ROCHESTER, Minn.- When it comes to managing insects in alfalfa, Bruce Potter suggests a heavy duty sweep net for scouting, one with replaceable parts.
"If you don't tear them up, you're not using them," he said Feb. 12 during Minnesota Forage Days in Rochester.
Potter, a University of Minnesota Extension integrated pest management specialist, said bugs enjoy alfalfa because it's a perennial crop, providing a more stable habitat than those in rotation.
Alfalfa weevils primarily cause damage in the first cutting and in re-growth of the second cutting. They can reduce yield and forage quality, destroy new seedlings and lower the overall stand health. Parasitic wasps were thought to be a good biological control, but now the weevil is becoming more of a widespread problem in the state.Growers should avoid using insecticide as insurance to maintain natural weevil enemies.
"In southeast Minnesota, it's something people should keep an eye on," Potter said during a telephone interview with Agri News.
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Eggs are laid in spring at the base of stems. Larvae, with chewing mouth parts, molt four times and are active until mid-June. Pupae form cocoons at the plant's base and emerge as adults, which overwinter. One generation exists per year.
Growers typically need to start looking in mid-May for alfalfa weevil and it's look-alive, clover leaf weevil. Treat them as a group because they can do the same damage, Potter said .
Use temperature to track their development.Eggs hatch around 300 degree days based on a 48 degree F base temperature. Larvae feed lightly on leaves between 301-438 degree days, but really get chomping between 439-595 degree days. Feeding stops by the time they're pupae, at 596 to 810 degree days.
A sweep net can be used to determine if you have an alfalfa weevil problem. If 25 larvae or more are caught per 180-degree pendulum sweep, which means swinging the net as if you were using a broom, investigate whether or not the pests are reaching economic thresholds. For this, Potter suggests collecting 30 to 50 stems randomly from the field before the first cutting. Shake them off in a bucket - a use a white bucket to spot them easily. Treat the first crop if 30 percent to 50 percent of 50 stems have any feeding at all, of if they're averaging 1.5 to 2 larvae per stem.
If the threshold is met close to bloom or during bloom, it's better to harvest rather than treat it, then watch re-growth. Once cut, the threshold is based on 20 one-square-foot areas, particularly under windrows. Treat when eight larvae per square foot is found or if re-growth is slowed.
About three weeks before the first cutting, start looking for potato leafhoppers while scouting for alfalfa weevils. Potato leafhoppers can arrive as early as May and scouting for them should continue until the season's end.
A culprit of frequent, significant losses in Minnesota, potato leafhoppers are a statewide pest, with populations typically higher in the south and east. Thresholds are normally met in the second cutting. Piercing-sucking mouth parts remove nutrients from leaves. A generation lasts about 28 days, so they can overlap in one growing season.
"This is probably our number one alfalfa insect," Potter said.
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Move the net in a pendulum sweep to make 20 sweeps at five different places in the field, then count how many are caught in each sweep. Economic thresholds depend on plant height: 0.2 adult leafhoppers per sweep if the alfalfa is less than three inches tall, 0.5 adults when alfalfa is four to six inches tall, 1.0 adult or nymphwhen it's 8 to 11 inches tall and 2.0 adults or nymphs when alfalfa is 12 to 14 inches tall.
If the threshold is reached close to the cutting schedule, it's best to cut early, Potter said. Check for eggs or re-infestation in re-growth.Don't go cheap on insecticide rates if the goal is to have season-long control. Residual effects are needed to control eggs.
Glandular haired alfalfa can effectively resist potato leafhopper, but Potter warned against assuming they won't need treatment. Growers could end up spraying if the pests feed on new seedlings or if high populations are present.
No good thresholds exist for specific aphid species, he said, but a general rule is 100 aphids per pendulum sweep.
The event was hosted by U of M Extension, Minnesota Forage Association and the Southeast Minnesota Forage Council.
