How to Manage Pests
UC Pest Management Guidelines
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.
Organically Acceptable Methods
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.
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.
UC IPM Pest Management Guidelines: Rice