Potential Alfalfa Weevil Problems in 2010

Published: April 1, 2010

The alfalfa weevil is an insect which benefits from mild weather during fall, winter, and spring months. When temperatures rise above 50OF for several consecutive days during these seasons, adult weevils will deposit eggs in old stubble or in new spring growth of alfalfa. 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 48OF. 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. Producer 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 1st and possibly 2nd larval (worm) stages called instars. As larvae grow larger (3rd and 4th instars) they cause very visible foliage feeding which often results in substantial economic loss.

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. 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 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 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% 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.

Insect Larvae in Early Season Alfalfa

The larvae of two weevils, the clover leaf weevil and the alfalfa weevil, may be found in Missouri alfalfa fields prior to first harvest. Although they both feed on alfalfa foliage and look similar in appearance, only the alfalfa weevil is considered a severe pest of alfalfa in this state. How do we tell them apart in the field so we don't apply unneeded insecticide applications? The following list compares identifying characteristics of larvae of these two alfalfa weevils:

Clover Leaf Weevil (CLW)

• Appear in fields in early season, often March and April
• Larvae look similar to AW larvae, but are larger in size, have a white stripe running down the back that is often bordered by patches of pink or rosy pink areas or flecks.
• Head capsules are brown in color
• Larvae feed at night and generally spend the day on the ground near the plant or in the plant crown.
• A large majority of CLW larvae are often parasitized by one to several larvae of a parasitic wasp species.
• Feeding damage is seen as circular holes cut into the alfalfa leaflets
• Populations of CLW rarely reach economic levels, so they rarely require insecticide applications to reduce or prevent damaging populations

Alfalfa Weevil (AW)

• Larvae appear in fields about two weeks later than CLW (late March, April and May depending on location in state)
• Larvae have brown to black head capsules (sometimes difficult to distinguish from CLW larvae)
• AW larvae always feed on the alfalfa plant foliage and are not found on the ground
• Parasitism rates rarely exceed 15% in most areas of the state
• Small larvae (1st and 2nd instars) feed inside growing plant terminals which produces shothole damage on plant leaflets as they grow out of the terminal. Older larvae (late 2nd, 3rd, and 4th instars) substantially reduce forage quality of the alfalfa by skeletonizing and defoliating plant leaflets
• Common pest of first cutting alfalfa in Missouri, especially in the southern half of the state.