alfalfa weevil closeup Frank Peairs, Colorado State Univerity, Bugwood.org
CROP MONSTER: For alfalfa producers, the arrival of weevils is far from welcome news. While this photo shows a more mature weevil, there are ways to spot younger bugs for better control.

Weather conditions right for alfalfa weevil

Colorado State University offers scouting tips for detecting this field-crop yield robber.

It's an annual ritual for alfalfa producers — the hunt for alfalfa weevil. Depending on the state where you raise the crop, the plant-eating critters can arrive at different times. The time is right in Colorado as temperatures rise. Assefa Gebre-Amlak, Extension specialist at Colorado State University, offers a refresher on these pests and some tips for scouting them in your fields.

When they hatch, alfalfa weevils are about 1/20 of an inch long. They can range from cream to pale green, and they’re curved with shiny black heads. A white stripe running down the middle of the back may be visible and becomes more distinctive as larvae mature. You need a hand lens that magnifies 10 times to identify weevil larvae at this stage, Gebre-Amlak says.

The bug has four larval, or instar, stages. Fully grown larvae are up to 3/8 inches long and wider in their midsections.

Spotting field damage on heavily infested stands is relatively easy. A grayish, frostlike appearance shows up because leaves are dried and stems defoliated. With a big infestation, yield losses can be as high as 40% of the standing hay crop.

Here are some monitoring techniques Gebre-Amlak recommends:

Sweep sampling using a standard-size 38-centimeter-diameter net is efficient for estimating larval populations. Sampling should begin when 148 growing degree days have accumulated. This is when larvae are expected to be primarily second instars and when alfalfa hay has reached at least 10 inches in height.

When sweeping, taking 10 180-degree sweeps as you walk will result in a good representation of the field area. Count the larvae per sweep and repeat the procedure for a minimum of three samples in fields up to 20 acres, four samples in fields up to 30 acres, and five samples in larger fields.

The bucket method, or stem count method, may also be used. Take three six-stem samples in fields up to 19 acres, four samples in fields up to 29 acres; and five samples in fields 30 acres and higher. This method involves using a 3- or 5-gallon bucket, a white cloth, a hand lens, paper and pencil.

Economic threshold for the sweep sample is 20 larvae per sweep. For stem sampling, the threshold is 1.5 to 2 larvae per stem. The High Plains Integrated Pest Management Guide at wiki.bugwood.org/HPIPM has more information.

Managing weevil
Gebre-Amlak offers a few tips for keeping weevil under control:

• Cultural control. A noninsecticide control measure for alfalfa weevils is taking an early first harvest if an economic infestation is not detected until late in the growth of the first cutting. Harvesting alfalfa in an immature stage provides good control of larvae for the first crop. Rapid removal of hay will accelerate larval mortality due to desiccation by direct sunlight.

An early first cutting tends to cure more rapidly because lighter windrows dry quickly, and forage quality is enhanced by higher crude protein and lower fiber content.

Additional steps should be taken to ensure that surviving larvae do not cause economic damage to the regrowth. If larval survival under the windrows is high and baling is delayed (e.g., due to rainfall), damage to regrowth may be exacerbated. Regrowth should be inspected at a height of 1 to 2 inches to determine larval density.

• Chemical control. If damage becomes unacceptable as harvest approaches, an early harvest or “rescue” insecticide treatment may be necessary. Use care in applying insecticide when alfalfa is approaching bloom; refer to the “Protection of Pollinators” section of the High Plains IPM link above for guidelines on minimizing insecticide contact of pollinators.

Also, consider the waiting period before harvest for different insecticides. Generally, harvest or insecticide applications should happen before bloom if weevils are a problem.

Source: Colorado State University

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