Little weevils make Big Impact on alfalfa fields
By Kylene Scott
"This is the largest population of weevils that I've seen in at least 10 or 12 years in Kansas," Jeff Whitworth, entomology specialist with K-State Research and Extension, said.
What makes this year different? Whitworth has a couple of ideas, and he thinks the infestation has something to do with the weather patterns this spring and in late 2009.
"We probably had a relatively intense infestation this year because we enjoyed relatively mild, damp weather in the fall of 2009," he said. "Weevils start laying eggs in the fall, and they enjoyed relatively mild conditions until December. This provided considerable time for oviposition."
Whitworth thinks there may be other environmental factors, but he hasn't determined exactly what they are.
Typically, it is common to find one larvae per two or three stems, but in 2010 Whitworth is finding two to three larvae per stem. And that is out of the ordinary, he said.
Traditional insecticides still worked, Whitworth said, but they seemed to provide less consistent results than they have in the past.
"This is probably partially due to the intensity and duration of the infestation," Whitworth said. "Some reports indicated there were no problems with insecticidal control. It's just that we did see and heard about some fields that the insecticides treatment provided less control than expected."
To Whitworth those fields were somewhat troubling. It was hard to discern what exactly caused the less-than-expected control.
"I think there are enough insecticides available with different chemistries that they will continue to be utilized and provide acceptable results, but growers may have to switch insecticides instead of just using the same ones year after year," Whitworth said. "There are no good alternatives to insecticides for weevil control at the present time."
What they are
According to Extension sources, the alfalfa weevil was thought to have been introduced to the United States from southern Europe and was discovered in Utah in 1904. It is now present in all 48 mainland states. Whitworth believes Kansas may be one of the only states experiencing such a high infestation of alfalfa weevils in 2010.
"As for the areas of Kansas that are most affected, I have most reports from the south-central part of the state," Kent Martin, KSU Southwest Area crops and soils specialist and certified crop advisor, said. "However, as I understand, the state as a whole is having increased levels of the alfalfa weevil."
Both experts said the alfalfa weevil is one of the primary insect defoliators of alfalfa.
"Early in the growing season, the alfalfa weevil damage will look like small pinholes in the leaves," Martin said. "As the plants progress and as the larvae get larger, they will increase feeding on the tissue between the veins on the leaves."
The weevils will then move to the buds and growing points to feed, and the adults will feed on the leaf margins.
"So, the field appearance will begin as slightly noticeable pinholes to defoliation of the leaves and increase to defoliation of much of the plant," Martin said. "Then, after the adult feeding, the plants will take on a fuzzy or feathery look. This will increase the drying of tissue causing the affected areas to have a gray or white cast to it."
According to a University of Illinois Extension fact sheet, insect pests of alfalfa, including the alfalfa weevil and potato leafhopper, cause hundreds of millions of dollars in losses annually. Though difficult to estimate, these same insects are estimated to reduce yields by 10 to 15 percent annually (forage quality not taken into account). Introduced biological control agents and natural enemies have reduced alfalfa weevil populations in many areas, though it continues to be a sporadic pest of alfalfa.
Weevils will normally appear in alfalfa production cycles.
"The larvae always affect the first cutting. Adult weevils lay eggs in alfalfa stems and in the leaf litter, starting in the fall, and those eggs overwinter as do adult weevils," Whitworth said. "In late winter/early spring then these eggs will start hatching and the adults will continue to lay eggs."
To the alfalfa plant, weevils damage primarily leaves.
"The larvae can cause serious defoliation," Whitworth said. "If the infestation is severe enough, they can completely denude the plant, but usually they get treated before reaching that level of destruction."
That destruction can hinder the primary function of alfalfa, and that is as a livestock forage.
"They (the weevils) can cause serious yield reductions and since alfalfa is a perennial, it will affect the growth of the plant for the rest of the growing season," Whitworth said.
The resulting alfalfa that can be cut will be less than desirable. But the resulting alfalfa condition has nothing to do with the treatment effects, but instead, the weevil's destructive behavior on the plant.
"There won't be as much of it, or it may be more stemy--no effect due to the spraying itself."
"Often, it will seem as though all the damage is done by the first cutting, but close evaluation will show that regrowth and production is slowed on subsequent cuttings," Martin said. "Some studies show that protein content and digestibility are decreased as a result of the alfalfa weevil, so an infestation does more than reduce yield--it reduces quality as well."
Plan of attack
Like most crops, insect control in alfalfa is achieved through spraying, but since alfalfa flowers, pollinators are an important part of the growth cycle of the plant.
"Alfalfa is usually either sprayed by ground rigs or aerial applicators," Whitworth said. "The most commonly used insecticides are generally toxic to all insects, including pollinators."
However, applicators are usually considerate of area bee keepers and try to avoid treating when bees are active. But, weevils tend to appear before the flowering stage.
"Weevils often are problematic before the plants start flowering anyway," Whitworth said. "However, it does take a toll on lady beetles and other beneficial insects if they are present in the fields during or soon after treatment."
Kylene Scott can be reached by phone at 620-227-1804 or by e-mail at firstname.lastname@example.org.
--Early spring pest
Brown, distinctive snout
Light brown with a dark line extending down the middle of the back
Readily fall to the ground when disturbed
--Eggs are laid inside alfalfa stems in fall or spring
Small in size
Light green, black-headed with a distinct white stripe down the center of the body
Feed on the terminal and upper leaves of the plant early in the spring
Reach a quarter-inch in length in about three weeks
--Most damage occurs before the first cutting, but damage by larvae and adults can suppress yields by delaying regrowth.
Treatment: To decide if an alfalfa field should be treated for alfalfa weevil, use the stem-count decision method. Carefully break off 30 to 50 stems selected at random from across the field and shake them individually into a deep-sided bucket.
Refer to the Alfalfa Weevil Stem Count Decision Guide to determine the suggested action.
November: some initial egg laying
Winter: some egg laying as weather permits
Late March/early April: Weevils start to become active. Females insert two to 25 eggs into alfalfa stems.
April: Larvae become noticeable on plants.
Early June: Larvae are readily observable on plants. Green larvae feed roughly three to four weeks depending on quality of alfalfa and temperature. Larvae molt and shed at least three times. Following the last instar, cocoons are spun and implanted upon fallen leaves or plant. Cocoons last about one to two weeks. Once they emerge from cocoon, adults spend an inactive summer. Some older adults will continue to lay eggs. All stages of the life cycle can be found during different times of the year.
Cold weather: Overwintering occurs.