Recent rainfall events have been a major problem this summer. Fall armyworms have been hitting hay field heavily.

There are two strains of fall armyworms (FAW): the corn strain and the grass strain. The corn strain usually appears in the spring and early summer and feeds on crops such as corn, sorghum, and cotton. The grass strain, which is the strain that infests hay fields and pastures, generally shows up after significant rain events from mid-July through fall. The corn strain is known for being resistant to pyrethroids, while the grass strain is susceptible to pyrethroids. FAW caterpillars survive and develop better in areas with fertilized or well-watered grasses which can lead to outbreak scenarios.

FAW caterpillars are primarily identifiable by two features. The head will have apparent white markings that form an upside down “Y” pattern. The second feature is that the last couple segments of the caterpillar will have black bumps that form a square or rectangle.

If the grass is being used for hay and is near harvest, then harvest early to prevent extra feeding damage. However, be aware that the caterpillars may consume the cut hay, so this is not always a wise option.

Insecticide applications should be made early in the morning or late in the evening if possible, to ensure caterpillars come into contact with insecticide. During hotter parts of the day, caterpillars may be out of the canopy and avoid maximum insecticide exposure.

There are many insecticides that can be used to control FAW caterpillars. Pyrethroids are relatively cheap and readily available. These insecticides take roughly three days to achieve maximum effectiveness against small and large caterpillars. Pyrethroid insecticides tend to have a short residual period and can be washed off by rains. This lack of rain fast protection can be a problem with the recent weather and possible overlapping generations of fall armyworm caterpillars.

Addition of a product like Dimilin (or generic products with diflubenzuron) can increase the residual control period to 10-12 days, eliminating caterpillars that emerge in that timeframe. Neither pyrethroids nor Dimilin will continue providing control if rain occurs though. Another fairly inexpensive option in Intrepid (or generic products with methoxyfenozide). This product will provide residual control for about seven days, but it must be eaten to kill the caterpillar and is not rain fast.

If rain is a continuous issue, the only truly rain fast options are products such as Prevathon, Vantacor, or Besiege. All of these products contain the active ingredient chlorantraniliprole, but Besiege also contains a pyrethroid. These products are absorbed by the leaf tissue and are rain fast upon drying. While these products are more expensive, they do provide excellent residual activity and will persist longer at the higher rate. For example, Prevathon at 14 fl-oz/ac will typically provide 14 days control, and a 20 fl-oz/ac rate will provide 20-21 days of control.

Fenceline Chatter is written by Lonnie Jenschke, Erath County Extension Agent – Ag/NR. He may be contacted at l-jenschke@tamu.edu.