Information from 2000 Missouri Rice Research Update, February 2001.

Rice Water Weevil Management In Missouri

Michael L. Boyd and Jeff L. House1

News:
Aventis CropScience has expanded its Icon label to target other rice pests (ex. grape colapsis beetle). FMC Corporation received EPA approval for use of its Fury 1.5E insecticide for broad-spectrum insect control in rice. Uniroyal Chemical Company also has received EPA approval for use of its Dimilin 2L insecticide for RWW (egg stage) control.

Introduction:
The rice water weevil (RWW) is occasionally an important insect pest in Missouri rice fields. Adult feeding damage to developing leaf tissue is rarely of economic importance; however, larval damage to developing plant roots can reduce plant vigor and yields by interfering with the plant's ability to take-up nutrients and water. Severely damaged plants also are more prone to wind damage because the pruned roots no longer provide anchorage to the soil. In 2000, we conducted two trials (insecticide efficacy and varietial response to RWW infestations) in drilled-seeded rice at the Missouri Rice Growers' Research and Demonstration Farm.

Materials and Methods:

Insecticide Efficacy Trial
Field plots were established with 'Bengal' on 01 June at 80 pounds per acre. Plots measured 5 feet by 20 feet and were flooded on 29 June. Metal barriers were erected around all plots before foliar insecticide treatments (except seed treatments) were applied on 07 July or 8-days postflood (DPF). The barriers help to prevent the mixing of the different insecticides within the water column. The treatments were: an untreated check; Icon 6.2FS (0.037 lb. AI / 100 lb. seed weight)-treated seed at planting; Dimilin 2L (0.187, and 0.25 lb. AI/ acre) 8 DPF; Furadan 3G (0.6 lb. AI/ acre) 8 DPF; Fury 1.5E (0.04 lb. AI/ acre) 8 DPF; and Karate Z 2.08CS (0.025 and 0.033 lb. AI/ acre) 8 DPF. Foliar insecticides were applied with a CO2, backpack sprayer calibrated to deliver 10 gallons per acre at 30 PSI through TSX-4 hollowcone nozzles. Furadan was applied with a hand-held, whirly-gig spreader. Treatments were arranged in a randomized complete block design with four replications each.

Plots were first evaluated on 26 June when two 3-row feet samples were taken per plot to determine plant stand counts. Five randomly selected plants per plot also were examined for height growth and number of adult RWW feeding scars. Insecticide treatments were evaluated for RWW control by taking three, 4-inch core samples per plot on 20 July and 02 August. Core samples were washed over a 40-mesh screen to remove the mud from the roots, placed in a concentrated salt solution, and the number of RWW larvae were counted. The center 5-feet of each plot was harvested on 18 October. Data was subjected to analysis of variance (ANOVA) and means separated with Duncan's multiple range test (DMRT).

Varietial Response to RWW Infestations
Four commercial varieties (Bengal, Cypress, Drew, and Jefferson) were selected from representative maturity groups (very short, short, and medium). Rice was planted on June 01, 2000 at a seeding rate of 80 lbs. per acre. Strips (10 ft. in width x 100 ft. in length) were randomly arranged and replicated three times.

The plots were fertilized (145 lbs. nitrogen per acre) on June 28, 2000 and flooded on June 29, 2000. RWW larval populations were evaluated by taking three 4-inch core samples per plot on August 02, 2000. On 18 October a 100-square ft. area (5 ft. in width x 20 ft. in length) of each plot was harvested with a small-plot combine.

Results and Discussion:

Insecticide Efficacy Trial
No significant differences were observed for number of plants per 3-row feet and adult RWW feeding scars per plot (Table 1). This information indicated we had a fairly even stand, and that RWW adult activity was evenly distributed throughout the test area before foliar insecticides were applied. We did observe slightly fewer adult RWW feeding scars in Icon-treated plots than in the remaining treated and untreated plots.

Following the application of foliar applications, significant reductions in RWW larval populations were only observed among treatments on 20 July (Table 2). All insecticide-treated plots had significantly lower RWW infestations than in the untreated plots on the first sampling date. Dimilin (0.25 lb. AI/A) also was observed to have significantly higher RWW infestations than in plots treated with Fury, Icon, and both rates of Karate. On 02 August RWW infestations still remained substantially lower in treated versus untreated plots.

Despite the presence of RWW infestations, untreated plots actually had slightly higher yields than plots treated with insecticides (except Dimilin at 0.25 lb. AI/A). The lack of significant yield enhancement from insecticide treatments as compared to the untreated plots may have been due to favorable growing conditions that allowed infested plants to overcome any root damage by RWW.

Varietial Response to RWW Infestations
There were no significant differences among the varieties with respect to RWW infestations and yield (Table 3). This may have been partially due to variability among the three replications for each variety. We did observe that Drew and Cypress had the highest RWW infestations, whereas, Bengal and Jefferson had the lowest infestations. The yield impact of higher RWW infestations did not have a direct correlation to lower yields; however, Jefferson had slightly higher yields than the long-season Cypress and Drew varieties.

Acknowledgments:
We would like to thank the Missouri Rice Research and Merchandising Council and Dr. Donn Beighley, Missouri Rice Research and Demonstration Farm manager, for the use of land and their assistance with this work. We also appreciate the support of our industry sponsors (Aventis CropScience, FMC Corp., Uniroyal Chemical Co., and ZENECA Corp.) to carry out the insecticide evaluation trials. We also want to acknowledge the assistance of student workers (Josh Chipman, Shelly Kellams, Raymond Nabors, and Gus Recker) in conducting this research.

1Assistant Professor of Entomology / State Extension Entomology Specialist, and Senior Research Specialist at the University of Missouri-Columbia Delta Center

Table 1.  Plant stand counts and height growth plus adult rice water weevil 
(RWW) feeding scars in Missouri (2000).

Treatment    Rate       Time1       # plants       plant height      # RWW feeding
          (lb.AI/A)                 /3-row ft.         (in.)          scars/plant 
Dimilin 2L        0.188      6 DPF      30.75 a          9.67 a            1.75 a
Dimilin 2L        0.25       6 DPF      32.00 a          9.90 a            1.60 a
Furadan 3G        0.6        6 DPF      28.88 a          9.51 a            2.10 a
Fury 1.5E         0.25       6 DPF      30.50 a          9.83 a            2.45 a
Icon 6.2 FS       0.037      planting   27.63 a          9.62 a            1.38 a
Karate Z 2.08CS   0.025      6 DPF      27.63 a          9.44 a            1.53 a
Karate Z 2.08CS   0.033      6 DPF      32.13 a         10.00 a            1.58 a
UTC                 ---        ---      24.13 a          9.35 a            2.23 a    
1Timing of insecticide applications (DPF = days postflood).
Data in columns followed by the same letter were not significantly different 
(P < 0.05, DMRT).


Table 2.  Rice water weevil (RWW) control and rice yields in Missouri (2000).

                                                 Avg.# RWW/3 core
Treatment       Rate(lb.AI/A)      Time1      20 July      02 August      Yield(bu/A)
Dimilin 2L           0.188          6 DPF      11.50 bc      10.25 a          82.00 a
Dimilin 2L           0.25           6 DPF      16.00 bc      12.75 a         116.25 a
Furadan 3G           0.6            6 DPF       6.75 bc      13.75 a         102.95 a
Fury 1.5E            0.04           6 DPF       3.50 bc       4.00 a          95.03 a
Icon 6.2 FS          0.037          planting    1.00 c        4.94 a          93.45 a
Karate Z 2.08CS      0.025          6 DPF       3.50 c        2.50 a          88.76 a
Karate Z 2.08CS      0.033          6 DPF       2.25 c        2.04 a          92.67 a
UTC                   ---            ---       32.50 a       28.00 a         110.91 a 
1Timing of insecticide applications (DPF = days postflood).
Data in columns followed by the same letter were not significantly different 
(P < 0.05, DMRT)


Table 3.  Rice water weevil (RWW) populations and varietial yield response 
in Missouri (2000).

               Avg.# RWW/3 core samples                  Yield
Variety        Larvae      Pupae      Total      Lbs./plot       Bu/acre 
Bengal        10.00 a     1.33 a     11.33 a      13.50 a        92.31 a
Cypress       21.33 a     1.00 a     21.33 a      16.57 a       113.40 a
Drew          23.00 a     1.33 a     24.33 a      17.03 a       117.08 a
Jefferson     10.67 a     0.67 a     11.33 a      17.50 a       118.70 a 
Data in columns followed by the same letter were not significantly different 
(P < 0.05, DMRT).

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