RWW flight and biology

Studies of RWW biology continued to focus on adult infestation levels, relative susceptibility of commonly grown rice varieties, and the influence of rice seedling establishment methods on RWW population severity.

The 2009 RWW infestation was one of the most significant in the last 10 years. About 6,150 adult RWW were captured in the blacklight trap at the Rice Experiment Station. That compares with 2,300 RWW adults the previous year. The RWW flight was concentrated in two periods: April 17-21 and May 8-15. On April 17 alone, more than 2,500 RWW adults were captured, more than twice the entire season total for 2002 and 2004.

Studies continued on 12 commercial rice varieties for their susceptibility to RWW infestation and damage. Soon after seedling emergence, plant scarring by RWW adults varied from 10.5% in Calmati-202 to more than 30% in M-208, M-104, M-205, and M-202. However, just a week later, scarring severity ranged from 30% to 40% for all varieties. The RWW most likely went after the more vigorous varieties first. These varieties are most likely of equal susceptibility to adult RWW.

Numerically, M-104 had the highest larval infestation. Overall, the medium grains tended to have more weevil larvae than Calhikari-201, Calmochi-101, Calamylow-201, and S-102. RWW populations declined significantly from the first to the second sampling (about two weeks later) in M-104 and Calmochi-101. This could be an anomaly or an indication of something peculiar to these varieties.

Efforts to control RWW resulted in yield increases compared to untreated control plots in five of the 12 varieties. Calmati-202 displayed the greatest advantage: 1,350 pounds/acre. A significant correlation was observed between RWW density and grain yield in some varieties. Estimated percentage yield loss per larva was 13.9% in Calamylow-201, 25.1% in Calmati-202, 12.2% in M-206, 1.3% in M-208, and 12% in S-102.

Alternative seeding and establishment methods for rice production have implications for weed management, water quality, and water usage. Most invertebrate pests of concern to rice growers are also influenced by water levels. These include seedling pests such as seed midge and tadpole shrimp. RWW adults always lay their eggs in the rice sheath just below the water line. Armyworms, on the other hand, are probably not as directly affected by rice field flooding.

In 2009, three different stand establishment methods were studied for impact to RWW populations. These included drill-seeded, no-till stale seedbed; water-seeded conventional; and water-seeded, no-till stale seedbed. The drill-seeded option had the highest larval population on July 14 at 1.3 per sample. The other two treatments had about half as many larvae. However, by July 28 the levels were comparable to each other.

The influence of water depth on RWW populations during the first six weeks after seeding was also examined. RWW populations were four times higher in rice grown with three inches of flood, compared to six to nine inches. Interestingly, similar studies in the southern U.S. rice areas showed the opposite result with more RWW larvae in the deep-water flood than in the shallow flood (in the drill-seeded, delayed flood production system used in this area).

RWW chemical controls

Studies in 2009 were conducted in ring plots, small basins, and quarter-acre plots to evaluate experimental and registered insecticides for RWW control.  Seven different active ingredients were examined in 24 treatments. The unusually high RWW infestation in 2009 allowed for a real “acid test” of the different treatments.

The standards – three-leaf applications of Mustang®, Warrior®, and Dimilin® – all performed well. Warrior® and Mustang® applied day of flooding were also highly effective. “Stressing” the product by applying three days prior to flooding showed the performance of Mustang® may be hindered more than Warrior®. (The pre-flood treatment is a registered use for Warrior® but not Mustang®.)

Of the experimentals, clothianidin (V10170) appears to have significant potential for RWW management. Applied preflood and at three-leaf, it afforded 95% to 98% RWW control. It would also be worth evaluating at five-leaf stage as a “rescue” treatment.

Results of etofenprox (Trebon®) applied at three-leaf and in ring plots are promising. However, this product should be evaluated under grower field conditions before efficacy can be conclusively established.

Seed treatments of rynaxypyr (Dermacor®) and cyazypyr (DPX-HGW86) were moderately effective, at best. However, cyazypyr has been evaluated only at two rates for two years, with contrasting results. Thus, conclusions about its efficacy would be difficult to draw. Rynaxypyr applied preflood in 2009 was effective, so the seed treatment method seems to limit its performance. The performance of seed treatments is additionally compromised by the use of a sodium hypochlorite seed soak, which is used to manage for the Bakanae fungus.

In quarter-acre plots with a natural weevil infestation, Dermacor® was not effective, V10170 was moderately effective, and Trebon® was very effective. Given the reevaluation of pyrethroid registrations, it is important to continue developing alternative active ingredients and classes of chemistry. These unregistered active ingredients have some favorable properties in regards to non-target organisms.

Tadpole shrimp

Seven insecticide treatments were compared for control of tadpole shrimp. Rice stands in ring plots ranged from 83 for a copper sulfate treatment to 58 in a PyGanic® treatment. Grain yields ranged from 4,780 pounds/acre to 7,300 pounds/acre. Yields in untreated plots were intermediate and did not differ significantly from the other treatments. Yields were higher numerically in the two Warrior®-treated plots (pre- and post-flood). Yield reductions likely came from a combination of RWW damage and tadpole shrimp damage. Tadpole shrimp were introduced into the plots; but with a heavy RWW flight in 2009, there was also a moderate naturally occurring weevil
infestation.

Non-target studies

Nine insecticide treatments were compared for their effects on populations of non-target invertebrates and their potential to upset naturally occurring mosquito management.

Three separate preflood applications of Warrior®, V10170, and DPX-HGW86 were evaluated. Warrior® greatly reduced populations of aquatic insects and other invertebrates, but the effect was short term. By 24 to 31 days after application, population levels rebounded. The other two products had no effect on populations of non-target insects. In postflood applications at three-leaf stage, impacts on aquatic invertebrates were greater than on aquatic insects.

At 10 days after treatment, all five products significantly reduced aquatic invertebrate levels by an average of 63%. Later samplings showed no strong reductions. For aquatic insects, the greatest reductions were at 17 and 24 days. These reductions were moderately severe but were not seen in later samples.

Warrior® was also evaluated for its fit as an armyworm control material. At two and three weeks after application, this treatment was very damaging to populations of aquatic insects. Other invertebrates were similarly affected at two weeks after treatment.

Panicle rice mite

In January 2009, panicle rice mite was discovered in greenhouses at UC Davis. This is a serious pest in other parts of the world that is currently not established in California or other parts of the U.S. Federal, state, and university officials worked quickly to eradicate this pest. A rice-free period on campus, treatment of existing rice in greenhouses, and new procedures were enacted. The infestation in the greenhouses was eliminated, but vigilance against this pest will be required to keep it out of California.