Agriculture: Rice Pest Management Guidelines


  • Armyworm: Mythimna (= Pseudaletia) unipuncta
  • Western yellowstriped armyworm: Spodoptera praefica
  • Description of the Pest

    Two species of armyworms, the armyworm and the western yellowstriped armyworm, are found in rice fields in mid-summer. Spring and early summer generations are spent on other plants. When other food sources are depleted, larvae of either species may migrate into rice paddies, or adult moths may fly into the rice field to lay eggs.

    The armyworm moth lays its eggs in linear masses with the leaf tied around the eggs in a roll on either rice or other grass species in the field.

    The western yellowstriped armyworm moth is believed to restrict its egg laying in rice fields to broadleaf weeds. Eggs occur in the form of a flattened mass that is covered by body scales. Both larvae are striped and vary in body color but the older worms can be distinguished by markings on their sides. The western yellowstriped armyworm has a black spot on the side of its first legless segment, and the centers of spiracles on each body segment are white. The first legless segment of the armyworm does not have a dark spot and the center of each spiracle is black.

    Larvae feed predominantly at night or during cloudy days. They develop to full size and pupate in about 3 to 4 weeks in summer. Pupation normally takes place in the upper surface of the soil or in debris, consequently most mature larvae drown in flooded paddies before reaching a suitable pupation site. Usually only one generation a year will be spent on rice.

    Adult moths of both species have a wingspan of about 1.5 inches (about 35–45 mm). The western yellowstriped moth has mottled forewings and silver and gray hindwings. The armyworm adult has a single white spot in the middle of its buff-colored forewing. Both moths fly at night.


    Injury by armyworms is most serious during periods of stem elongation and grain formation. Larvae defoliate plants, typically by chewing angular pieces off leaves. They may also feed on the panicle rachis near the developing kernels causing these kernels to dry before filling. This feeding causes all or parts of the panicle to turn white. If the entire panicle is white, injury may also be due to low nighttime temperatures during panicle differentiation, stem rot or feeding by rats. The seriousness of armyworm injury depends on the maturity of the plant and the amount of tissue consumed. Significant yield reduction can occur if defoliation is greater than 25% at 2 to 3 weeks before heading.


    Early broadleaf weed control and biological control can be important in limiting the numbers of armyworms. Monitor throughout the summer to assess the need to treat.

    Biological Control

    Various natural factors cause mortality of armyworms in the rice field. Many caterpillars drown or 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 wasp Hyposoter exiguae often parasitizes the western yellowstriped armyworm, and Apanteles militaris is the most commonly seen parasite of the armyworm. The larvae of these wasps live within the armyworms until they emerge to form white silk cocoons on tillers and leaves.

    Cultural Control

    The western yellowstriped armyworm is believed to limit its egg laying to broadleaf plants; thus, the early control of broadleaf weeds in rice fields may be important in limiting its populations.

    Organically Acceptable Methods

    Biological and cultural controls are acceptable for use on an organically certified crop.


    Foliar Injury

    Monitor for foliar injury from panicle differentiation to heading by looking for signs of armyworms feeding on leaves. Once you begin to observe injury, start taking samples twice a week until grain start maturing or larvae are no longer present. To sample, choose a part of the field where you have observed injury. Select a plant at random and pull it up or move all the surrounding foliage away and check for defoliation. Check the plant from the top of the leaves to the base of the plant and the water surface for armyworms. Determine if 25% or more of the foliage has been removed by armyworms; also note if you find armyworms on neighboring weeds or rice plants. Record your observations on a monitoring form. Repeat this procedure every 5 to 10 feet (1.5–3 m) across a transect until 10 plants have been examined. Move to a different part of the field where feeding is evident and examine 10 more plants in the same manner. Repeat this procedure at several areas of the field until you are confident that you have an estimate of the average field condition.

    Panicle Loss

    Monitor for panicle loss after panicle emergence by checking for entire panicles or parts of panicles that have turned white; these indicate armyworm feeding. Be sure to differentiate this injury from stem rot, which may kill the entire panicle and darken the stems. Once you begin to observe armyworm injury to the panicle, take samples twice a week to determine the need for treatment. Use a sampling ring made of plastic tubing that encloses 1 square foot. Select your sampling sites in parts of the field with white panicles. Drop the ring at your side without looking. Examine all the plants within the ring down to the water level for armyworms; at the same time check for stem rot. Record the number of panicles and the percentage of them that are white and the presence or absence of armyworms within the ring. Move on 5 to 10 feet and repeat the procedure until 10 samples have been taken. Move to another area of the field with signs of panicle injury and take 10 more samples. Repeat the 10-sample procedure until you feel that you have a good estimate of the field condition.

    Treatment Decisions

    From panicle differentiation through heading, treat for foliar damage only in those areas of the field where 5 or more of the 10 samples taken have over 25% defoliation and armyworms are present on the plants. If you observe a few or no armyworms, come back at night to check for the larvae, which are more active after dark. Do not treat if armyworms are not present, especially during late August, because they have probably completed development.

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

    Common name Amount per acre REI‡ PHI‡
    (Example trade name)   (hours) (days)
    Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees, and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide's properties and application timing. Always read the label of the product being used.
      (Warrior II with Zeon) 1.6–2.56 fl oz 24 21
      COMMENTS: Can be used safely when propanil products are being used for weed control. Notify the mosquito vector control district personnel so that they can monitor populations of mosquito fish after use of this product. Do not apply more than 0.96 pts/acre per season.
      (Mustang) 3.4–4.3 oz 12 14
      COMMENTS: Make applications at least 7 days apart and do not release flood water within 7 days of application. Do not apply more than 1.1 pt (0.2 lb a.i.)/acre per season. Notify the mosquito vector control district personnel so that they can monitor populations of mosquito fish after use of this product.
      (Sevin 4F) 1-1.5 qt 12 14
      (Sevin XLR Plus) 1-1.5 qt 12 14
      COMMENTS: Do not apply 15 days before or after application of the herbicide propanil.
    Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.
    1 Rotate chemicals with a different mode-of-action Group number, and do not use products with the same mode-of-action Group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a Group number of 1B; chemicals with a 1B Group number should be alternated with chemicals that have a Group number other than 1B. Mode-of-action Group numbers are assigned by IRAC (Insecticide Resistance Action Committee).
    * Permit required from county agricultural commissioner for purchase or use.
    Text Updated: 02/09
    Treatment Table Updated: 10/15