Friday, September 24, 2021

Watch for armyworms and stinkbugs invading rice fields

• By Nick Bateman, Gus Lorenz and Ben Thrash •

fall armyworm feeding on rice
Armyworms defoliating rice — photos courtesy University of Araknsas

Armyworm calls have picked up big time this week. A majority of the fields with armyworms in them is row rice at various stage, spanning from early tiller all the way up to half-inch joint movement.

Some of these fields have high numbers of worms in them, 2-4 per plant, and a large amount of defoliation occurring.

Our threshold for defoliation in rice is: 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.

Our work over the past four years has shown that the later we plant the worse yield is impacted from defoliation (Table 1). Armyworms have the potential to do large amounts of defoliation in a short amount of time.

fall armyworm at green ring
Table 1. Yield and maturity delays caused by defoliation at green ring.

Fields with armyworms in them should be monitored closely to ensure we do not exceed these defoliation thresholds. If applications are warranted, we are recommending lambda-cyhalothrin (Warrior II, Lambda-Cy, ect.) plus Dimilin 2 L. We have seen a lack of control in some cases with straight lambda. The Dimilin will also provide some residual.

Rice stink bugs

Now that we have rice reaching 50%-75% heading, we are starting to spray for rice stink bugs. Numbers have ranged from right at threshold to 30-40 on 10 sweeps.

lambda efficacy shart for stink bugs
Table 2. Control of rice stink bug with lambda-cyhalothrin. (Click on chart to enlarge.)

Based on our assays from this year (Table 2), we are getting adequate control from lambda. With that said, we need to follow up these applications within three to four days to determine the efficacy.

It is common to see adults shortly behind an application, and this is typically new stink bugs moving in. If nymphs are found shortly behind an application, that is a sign of potential resistance issues.

We have seen some issues with lambda late in the heading season (late August through September) but none during July and August. If you have concerns that you missed stink bugs with lambda, please contact one of us so we can conduct assays.

Drs. Nick Bateman, Gus Lorenz and Ben Thrash are University of Arkansas Extension entomologists.

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