Potato Leafhopper in Alfalfa
By Wayne Bailey

Potato leafhopper (PLH) adult numbers continued to increase throughout the state during the past two weeks as recent storms transported additional PLH adults into the state from more southern locations. Alfalfa producers should scout fields at least twice per week to determine whether PLH numbers have reached the economic threshold level. If treatment is justified, producers should consider an insecticide application if the alfalfa is not ready for second harvest. From research conducted in Missouri, it was found that mechanical harvest of alfalfa (disc mower conditioner) substantially reduced numbers of PLH nymphs by greater than 90 percent. Adult numbers were reduced by about 60 percent because of their ability to fly out of the path of harvesting equipment. In situations where adult PLH return to feed on alfalfa following harvest, a stubble spray may be necessary to protect alfalfa regrowth. Fields most at risk from PLH damage are field seeded this spring and fields which have recently been harvested. Threshold levels for potato leafhopper in alfalfa vary with plant height and whether or not the alfalfa is a potato leafhopper resistant (PLH-resistant) variety. For traditional varieties (not PLH-resistant) the economic thresholds are as follows:

Average Alfalfa Height (inches)	Number of PLH per Sweep (15-inch diameter net)
0 to 3	0.2 (10 per 50 sweeps)
3 to 6	0.5 (25 per 50 sweeps)
6 to 12	1.0 (50 per 50 sweeps)
12 or taller	2.0 (100 per 50 sweeps)

The economic thresholds for PLH-resistant varieties are 3 times higher because PLH-resistant alfalfa varieties can withstand greater numbers of potato leafhoppers than not resistant varieties. This resistance is accomplished by the presence of small hairs on the leaflets and stems of resistant alfalfa plants. These hairs act a as physical barriers which holds the insect away from resistant plants and generally prevents the leafhoppers from probing and feeding on plant juices using their piercing-sucking mouthparts. Typically, migrating leafhoppers may land on resistant varieties at the same rate as traditional non-resistant alfalfa varieties, but most will leave the PLH-resistant varieties within 24 hours to find more suitable hosts. If a producer is growing a PLH-resistant variety, the following economic thresholds should be considered when assessing leafhopper numbers.

Average Alfalfa Height (inches)	Number of PLH per Sweep (15-inch diameter net)
0 to 3	0.6 (10 per 50 sweeps)
3 to 6	1.5 (25 per 50 sweeps)
6 to 12	3.0 (50 per 50 sweeps)
12 or taller	6.0 (100 per 50 sweeps)

PESTICIDES LABELED FOR USE ON POTATO LEAFHOPPER
Chemical Name	Product Name	Rates Amount of Product	Preharvest Interval (PHI)
Cyfluthrin	*Baythroid 2	0.8 to 1.6 fl oz / acre	7 days
Methyl Parathion	*Chemnova Methyl 4EC	see label rates	15 days
Carbofuran	*Furadan 4F	1 to 2 pts / acre	14 - 28 days depending on rate
Chlorpyrifos	*Lorsban 4E	0.5 to 1 pt / acre	7 - 14 days
Chlorpyrifos	*Nufos 4E	0.5 to 1 pt / acre	7 - 14 days
Chlorpyrifos	*several formulations	see specific labels	see specific label
Malathion	Malathion	1.0 to1.25 lb a.i. / acre	See specific label for rates/PHI
Zeta-cypermethrin	*Mustang Max	2.24 to 4.0 fl oz / acre	3 days
Permethrin	*Pounce 3.2EC	2 to 4 fl oz/acre	7 to 14 days, depending on rate
Gamma-cyhalothrin	*Proaxis	1.92 to 3.2 fl oz / acre	1 day forage, 7 day hay
Lambda-cyhalothrin	*Warrior	1.92 to 3.2 fl oz / acre	1 day forage, 7 day hay
Lambda-cyhalothrin	*Silencer	1.92 to 3.2 fl oz / acre	1 day forage, 7 day hay
Lambda-cyhalothrin	several formulations*	see specific labels	see specific labels
* indicates Restricted Use. a.i. = active ingredient

Wayne Bailey
573-882-2838 office
573-864-9905 cell