sampling a field by using the following formula. The formula for determining the economic threshold is (C * 200)/(EY*MV), where C = control costs per acre (insecticide plus application, EY = Expected yield per acre, and MV = Market value per bushel. For example if Control costs = $15 per acre, EY = 40 bushel per acre, and MV = $4.50 per bushel the economic threshold = (15 *200)/(40*4.50), which equals to a minimum of 16.6% of tillers infested. So, when sampling the field and the percentage of tillers infested is less than 16% the field does not need to be sprayed now. But, if the percentage of tillers infested are > to 16% then it is cost effective to treat the field. If wheat is flowering then substitute 500 for the value 200 in the formula. If an individual does not want to calculate a threshold based on economics, they can use the following action threshold table for making control decisions.

Spring Action Threshold Table for Russian Wheat Aphid
Growth stage
% of damaged and infested plants
Regrowth to early boot stage
5% to 10%
Early boot to flowering
10% to 20%
After flowering
More than 20%
Plants with even a single infested or damaged tiller should be considered infested.
Peairs, F. B. Aphids in Small Grains, Colorado State University, no. 5.568

Army cutworms 
This past week Dr. Tom Royer, Extension Entomologist for Oklahoma State University, wrote an article (http://osuwheat.com/2015/03/10/army-cutworms-reported-in-some-oklahoma-wheat-fields/) about army cutworms being found in a wheat field in Cimarron County. The producer had taken the larvae to Sug Farrington, Extension Educator, for identification. The article by Dr. Royer provides excellent information about the life history, damage potential, and sampling methods, and treatment thresholds for the army cutworms in wheat and canola.
J. R. Sprague, County Extension Agent AG/NR, from Lipscomb County had seen an email about army cutworms in the Oklahoma Panhandle. He found an infestation of army cutworm larvae in a drylands corner of an irrigated wheat fields. The infestation levels were not at treatable levels. So, if you notice wheat stand loss or as Dr. Royer stated, “If you notice a field at this time of year with a number of starlings or black birds feeding in a concentrated area of your wheat field, they are likely feasting on army cutworms”.
Army cutworm larva, Photo Whitney Chanshaw, Bugwoo.com