Protection of Rice from
Invertebrate Pests-99

 

 

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Project Leader and Principal UC Investigators

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

Terry D. Cuneo, Postgraduate researcher, Dept. of Entomology, UC Davis

Research to protect rice from invertebrate pests is concentrated on the Rice Water Weevil (RWW). With the insecticide FuradanŽ slated for elimination this year as the standard treatment for RWW, there is a heightened interest in alternative insecticidal products, refined cultural controls and knowledge of this pest’s biology. Researchers report significant progress toward all objectives in 1999. Highlights of their work is summarized below.

RWW Flight

The timing of the adult flight of Rice Water Weevil has been monitored at the Rice Experiment Station for more than 35 years. This information helps to track yearly intensities and intervals between peak flight periods. Cool spring weather affected RWW populations in 1999, as it had in 1998. However, rainfall was much lower in 1999.

Peak RWW flights at the RES occurred April 20, May 3 to May 5, May 12 and May 21. A total of 1,069 weevils were captured, slightly lower than the 1,206 captured in 1998. At additional monitoring sites in Butte and Colusa counties, peak flights occurred between May 3 and May 7. Ninety percent of the flight was completed by May 30 compared to an April 30 date in 1998. This relatively late date reflects the prolonged flight period in 1999 caused by abnormally cool weather. The first peak occurred when no rice had been planted in the area. The second was during the middle of planting time.

Chemical Controls

rice 36a.jpg (31517 bytes)Twenty treatments involving numerous formulations of five synthetic chemicals and one biorational insecticide were tested on M-202 rice in aluminum ring plots at the Rice Experiment Station.  Two of the materials – DimilinŽ and WarriorŽ – are newly registered as replacements for FuradanŽ 5G. Numerous formulations and application timings of the six active ingredients were used. Several of these treatments were a continuation of testing conducted in 1998 to further refine timing and formulation.

DimilinŽ 2L and WarriorŽ are applied post-flood to either the full basin or along borders only. Since the heaviest rice water weevil concentrations occur within 30 to 50 feet of fields adjacent to levees, researchers sought, as in 1998, to determine the effectiveness of border-only applications. Tests were conducted in 10 locations of several acres each.

Unseasonably cool spring and early summer weather made predicting the behavior of pest infestations very difficult. With the exception of a DimilinŽ+WarriorŽ tank mix, treatments of each chemical alone exceeded acceptable economic thresholds for RWW damage and would have required a second treatment for optimal control. Larval numbers were clearly higher in the mid-July samples compared to early July. Researchers attribute these higher larval counts to cooler temperature that delayed the RWW lifecycle and caused a subsequent invasion into the field after application of post-flood treatments. The egg-laying period was thus prolonged.

Machine yields for these treatments (pounds/acre) averaged 9,021 (DimilinŽ-full), 8,350 (DimililnŽ border), 7,720 (FuradanŽ), 9,021 (DimilinŽ+WarriorŽ) and 7,984 (untreated). WarriorŽ treatments on borders only at three grower fields saw slightly higher RWW larvae concentrations and would have required another application for better control. IconŽ results were comparable to FuradanŽ 5G in small plot studies. 

Overall, larval populations for most grower field sites in 1999 exceeded economic threshold limits. More studies are needed to refine timing of post-flood applications and to examine the number of necessary applications.

pest 26.jpg (15798 bytes)MustangŽ, NovodorŽ and IconŽ — products that may in the future afford additional flexibility for RWW control — were also tested in aluminum ring plots. They provided excellent control. NovodorŽ, a biorational agent, was less effective than in 1998 but researchers believe it was a temporary product quality problem. The influence of application timing on DimilinŽ 2L was intensively examined.  Applications one day after rice emergence through the water performed the best out of six treatments, but all scored below economic thresholds.

Resistance Study

Insects have the ability to rapidly develop resistance to insecticides. Several factors influence how fast this occurs, including characteristics of the insect (number of generations per year, mobility etc.) Carbofuran (FuradanŽ) has been the only insecticide used for rice water weevil control since 1974. The product still provides good control, although some growers report that the control is not as good as it used to be. A dose-response study confirmed that it took twice as much carbofuran to kill RWW as it did in 1969.

RWW Biology

The pattern of RWW egg-laying is important for determining the timing of post-flood applications. Since DimilinŽ and WarriorŽ have minimal activity on RWW larvae, they must be applied before significant egg deposition. RWW egg laying patterns were monitored in 1998 and 1999 in 12 and nine grower rice fields, respectively. Seedlings were collected two to three times a week and taken to a laboratory, where they were analyzed for the prevalence of RWW eggs. Water depth and rice leaf stage were recorded. Egg deposition began as early as the 1-2 leaf stage and peaked from the 3-4 leaf stage in 1998 and 3-6 stage in 1999. The cool spring temperatures in 1999 allowed the RWW adults to survive and to deposit eggs for a longer period. With an egg-laying period in excess of three weeks, a short-lived chemical such as DimilinŽ or WarriorŽ would not persist long enough to control late egg laying. However, larvae arising from eggs deposited after plants have obtained some growth will probably not significantly affect plant growth and yields. This, as well as the timing of the beginning of egg-laying, will be critical for the effectiveness of post-flood treatments.

Cultural Controls

Previous small plot research indicated that lower RWW larval populations occurred in fields that had been winter flooded, so researchers expanded the scope of the study to grower field locations.

The original plan was to find paired (flooded and non-flooded) fields. As in 1998, however, spring precipitation in 1999 resulted in many rice fields being inadvertently "winter flooded." One of these studies took place at the long-term straw management study site near Maxwell. Fewer RWW were evident on the winter-flooded site compared with the treatment not flooded during the winter. Similar results — fewer RWW larvae in the winter-flooded field — occurred in four of the seven paired grower field comparisons. Two of the seven comparisons showed no differences in RWW level between winter-flooded and non-flooded fields.

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