Heavy alfalfa weavil feeding already beginning in western Pennsylvania.
NEW CASTLE, Pa. – Heavy alfalfa weevil feeding is showing up in many fields in the area.
According to western Pennsylvania Extension Agent Ryan Hockensmith, the weevils target established alfalfa fields, particularly fields that are well drained with a south-facing slope.
Hockensmith suggests that you scout alfalfa for this potentially damaging pest.
The larvae stage of the weevil is the most damaging to first cutting. This small (1/4 inch to 3/8 inch) worm-like larvae is green with a white strip and black head. You will usually find the weevils feeding in the tip of the alfalfa stem.
As the feeding progresses, the leaves will become skeletonized leaving the field with a gray cast.
In most years, biological control by a complex of beneficial parasites generally maintains weevil activity at sub-economic levels of activity.
But in some years, such as in 2000, the parasites may not maintain weevil populations at sub-economic levels, and application of a timely insecticide treatment may be warranted to limit economic losses from excessive defoliation, Hockensmith said.
Check for tip injury.
When weevil activity appears to be above normal, tip injury and weevil larval counts in the alfalfa should be made. Select a number of stems at random to determine the percent of tips exhibiting feeding injury.
If tip-feeding injury indicates a potential problem, then walk across the field and select an additional 30 stems at random to determine the number of larvae per stem. Hockensmith suggests placing the stems in a bucket as you collect them and then shaking the weevils off to facilitate counting.
Walk into the field at least 30 to 40 feet before collecting stems. Field margins often have inflated counts that are not representative of the majority of the acreage.
You will also need to note the average height of the alfalfa.
Penn State’s reference guide, “A Pest Management Program for Alfalfa,” provides tables for determining economic thresholds for weevil based on the value of the hay and the cost of the treatment.
Ohio State also makes some generalizations.
* Six-inch alfalfa stand height with 25 percent tip feeding and 1 weevil per stem indicates the stand should be rechecked in seven days.
* Nine-inch stand height with 50 percent tip feeding and greater than 1 weevil per stem indicates a rescue treatment should be applied.
* Twelve-inch stand height with 75 percent tip feeding and greater than 2 weevils per stem indicates a rescue treatment or early harvest to control weevils (harvesting at this stage will give low yields).
* Sixteen-inch stand height with 100 percent tip feeding and greater than 4 weevils per stem indicates the crop should be harvested early.
Watch for regrowth carefully after early harvesting, as a stubble treatment may be necessary.
Stem count critical.
Because environmental conditions and parasites can dramatically reduce alfalfa weevil larvae, the most important variable in making a decision to treat or not to treat is the larvae per stem count. Economic thresholds assume that a larvae count is based on a random sample of stems, not just the stems exhibiting injury. Too often, attention is focused only on plants exhibiting injury.
If diseased larvae are readily observed on the foliage, it may be assumed that additional larvae are infected and that a decline in feeding activity is occurring. Treatment of a declining population will not likely achieve an economic return on the cost of an insecticide application, and will also kill beneficial parasites needlessly.
While rare, Hockensmith reports observing fields that would have benefited from a stubble spray following harvest in previous years. Use two weevils per crown as your treatment threshold for stubble applications.
Any of these products can be used to control alfalfa weevils: Sniper 50W, Furadan 417, Lorsban 4E, Baythroid 2, dimethoate, Warrior 1EC, Malathion, Lannate 90SP, Pounce and Ambush.
* * *
Potato leafhoppers cause damage to fields from the Midwest through the East.
DES MOINES, Iowa – Potato leafhopper infestation levels in alfalfa fields can vary from year to year. What makes this pest so dangerous is that early symptoms are very subtle and can easily be missed.
The best defense against this costly pest is scouting fields to look for leafhopper activity.
“Potato leafhoppers damage alfalfa plants, reduce yield, lower forage quality and sometimes reduce stand persistence,” says Carl Bannon, technical information manager for Pioneer Hi-Bred in Mt. Joy, Penn. “Economic losses from severe leafhopper infestations can reach up to $50 per acre per cutting or one ton or more per acre a year.
“Also, potato leafhopper damage can stunt the plants and make them less competitive, allowing weed infestations into the stand.
“By the time producers see the first signs of damage, which is V-shaped yellowing of the leaves, it’s too late to recapture lost yield or quality.”
Each year, potato leafhopper frequently causes damage to alfalfa fields from the Midwest through the East.
Scouting alfalfa fields with a sweep net is the only effective method of monitoring leafhopper activity. Bannon recommends scouting weekly beginning in late May or early June. Fields should be scouted at least two to three times before the first cutting.
After cutting, once regrowth reaches 2 to 3 inches, Bannon recommends scouting again for at least three consecutive weeks. New stands of alfalfa are more sensitive to potato leafhopper damage and should have the highest priority for scouting.
Most damage takes place on second and third cuttings, after potato leafhopper populations have had a chance to increase. However, new seedlings, which are especially susceptible to leafhoppers because of underdeveloped root systems, may require earlier and more frequent inspection.
Sets of sweeps.
To measure leafhopper activity, Bannon says to make 10 sets of sweeps (10 sweeps per set) from different areas of the field. Make sweeps while walking through an area by moving the net from side to side in a sweeping motion through the foliage. Count both adult and nymph leafhoppers. After each set of sweeps, measure the height of at least two stems in each area.
If leafhopper numbers are at or above threshold levels, the pest can be managed by cutting immediately, provided the crop is close to the normal harvest time.
If threshold levels are present, but the alfalfa isn’t ready to harvest, Bannon recommends producers consider insecticide.
“A timely treatment will prevent a yield and quality loss, but early detection is essential,”
He adds that if infestations are allowed to continue, yield and quality losses begin to occur. Producers can see significant reductions in crude protein from heavy infestations. Leafhopper damage makes it harder for the alfalfa plant to survive the winter and can reduce the overall life of the stand.
Potato leafhoppers thrive in dry weather and when temperatures are between 70 to 90 degrees.
Further information is available through Ohio State University Extension at www.ag.ohio-state.edu/~ohioline/b545/index.html.