Research into the biology and management of invertebrate pests of California rice is the mission of this ongoing project. The major pest of concern continues to be the Rice Water Weevil (RWW). However, research is intensifying on two early season pests – seed midge and tadpole shrimp. Studies of armyworm, an important pest in recent years, were hindered by the lack of armyworm infestations in 2008.

Effective integrated pest management practices are developed with an awareness of and concern for the environmentally sensitive nature of the rice agroecosystem and for the cost effectiveness of pest management recommendations. Major influences on the direction of the research program in 2008 were the need for alternatives to pyrethroid insecticides;  mitigation of mosquitoes without harm to beneficials; control of new early season pests; and grower education about new exotic pests.

RWW flight and biology

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

The 2008 RWW infestation was significant – 2,300 RWW adults at the Rice Experiment Station, comparable to the previous year. The RWW flight was fairly spread out in 2008. The first flight peak was on April 11, followed by April 25, and May 2-16. A few RWW adults were trapped even as late as June 12. The last three years the population has been fairly constant.

Studies continued on rice varieties for their susceptibility to and yield loss from RWW in field and ring plots with moderate infestations. Larval infestations were lowest in an experimental line under development for resistance to RWW. Control of RWW resulted in a yield increase in eight of the 12 varieties. M-205 showed the greatest yield advantage at about 1,800 pounds/acre over other varieties. Overall, control of RWW resulted in a 240 pounds/acre grain increase averaged over varieties.

In the more controlled ring study, RWW larval populations were lower in L-206 than in M-202 and in the RWW-tolerant experimental line.

Alternative seeding and establishment methods are being designed to open up new opportunities for weed management in rice. Monitoring these changes is important to ensure there is no impact on pest management. At one RES study site, RWW infestation was low. The highest plant scarring was 27 percent, but 18 of the 20 plots had less than 5 percent scarring. Larval populations were also low. Mosquito populations on September 4 were lower in both no-till, stale-seedbed treatments than in standard water-seeded treatments.

RWW chemical controls

Ring plot studies in 2008 evaluated experimental and registered insecticides for RWW control, as well as efforts to refine application methods. Eight different active ingredients were examined in 24 treatments.

There were no significant differences among treatments in seedling vigor and emergence. Nor was any phytotoxicity observed in any treatments.

Research continued on four experimental products – etofenprox (Trebon®), indoxacarb (Steward®), rynaxypyr (Dermacor®), and clothianidan. The first two products were applied at the three-leaf stage of rice and the latter two products as seed treatments. A new experimental product, HGW86, was very effective against RWW as a seed treatment.

V10170 (clothianidin) is a promising experimental product that practically eliminated the RWW infestation when applied preflood and at three-leaf stage, regardless of rate. As a seed treatment, it was moderately effective.

In summary, etofenprox, indoxacarb, rynaxpyr, and clothianidan all appear to have significant potential for RWW management. However, the manufacturer of indoxacarb is not pursuing registration of this product. A sodium hypochlorite seed soak for Bakanae control reduced the effectiveness of rynaxpyr and clothianidan. Etofenprox may be registered in time for the 2010 season.

The pyrethroid product Warrior® applied immediately before rice field flooding was very effective for RWW control. The label stipulates application at five days or less before flooding.

Rynaxpyr seed treatment was more effective against RWW in a water-seeded system than in a drill-seeded system.

Grain yields were significantly higher in treatments with Warrior® at three-leaf, clothianidan at three-leaf, and with Steward® and rynaxpyr than in untreated plots. Yields in several other treatments were 1,000 pounds/acre higher than in untreated plots.

Non-target studies

The effect of insecticide treatments in rice on non-target invertebrates and mosquitoes continues to be an important area of research because of the serious human health threat posed by West Nile virus. The diverse organisms present in rice fields help keep mosquitoes under control by feeding upon their aquatic stages. Agronomic activities such as field draining, straw incorporation, fertilization, and insecticide application can alter the populations of these beneficial insects.

Several insecticides and timings, including two preflood, six three-leaf stage, and one mid-season application, were investigated. After initially reducing aquatic insects in the first two weeks after application, populations bounced back in five of the six treatments – as much as five times more than in untreated plots. At four weeks after treatment, mosquito levels were higher in four of the treatments and were equivalent in two treatments compared to untreated.

Work will continue in this area to modify best management practices to minimize mosquito populations as new research findings suggest.

Early season pests

Tadpole shrimp problems appear to be on the increase as copper sulfate (bluestone) use has declined. Tadpole shrimp populations tend to build up gradually without control measures.

Three studies were conducted on tadpole shrimp in 2008. A bioassay of an organic formulation of pyrethrins (from chrysanthemum flowers) called PyGanic® 5% EC was conducted as a first step to evaluating its efficacy. In the laboratory, this product definitely showed activity on tadpole shrimp. Two ring-plot studies were also conducted with preflood and post-flood products. Shrimp damage was observed, although grain yields did not differ significantly among treatments.

Seed midge populations are spotty, but this pest can be devastating to rice stands under the right set of circumstances. Cool air and water temperatures and a period of delayed rice germination and establishment are conducive to midge growth. Swarms of midges were common in 2008. However, that did not necessarily translate into field damage. Because of its unpredictability, this pest is difficult to study.

Two standard midge treatments – Mustang® and Warrior® – were examined, as well as an experimental seed treatment and an experimental post-flood material. However, there was no evidence of an infestation.

Vigilance for exotics

Researchers continue a vigilant watch for exotic pests through numerous visits to rice fields through the season. The panicle rice mite, a potentially serious pest, was discovered in Texas in July 2007 and later in the season in Louisiana, Arkansas, New York, and Puerto Rico. Some greenhouses at UC Davis were found to be infested with the panicle rice mite in early 2009 and were sterilized. However, this pest has not been identified in any California rice fields.