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Look out for armyworms after recent rains

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If you are in the forage business you need to be on the watch for armyworms. I have already had reports of armyworm damage. With the recent rain, producers need to be diligent in watching their pastures and fields.

The armyworm has four life stages: egg, larva, pupa and adult. The armyworm has not shown the ability to diapauses so its ability to survive winter depends on the severity of the temperature. The armyworm does overwinter in the southern regions of Texas in the pupa stage. The adult is a moth that migrates northward as temperatures increase in the spring. The adult moth has a wingspan of one and one half inches. The hind wings are white; the front wings are dark gray, mottled with lighter and darker splotched. Each forewing has a noticeable whitish spot near the extreme tip.

Eggs are very small and are laid in clusters of 50 or more and covered with grayish, fuzzy scales from the body of the female moth. The eggs are seldom seen and are laid at the base of appropriate host plants.

Larvae hatch from the eggs and when full-grown larvae are green, brown or black and about one to one and one half inches long when full grown. The larvae have a dark head capsule usually marked with a pale, but distinct, inverted “Y”. The larvae have five stages or instars and usually hide in debris on the soil surface in the middle of the day. When full grown larvae will enter the soil and form the pupa stage. Adult moths emerge from pupae. Moths mate and lay eggs, thus starting the life cycle over again. Lush plant growth is preferred by the adults for egg laying.

Several generations (a generation is the development from egg to adult stage) occur each year and typically the life cycle from egg to adult takes 28 days. The life cycle can be extended if cooler temperatures occur and can last up to several months. Armyworms in the spring and summer occur in more distinct groups than later in the season. Populations of larvae often blend together several generations and may appear to be continually occurring.

When feeding larvae strip foliage and then move to the next available food. High populations appear to march side by side to the new food, thus the name armyworms have been applied. Armyworms attack many different kinds of plants. When food is scarce they will move to plants that are not normally attacked. Armyworms can be found on nearly any plant as they migrate in search of edible foliage. Plants attacked by armyworms include, Bermuda grass, grain and forage sorghum, corn, small grains, sweet potato, beans, turnips, clover, spinach, cucumber, potatoes, tomatoes and many more.

Damage consists of foliage consumption. The small larvae will chew the green layer from the leaves and leave a clearing or “window pane” effect. The first three instars do very little feeding while the last two instars consume 85% of the total foliage consumed.

Armyworms should be controlled when they occur in large numbers of plant damage is becoming excessive. Preventive treatments normally are not justified because attacks are sporadic and egg mortality is usually high. During favorable seasons, a number of parasitic enemies keep fall armyworm larvae down to moderate numbers. Early detection works best and is achieved by frequent, thorough inspection of plants. Outbreaks seem to occur shortly after a rain or supplemental irrigation. Armyworms feed any time of the day or night, but are most active early in the morning or late in the evening. Susceptible fields or lawns should be scouted by counting the number of armyworms in a square foot area in eight different sites. Divide the total worm county by eight to find the average number of armyworms per square foot. Be sure to take samples in the interior of the field because this pest is often heaviest near the field margins. Sometimes only the field margins require treatment.

The threshold level ranges from two to three larvae per square foot for young tender growth. For older plants, three to four larvae and obvious foliage loss justify control measures. Thresholds in improved pastures and lawns vary with condition but treatment should be considered when counts average three or more small worms per square foot.

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