Recent rainfall could cause spike in the fall armyworm population

Randi Williams, Texas AgriLife Today

October 31, 2023

Many producers throughout Texas are noticing an increasing fall armyworm population in their rangelands and forages in areas after receiving rainfall over the last few weeks. Texas A&M AgriLife Extension Service experts recommend producers be prepared to protect their valuable forages.

Vanessa Corriher-Olson, Ph.D., AgriLife Extension forage specialist and professor in the Department of Soil and Crop Sciences, Overton, said the most important thing for producers right now is to realize the areas receiving rain will see grass growth that could fuel armyworm populations. Damage from infestations could be problematic for cattle operations dealing with short hay and forage supplies due to drought.

“Armyworms can be picky eaters,” said Olson-Corriher. “These insects are going to select well-fertilized or irrigated forages to feed on.”

Scouting valuable forages for armyworms

Forage producers planting cool-season forages, such as winter wheat, annual ryegrass or any small grains, are at higher risk of armyworm infestations. Young seedlings are subject to more damage than more mature perennial warm season forages.

Scouting is the most effective way to know if you’re dealing with armyworms and deciding how to manage populations.

“After a rain, producers should walk through their fields at dawn, when armyworms are most likely to be active and see if the population size warrants treatment with an insecticide,” Corriher-Olson said.

Producers should be looking for armyworms that are green with brown or black colorations and are identified by the white inverted Y on their heads. Mature armyworms can grow up to 1.5 inches in length.

Controlling armyworm populations

When scouting forages and seeing a spike in armyworm population, if there are more than three armyworms per square foot, producers are advised to determine which insecticide to spray and do so immediately.

“Most producers keep insecticides on hand that are labeled for different scenarios,” said Corriher-Olson.

David Kerns, Ph.D., AgriLife Extension state integrated pest management coordinator and professor in the Texas A&M Department of Entomology, said the bigger the worm, the more they can eat.

“If you have a large population of large larvae, you’ll need to handle them immediately because they can consume an entire field overnight,” Kerns said.

Most producers are likely to use a pyrethroid insecticide due to the low cost, but that might not get rid of the majority of the population because they only last up to five days. Additionally, the armyworms must be directly exposed to the pyrethroid by touching or eating the plant.

“We have translaminar pesticides that soak into the leaf tissue of the plants,” Kerns said. “This means it lasts longer and the rain won’t wash it off, but the translaminar pesticides are higher in price.”

He added that a year like this that has been droughty “is going to suppress the armyworm population but be on the lookout for armyworms since the recent rain gives them something to build on.”

AgriLife Extension district reporters compiled the following summaries:

Dry conditions continued with a dire need for rain. Cooler weather arrived, and while it brought some greening to the pastures and much-needed moisture to the fields, the lack of significant rainfall remained a concern. Mild conditions and cooler temperatures allowed fieldwork to continue. Nighttime temperatures were expected to be in the 50’s, which would cause the warm-season grasses to slow their growth. Most cotton acreage, including irrigated, that was not already harvested was abandoned due to lack of rainfall or running out of irrigation water. Stock tank levels were good. Some moderate leaf spot disease pressure showed in Jiggs Bermuda. Wheat planting was delayed as much as possible due to Hessian fly concerns. Few fields were planted to resistant varieties. Small grains were going in and hay was being baled. The pecan harvest began, but yields were very low and quality was only fair. Livestock numbers were still holding with the uptick in green grass. Some culling was taking place before winter. Cattle remained in good body condition, with producers feeding limited hay.


Winter wheat planting was in full swing across the Rolling Plains. Most producers were hopeful that the recent moisture would help the wheat come up, but more rain will be needed shortly to keep soil moisture at minimal levels going into the fall. Most dryland cotton was turned into insurance as a loss. Rain was needed for pastures and livestock drinking sources.


Cooler weather and scattered showers helped with soil moisture retention. Some counties received over 2 inches while others only got two-tenths of an inch. Grain and cotton producers were planning to fertilize soon. Winter pasture planting continued. Some producers were hoping to get a final hay-cutting before the first frost. Some cattle producers were still feeding hay and supplements. Markets were still strong, and many producers took advantage of the high prices by weaning and selling off calves earlier than usual.


Although conditions had improved, the drought continued. Pasture and rangeland conditions were fair to good. Subsoil conditions were adequate to short overall, while topsoil conditions were adequate. Producers were getting another cutting of hay when possible. Others were planting winter pastures to try and offset low hay supplies. Shortages remained a significant concern going into the cooler seasons. Livestock were in fair to good condition, with some supplementation taking place. Producers noticed larger populations of armyworms. 


Cool nighttime and moderate daytime temperatures allowed a few heat units to be picked up and slowly finish off the cotton crop. Producers sprayed defoliants on the cotton fields, and a few started stripping cotton. A few gin yards were beginning to receive cotton and will kick off ginning soon. The pumpkin harvest finished up this past week. Winter wheat has emerged with the rains from last week. Rainfall totals ranged from half an inch to 1.4 inches. Silage was being cut. The grain was still drying down, and harvest had slowly begun. Cattle reports were in good condition.


There were high winds and arid conditions. Some producers dusted in wheat crops in hopes of some moisture. Irrigated wheat progressed and several producers started wells for grazing. Fieldwork continued as producers prepared for the harvest of corn and sorghum. Silage choppers continued, with some producers wrapping up the year very quickly. Dry conditions were affecting pastures and rangelands.


Pastures and rangelands were reported as fair to poor from most counties. Subsoil and topsoil moisture levels were average, with few counties reporting very short or adequate moisture. Temperatures overnight ranged from the 30s to 50s and slowed the growth of vegetation and grasses. A few counties received rainfall, but most did not. Winter pasture planting continued. The ground was prepared for wheat, but few forage-use acres were planted. Hay harvest continued. No grain crops were planted. There were no significant insect or disease issues, and livestock were doing well.


Temperatures steadily decreased with highs in the low 80s and lows in the high 50s. Scattered thunderstorms continued across the region. Cotton harvest was picking up, with almost everyone in the field either defoliating or harvesting. Defoliation was taking longer with the cooler temperatures and rates of harvest aids being increased. Yields were proving to be well below expected due to very small bolls and fewer of them on the plants. The harvest should not last long because of the dry weather and no rain in the forecast. Gins were letting modules stack up to try and keep the ginning season in as short of a window as possible. Wheat planting continued and planted acres were expected to be higher than average this season, but emergence was poor due to a lack of soil moisture. Watermelons and cantaloupes were still producing well. Chilies were growing well. Onions were in the ground, and pumpkins were being harvested. Pawnee pecans were starting to get harvested as well. Livestock were in fair condition as supplemental feeding continued. Cattle have a body score of 4.


Temperatures cooled significantly, with lows in the 45-50 range and highs in the 75-85 range. Conditions remained dry as rain fell sporadically in small amounts. Producers continued planting oats and wheat. Cotton defoliation continued and harvest started. Pecan trees were dying in some orchards and even irrigated orchards were showing stress. Insect pest problems continued and fall armyworm populations were increasing. Drought was still a severe issue as stock tanks were low. Grasses slowed down due to the cool fronts. Rangeland and pasture conditions needed more rain. Many producers were selling livestock due to the lack of grass and water. Cattle prices were still high. Supplemental feeding continued for the cattle producers kept.


Some producers managed to harvest the last hay/forage cutting, which will be a tremendous help going into winter. Dry conditions persisted as the cold front blew in. Rangeland and pasture ratings varied from excellent to very poor, although rainfall improved pastures and crop fields and new growth was emerging. Producers have noticed fall armyworms and most pastures were treated for them, as well as Bermuda grass stem maggots. Supplemental feeding had slowed down, and producers continued culling the herds. Some producers started spreading lime. 


The district had variable precipitation. Despite the rainfall and cooler temperatures, drought conditions persisted. Farmers were preparing for wheat plantings, and oats were being planted. Supplemental feeding continued for livestock. Deer body conditions were expected to improve with the growth of greener forages. 


The recent cool down and rainfall improved rangeland and pasture conditions. With the recent rain, producers noticed fall armyworm populations rising. Cotton harvest was completed, but peanuts were still being harvested. Plastic was being put down for strawberries, and harvest was completed for the season on vegetables and coastal Bermuda. Supplemental feeding continued for livestock. Wildlife benefitted from the moisture, quality of forage and cover. Cattle sales slowed due to improved pasture conditions and less cow culling. Preparation for hunting season began, with many landowners preparing for wildlife population surveys.

Southern Livestock

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