How producers control the alfalfa weevil depends on how they use the crop.
Insecticides are the best bet in alfalfa seed crops, while cutting is the way to go if it is to be used as forage.
Manitoba Agriculture entomologist Brent Elliott said the alfalfa weevil overwinters on the Prairies as an adult beetle in plant debris within the field.
Beetles emerge in spring to feed on new growth. In mid to late May, adult females deposit eggs in alfalfa stems. The eggs hatch shortly after being laid and the young larvae move to the tops of plants to feed on buds and new foliage.
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This feeding can occur for six to eight weeks and damage seed crops.
When the larvae are fully developed, they drop to the ground and form a cocoon for pupation. They remain in this stage for a week or two and then new adults emerge, usually in early July.
Adults may feed in July but do not cause economic damage. They will overwinter to start the cycle again the following spring.
Jack Feenstra, an Agri-Trend consultant with Chinook Crop Care in Nobleford, Alta., said normally the alfalfa weevil doesn’t become a problem in his area until the temperature warms, typically around early June.
“By then, we have a foot of growth and the alfalfa outgrows the problem.”
Adult weevils are small, less than a quarter inch long, and tend to be dark grey to black in spring. Adults found during the summer are usually dark brown, with a conspicuous dark stripe extending most of the way down the centre of the back. They have a pronounced “snout.”
Larvae will reach nearly half an inch in length when mature, but are tiny just after hatching. They do not have legs and have a dark head capsule, which helps distinguish them from predatory larvae that are in the field at the same time.
Young larvae at the first two instar development phases are light brown to brown. More mature larvae, at instars three and four, are green with a white line down the middle of the back.
“(The larva) are generally in the tip of the plant, where the young leaves are,” Feenstra said.
“You can find them if you take a handful of plants and whap them on the hood of your truck. They’re a similar size to diamondback moth larvae – one or two millimetres. They’re visible, but just.”
Early feeding damage is not noticeable. Adult beetles will cut shot holes in the foliage. Unlike shot holes caused by flea beetles, these are larger, quite visible and seldom round or consistent in shape.
Larval feeding begins as pinhole size damage and increases in size as the larvae grow. Notches in leaves soon appear and in severe damage, the larvae will skeletonize the leaves by feeding on all the leaf material except the veins.
Heavy infestations cause considerable defoliation and may result in the field appearing frosted or bleached out.
Forage field damage includes loss of protein and relative feed value. In seed fields, larval feeding can also cause substantial damage to buds, significantly reducing seed set when populations are high.
If a field that should be blooming is not, there’s a strong chance significant alfalfa weevil damage has occurred.
Early scouting should focus on parts of fields that warm up faster such as dry, unshaded areas. In Manitoba, peak damage tends to be around the timing of the usual first cut of alfalfa for hay.
Cutting is not an option in seed fields, but this gives an accurate idea of timing for the beetles’ most significant damage. Look for shot holes initially, then clipping along the edges of leaves and pinhole damage.
Economic thresholds
For seed alfalfa, several thresholds have been established. Thresholds are generally based on either the number of larvae per sweep net sample or the percentage of damage to foliage tips.
The field may be at the economic threshold for an insecticide treatment if 35 to 50 percent of the foliage tips show feeding damage or there are 20 to 30 third or fourth instar larvae per 180 degree sweep sample.
If hot spots are found in fields, producers can treat only the areas with significant populations of weevils. It’s not necessary or economical to treat the entire field in all cases, because these insects, especially larvae, are not active or mobile.
Insecticides are rarely economically justified if alfalfa is grown for hay. The best option in hay fields is to cut the field when possible.
General economic threshold for hay operations is based on crop height:
- Thirty centimetres – one larvae per stem.
- Forty cm – two larvae per stem.
- Three larvae per stem requires immediate action regardless of crop height.
Larvae may occasionally remain feeding after a first cut, which could significantly affect regrowth of the second crop and lead to stand loss.
Producers with severe infestations should monitor stubble regrowth. The field will require insecticide application if two or more active larvae are present per crown or four to eight larvae are found per sq. foot.
In some situations, alfalfa weevils can become concentrated into the swath during the first cut.
“When the regrowth starts, there’s a concentration of alfalfa weevil larvae under the swath and the threshold numbers are extreme at this location. They can do a lot of damage,” said Feenstra.
“Last year I was in a producer’s field, where he used a double swath swather. Three weeks after silaging, the field was four inches tall in the swath row and 18 inches tall between the swaths. All the larvae were in the centre of the double swath.”
The small regrowth plants could never outgrow the damage because of the high pressure from the weevils, but the crop stayed ahead between the swath rows once the rest of the field got ahead of the insects.
In that case, Feenstra said the producer sprayed the entire field from the air.
Seed fields can’t be cut so in most instances insecticides are advised once the economic threshold has been exceeded.
Little useful information is available for controlling the weevil in seed fields on which conventional insecticides can’t be applied.
Alternative controls such as cultivation or flaming with propane have been geared toward hay producers rather than seed operations.
None of the insecticides acceptable under organic standards show any registrations for alfalfa grown for seed, so there appears to be little help available there.
