UNL CropWatch June 23, 2010: Scouting for and Treating Potato Leafhoppers in Alfalfa
June 23, 2010
Potato leafhoppers have the potential to hurt alfalfa in Nebraska every year. This is generally a second and third cutting pest and has been common in Nebraska recently. This pest doesn’t overwinter in Nebraska but rather is brought in on southerly winds
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So far this year we have not seen a lot of potato leafhoppers, at least not in northeast or south central Nebraska. There have been a lot of strong southerly winds with the recent rains so it is probable we have potato leafhoppers in the state. So, it is time to begin scouting.
These small (1/8 inch long), bright green, wedge-shaped insects may cause severe damage to alfalfa by injecting a toxin into the plant as they feed. This feeding results in a distinctive yellow or purple triangle shape at the leaf tip. First year, spring planted alfalfa fields are particularly attractive to and vulnerable to potato leafhoppers, as are fields planted last year. In older fields, these insects are usually a problem on second and third cuttings.
Newly developed resistant varieties provide fairly good protection from potato leafhoppers, but alfalfa in the seedling stage may still be damaged. All fields should still be scouted, as large numbers of leafhoppers may still cause a problem, even in resistant variety fields.
Treatment Thresholds and Insecticides
Treatment decisions are based on numbers captured by a sweep net. (A sweep net is the only reliable way to scout for potato leafhoppers.) See Tables 1-3 for decision-making help. Note that there do not have to be many to cause a problem.
Many insecticides are registered for control, and all will provide good results when applied properly. Commonly used insecticides include Mustang, Warrior, Baythroid, and Lorsban. If possible, refer to the UNL Department of Entomology website at http://entomology.unl.edu/instabls/ for a list of suggested insecticides.
Keith Jarvi
Extension Educator in Dakota, Dixon, and Thurston Counties
Tom Hunt
Extension Entomologist, Haskell Agricultural Laboratory, Concord
Robert Wright
Extension Entomologist, Lincoln
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Table 1. Dynamic treatment thresholds for potato leafhoppers (average number per sweep) on alfalfa that is 1 to 4 inches tall. |
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Value of hay (per ton) |
Cost of insecticide application (per acre) |
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|
|
$8 |
$10 |
$12 |
$14 |
$16 |
$20 |
|
$ 60 |
0.4 |
0.5 |
0.6 |
0.7 |
0.8 |
1.0 |
|
$ 80 |
0.3 |
0.4 |
0.5 |
0.5 |
0.6 |
0.75 |
|
$100 |
0.25 |
0.3 |
0.4 |
0.4 |
0.5 |
0.6 |
|
$120 |
0.2 |
0.25 |
0.3 |
0.35 |
0.4 |
0.5 |
|
$140 |
0.2 |
0.2 |
0.25 |
0.3 |
0.3 |
0.4 |
|
$160 |
0.15 |
0.2 |
0.3 |
0.3 |
0.3 |
0.4 |
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Table 2. Dynamic treatment thresholds for potato leafhoppers (average number per sweep) on alfalfa that is 4 to 8 inches tall. |
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Value of hay (per ton) |
Cost of insecticide application (per acre) |
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|
|
$8 |
$10 |
$12 |
$14 |
$16 |
$20 |
|
$ 60 |
0.7 |
0.8 |
1.0 |
1.0 |
1.3 |
1.7 |
|
$ 80 |
0.6 |
0.6 |
0.75 |
0.9 |
1.0 |
1.3 |
|
$100 |
0.4 |
0.5 |
0.6 |
0.7 |
0.8 |
1.0 |
|
$120 |
0.3 |
0.4 |
0.5 |
0.6 |
0.7 |
0.8 |
|
$140 |
0.3 |
0.35 |
0.4 |
0.5 |
0.6 |
0.7 |
|
$160 |
0.25 |
0.3 |
0.4 |
0.4 |
0.5 |
0.6 |
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Table 3. Dynamic treatment thresholds for potato leafhoppers (average number per sweep) on alfalfa that is 8 to 12 inches tall. |
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Value of hay (per ton) |
Cost of insecticide application (per acre) |
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|
|
$8 |
$10 |
$12 |
$14 |
$16 |
$20 |
|
$ 60 |
2.0 |
2.4 |
2.8 |
3.0 |
3.9 |
5.0 |
|
$ 80 |
1.8 |
1.9 |
2.2 |
2.7 |
3.0 |
4.0 |
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$100 |
1.2 |
1.5 |
1.8 |
2.1 |
2.4 |
3.0 |
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$120 |
0.9 |
1.2 |
1.5 |
1.8 |
2.1 |
2.4 |
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$140 |
0.9 |
1.0 |
1.2 |
1.5 |
1.8 |
2.0 |
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$160 |
0.8 |
0.9 |
1.0 |
1.2 |
1.5 |
1.8 |
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