Fall Armyworms Inundate Pastures and Landscapes

By Frank Escobedo, Texas A&M AgriLife Extension Service

Larva stage (caterpillar)

Inverted Y on the head with 4 bumps on the end indicates Fall Army Worm

KINGSVILLE - The South Texas area does not display much of the characteristics of the four calendar seasons like other parts of the country. Most residents are still wearing shorts and short sleeve shirts in December. We are typically guided by a calendar to determine the end of one season and the start of the next.  In the insect world, their internal clock suggests their migration. One pest in particular that has made a comeback to the area is the Fall Armyworm. Favorable conditions and time of the year (internal clock) have contributed to the return of this top forage feeder in pastures and landscapes. This tiny pest can consume more vegetation overnight than most insects.

The armyworm is named for its habit of moving across pastures and landscapes in large numbers like legions of an advancing army, devouring grasses in their wake, according to Allen Knutson, AgriLife Extension Entomologist. The fall armyworm has four stages: egg, larva, pupa, and adult. The moth (adult stage) has a wingspan of about 1½ inches. The hind wings are white while the front wings are dark gray, mottled with light and dark splotches. Moths are active at night and can be seen flying around lights. Adults migrate north as temperatures increase in the spring to lay their eggs. The female adult can lay as many as two thousand eggs.

The eggs hatch within a few days producing the larva stage or caterpillar. Eggs and larvae can be transported from one part of the state to another on grass or sod used for residential or commercial landscapes. The caterpillar can be green, brown, or black. Fall Armyworms are very distinct in their identification. They can be identified by an inverted Y pattern between their eyes. On the top segment near the back end, there are four spots (bumps) aligned in a square. The caterpillars feed for 2 to 3 weeks until they are 1 to 1½ inches in length.  With their great appetite, high numbers, and marching ability, armyworms can damage entire fields, pastures, and landscapes in a few days. Once they are done feeding, they tunnel into the soil and transform into the pupa stage. After 10 days in this non-feeding stage, the moth emerges and repeats the life cycle. The entire developmental life cycles take about one month with several generations throughout the year.  Typically cycles end in the fall when cold weather persists.  

Adult Stage (Moth) 

Although armyworms prefer grasses, they are not shy from entering landscape plants like prized roses. Early detection is the key to controlling or managing Fall Army worms. Birds, frogs, or other vertebrae around the area can provide early signs of infestations or concerns. In pastures or crop land, early detection is significant to avoid economic damage. The larva stage primarily feeds at night and on cloudy days. During the day, armyworms can be found on the ground under loose soil and fallen leaves. Chewed leaves are also a good indication of army worms.  

Beneficial insects such as wasps, flies, beetles, and other predators can decrease numbers. Control measures vary from the area being considered. Insecticides labeled for Armyworms such as products containing the following chemicals: malathion, carbaryl, trichlorfon dimethyl phosphonate, chlorpyrifos, bifenthrin, lambda-cyhalothrin, acephate, bacillus thuringiensis (organic), and Spinosad (organic) are available for both residential and commercial use. As always, read and follow label directions for any of the products listed above especially when considering pasture and row crop restrictions, mixing with other products, residential and commercial use, cutting hay, and other conditions that can apply to a certain product.  

Although armyworms struggle to overwinter in the northern part of the state, they can certainly survive an extended period in South Texas.