Efficacy of insecticides to control alfalfa weevil larvae in Southern Illinois, 1996

John T. Shaw, Steve Roberts, James W. Finger and Edward Armbrust

Summary

Plot Information and Methods

Location

Southern Illinois University Belleville Research Center, St. Clair County, Illinois.

Plot Size

40 feet x 50 feet for each treatment in each replication.

Experimental Design

A randomized complete block design with 4 replications was utilized. An untreated control block was included in each replication.

Alfalfa Cultivar

'Vernal'

Pre-treatment Infestation

All plots were sampled on May 13, just prior to treatment. An average of 3.12 (3rd and 4th instar) alfalfa weevil larvae per sweep (n = 80) was recorded. The average stem height at this time was approximately 12 inches.

Insecticide Application Date and Environmental Conditions at Time of Application

May 13, 1996; wind calm, 3 to 5 mph; canopy wet; air temperature 65°F.

Method of Insecticide Application

A 20-foot boom was mounted by a 3-point hitch to the rear of a 3020 John Deere tractor (boom height = 17 inches above the crop canopy). Flat fan spray tips (XR TeeJet 8002VS, Spraying Systems Co.) were spaced 20 inches apart on the boom. A compressed-air system was calibrated to deliver 19.7 gpa at a pressure of 30 psi at a speed of 2.5 mph.

Post-Treatment Samples

Twenty sweep-net samples were taken from each plot 7, 16, and 22 days after treatment. The standard 15-inch diameter sweep-net was used for all sampling.

Weather Information

See Appendix A, Table A.1

Statistical Analysis

Data were analyzed with the Pesticide Research Manager version 5.0, from Gylling Data Management, Inc.(GDM). Means were separated by Fisher's LSD test (P=0.05).

Results and Discussion

The initiation of this test was later than usual due to the low densities of alfalfa weevil larvae in 1996. Prior to the application of insecticides, the mean numbers of alfalfa weevil larvae per sweep ranged from 3.6 to 2.7 for the six treatments. These pre-treatment means were not significantly different from one another. The mean numbers of alfalfa weevil larvae per sweeps for all treatments throughout the duration of the study are presented in Table 1.1. Seven days after treatment (7 DAT), all compounds had reduced the alfalfa weevil larval density by more than 90% (Table 1.2). All treatments performed well even though the larvae were quite large at the time of application.

Table 1.1: Mean number of alfalfa weevil larvae (all instars combined) per sweep for three sampling dates after treatment (DAT) in St. Clair County, Illinois, 1996

Mean larvae per sweep²

Treatment

Rate¹

7 DAT

16 DAT

22 DAT

Baythroid 2EC

0.031

0.08 b

0.01 a

0.00 b

Baythroid 2EC

0.025

0.05 b

0.01 a

0.01 b

Warrior 1EC

0.02

0.06 b

0.05 a

0.01 b

Mustang 1.5EC

0.024

0.04 b

0.00 a

0.01 b

Mustang 1.5EC

0.037

0.13 b

0.03 a

0.05 b

Furadan 4F

0.5

0.05 b

0.14 a

0.11 a

Control

1.90 a

0.01 a

0.03 b

¹ All rates are specified as lb (AI)/acre.
² Means in same column followed by the same letter are not significantly different (P=0.05;Duncan's New MRT).

Table 1.2 Mean percentage control of alfalfa weevil larvae (all instars combined) for three sampling dates after treatment (DAT) in St. Clair County, Illinois, 1996

Mean percentage control²

Treatment

Rate¹

7 DAT

16 DAT

22 DAT

Baythroid 2EC

0.031

96

0

100

Baythroid 2EC

0.025

97

0

50

Warrior 1EC

0.02

97

0

50

Mustang 1.5EC

0.024

98

100

50

Mustang 1.5EC

0.037

93

0

0

Furadan 4F

0.5

97

0

0

Control

0
(1.90)

0
(0.01)

0
(0.03)

¹ All rates are specified as lb (AI)/acre.
² Percentage control is based on the reduction in larval numbers per sweep from the untreated control; numbers in (brackets).
³ Means in same column followed by the same letter are not significantly different (P=0.05;Duncan's New MRT).

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