Several producers have asked about alfalfa weevils this spring. These pests have commanded the attention of alfalfa growers, and for good reason. The cool spring that has delayed everything has also delayed both alfalfa development and alfalfa weevil activity.
I have been stopping at a few alfalfa fields periodically as I travel the area, and have yet to see either the shot holing damage in the new growth that is characteristic of alfalfa weevils, or located eggs that have been laid. As Anitha Chirumamilla, Entomology Field Specialist in the Rapid City Extension Center reports in a recent article however, once we have accumulated 200-300 growing degree days, it’s time to be scouting. The SDSU Climate and Weather website: http://bit.ly/1tv2zYM indicates that much of southern South Dakota is at that level or about to be in the near future.
It is important to recognize that the mere presence of alfalfa weevils in the field does not warrant pesticide applications. The general threshold (and least precise) is to treat if there are 1.5 to 2 larvae per stem or 30-40% of tips are damaged by the weevils and early harvest is more than one week away. Anitha’s article: http://bit.ly/1m3vo9P goes into greater detail on how to scout for alfalfa weevils and make more precise control decisions.
Early cutting of fields is recommended when possible to manage alfalfa weevils, and regular scouting is crucial in making sustainable management decisions. The key to managing alfalfa weevils with early cutting is to take away their source of food and water and expose them to the sun and wind. If several days of warm, sunny weather follows cutting the alfalfa, and timely harvest and removal of the crop from the field is accomplished, most of the weevils will not likely survive. Cloudy conditions, rain, and/or delayed harvest might allow them to survive long enough for regrowth to occur, and they will feed on the new growth, holding it back and allowing them to survive and continue to damage the second cutting.
Some producers choose to take the first cutting as haylage, immediately exposing the weevil larvae to the sun and wind, rapidly desiccating them. Early cutting does not guarantee there will be no further problems from alfalfa weevils, but when conditions work against the weevils’ survival, it can be an excellent management strategy.
If damaging levels of alfalfa weevil larvae are present and early harvest is more than one week away, insecticide application may be justified. The list of insecticides available to control alfalfa weevil, the rates to apply and pre-harvest intervals, along with additional information is available at High Plains Integrated Pest Management Guide: http://bit.ly/1kbM7di.