Protection of Rice From Invertebrate Pests-97



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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

Weevil plot.jpg (56304 bytes)Current research to protect rice from invertebrate pests is targeted primarily on efforts to control the rice water weevil (RWW). A major focus of this work is the evaluation of three insecticides proceeding toward registration. Because proper application timing is critical to the ability of these chemicals to control rice water weevil, researchers are also emphasizing aspects of the pest's biology and are examining yield losses under grower field conditions. The narrative below summarizes some key findings from last year's work.

RWW Flight

Entomologists have been monitoring RWW infestations at the Rice Experiment Station since 1962. Population levels have varied widely over the years. In 1997 a total of 2,556 weevils were captured at the RES about 53 percent lower than the previous year and very similar to the number caught five years ago. Peak flights occurred on April 15-16, April 26-27 and May 7. Secondary flights occurred April 6, April 30 to May 2, May 12 and May 19. The period of capture was about three weeks shorter than in 1996. Although the flight was 90 percent complete by May 7, entomologists caution that the remaining 10 percent can still create a significant infestation.

New Chemical Controls

Rice water weevil has been in California for more than 40 years. Treatment for this pest has consisted of preventive, preplant controls with FuradanŽ and other chemicals. With the uncertain status of FuradanŽ, attention has turned to three new chemicals – DimilinŽ, KarateŽ and IconŽ. The first two of these have no effect on RWW larvae, instead targeting the adult. Proper timing is imperative for these products to work adequately. Thus knowledge of when the adult RWW lays its eggs is critical in a postflood strategy. Researchers determined that RWW egg laying begins much earlier than previously thought - at the two- to three-leaf stage. This finding will influence timing of postflood applications.

Researchers tested several formulations and application timings of the new chemicals in ring plots, medium-sized basins and two to three-acre grower field blocks. In each plot researchers examined a number of indicators of RWW activity, such as larval density, leaf scarring from adult weevils and rice grain yield.

In the ring test all three new chemicals provided good control of RWW larvae, with the most effective treatment of each product reducing larval density by at least 95 percent. As indicated by plant scarring, KarateŽ provided control by killing adult RWW. DimilinŽ controlled RWW by sterilizing adults. IconŽ controls RWW larvae directly. The IconŽ ring plots produced the highest grain yields but the others also produced well.

Entomologists also set up a larger "basin" test in 1997. The tests were unsuccessful, however, because a severe crayfish infestation annihilated the rice stand.

Grower field tests were conducted in cooperation with UC farm advisors in Butte, Sutter, Placer and Colusa counties. The standard preplant FuradanŽ 5G apphcation was used for comparison (as well as untreated areas). DimilinŽ afforded moderate RWW larval control and yielded about 200 pounds/acre more than FuradanŽ and 1000 pounds/acre more than an untreated control. IconŽ provided better larval control than Furadan" but grain yield was lower. KarateŽ (also called Warrior) was not effective and had larval levels and grain yields comparable to the untreated. Delayed application of KarateŽ because of weather conditions and other factors greatly hindered its performance.

Research in 1998 will continue to refine the use of these products under grower field conditions. The activity of IconŽ, KarateŽ and DimilinŽ has been outstanding in small plots and the potential is there for a high level of efficacy in grower fields. In addition, the effectiveness of postflood treatments with a border treatment will be examined.

Yield Loss Studies

Entomologists completed a five-year study on the influence of rice water weevil on the growth, development, physiology and yield of the most widely grown California rice cultivar, M-202. Three years of intensive aluminum ring studies showed a grain yield loss of 80-100 pounds/acre for each RWW larva. Larval feeding primarily reduced plant tillering and root volume, resulting in less leaf area and, of course, reduced grain yield. RWW damage and yield loss was studied in grower fields in 1996 and 1997. Generally, RWW damage decreased as distance from levees increased. None of the eight fields had damaging RWW larval populations at distances greater than 50 feet. This bodes well for a perimeter insecticide strategy. In addition to the important yield loss information this study provides growers, this information is also used to support emer- gency pesticide registrations; by agrichemical companies investigating product development; by EPA regarding the need for registrations; and for investigating thresholds needed for possible postflood applications.

Winter Flooding Examined

Researchers also examined the influence of straw management treatments on rice water weevil populations. Winter flooded plots consistently reduce RWW larvae compared to unflooded plots. Straw tillage - burning, incorporation, rolling or baling - did not, however, influence larval density. The differences in the flooded plots could be important because they reduced RWW below economically damaging thresholds. The reason for these differences is uncertain but will be investigated in future studies. Also, straw management treatments had no effect on other early season seedling pests such as tadpole shrimp, crayfish and seed midge.

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