Insect Strip

Insect Strip

Wednesday, October 5, 2016

Fall Wheat Pest Report

Fall Armyworm
The fall armyworm, Spodoptera frugiperda (J.E. Smith), which has been a pest in sorghum on the Texas High Plains is now causing problems in seedling wheat. When scouting there may be small to large “worms” due to overlapping generations and moth activity for egg laying. The potential of continuing to have fall armyworms is very likely until there is a killing freeze. 

Description and Biology
The larvae are usually shades of brown but may also be greenish to nearly black. Four distinct black spots are on top of the eighth abdominal segment, and a white inverted Y is on the front of the head. Mature larvae are 11⁄2 inches long. In addition to small grains, fall armyworms feed on corn, grain sorghum, sorghum grass hybrids, peanuts, alfalfa, cowpeas and cotton.
Fall armyworm moths may deposit egg masses on leaves of seedling small grains. There are 6 caterpillar life stages (instars) from when the larva hatches to when it pupates. This will take approximately 14 days under summer temperatures to 30 days under cooler conditions. Small larvae feed on the leaf tissue, creating tiny “window panes” in the leaves. Larger larvae consume entire leaves and can completely eat the plant to the ground causing severe stand losses. The larvae are generally more active on the plant in the morning and evening. 

Photo by: Pat Porter

Photo by: Pat Porter


Fall Armyworm Larval Consumption Rate
Life Stage
Larval Length
(approximate)
% Total Consumption
1
1/16 in.
0.1
2
1/8 in.
0.6
3
1/4 in.
1.1
4
3/8 in.
4.7
5
5/8 in.
16.3
6
1 1/4 in.
77.2

When making decisions on whether to treat consider the following questions?
  • Do you already have a marginal stand?
  • Are you growing wheat for grazing or for grain? If it is for grazing then preventing stand losses will be important.
  • What is the cost of replanting compared to application costs?
  • What is the outlook for a killing freeze?
  • What is the infestation level, the size of the larvae, growth stage of the wheat, and damage symptoms? If larvae are small they will not be consuming as much as large larvae and there may be a few days before there is much leaf or any stand losses. But if the larvae are larger the amount of leaf loss or stand losses will occur very quickly. Of course these losses should occur sooner to seedling wheat than to wheat that was planted earlier. Scouting becomes very important to know what level of damage there is and what is the size of the larvae from one sample time to the next. Therefore, it is difficult to use a set number of worms per length of drill row as a threshold guideline. Still most individuals want something to go by. One might consider using two to three fall armyworms per linear foot of row, but keep in mind the factors just mentioned.
  • Control is more effective when the larvae are less than 1/2 inch in length. For control of fall armyworm in wheat, the choices in terms of active ingredient are primarily:
Chlorpyrifos (Lorsban, Nufos, and other generic products)  
Pyrethroids, including Baythoid XL, Karate or Warrior with Zeon , Proaxis or Declare, Fastac, Mustang Max
Combination of a Pyrethroid plus chlorpyrifos.  These include Cobalt and Stallion
Prevathon, Spinosad (Blackhawk replaces Tracer).

Of these, Prevathon would provide the longest residual control but will be the most expensive.  There is some concern that the  Pyrethroid alone may not be effective against fall armyworm as they are not very effective in sorghum.  However, the pyrethroids are effective against FAW in bermudagrass hay and on small grains in other areas in Texas.   The pyrethroid alone would be the least expensive.  The combination products, Cobalt and Stallion, would be a good choice if the pyrethroid alone did not do well.  These combination products would also provide control of any greenbugs that might be present.  However, some of the pyrethroids, Declare, Proaxis, etc, are labeled for greenbug control, so alone these could be effective against both pests. The labeled rate of chlorpyrifos may not be as effective as the chlorpyrifos combination products.
Since fall armyworm infestations cause sporadic damage to wheat, it is difficult to conduct efficacy trials with the products and, therefore, we do not have data for how effect these products are on the Texas High Plains. But, the following are what products/rates some individuals are using with a few comments about control. Chlorpyrifos (Lorsban) has been applied at 1 qt./A because lower rates were not effective, but this is an off label rate. Also, one individual stated the 1 qt. rate used on a field did not kill larger sized larvae. Another individual is using Lorsban @ 4 oz/A (1/4 pt) plus Mustang Max @ 4 oz/A. No comments about control. Prevathon has been used at 12 oz/A and 14 oz/A with what is considered good control of all size larvae. And an individual is considering using Prevathon @ 8 oz/A for small larvae and 10 oz/A for larvae larger than 1/2 inch. We have no other information about other products.