Invertebrate Pests-92




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Project Leader and Principal UC Investigators

Robert K. Webster, professor, Department of Plant Pathology, University of California, Davis


Rice water weevil continues to cause problems for California rice growers. Research into management of this pest focuses on furthering knowledge of the weevil's biology and chemical, cultural and biological strategies for its control. Following is a summary of researchers' principal findings from 1992.

1992 RWW flight

The 1992 spring migratory flight of the rice water weevil - as measured by light traps in Butte, Glenn, Colusa and Sutter counties - began April 11 and reached its first peak between April 21 and April 24, depending upon location. Researchers characterized the '92 flight as "moderately heavy" and highly variable in both intensity and timing between trap locations. Several peaks occurred at each monitoring site during April and May. The flight was 90 percent complete by May 11 at each of the sites.

The variability in the weevil flight underscores the need to monitor the pest locally rather than regionally to get a better idea of where it is more likely to become a problem each season. Knowledge of rice water weevil flight patterns may allow growers to manipulate cultural practices or more efficiently apply carbofuran to reduce weevil damage.

Chemical Controls

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Project Leader Larry Godfrey, a UC Cooperative Extension Entomologist, sets up an "aluminum ring" experiment to test for water weevil damage at the Rice Experiment Station

Researchers tested a flowable formulation of FuradanŽ, as well as two experimental chemicals, against weevil adults and larvae. All materials were applied preflood in "aluminum ring" tests at the Rice Experiment Station.

Granular FuradanŽ and the high rate of flowable FuradanŽ reduced adult feeding scars by 44 percent and 35 percent, respectively. Only one of the experimental compounds showed encouraging results, with weevil feeding scars reduced by 57 percent.

Against weevil larvae, both experimental insecticides provided control equal to or better than the standard granular formulation of FuradanŽ. The high rate of flowable FurdanŽ provided intermediate control.

The experimental products, both from Rhone-Poulenc, are several years away from possible registration but, as the research team concluded, "their new chemistry does appear promising."

Biological Controls

On the biological front, researchers are-somewhat optimistic about a fungus, Beauveria bassiana, that has shown potential for controlling soil-borne insect pests.

Researchers tested a granular formulation of this product against weevil larvae in a greenhouse study at UC Davis. The highest rate of fungi provided about 25 percent larval control. Additionally, treated plants had larger roots and more tillers and leaves than untreated plants, indicating less larval-induced plant stress.

Cultural Controls

Research into delayed planting dates and vegetation management on the levees may offer growers other options for limiting weevil damage.

In field tests conducted in Butte, Colusa and Sutter counties, researchers found that delayed planting generally resulted in lower weevil infestations. The results were less variable than in the preceding year and tend to support earlier findings.

The pattern scientists are beginning to see emerge is that in years with earlier than average weevil flights, fields that are planted in late May or in June will see weevil. infestations below economic thresholds. However, the occasional exception to this rule indicates that infestation levels are influenced by other factors as well.

The vegetative condition of levees and other areas adjacent to rice fields is one of those other factors. Researchers again examined how the removal of levee weeds affected water weevil damage. Five of seven comparisons showed higher weevil infestations in fields with vegetation on the levees. However, for unknown reasons one test showed just the opposite effect.

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Scientists use light traps, such as the one pictured here, to monitor the timing and intensity of the rice water weevil spring migration

Another aspect of this ongoing research project concerns drill-seeded rice. Scientists compared weevil I damage in both drill-seeded and water-seeded ricewith and without a treatment of the insecticide FuradanŽ at the Rice Experiment Station. They found that drill-seeding of rice has the potential to reduce rice water weevil damage.

As indicated in previous research, the feeding activity of rice water weevil is directly affected by the presence or absence of standing water. Flooding is delayed in drill-seeded rice, which reduces adult feeding and slows down the pest's reproductive cycle. Adult weevil feeding was 70 percent lower in drill-seeded plots than in waterseeded plots. However, this marked difference disappeared within two weeks of flooding the drill-seeded field. Drill seeding also lowered larval infestations by 58 percent, most likely because of the delayed application of field water and reduced egg laying by the adult weevil.

Researchers determined that drillseeded rice without a treatment of FuradanŽ gave protection comparable to that of water-seeded rice with FuradanŽ Drill-seeded rice with an application of FuradanŽ provided the best protection of all, with sharply reduced, larval infestations -and the lowest weevil level of the four treatment applications.

Economic injury

Just what does rice water weevil do to growing rice plants and, consequently, to growers' yields? The answer to those questions will help in the determination of control thresholds and will also prove useful to plant breeders.

With a portable instrument that measures photosynthetic rate, researchers examined how three different levels of rice water weevil larvae stressed rice plants. They also looked at how plant growth is affected by weevil-induced stress. They found that rice water weevil reduced photosynthetic rate - the plant's capacity to create and store energy - by 25 to 30 percent. Similarly, weevil-infested plants grew smaller leaves and roots than uninjured plants.

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