Potential Problems with Alfalfa Weevil By Wayne Bailey

Alfalfa weevil larvae may occur in high numbers this spring. The alfalfa weevil is an insect which benefits from mild weather during fall, winter and spring months. When temperatures rise above 50 F for several consecutive days during these seasons, adult weevils will deposit eggs in old stubble or in new spring growth of alfalfa.

During the past few months, warmer than normal winter temperatures have allowed for greater numbers of eggs to be laid and for better survival of these eggs. Although high numbers of alfalfa weevil eggs present in the spring do not always result in economic infestations, the potential for damage is greater in years when this condition exists.

Alfalfa weevil eggs develop and eventually hatch after accumulating about 300 degree day heat units based on 48 F. This means that infestations of alfalfa weevil larvae often occur first on south-facing slopes of alfalfa fields, because these slopes warm faster in spring.

Although problems with alfalfa weevil have yet to occur this spring, producers in the southern counties of Missouri should scout fields on a weekly schedule beginning now and continue through first harvest. Producers in central and northern counties should begin scouting for alfalfa weevil within the next two weeks. The first damage observed will be small feeding holes in alfalfa leaflets as they grow out of the terminals of plant stems. This minor foliage damage is caused by the first and possibly second larval (worm) stages called instars. As larvae grow larger (third and fourth instars), they consume greater amounts of plant tissue and may cause economic loss.

Scouting for alfalfa weevil is accomplished by randomly collecting 50 alfalfa stems (10 stems at five different locations) and tapping them into a white bucket. Larvae will generally be dislodged by this action and allow for an average number of larvae per alfalfa stem to be calculated. Caution should be used when collecting stems as larvae can be easily dislodged from the growing tip of the plant stem by rough handling. It is recommended that the top of the alfalfa stem be cupped in one hand while the plant stem is removed by cutting with a knife near the base of the stem. If an average of one or more larvae per stem is found, then the economic threshold has been reached and control is justified.

Management Options: The main management option for early infestations of alfalfa weevil larvae on small alfalfa is an application of a labeled insecticide. Early harvest of the alfalfa by either machine or livestock may be a viable option for some producers in Missouri. If early harvest of alfalfa by machine is selected as a control strategy, then the crop is harvested approximately 7-10 days prior to the normal plant growth stage of 1/10th bloom.

Data from a recent study in Missouri indicate that alfalfa weevil larval numbers are reduced by about 98 percent with mechanical harvest and about 90 percent by cattle grazing in a management intensive grazing system. Producers using grazing as a control strategy must be aware of the bloat risk to cattle grazing green alfalfa and risk to the alfalfa stand due to trampling during wet conditions.

If an insecticide application is selected, a list of insecticides recommended for alfalfa weevil control follows. Rates are given as amount of product applied per acre. The preharvest interval lists the minimum number of days before harvest that an insecticide application can be applied. If an insecticide application is selected, a list of insecticides recommended for larval alfalfa weevil control follows. Rates are given as amount of product applied per acre.

ALFALFA WEEVIL LARVAE INSECTICIDE RECOMMENDATIONS
PREHARVEST
CHEMICAL NAME	PRODUCT	RATES	INTERVAL
Carbofuran	Furadan 4F*	½ to 1 pts/A	7-21 days
Chlorpyrifos	Lorsban 4E*	1 to 2 pts/A	14-21 days
	Nufos 4E*	1 to 2 pts/A	14-21 days
Cyfluthrin	Baythroid 2*	1.6 to 2.8 fl oz/A	7 days
Gamma-cyhalothrin	Proaxis*	2.56 to 3.84 fl oz/A	1 day forage, 7 hay
Lambda-cyhalothrin	Warrior*	2.56 to 3.84 fl	1 day forage, 7 hay
Malathion		1.25 lb a.i./A	see label for specific Rate and PHI
Methyl parathion	Cheminova Methyl	see label rates	15 days
	4EC*
Permethrin	Pounce 3.2EC*,	8 fl oz/A	14 days
	Ambush 2E*	12.8 fl oz/A	14 days
(Results variable for permethrin formulations in Missouri trials)
Phosmet	Imidan 70-W*	1 to 1.3 lbs/A	7 days
Zeta-cypermethrin	Mustang Max*	2.24 to 4.0 fl oz/A	3 days

Table information
* Designates a restricted use insecticide
a.i. refers to active ingredient
Read and follow all label instructions