Check Pastures and Sorghum for Armyworms

It is time to crank up our annual fall armyworm alert for Oklahoma. Brian Freking, Area Livestock Specialist in Ada reported fall armyworms infesting some grass pastures. It is likely fall armyworms will begin infesting the whorls of later-planted sorghum.  I encourage producers to scout their sorghum and grass pastures regularly over the next month.

 

For those wanting to put up grass hay, scout your fields by looking for caterpillars and “window paned” or chewed leaves. Scouting for caterpillars in pasture is easy.  Get a wire coat hanger, bend it into a hoop, place it on the ground, and count all sizes of caterpillars in the hoop.  Take samples in several locations, along the field margin as well as in the interior. The hoop covers about 2/3 of a square foot, so the threshold in pasture would be reached when you find an average of two or three ½ inch-long larvae per hoop sample (3-4 per square foot. It is much easier to control them with an insecticide when they are small (less than ½ inches).  

 

Figure 1. Windowpaning Caused by Fall Armyworm

 

Figure 2. Scouting Tool for Fall Armyworms in Pastures   

 

For control guidelines and information on registered insecticides for fall armyworm, consult OSU Fact Sheet CR-7193 Management of Insect Pests in Rangeland and Pasture.

 

Sorghum “whorl worms” are a different problem.    If plants are very small (2-4 leaf stage), it is important to scout and treat if fall armyworms are feeding, because they can literally kill small, newly emerged plants by eating the growing point. Treat if 25% of small (2-4 leaf stage) plants are infested.

 

Extension entomologists rarely recommend controlling fall armyworm in the whorl of larger sorghum plants for three reasons.  First, they often go unnoticed until they have caused major damage to the leaves and they have either pupated, or are within a day or two of turning in to a pupa.  Secondly, even when found in time, an insecticide application typically provides poor control because most registered insecticides require that the caterpillar come in physical contact with the insecticide.  Fall armyworm caterpillars feed deep within the whorl, and most applications simply can’t deliver enough insecticide into the whorl to kill more than 40-60% control.  In both cases, the money spent for control is wasted. Finally, the jury is still out on how much yield is saved by controlling them in the whorl. 

 

Figure 3.  Fall armyworm damage to sorghum whorl  

 

Figure 4.  Fall armyworm buried deeply in sorghum whorl.

 

However, several new insecticides have been registered, and recent research results from irrigated sorghum in Lubbock suggest that Beseige and Coragen can provide acceptable control (> 75%) of the caterpillars in the whorl (even large caterpillars), but the actual yield savings that may result is still being studied. Consult CR-7170, Management of Insect and Mite Pests in Sorghum for more information on control of sorghum pests.