Alfalfa Weevil Update By Wayne Bailey

Alfalfa weevil larvae are causing problems in southern and central regions of the state. Although larvae are variable in number depending upon the field being sampled, most fields are at or above the economic threshold of 1 or more larvae present per alfalfa stem. Many fields have been sprayed for this pest in southern counties with spray applications beginning in central counties. Larvae are present in low numbers in north Missouri fields, but numbers are expected to increase during the next week as additional larvae emerge from eggs and begin feeding. As larvae grow in size, they consume more alfalfa foliage and damage becomes more apparent. Rapid alfalfa growth has help limit the expression of alfalfa weevil damage in central Missouri to this point in the season. Producers are encouraged to scout alfalfa fields every few days to determine the status of alfalfa weevil larvae. Although insecticide applications are used to manage alfalfa weevil in about half of Missouri’s alfalfa fields each year, early harvest of the crop may be an effective option this season due to the rapid growth and size of alfalfa plants at this time. If alfalfa is within 7 - 10 days of normal harvest, early harvest by mechanic removal or grazing with livestock may produce effective control of alfalfa weevil larvae.

Data from a harvest study conducted in northeast Missouri over a three year period indicate that mechanical harvest often reduces larval numbers by up to 97%, whereas, grazing with cattle in an intensive, short-term grazing system reduced alfalfa weevil larval numbers by about 93%. Both harvest methods were effective at reducing weevil numbers to below economic threshold levels. Bloat of cattle and damage to alfalfa crowns by cattle hooves during wet conditions should be considered before use of early harvest methods. Wet conditions predicted for the next few days may help with control of alfalfa weevil larvae this season. A fungal pathogen is present in very low numbers of larvae in fields throughout the state. Several days of cool wet conditions could allow for the rapid increase in the pathogen and decrease in larval numbers. Infected larvae will turn from green to yellow in color and die about 2-3 days after being infected by the pathogen.

ALFALFA WEEVIL LARVAE INSECTICIDE RECOMMENDATIONS
CHEMICAL NAME	PRODUCT	RATES	PREHARVEST INTERVAL
Cyfluthrin	Baythroid 2*	1.6 to 2.8 . oz/Acre	7 days
Malathion	1.25 lb a.i./A	see label for specific formulations
Methyl parathion	Cheminova Methyl	see label rates	15 days
	4EC*
Carbofuran	Furadan 4F*	½ to 2 pts/Acre	7-28 days-check rate
Phosmet	Imidan 70-W	1.3 lbs/Acre	7 days
Chlorpyrifos	Lorsban 4E*	1 to 2 pts/Acre	14-21 days-check rate
	Nufos 4E	1 to 2 pts/Acre	14-21 days-check rate
Zeta-cypermethrin	Mustang Max*	2.24 to 4.0 oz/Acre	3 days
Permethrin	Pounce 3.2EC*	see label (variable results in Missouri trials)
	Ambush 2E*	see label (variable results in Missouri trials)
Gamma-cyhalothrin	Proaxis	2.56 - 3.84 oz	1 day forage, 7day hay
Lambda-cyhalothrin		2.56 - 3.84 oz	1 day forage, 7day hay
* Designates a restricted use insecticide. Read and follow all label instructions

Wayne Bailey
Entomology
573-882-2838