Scout area alfalfa fields for alfalfa weevil. Female weevil lay eggs in the stubble and stems of growing alfalfa. The eggs hatch and a lava with a black head will emerge and feed on the terminal growth of the alfalfa plant.

The alfala weevil which are yellowish initially will become green as it grows. A full-grown larva is about 3/8” long. Typically it takes about three weeks to go through the larval stages.

When fully grown, the larva makes a cocoon on the leaves or debris on the ground. The larva pupates in this cocoon and the adult weevil emerges in about 10 days. After feeding for a while, it flies from the field and is inactive during the summer. The adults will return to lay eggs in fall.

The initial damage is tiny holes in the terminal leaves. As the larva growth, they will move down the stems and feed on the leaves. If many leaves are eaten, the field will look gray.

Management decisions are based on stem sampling. There are detailed tables that use height of alfalfa and number of larvae present as economic thresholds. If you are not able to find tables, if an average of one or more larvae per stem is found in the 10-stem sample, then the economic threshold is justified and control is justified.

Many insecticides are available for control. Apply insecticides when temperatures are warm and avoid spraying when less that 40°F. Contact your supplier for availability of products.

For more information, contact Wayne Flanary, Field Specialists in Agronomy, University of Missouri Extension at 816-279-1691.