Manhattan, Kansas, USA
April 15, 2011
Alfalfa has been under stress in many parts of Kansas during much of March and April. Drought, alfalfa weevil and various aphids are causing stunted growth and damaged leaves, said Jim Shroyer, K-State Research and Extension crop production specialist. This will affect management plans.
“Normally, the first cutting of alfalfa should be made when regrowth at the crown is apparent. In the spring, this occurs prior to bloom. But this year, the combination of drought and heavy insect pressure has stressed the dryland alfalfa,” Shroyer said.
Producers may have to consider taking their first cutting earlier than they’d like, he said, even if regrowth at the crown has not yet begun. Leaves contain more nutrients than stems, and it’s important to retain as many of the leaves as possible to produce high-quality forage, he explained.
“If producers need to make the first cutting before the optimum time, root reserves on newly-established stands or even older stands may not be satisfactory to permit rapid regrowth. But if the alfalfa is not cut, the hay crop may be lost and damage to the stand may occur,” Shroyer said.
If producers are forced by drought stress to make the first cutting earlier than the ideal time, it’s important to delay the second cutting enough to allow nutrient reserves in the roots to replenish, he added.
Where weevils are present, producers have to decide whether to spray with a pesticide first and then cut, or to forgo spraying and make the first cutting, said Jeff Whitworth, K-State Research and Extension entomologist.
“If growers decide to cut instead of spray, they need to watch the fields right after making the cutting to make sure adult weevils don’t attack the stems. If there’s little or no growth a few days after cutting, that’s an indication that the stems are under attack,” Whitworth said.
“The adult weevils chew around the bark – a condition called barking – and that restricts growth. If producers determine the weevils are still a problem, growers should go ahead and spray.”
Producers should make sure alfalfa weevils and aphids are controlled, since this will have long-lasting impacts on productivity, the K-State entomologist stressed.
If producers need to spray and swath early they can refer to the K-State Research and Extension publication, “Alfalfa Insect Management 2011” (MF-809) www.ksre.ksu.edu/library/entml2/MF809.pdf for insecticides with a very short pre-harvest interval (PHI), some with 0-1 day after treatment.
With warm weather in April, such as we’ve had this year, insect development will progress rapidly, Whitworth added. A field that may not have shown much damage three or four days ago can start to take on a silver or frosted appearance from a distance as alfalfa weevil larvae emerge from eggs, grow larger and inflict more damage, he said.
“Treatment of alfalfa three to seven inches tall may be justified when feeding is evident on the top inch of growth and one to two alfalfa weevil larvae per stem are present. When alfalfa is eight to 14 inches tall, growers should consider treatment when larvae are found causing significant feeding damage to the top one to two inches of growth on 30 to 50 percent of terminals or about one-and-a-half to two (or more) larvae are present per stem,” he said.
With luck, one treatment will be all that is required in most locations, but growers are advised to keep watch as their hay crop develops, he said.
In addition to weevils, producers should pay attention to aphids, he added. Pea aphids have a dark band encircling the base of each antennal segment, but blue alfalfa aphids do not. Cowpea aphids are much darker -- sometimes almost black -- and they frequently seem to congregate near the tips of infested stems, he explained.
Evaluate plant vigor and aphid densities to determine if treatment is warranted. “Fifty pea aphids per 10-inch-tall alfalfa is thought to justify treatment, whereas 20 blue alfalfa aphids should be considered threatening. Although there is less research data regarding the importance of cowpea aphids, recommendations suggest that treatment thresholds for pea aphids be followed if these dark aphids are present in significant numbers,” Whitworth said.
In most cases, fewer aphids are required to trigger concern about smaller alfalfa, he said.
“In past years, we have sometimes seen aphids develop to significant levels in Kansas when dry weather held back the initial alfalfa growth,” he said.
At low levels, however, aphids may not cause the crop significant harm, and they can serve as important food sources for beneficial insects that overwintered in the field before dispersing into other crops, Whitworth added.
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