UC Cooperative Extension has augmented efforts to monitor armyworm populations in rice fields since 2015 when we observed large populations earlier in the year (June) than previously observed. Here in San Joaquin County, we began monitoring in Delta rice fields a couple weeks ago. We are cooperating with rice farm advisors in the Sacramento Valley to get a more comprehensive assessment of the populations. Figure 1 shows moth counts from last week. The Delta count is the single orange data point at about 36 moths/day. Compare that to the Sacramento Valley where there have been similarly high moth catches in Colusa and Glenn counties.
These counts illustrate that it is important to be monitoring fields for worm feeding. The UC IPM guidelines for monitoring and treatment are excerpted below. In general, at the foliar stage, if half of the sampled plants have at least 25 percent defoliation from feeding, and worms are visible, then treatment is probably warranted. At the panicle stage, if 10 percent of the panicles are damaged and worms are present, then treatment may be warranted.
Armyworm larvae will grow to full size and pupate in about 3 to 4 weeks. That said, it is important to monitor fields now and later in the summer in case the current generation reproduces and a new generation develops when panicles are present.
In 2016, UC Cooperative Extension worked with the California Rice Commission to get an emergency approval for the use of Intrepid 2F (methoxyfenozide) in affected rice counties, including San Joaquin County. The 2017 application has not yet been approved, so we do not currently have this product available for rice. Please see the UC IPM guidelines for a list of registered products. I will provide an update on future moth trapping counts and product approvals as the information becomes available.
From UC IPM: Rice Armyworm Monitoring and Treatment
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.