Invertebrate pest research seeks effective, yet environmentally sensitive, management for pests of California rice. The major pest of concern is Rice Water Weevil (RWW). Research was also conducted on two early-season pests of seedling rice – seed midge and tadpole shrimp. Limited studies also continued in 2007 on armyworm. Four overall themes influence current research on invertebrate pest research:

• Pyrethroids—The California Department of Pesticide Regulation is reevaluating pyrethroid insecticides because of their propensity to move off-site in sediment. Studies continued on insecticides with alternative active ingredients and chemistries.
• Mosquitoes—Best Management Practices to help control mosquitoes have been developed for rice farmers. A study continued on the effects of rice insecticides on non-target invertebrates, which is important for mosquito control. Changes in seeding methods were studied for their impact on mosquito populations.
• Early-season pests—Early season invertebrate pests were more common in 2007 than in recent years. To respond to grower questions about control of these pests, studies were done on seed midge and tadpole shrimp control.
• Exotic pests—Exotic pests moving into California are becoming a more common occurrence. The rice panicle mite, a potentially serious invertebrate pest, was found in rice grown in the Southern U.S. in 2007. Thus far the mite has not been found in California. Grower education about this pest is a priority.

RWW flight and biology

Studies of RWW biology 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 2007 RWW infestation was significant – 2,700 RWW adults at the Rice Experiment Station. This was 15% increase over the previous year. RWW flight was recorded on only 13 of 76 nights and was concentrated between April 26 and May 8. It’s worth noting that two-thirds of the seasonal total occurred on May 7 alone, and another 30% was captured on April 26-27.

Twelve rice varieties were compared for their susceptibility to and yield loss from RWW in field plots with moderate infestations. M-104, M-206, M-202, and S-102 had the highest levels of RWW leaf scarring, an indication of infestation levels. These varieties, along with Calamylow-201, also had the highest larval populations. S-102 was most affected, with a severe negative yield loss to RWW infestation. Calhikari-202 and Calamylow-201 were moderately affected. The PI line, M-206, and M-202 showed slight yield loss. The other six varieties showed no yield losses across the levels of RWW present in these plots.

In ring plots infested with RWW, one third as many larvae survived on Calmati-202 compared to the other three varieties, but grain yield losses were highest in Calmati-202 on a loss per larva basis.

Refined rice seedling establishment techniques are being investigated at the RES, primarily for improved weed control. However, these techniques will likely affect insect pest populations and mosquitoes. Leaf scarring from RWW adults was higher in all the other seedling establishment treatments than conventional water seeding. Larval populations were much higher in the drill-seeded conventional than the water-seeded conventional treatments. Both stale seedbed treatments (drill- and water-seeded) had slightly fewer larvae than conventional water-seeded rice.

There were about 20 percent more mosquito larvae in the drill-seeded, stale seedbed, no-till treatment than conventional water-seeded treatment and only half as many larvae in the water-seeded, stale seedbed, spring-till treatment.

RWW chemical controls

Ring plot studies in 2007 evaluated experimental and registered insecticides for RWW control, as well as efforts to refine application methods. Eight different active ingredients were examined in 20 treatments. Rice seeds were soaked in Clorox® to control Bakanae disease.

Research continued on four experimental products – etofenprox (Trebon®), indoxacarb (Steward®), rynaxypyr, and clothianidan. In summary, etofenprox, indoxacarb, and clothianidan all appear to have significant potential for RWW management but are a few years away from possible registration. Indoxacarb is applied post-flood. Clothianidan has the most flexible application timing. Given the environmental concerns regarding pyrethroids, it is crucial that research continue on alternative active ingredients.

In these studies, Dimilin®, Warrior®, Trebon®, V10170, Mustang Max® and Steward® significantly reduced RWW population (more than 90%) compared to untreated plots. The best control was a 95% reduction in larval populations.

The Clorox® seed soak greatly affected the activity of V10170. The soaked treatment was largely ineffective. Without the Clorox® soak, activity was good. This had not been observed previously.

The E2Y45 (rynaxypyr) seed treatment was largely ineffective, although these seeds were also soaked in Clorox®. Further evaluation of active ingredients without seed soak treatment is warranted.

Among the pyrethroids, only Warrior® was evaluated as a preflood application in 2007. Warrior® was registered for use in 2007 with preflood applications. Early application of Warrior® (one week before flooding) provided about 50% control, whereas application immediately prior to flooding was as effective as application at three-leaf stage of rice. Environmental conditions between application and flooding undoubtedly influences control and may have been a factor in these trials.

Aza-Direct®, a liquid form of the neem-based product containing azadirachtin, was evaluated and provided RWW control in the 50% range.

Non-target studies

The effects of insecticide treatments in rice on non-target invertebrates and mosquitoes are also being studied. Research in this area is important because of the serious human health threat posed by mosquito-vectored West Nile virus. The diverse organisms present in rice fields help keep mosquitoes under control by feeding upon their aquatic stages. Research in this area is developing a good understanding of the effects of registered insecticides on non-target organisms in rice fields.

A study was conducted in 2007, but sorting and counting results were not yet available at the time of this report. From 2006 studies, a preflood Warrior® application reduced populations of aquatic insects compared with the untreated for the first week after application but not thereafter. Seven treatments applied at three-leaf stage were compared and V10170 and Mustang® reduced aquatic insect populations at two and three weeks after application. Averaged over the five-week period following application, all the treated plots had levels of aquatic insects equal to or greater than in the untreated. Warrior® was evaluated as a material that could be applied against armyworms in July. This treatment was very damaging to aquatic insects at one and two weeks after treatment, but populations bounced back to levels of the untreated plots.

Armyworm biology

Two species of armyworms – the western yellow-striped armyworm (Spodoptera praefica) and the “true” armyworm (Pseudaletia unipuncta) – seem to be adapting to the rice agroecosystem. Larval populations and damage were monitored in two studies at the RES but were too low to draw meaningful conclusions. Moth flights were monitored using pheromone traps. True armyworm flight peaked in early August. The Western yellow-striped armyworm flight was low overall with no real peaks.

Early-season pests

Tadpole shrimp occur in many rice fields with damage running from mild to severe. In recent years, there have been reports that copper sulfate is not reducing shrimp numbers as well as it had previously. This may be caused by the interaction of rice straw residue with copper sulfate.

Seven registered and experimental insecticide treatments were evaluated in ring plots infested with tadpole shrimp. Stand counts were lowest in the copper sulfate treatment and highest in the Dimilin® treatment. There were no significant yield differences among treatments.

Seed midge populations were unexpectedly high in spring 2007. This pest is generally worse during cool springs when rice is struggling to germinate, conditions that were not present last year. The midge is fairly common in and around rice fields. Once a field is flooded, the midge adults drop eggs into water. Within a few days of hatching, larvae drop to soil, form a mud tube and begin feeding on germinating seeds. Larvae can hollow out seed and also feed on emerging shoots, leaves, and roots. Once seedling spike is several inches long, rice is tolerant of damage.

The unpredictable nature of the seed midge makes it difficult to study. In one ring-plot experiment a field was flooded and seeding delayed to facilitate midge populations. There were no significant differences in stand counts or yields among treatments in this study and no clear sign that the pest was present.

Vigilance for exotics

Researchers maintain a vigilant watch for exotic pests through numerous visits to rice fields through the season. The rice panicle mite is a potentially extremely serious pest that is present in Asia and Central and South America. It was discovered in Texas in July 2007 and later in the season in Louisiana, Arkansas, New York, and Puerto Rico. Some 20-30 suspect samples from California rice fields were examined for presence of the panicle mite from August through October but none was found.I