Over the past week we have gotten multiple calls on armyworms, specifically fall armyworm, showing up in crops ranging from soybean to pastures, and of course rice. There doesn’t seem to be a pattern on which fields these armyworms are targeting.
Our calls are ranging from 2-3 leaf rice up to joint movement. We have done a lot of work over the past 5 years to determine when applications need to be made for armyworms in rice.
The bottom line is rice can take a lot of defoliation before we get significant yield losses. At the 2-3 leaf stage, even with 100% defoliation, where plants were cutoff flush with the soil, we do not see a yield decline across any planting date. We have seen a similar trend at early tiller (Table 1).
Things change once we move into late tiller and green ring growth stages. We can get significant yield losses and heading delays if large amounts of defoliation (Fig 1-3) occur at these growth stages (Table 2). Keep in mind that the severity of the yield loss will be worse for later planted rice.
If you notice, the May planting date has a higher percentage of yield loss than April or June. This is related to the amount of heat units this planting received shortly after the green ring defoliation occurred. For the June plantings, it was milder at the green ring timing allowing the rice to recover more of its yield potential.
Based on this work we have set our thresholds for defoliation in rice at: No treatment is warranted for rice between the seedling and 2-3 tiller growth stages unless armyworms are able to feed on the growing point. For May and June plantings, armyworms should be treated when defoliation exceeds 40% at 5-6 tiller and 20% at green ring. During heading, treat if head clipping is occurring and armyworms are present.
If applications are needed, Lambda-cyhalothrin (Warrior II, Lambda-Cy, Silencer, ect.) is the product of choice. Keep in mind our counterparts in surrounding states have seen a few misses with lambda due to having some corn strain fall armyworms in the field, which is resistant to lambda.
We recommend spraying lambda first and scout withing 3-4 days after application. If there is still a large population of armyworms in the field, then we can come back with Dimilin.
Rice can handle a lot of defoliation before yield loss or delays in maturity occur. Let’s not spray when we don’t need to, and if we do have to let’s try to go as cheap as possible. Let us know if you need anything or need help making decision on making applications.
Table 1. Yield and days in delayed heading for different defoliation levels in April planted rice. Click Image to Enlarge
Table 2. Yield and maturity delays caused by defoliation at green ring. Click Image to Enlarge
Fig. 1. Non-defoliated rice at green ring.
Fig. 2. 33% defoliation at green ring.
Fig. 3. 66% defoliation at green ring.
