Potential for True Armyworm Problems in Grass Crops

Published: April 21, 2009

Present Situation: Armyworm moth numbers are elevated in a couple areas of the state and relatively low in many others. This could change rapidly if additional moths either emerge from overwintering sites in Missouri or migrate into the state from more southern states. In those areas where moth numbers are elevated, producers are encouraged to scout grass, wheat, and corn fields to determine true armyworm larval numbers. If thresholds are reached or exceeded, then specific insecticides are recommended for treatment of this pest.

About every four years the True armyworm, Mythimna unipuncta (formerly Pseudaletia unipuncta), is a moderate to serious pest of many grass crops in Missouri. Although larvae of this pest may attack a wide range of host plants, most problems in Missouri occur in tall fescue seed and forage fields, wheat and other small grain crops, and occasionally to field corn. Economic important infestations of true armyworm often develop when several factors occur at the same time to favor armyworm distribution and population increases. These factors include (1) emergence of high numbers of moths from overwintering sites in the state, (2) early spring migration of moths into Missouri from more Southern states, often helped by spring storms arriving from more southern locations, (3) reduced numbers of beneficial insects which allow for better survival of armyworm larvae, (4) the occurrence of cool, wet spring conditions which favor lush growth of tall fescue and wheat plants which serves as good egg laying and larval feeding sites, and (5) the natural rhythm of true armyworm populations which often peak about every four years in Missouri. All of these factors are present to some degree this spring and certainly increase the possibility of economic infestations of true armyworm developing in areas of the state where moth captures are occurring in relative high numbers.

Monitoring of true armyworm larvae are provided by the University of Missouri IPM Network with the assistance of 33 regional extension specialists who trap armyworm moths across the state and from meteorological data provided by the statewide system of commercial agriculture weather stations. Male true armyworm moths are captured using traps baited with a synthetic version of pheromones emitted naturally by the female armyworm moths to attract males. When numbers of male moths captured in traps reach levels of 100 to 200 or more moths per night for several nights, the potential for true armyworm problems in fields located within a few miles of the trap is elevated. If several traps within a region capture high numbers of moths for several nights in the spring, then true armyworm problems may cover several thousand acres of forages in the state similar to problems experienced with this pest a few years ago in northcentral Missouri, where over 500,000 acres of tall fescue were heavily infested. Wheat fields may also experience problems with true armyworm as larvae feed on foliage and occasionally will cut seed heads from developing wheat plants. In field corn, economic infestations of this pest are uncommon, but often cause severe defoliation when they occur. True armyworm moth capture counts can be found online at www.ipm.missouri.edu. At the site go to pest monitoring and select true armyworm. At this site you can view all moth capture numbers in the state or search for numbers by specific regions of the state. NOTE: Although elevated numbers of moth captures often result in economic infestations of true armyworm, in some years other factors (insect pathogens, predators and environmental conditions) may limit the development and growth of this pest. To determine risk from true armyworm larvae in specific fields, producers in areas of elevated moth captures are encouraged to scout for the presence of true armyworm larvae to determine larval armyworm numbers in their forage and crop fields.

Scouting for armyworm larvae is best accomplished during the night, early morning or late evening as newly hatched larvae avoid light and feed on the lower leaves of grasses during hours of reduced light intensity. As larvae grow in size, they will feed higher on the host plant even during daylight hours. True armyworm larvae grow through 7 or more larval or worm stages often referred to as "instars". Larvae newly emerged from eggs are very tiny, but quickly grow to about 1 ½ inches in length when full grown. Larvae are greenish-brown in color with a pale stripe running the length of the back and an orange line running the length of each side of the larva. The head capsule is light brown in color and the body is generally smooth and mostly hairless. A good identifying characteristic for this insect in the larval stage is the presence of a dark brown to black triangle located on the outside of each of the four pairs of prologs found on the middle to back part of the insect body. The tip of the foot on each of these prologs is also dark in color if viewed from under the larvae looking outward.

If true armyworm larvae are present in grass, small grain, or field corn crops, use the following thresholds to determine if treatment is justified and recommended insecticides for each specific crop.