Managing alfalfa weevils with natural controls

A producer recently called to ask when was the best time to spray alfalfa weevil adults. There are a few alfalfa producers who seem to have discovered that spraying the adults reduces the incidence and feeding damage of the larvae.

While there may be some truth to it, there are some inherent problems with this practice. SDSU Extension Entomologist, Ada Szczepaniec, reports that a number of things can happen to adversely affect the adults laying eggs, the eggs hatching, the larvae surviving, etc. Warm and wet springs promote the growth of pathogens that attack the larvae so weather conditions and soil moisture play a role in the severity of alfalfa weevil infestations. There are also several predatory insects that offer a bio control alternative.

These natural controls can result in larval populations being low enough that insecticide applications may not be economical. If you apply insecticides with the intention of controlling the adult weevils, you will never know if the population of larvae would have justified insecticide treatments or not. The larva is the damaging life stage and the target for control, if needed. Routine insecticide applications are detrimental to the predatory insects that are typically abundant in alfalfa fields. There is also concern that consistent, routine insecticide applications may lead to resistance of alfalfa weevils to insecticides.

SDSU Extension’s recommendation is to scout for alfalfa weevils and make management decisions based on numbers of weevils, the growth stage and/or height of the alfalfa, and other factors. The general threshold (and least precise) is to treat if 30-40% of tips are damaged by the weevils, larvae are present, and early harvest is more than one week away. The bucket method is a more precise sampling method and is the preferred technique to sample alfalfa weevils to determine whether pesticide applications are warranted. An explanation of the bucket method, along with other good information about alfalfa weevils can be found in the iGrow article, “Entomology Update: Alfalfa Weevil Scouting Notes” at: http://tinyurl.com/p3opvpr

Early cutting can be a highly effective strategy in managing alfalfa weevils if the weather cooperates. Ideal conditions for early cutting in alfalfa weevil management are good drying conditions, i.e. warm temperatures, low humidity, sunshine, and wind. The idea is to cut the alfalfa and get it baled and out of the field to expose the larva to the drying conditions, which will lead to a lot of mortality. With early cutting, producers need to monitor the regrowth after the first cutting to make sure enough larva didn’t survive to keep the second cutting from regrowing. Regular scouting is crucial in making sustainable management decisions.