Efficacy of insecticides to control potato leafhoppers on second growth alfalfa 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 June 19, just prior to treatment. An average of 6.65 potato leafhoppers per sweeps (n=80) was recorded. Average stem height at this time was approximately 10.2 inches.

Insecticide Application Date and Environmental Conditions at Time of Application

June 19, 1996; wind calm, 0 to 2 mph; canopy dry; air temperature 89°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, 14, and 21 days after treatment (DAT). The standard 15-inch diameter sweep-net was used for all sampling.

Weather Information

See Appendix A, Table A.2

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

Effects of insecticides on potato leafhopper populations

Before the insecticides were applied, the mean numbers of potato leafhoppers per sweep averaged 6.65. The pre-treatment means were not significantly different from one another. Seven days after treatment (7 DAT), all treatments but the two Lorsban 4E treatments had reduced the potato leafhopper density significantly from that of the untreated control (Table 2.1). By 14 DAT all treatments were significantly better than the untreated control and remained that way through 21 DAT. Potato leafhopper adults increased dramatically from 14 DAT through 21 DAT. At 14 DAT only Lorsban 4E at 0.5 and 0.25 lb(AI)/A, and Cygon 400 at 0.5 lb(AI)/A had the number of adults per sweep exceeding the 2.0 threshold, yet all were significantly better than the untreated control. At 21 DAT all treatments had exceeded the per sweep threshold but all were significantly better than the untreated control.

Effect of PLH control on alfalfa stem length

On June 19 (pre spray), the mean plant height was 10.2 inches (all plots combined). On July 10 (21 DAT), the average plant height for all treated plots was 27 inches, a 62% increase in growth. The untreated control plots had a mean plant height of 14 inches, only a 27% increase in plant growth. It was also noted that all treatments were still quite green in color on 21 DAT, while the untreated control was severely damaged and yellow in color.

Table 2.1: Mean number of potato leafhopper adults per sweep for three sampling dates after treatment (DAT) in St. Clair County, Illinois, 1996

Mean number of adults per sweep²

Treatment

Rate¹

7 DAT

14 DAT

21 DAT

Baythroid 2EC

0.025

0.05 b

0.61 d

2.59 c

Mustang 1.5EC

0.0375

0.40 b

0.85 d

2.81 c

Baythroid 2EC

0.0125

0.26 b

0.77 d

2.99 c

Furadan 4F

0.5

0.70 b

1.11 d

3.14 c

Pounce 3.2EC

0.15

0.38 b

1.36 d

4.34 c

Lorsban 4E

0.5

3.04 a

2.56 cd

5.31 c

Cygon 400

0.5

1.02 b

4.44 c

8.48 bc

Lorsban 4E

0.25

3.89 a

10.46 b

19.36 b

Control

3.96 a

13.46 a

64.36 a

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 2.2 Mean percentage control of potato leafhopper adults in alfalfa for three sampling dates after treatment (DAT) in St. Clair County, Illinois, 1996

Mean percentage control²

Treatment

Rate¹

7 DAT

14 DAT

21 DAT

Baythroid 2EC

0.025

99

95

96

Mustang 1.5EC

0.0375

90

94

96

Baythroid 2EC

0.0125

93

94

95

Furadan 4F

0.5

82

92

95

Pounce 3.2EC

0.15

91

90

93

Lorsban 4E

0.5

23

81

92

Cygon 400

0.5

74

67

87

Lorsban 4E

0.25

2

22

70

Control

0
(3.96)

0
(13.46)

0
(64.36)

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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