Protection of Rice from
Invertebrate Pests - 96



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Project Leader and Principal UC Investigators

Larry D. Godfrey - Extension Entomologist Associate Entomologist Department of Entomology UC Davis

Terry D. Cunco - Postgraduate Researcher Department of Entomology UC Davis

Most research in this ongoing project is directed toward efforts to control rice water weevil. In 1996 entomologists conducted tests on three insecticides that are moving toward registration – good news with the loss of Furadan for RWW control. A second important objective is the study of rice water weevil damage on M-202 growth, development and yield. These researchers also monitor seasonal trends in the flight activity of rice water weevil. This section of the report summarizes some of their important findings.

A Large RWW Flight

The timing of the RWW flight has been monitored with a light trap at the Rice Experiment Station since 1962. Monitoring the levels and intervals of peak flight periods over the years in important to establish a baseline on the intensity of the spring weevil flight.

In 1996 peak flights occurred at the RES on April 25 and again from April 29 to May 1. Secondary flights also occurred on April 5, April 22 to 24, May 10 to May 12, May 19 and 20, and from May 24 through May 27. Although 90 percent of the weevil flight at the RES was complete by May 1, researchers noted a 40 percent increase in the flight over the previous year. A total of 5,420 weevils were captured at the RES during 1996 30 times more than that caught just five years ago.

The flight timing in 1996 was much earlier than in the previous year, but this may be somewhat misleading since the extremely heavy flight meant that the remaining weevils could still create a significant infestation. The period of flight was very long - extending from April 5 to June 6. The cool, wet spring weather in 1996 delayed the emergence of some RWW adults from overwintering sites and therefore a significant portion of the spring flight.

New Chemical Controls

Entomologists conducted several studies during 1996 to evaluate the efficacy of several chemicals for rice water weevil control. Given the pending cancellation of Furadan usage, there is an urgent need for alternative methods of managing this insect pest.

Tests were done in ring plots and small basin studies at the Rice Experiment Station and in grower fields in Butte, Placer and Sutter counties. Researchers examined numerous formulations of four chemical insecticides - Regent (Fipronil), Dimilin (diflubenzuron), Karate (Lambda-cyhaloffirin), and Furadan (carbofuran). All showed good RWW control.

Invert1.jpg (103238 bytes)In each plot researchers examined a number of indicators of rice water weevil activity - larval density, leaf scarring from adult weevils, seedling vigor and emergence, rice plant growth characteristics, and rice grain yield. Twenty-one of 23 treatments in ring tests provided good to excellent control with larval densities less than one larva per plant. Treatments of Regent 7OFS and postflood Karate 1E were among the most effective formulations for reducing larval populations. Grain yields ranged from an estimated 6,350 pounds/acre to 9,000 pounds/acre. Yield was lowest in untreated plots and highest in the Regent treatments.

Small basin tests were designed to evaluate viable treatments on a more commercial scale. RWW density was low. However, researchers could not augment the basins with more RWW adults as they did with the smaller ring plot studies because of the large plot size. Larval density ranged from an average 1.1 in the untreated plot down to zero in various Regent, Dimlin and Karate treatments. There were no significant differences in rice grain yield among the various treatments.

In six grower fields researchers compared Dimilin and Furadan 5G. On average Dimilin performed equal to or better than Furadan in terms of larval control and yield. These results were more favorable than in the previous year because researchers have refined rate, formulation and timing for this product. In one grower field test, performance of Regent was comparable to Furadan. Scientists noted substantial larval control and yield protection compared to an untreated field. Knowledge gained from a greenhouse study at UC Davis contributed to the improved use pattern for Dimilin. The "window" of application with Dimilin, for instance, is longer and applications should be timed earlier than previously thought.

Application Timing "Imperative"

Entomologists examined the egg-laying habits of RWW in relation to plant growth, noting that this aspect of the pest's biology is taking on increased importance. Two of the possible three replacements for Furadan would be applied postflood. Thus it is imperative that the treatment be properly timed in order to achieve effective control.

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UC Entomologist Larry Godfrey

From samples taken at the three-leaf rice stage a significant number of RWW eggs were found - an indication that egg laying occurs much earlier than previously thought. This was a surprising finding because the common belief is that migrating RWW adults move into rice fields at the time of plant emergence through water. A three- to four-day period is needed for the females to mature eggs before laying. Therefore, entomologists would not expect to find eggs in the field until about the five-leaf stage. Based on this one-year study, however, it appears that RWW is on the move much earlier.

Another study examined the relationship between larval density and plant density - 96 vs. 150 plants per eight-foot ring plot. Larval densities were greater in plots with higher plant density. Researchers believe the lower number of roots in the lower plant density plots inhibited RWW survival.

Yield Loss Studies

Entomologists also examined the relationship among scar counts, larval density and grain yield of M-202. The majority of this work was done in grower fields with a limited number of ring plots at the Rice Experiment Station. Information on how RWW damage affects yield is useful in a number of ways. It helps support emergency pesticide registrations; agrichemical companies investigating new products; the federal Environmental Protection Agency in establishing the need for pesticide registration; growers concerned about possible yield losses; and for investigating thresholds needed for possible postflood applications.

In the ring plots researchers used field-collected RWW. Natural infestation levels were used in grower fields. Yield losses from varying levels were much greater in the range of 425 to 650 pounds per acre - than in similar investigations conducted from 1992 through 1995. Prior losses were about 200 pounds per acre. Reasons for the difference are unclear, but researchers say there appears to be significant year-to-year variation in rice plant response to RWW injury.

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