Armyworm

The adult true armyworm is a nocturnal moth with a wingspan of about 1.5 inches and a single white spot in the middle of its buff-colored forewing. (See photo of adult Western yellowstriped armyworm for comparison.)

Females lay eggs on either rice or other grass species in or around rice fields. Eggs are laid in linear masses with the leaf tied around them in a roll, making them difficult to find.

Larvae go through 6 instar stages. Early stages are small, difficult to find, and consume very little foliage. As larvae grow, the amount they eat increases, and defoliation becomes apparent as larvae reach the fifth and sixth instars in early summer. Larvae feed predominantly at night or during cloudy days, spending the day hidden among tillers near the water level. Larvae pupate in 3 to 4 weeks during summer lodged between rice tillers. Moths emerging from these pupae may lay eggs in rice again.

Two true armyworm generations occur in rice; moths fly in spring and early summer, laying eggs in rice fields and surrounding vegetation. The larvae that emerge feed on rice foliage. A second moth flight occurs in mid-summer, followed by larvae feeding on rice foliage and panicles.

It is important to distinguish true armyworm from western yellowstriped armyworm which can also be present in rice fields. Western yellowstriped armyworm moths only lay eggs on broadleaf weeds. Eggs form a flattened mass that is covered by body scales. Larvae feed primarily on broadleaf plant species and usually don’t cause economic damage to rice. The adult moth has mottled forewings and silver and gray hindwings.

Only true armyworm causes economic damage to rice. Injury is most serious during periods of stem elongation and grain formation. Larvae defoliate plants, typically by chewing angular pieces off leaves. In severe cases, all plant foliage can be consumed, leaving only a portion of the stem above the water. Defoliation greater than 25% during early summer will result in lower grain yields. During flowering and grain filling, larvae may also feed on the panicle rachis near the developing kernels causing these kernels to dry before filling. This feeding causes branches of the panicle to turn white and dangle from the panicle. Yield reductions have been documented if more than 10% of panicles are affected. Defoliation during flowering and grain filling does not typically result in reduced yields.

Monitor throughout the summer to assess the need for insecticide application. Drowning and natural enemies kill many true armyworms in the rice field.

Many caterpillars are killed by natural enemies including predators, pathogenic microorganisms, and parasites. Natural enemies, especially when they limit spring and early summer generations in other crops and along field margins, often keep armyworms from becoming pests in rice. The wasps Hyposoter exiguae and Apanteles militaris are the most commonly seen parasites of the true armyworm. The larvae of these wasps live within the armyworms until they emerge to form white silk cocoons on tillers and leaves.

Viral diseases of armyworms are also important natural control agents under certain conditions of temperature and humidity. Diseased caterpillars first appear yellowish and limp. After dying, they hang from plants, appear deflated, and they ooze disintegrated body contents.

For information on protecting natural enemies, see Protecting Natural Enemies and Pollinators.

Early weed control in and around rice fields may limit the development of armyworms infestations in rice fields.

Use cultural controls and naturally occurring biological controls in a certified organic crop. Bacillus thuringiensis ssp. kurstaki is registered for use in rice, and applications generally achieve 50 to 60% mortality of true armyworm.

Foliar Injury

Monitor for foliar injury during late June and early July by looking for signs of armyworm feeding on leaves. Once feeding damage is observed, monitor twice a week until larvae are no longer present.

To monitor, choose a part of the field where you have observed injury. Walk into the field and inspect plants in a small area around you from the water level to the top of the leaves. Check the water surface for armyworms that may drop from the plants. Determine if 25% or more of the foliage has been removed by armyworms and if they are still present. Record your observations. Repeat this procedure every 5 to 10 feet across a transect until 10 stops have been examined. Repeat this procedure in several areas of the field to create a confident estimate of the average field condition.

Panicle Loss

Monitor for panicle loss after panicle emergence by checking for broken panicle branches that have turned white, indicating possible armyworm feeding. Be sure to differentiate this injury from stem rot, cool temperature blanking, and rat damage, all of which may cause blanking.

Once armyworm injury to the panicle is observed, take samples twice a week to determine if an insecticide application is warranted. Inspect panicles in a 1-square-foot area and determine the proportion injured by armyworms. Inspect the plants down to the water level for the presence of armyworms.

Record the percentage of injured panicles and the presence or absence of armyworms. Move 5 to 10 feet and repeat the procedure until 10 samples have been taken. Move to other areas of the field with signs of panicle injury and repeat the process until you have a good estimate of the field condition.

Treatment Thresholds

During late June and early July, consider application of insecticides only in those areas of the field with more than 25% defoliation, and where armyworms are observed. If few or no armyworms are observed, come back to check in the evening when larvae are more active. Do not spray if armyworms are not present, especially during mid to late July, because they have probably completed development.

From panicle emergence to grain maturity, consider application of insecticide if 10% of the panicles in the area sampled are damaged and true armyworms are observed. If true armyworms are not observed but panicle loss is 10% or more, check for the larvae in the evening. If larvae are not found, do not spray because they have probably pupated and will do no further damage. Limit insecticide application to those areas of the field with economic damage.