Enemy No. 1

Presentation tackles alfalfa weevil management at High Plains Crop Convention

TORRINGTON – “The alfalfa weevil probably is the No. 1 (insect) pest for alfalfa in Wyoming,” Scott Schell, assistant extension entomologist for the University of Wyoming, said Tuesday at the High Plains Crop Convention in Torrington.
In his presentation, Schell explored integrated pest management, including a six-step approach to battling the bug.
“At UW, we use integrated pest management … multiple approaches,” he said, adding complete eradication is unrealistic.
“Management is the goal,” he said. “To keep damage below the economic injury level.”

Step 1: Know your enemy
The alfalfa weevil was first introduced to the U.S. about 95 years ago, Schell said. There are two strains: eastern and western. Wyoming is home to the western strain, however, it is near the border of where eastern alfalfa weevils are found.
“They have four stages, or instars, from egg to adult,” Schell explained. “The thirds and fourths are bright green with a white stripe down the back” and the insects range in size from one to two millimeters at stage 1, to approximately 3/8-inch when
fully grown.

Step 2: Learn pest and host lifecycle and biology
Each female alfalfa weevil is capable of laying between 400 to 1,000 eggs, though not all at once,
Schell said.
“There is a certain threshold temperature to start their development – for the alfalfa weevil, it’s 48 degrees, for alfalfa growth itself, it is 42 degrees,” he said.
Each day that meets the temperature threshold is known as a “degree day.” It takes between 300 to 310 degree days for weevil eggs to hatch, and “noticeable visual damage to alfalfa plants occur by (degree day) 375.”
Producers can access a degree-day calculator at www. pnwpest.org/NW/ddmaps.html.
“It can help tell you when to start sampling your field,” Schell said. “Start scouting at (degree day) 375; peak second instar occurs at (degree day) 425.”
Step 3: Monitor or sample
environment for pest population
Schell said producers should take note of several circumstances, including whether the pest is present or absent, if it’s distributed all over the field or only in certain areas, and whether its population numbers appear to be increasing or decreasing.
See the box for Schell’s explanation of how to use the “bucket method” to determine alfalfa weevil larval densities.
“When the alfalfa hay has reached at least 10 inches in height, it should give you enough to time to” determine management strategies, he said.

Step 4: Establish action threshold (economic: cost < benefit)
“The cost to treat varies with chemical, application method, timing … (and) one year vs. multi-year benefit,” Schell said in his presentation. “Benefits (include) increased yield, nutrient content, stand re-growth speed, health and vigor.”

Step 5: Choose appropriate combination of management tactics
Schell emphasized implementing control tactics – whether early harvest or pesticide – in a timely
“If you are going to harvest early, don’t wait until the alfalfa weevil grubs have completed development and can pupate,” he said.
As far as cultural controls, Schell recommended early spring grazing over fall grazing or burning
in Wyoming.
Studies conducted in Montana show limited success with alfalfa weevil population suppression through sheep grazing from mid-January through the first week of May.
“The Montana study didn’t see reductions in yield between grazed and un-grazed fields,” Schell said, adding early grazing problems could entail risk of bloat, palatability of immature alfalfa, and the need to allow the first cutting to flower to replenish root reserves if grazed early and intensely.
Other management options include alfalfa weevil resistant cultivars: winter hardy, disease resistant, high-yielding varieties to fight the weevils; biological controls, such as wasps – however, Schell said the western strain weevil might have something in its blood that protects it from parasitoids, potentially making this practice less effective in Wyoming; bio-pesticides: either not labeled or not effective; and other insecticide treatments, with relatively new products showing promise, including Steward –Indoxocarb, which must be consumed by the weevil. Schell reminded producers to protect pollinators if they choose to use insecticide.
“Alfalfa, when in bloom, can be very attractive to pollinator insects such as honey and leaf cutter bees,” he said. “Choose insecticide
with care.”

Step 6: Evaluate with care
Schell encouraged attendees to determine if treatment was effective by comparing the yield and feed quality between treated and
untreated fields.
“Evaluate your control with your chosen method: your yield, losses and your costs,” he said. “Whatever tactic you (choose), you have to take paper, pencil and calculator to crunch
the numbers.”


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