Efficacy of insecticides to control alfalfa weevil larvae in Illinois, 2000
John T. Shaw and Kevin Steffey

Summary

Plot Information and Methods

Location

University of Illinois Crop Sciences Research & Education Center, Field NC-900, Urbana, Illinois

Plot Size

40 ft x 50 ft for each treatment in each replication, with 20-ft alleys between each replication.

Experimental Design

Randomized complete block with four replications, with an untreated control in each replication

Alfalfa Cultivar

'Target II'

Pre-treatment Infestation

All plots were sampled on April 29, just before treatment. We found an average of 1.3 alfalfa weevil larvae (second and third instars) per stem (n = 100 stems picked at random), and an average of 98.75 larvae per 20 sweeps (n = 80). The average stem height on April 29 was approximately 21 in.

Environmental Conditions at Time of Application

Wind-light with 6-8 mph gusts
Canopy-wet
Air temperature-72°F.

Method of Insecticide Application

A 20-ft boom was mounted by a 3-point hitch to the rear of an International 140 tractor (boom height = 17 in above the crop canopy). Twelve flat fan spray tips (XR TeeJet 8002VS, Spraying Systems Co.) were spaced 20 in apart on the boom. A compressed-air system was calibrated to deliver 14.4 gpa at a pressure of 28 psi at a speed of 2.5 mph.

Post-Treatment Samples

Twenty sweep-net samples were taken from each plot 3, 7 and 10 days after treatment (DAT). The standard 15-in-diameter sweep net was used for all sampling.

Phytotoxicity

By 7 DAT the NuFos 4E treatment had turned many of the alfalfa plants white. Most plants survived but were stunted by 14 DAT.

Weather Information

See Appendix A, Table A.1

Statistical Analysis

Data were analyzed with Agriculture Research Manager (ARM) version 6.1.6, from Gylling Data Management, Inc. (GDM). Means were separated by Duncan's New Multiple Range Test MRT (P = 0.05).

Results and Discussion

Before the application of insecticides, the mean number of alfalfa weevil larvae (second and third instars) per stem in the trial area was 1.3. Numbers of alfalfa weevil larvae in the designated plot areas did not differ significantly. The mean numbers of alfalfa weevil larvae per sweep for all treatments throughout the duration of the study are presented in Table 1.1.

Percentages of control of alfalfa weevil larvae are presented in Table 1.2. The numbers of alfalfa weevil larvae in the untreated control plots increased from 120.5 per sweep on 3 DAT to more than 220 per sweep on both 7 and 10 DAT (Table 1.1). The numbers of alfalfa weevil larvae per sweep in the untreated control plots were significantly higher on all three sampling dates than the numbers of alfalfa weevil larvae in all insecticide-treated plots. No significant differences in numbers of alfalfa weevil larvae occurred among insecticide-treated plots.

By 3 DAT, all insecticides had reduced densities of alfalfa weevil larvae by 86% or more (Table 1.2). Baythroid 2EC, Mustang 1.5EC, and Warrior T 1SC all provided greater than 90% control for the duration of the study.

Table 1.1 Mean number of alfalfa weevil larvae (all instars combined) per 20 sweeps 3, 7 and 10 days after treatment (DAT), Urbana, Illinois, 2000

Treatment

Rate¹

Larvae per 20 sweeps²

3 DAT

7 DAT

10 DAT

Warrior T 1SC

0.025

6.25 b

10.25 b

10.25 b

Mustang 1.5EC

0.044

8.00 b

8.75 b

7.75 b

Baythroid 2EC

0.037

8.50 b

3.00 b

6.00 b

NuFos 4E

0.500

16.75 b

49.00 b

35.75 b

Control

120.50 a

221.00 a

227.50 a

Table 1.2 Percentage control of alfalfa weevil larvae (all instars combined) for 3, 7, and 10 sampling days after treatment (DAT), Urbana, Illinois, 2000

Treatment

Rate¹

% Control²

3 DAT

7 DAT

10 DAT

Warrior T 1SC

0.025

95

95

95

Mustang 1.5EC

0.044

93

96

97

Baythroid 2EC

0.037

93

99

97

NuFos 4E

0.500

86

78

84

Control

0

0

0

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