Recent Rains Suggest it’s Time to Check for Fall Armyworms

 

Rain has been falling across parts of Oklahoma, and while that is typically a good thing for agricultural operations this time of year, the rainfall has signaled the importance of checking for the presence of fall armyworms.

 

“Since survival of fall armyworm eggs is highest following warm, humid climatic conditions following summer rainfall or supplemental irrigation, some serious problems could develop for seedling wheat fields, sod farms, golf courses and residential lawns,” said Tom Royer, Oklahoma State University Integrated Pest Management coordinator.

 

Royer and Jessica Lindenmayer, graduate researcher with the OSU department of entomology and plant pathology, have been collaborating with OSU Cooperative Extension agricultural educators Leland McDaniel of Carter and Jefferson counties, Casey Russell of Pontotoc County and Kassie Junghanns of Grant County to monitor the growth of migrating fall armyworm populations. Fall armyworm populations also have been detected at high levels at OSU’s Cimarron Valley Research Station at Perkins.

 

“We’re getting a lot of moths in traps right now, which means fall armyworms are laying eggs,” Lindenmayer said. “It is important to scout for the presence of early instars in the coming weeks. The earlier they are caught the better for treatment efficacy.”

 

Preventive insecticide treatments are not practical because outbreaks are sporadic and mortality due to natural enemies is usually high. Unnecessary insecticide applications can eliminate these natural enemies from the landscape, causing a worse armyworm problem following treatment.

 

“A major key to controlling potential fall armyworm problems is early detection because infestations of fully mature larvae feed voraciously and can completely consume a lawn overnight and significantly damage a field in as few as several days,” Royer said. 

 

Royer added the best way to protect one’s investment in his or her crop is to just plan on checking fields early and often.

 

Female fall armyworm moths lay up to 1,000 eggs over several nights on grasses or other plants. Within a few days, the eggs hatch and the caterpillars begin feeding in groups. Caterpillars molt six times before maturing, increasing in size after each molt. They can complete a generation in 18 to 28 days depending on temperature.

 

“Newly hatched fall armyworms are white, yellow or light green and darken as they mature,” Lindenmayer said. “Mature fall armyworms measure about 1.5 inches in length with a body color that ranges from green to brown or black. They can be distinguished by the presence of a prominent inverted white "y" on their head.”

 

Small larvae do not eat through the leaf tissue but instead scrape off all of the green tissue and leave a clear membrane that gives the leaf a "window pane" appearance. Large larvae can quickly denude a turf or forage canopy.

 

To scout for fall armyworms, examine turf or seedling wheat plants from eight locations measuring 1 square foot each. Examine turf along the field margin as well as in the interior since armyworms often move in from road ditches and nearby weedy areas. Look for “window paned” leaves and count all sizes of larvae.

 

“Total the number of larvae in each size class and divide each number by eight to calculate the average number per square foot,” Lindenmayer said. “Thresholds are not well developed for fall armyworm in turfgrass but treatment is suggested when average counts reach two or three half-inch larvae per square foot.”

 

Royer and Lindenmayer said it is crucial to target smaller caterpillars of a half-inch or less for two reasons. First, the caterpillars do not cause severe damage until they reach a size of one inch in length. Second, smaller caterpillars are much more susceptible to insecticide control than larger caterpillars.

 

“Any product labeled for caterpillar control in turf should be effective for fall armyworm control in sod fields, lawns and golf courses,” Royer said. “The treatment threshold for wheat is one to two fall armyworms per linear foot.”

 

Grass hay producers need to check their fields as well. An easy way for hay growers to determine if they need to treat their fields is to get a wire coat hanger, bend it into a hoop, place it on the ground and count all sizes of caterpillars in the hoop.

 

“A hoop will typically cover about two-thirds of a square foot, so a threshold in pasture would be an average of two or three half-inch-long larvae per hoop sample, essentially three or four per square foot,” Lindenmayer said. “If the treatment threshold is exceeded, it is much easier to control them with an insecticide when they are less than a half-inch long.”

 

Always follow label recommendations when applying any insecticide, paying extra attention to the current rates and restrictions listed on the label. “Never assume the rates have remained unchanged from year to year,” Royer said. “Always check and double-check.”