Monoriting Alfalfa Weevils

Monitoring Alfalfa Weevil Can Save Money

A 15-inch diameter sweep net is often used for monitoring
alfalfa weevil (top). An insect suction sampler (D-vac) was
used in a 2011 beneficial insect survey in Cache County,
UT alfalfa fields (bottom).

The alfalfa weevil is a frequent foe of Utah alfalfa growers. Traditionally, this early spring pest is controlled with broadspectrum insecticides regardless of whether pest larvae reach economic thresholds. Utah State University research suggests that developing a weevil monitoring plan in preparation for spring weevil activity could be economically beneficial.

Monitor and Save on Weevil Sprays

In 2008, USU Extension agents in Cache, Beaver, Box Elder, and Weber counties conducted a study to investigate whether yield differed in fields that were sprayed or not sprayed for alfalfa weevil (click here for the full report). All fields sampled that year had weevil larvae below the economic threshold level of 20 larvae per sweep. More importantly, yield measurements
were similar for fields that were treated for alfalfa weevils and those that were left untreated. Recreational sprays–those that are not necessary–can be costly but avoidable with proper monitoring.

Monitor and Save on Aphid Sprays

Weevil monitoring and not using pesticides when densities are below the economic threshold level can also be profitable later in the season. A study conducted in Sevier County by USU Extension examined insects in alfalfa fields one month after broad-spectrum insecticides were used for weevil control (click here for the full report). Fields that were sprayed for alfalfa weevil had significantly more pea aphids and fewer beneficial insects later in the season than fields that were not sprayed for weevil. Large populations of aphids can decrease yield and may require additional insecticide applications and added cost. In unsprayed fields, the higher number of beneficial insects may contribute to maintaining lower aphid populations.

Monitor and Save with Beneficial Insects

A 2011 USU study conducted in Cache Valley highlighted the importance of beneficial insect populations and their role in suppressing aphid populations. In a survey of alfalfa fields we found that alfalfa is home to a diverse community of beneficial insects. These beneficial insects included damsel bugs, big-eyed bugs, and lady beetles. Both adult and juvenile stages of these beneficial insects are predatory and were found simultaneously in these fields. We set up a complementary field cage experiment at USU’s Greenville Research Farm in Logan to examine whether it was important to have all of these beneficial insect species and life stages (adults and juveniles) to suppress aphid populations in alfalfa. We found that conserving multiple predatory insect species can decrease aphid populations compared to having a single species. Furthermore, beneficial insect communities made up of both adult and juvenile stages were most efficient at suppressing pea aphids in alfalfa. This suggests yet another reason to monitor weevil populations and avoid unnecessary sprays.

Alfalfa weevils can be destructive insects and unfortunately, numerous alfalfa growers have lost money to weevil damage. It is clear, however, that insecticide use when weevil populations are below thresholds can also be costly and can have unintended consequences adding to these costs. Monitoring and using thresholds can be effective tools to make informed decisions about when or whether insect management is necessary and can add profit to the bottom line.

-Ricardo Ramirez, Entomologist, and Erica Stephens, MS student in Dept of Biology, USU