Alfalfa Insects - A Year in Review
The use of organophosphate and pyrethroid insecticides has yielded mixed results in suppressing alfalfa weevil. In 2012, several growers in Utah used Steward (indoxacarb), an insecticide with a different mode of action. A number of growers reported that this product was successful in reducing weevil damage. The insecticide protects alfalfa for 5 to 14 days, but it does not kill larvae immediately. Instead, it causes them to stop feeding and become lethargic. Larvae then die within a few days. USU recommends that when using Steward, growers should evaluate weevil suppression 3 days after treatment.
In Idaho, the often overlooked clover root curculio, whose larvae feed below ground, received attention as a problematic pest. USU surveyed for adult life stages in northern Utah alfalfa fields throughout the 2012 season. Sweep net sampling revealed no more than five adult weevils total (<0.05 weevils/sweep) in each field sampled. In other words, adult clover root curculio were not abundant.
Larvae were not sampled specifically; however, one root sample was submitted to USU that had symptoms indicative of clover root weevil feeding. Adults cause a semi-circular chewed pattern, and larvae scar the roots. Feeding from larvae can expose roots to plant pathogens. It is noteworthy that roots infected with the fungus Rhizoctonia have lesions that can resemble injury of larval clover root curculio feeding.
Aphids and Predators
USU surveyed the abundance of insects in alfalfa fields with and without weevil insecticide applications and found that aphids were more abundant in insecticide-treated fields later in the season than in untreated fields.
In addition, predatory insects were more abundant in untreated fields and may have contributed to the lower aphid populations in those same fields.
Predatory insects are often more sensitive to insecticide treatments than the actual target pest. After treatment, it takes time for the beneficial insects to rebound. Predators, including lady beetles, damsel bugs, big-eyed bugs, and syrphid fly larvae, are important predators of aphids.
The warm spring of 2012 allowed the adult life-stages of beneficial insects to be active in May in northern Utah, with a peak at the end of June. The juvenile stages of predatory insects were evident at the beginning of June and present much of the season.
Monitoring alfalfa weevils early in the season, and treating when necessary, can aid in the conservation of predators that buffer against secondary pest outbreaks.
In 2012, pheromone traps were placed in alfalfa fields in Cache, Millard, Utah, and Weber counties for the Cooperative Agricultural Pest Survey (CAPS) program. USU surveyed for three invasive moth species on the National CAPS Priority Pest List that could harm Utah’s alfalfa production: old world bollworm, cotton cutworm, and Egyptian cottonworm.
The caterpillars of each of these moths can completely defoliate alfalfa and they have a broad host range. None of these pests were detected, and surveys are anticipated to resume in 2013.
-Ricardo Ramirez, Entomologist
Featured Picture of the Quarter
On aspen, cottonwood, and poplar twigs, galls formed by the poplar twig-gall fly are most noticeable in winter. Inside the galls, fly larvae are resting until spring, when they will pupate to emerge as adults. Adult females lay eggs within newly forming twigs, and the feeding of the larvae starts the formation of new galls. Empty galls will continue to swell after the insect has emerged, and appear as swollen bands around larger limbs or trunk.
-Image by Marion Murray,
IPM Project Leader