Potential for Alfalfa Weevil Problems in 2011

Published: April 4, 2011

The alfalfa weevil is considered an occasional to severe pest in Missouri. In most years problems with this pest are more severe in southern Missouri counties where adult beetles may lay eggs during mild fall, winter, and spring days when temperatures rise above 50°F for several consecutive days during these seasons. Eggs may be laid in dormant stubble or in new spring growth of alfalfa. Although harsh weather conditions may cause egg mortality during periods of freezing weather in fall and winter, snow cover on fields during these periods provide eggs with some protection from the cold. Although high numbers of alfalfa weevil eggs present in the spring do not always result in economic infestations of larvae, 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 in more southern counties of Missouri and sooner on south-facing slopes of alfalfa fields due to the faster warming of these slopes in early 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 to three 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 1st and possibly 2nd larval (worm) stages called instars as they feed in the terminal buds of alfalfa plants. As larvae grow larger (3rd and 4th instars) they can readily be found on plant foliage where their feeding causes very visible foliage damage which often result in substantial forage yield and quality losses. Scouting for alfalfa weevil is accomplished by randomly collecting 50 alfalfa stems (10 stems at 5 different locations) and tapping them into a white bucket. Third and fourth larval instars 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 near the base of the stem by cutting with a knife. If an average of one or more larvae per stem is found and 30% of plants exhibit larval damage, 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 viable options 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 1/10nth bloom. Missouri data indicate that alfalfa weevil larval numbers are reduced by about 95-98% with mechanical harvest and about 90-95% 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 hoof trampling during wet conditions. If an insecticide application is selected, a list of insecticides recommended for alfalfa weevil larval control follows.