Invertebrate pest research in 2006 concentrated mostly on rice water weevil (RWW) and armyworms, a pest of increasing concern. The goal is to refine Integrated Pest Management techniques for both of these pests. Three areas influenced the direction of 2006 research:

• Best Management Practices have been developed by the rice industry to aid in the mitigation of mosquito populations, an area of great concern with the arrival of West Nile virus in California. To address this issue a study continued on the effects of registered and experimental rice insecticides on non-target invertebrates in rice fields.
• The California Department of Pesticide Regulation is reevaluating pyrethroid insecticides because of their propensity to move off-site in sediment. Thus continuing studies to develop alternative active ingredients and chemistries for invertebrate pest control are crucial.
• Armyworm populations at damaging levels have become more common in recent years. Studies are investigating the biological reasons for this upswing.

RWW flight and biology

The 2006 RWW infestation was heavy – 2,300 RWW adults captured at the Rice Experiment Station. That’s more than double the previous year. The flight occurred steadily from April 25 to May 17. Periods of high flight intensity within that timeframe occurred April 25-28, May 1-3 and May 11-17. These initial flights occurred before much rice was seeded, but the weevil adults can feed and survive on grassy weeds, which were common.

Although incremental increases in rice plant tolerance to RWW have been made, there are currently no rice varieties that are resistant to RWW. A continuing study evaluated 12 rice varieties for susceptibility to RWW infestation and rice yield loss. There were significantly more larvae in M-206 than M-205, M-206, M-104, S-102, and the PI plant line than in Calhikari-201. Populations were not high enough to impact grain yield.

Refined rice seedling establishment techniques are being investigated at the RES primarily as a means to improve weed management through stale seedbed and dry seeding techniques. However, these techniques will also likely affect insect pest populations and also mosquitoes. In 2006 all three water-seeded treatments had 1.4 to two times more scarring than conventional water seeding. Larval populations were higher in the drill-seeded than the water-seeded treatments.

RWW chemical controls

Ring plot studies in 2006 evaluated experimental and registered insecticides for RWW control, as well as efforts to refine application methods. Altogether, 10 different active ingredients in 24 ring plot treatments were examined.

Three experimental products – etofenprox, indoxacarb and clothianidan – all show significant potential for RWW control but are a few years from registration. Indoxacarb is best in post-flood applications, whereas clothianidan has the most flexibility in application timing – good RWW control as a seed treatment, in soil, pre-flood and at three-leaf stage of rice. Results with a preflood application of etofenprox were more mixed but show consistently excellent RWW control at three-leaf.

Given the environmental concerns regarding pyrethroids, it is important to develop alternatives. In 2006 these experimental products applied with various rates and application methods provided 95 percent or greater RWW larvae control and excellent grain yield. Two additional active ingredients – DPX-E2Y455 and V10194 – were evaluated for the first time and show promise.

The registered standards – Warrior®, Mustang®, Dimilin®, and Proaxis® – all provided excellent RWW larval control. A soil application of pyrethroids would provide some flexibility to growers and may provide a greater buffer to non-target effects. Preflood applications of Warrior® and Mustang Max® were evaluated and found effective against RWW.

Work on a biological insecticide – Azadirachtin®, extracted from the seeds of neem trees – was deemphasized in 2006, with only one greenhouse test. This study sought to explain inconsistent 2005 results in field and greenhouse studies. Overall, plants treated with the highest rate had about 20 percent fewer scars than untreated plants. A foliar application of this compound was found more effective against RWW than the preflood application. Also, a liquid formulation (Aza-Direct) was more effective than a granular product. A biological product of this type would be advantageous if shown to be effective.

Non-target studies

Small plot studies also evaluated the effects of insecticide treatments in rice on populations of non-target invertebrates. Preliminary results from both 2005 and 2006 show that preflood applications of Warrior® had minimal effects on aquatic insects. In post-flood treatments, slight to moderate effects were observed during the first two weeks after application. Reductions were most severe with dinotefuron and Mustang Max® and intermediate with indoxacarb, etofenprox and Warrior®. Dimilin® and Azadirachtin® had little effect on aquatic insects. Dinotefuron was the only product that reduced aquatic insects for three weeks. Although some of the reductions were in the 70 percent range, insect populations rebounded during the season. Warrior®, evaluated for possible mid-season treatment against armyworms, reduced aquatic insects by 70 percent for the first week only.

Armyworm studies continue

Armyworms have developed into a significant rice pest in the last five years. In some areas a mid-season insecticide treatment is now common.

Two species of armyworms are present in the Sacramento Valley – the western yellow-striped armyworm (Spodoptera praefica) and the “true” armyworm (Pseudaletia unipuncta). They seem to be adapting to the rice agroecosystem and are becoming a more significant threat. Armyworms damage rice through defoliation and by feeding on panicles and rice kernels. The latter damage is much more important than is simple leaf removal.

Growers often report that the only way they know an armyworm infestation is occurring is when they see birds staying and feeding in an area. This is useful, but the presence of birds means that the armyworms may have already done some damage. This would be especially important if the armyworms were panicle feeding.

It appears that higher armyworm populations are present in fields with a higher incidence of weeds, particularly broadleaf weeds. Research investigated this relationship in plots with varying amounts and types of weeds but results were more inconclusive in 2006 than in the previous year.

In 2006 the true armyworm flight peaked in early July and again in mid-August. The Western yellow-striped armyworm flight was lower in 2006 than the previous year. Information from pheromone traps, coupled with knowledge of temperature influence on pest lifecycles, would be an effective forecasting tool. This would pinpoint when larval populations would develop. Research also examined parasitism of armyworm larvae with wasps.

Exotic pest education

California does not have to contend with many exotic pests found elsewhere in the United States and western hemisphere that could prove extremely harmful to Central Valley rice. Pests of particular concern include the rice stinkbug, Mexican rice borer, sugarcane borer, South American rice leafminer, and apple snail. All these pests are present in the Southern U.S. and an educational program is seeking to keep them out of California.