Get ahead of alfalfa weevils that harm crops

Frank Becker
OSU Extension
As forage crops begin to come out of dormancy, so too do the insect pests like the pesky alfalfa weevil.

Activity is starting to pick up in our forage crops. Alfalfa stands are starting to put on new growth, grass species are starting to green up and the fields are starting to look more productive with each passing day.

As the plants begin to come out of dormancy, so too do the insect pests. The alfalfa weevil is one of the first pests of the season for alfalfa growers to be concerned about.

Even though we ran into some cold temperatures, we are still running above average on heat units and accumulated growing degree days.

In a recent article published by the Ohio State Crop Observation and Recommendation Network (C.O.R.N.), the concern for early arriving alfalfa weevil was highlighted and the timeliness of the issue was a focus of discussion.

The over wintering adult alfalfa weevils begin to lay their eggs when the temperatures exceed 48 degrees.

In the article, it is pointed out the peak larval feeding and damage begins to occur between 325 and 575 accumulated growing degree days.

As the warmer days come, this number will begin to climb quickly.

Scout alfalfa early and consistently

It is important to scout your alfalfa early and consistently. Do not let your guard down with the cold weather. The damage from the alfalfa weevil will be first spotted on the leaf tips and will look like small holes or leaves with a tattered appearance. South facing slopes tend to warm up quicker and therefore should be given first priority when you begin scouting.

When scouting for alfalfa weevils, there are some important steps to follow.

Scout for alfalfa weevils by collecting a series of 10 stem samples from various locations. Place the stems tip down in a bucket.

After you have collected 10 stems, shake the stems vigorously into the bucket and count the larvae. Divide this number by 10 to get the average number of larvae per stem.

Do this procedure at least three times (for a grand total of 30 stems, in 10-stem units).

Alfalfa weevil larvae go through four growth stages (called instars). The shaking will dislodge the late third and fourth instar larvae, which cause most of the foliar injury.

Alfalfa weevil leaf injury is evident on these alfalfa plants.

Close inspection of the stem tips may be needed to detect the early 1st and 2nd instar larvae. Also record the overall height of the alfalfa.

The treatment threshold is based on the number of larvae per stem, the size of the larvae and the height of the alfalfa. When alfalfa is around 12-16 inches in height, growers can consider an early harvest rather than spraying, if they feel the current growth is sufficient to justify the cost of harvest or if spraying can't be done for some reason (e.g. organic production).

When alfalfa stem height is over 16 inches, it would be recommended to take an early cutting. In fields that are cut early for alfalfa weevil, the regrowth should be checked closely to make sure weevils that are still alive do not prevent good regrowth.

Weather highs and lows make an impact

Cold weather following above average temperatures also can be damaging to forages. Fortunately, it does not appear the forecast low temperatures actually came to fruition.

The forecast lows going into the cold snap were down around 25 degrees, and according to the NOAA weather station at the Wayne County Airport, we briefly hit 29 degrees, but maintained temperatures in the over night hours closer to freezing.

'While we did not experience the prolonged hard freeze that was forecasted, the temperatures we did experience can still cause damage to perennial and annual forage stands.

Mark Sulc, OSU Extension Forage specialist, gives this advice in his recent C.O.R.N. newsletter article:

"Growers should scout and evaluate their forage stands several days after the cold nights because predicting freeze damage is difficult to impossible. Freeze damage and plant recovery from it are influenced by many factors, including the absolute minimum low air temperature, soil temperature during the freeze event that can moderate near-surface air temperatures in the canopy, field topography, snow cover during cold nights (that provides insulation), age and stage of plant growth, and stand health and vigor as influenced by soil fertility and prior cutting management."

Health and vigor of crop determines tolerance

The stand health and vigor of the crop will help determine the tolerance that it has to freezing and the likelihood of recovery from a freeze injury. Stands that were not cut too late in the fall and have good soil fertility and pH have the best chance to tolerate and recover from freezing temperatures.

Sulc goes on to describe what to look for when scouting for damage in your forages.

"Damage will initially appear as a wilting of the forage legume leaves and stems, with the top of stems bending over into a "shepherd's hook" appearance. This can usually be seen within 24 hours in legumes. This initial wilting might be a little harder to see in perennial grasses.

"This wilting damage is either temporary if the freeze damage is not too severe, or it is the initial symptom of more severe damage that will appear in the next several days. Several days after a severe freeze event, leaf and stem death will become obvious. The shoots and their growing points might be completely killed, but it is rare that the entire crown is killed unless the stand vigor is very poor."

Scouting stand health

Take the opportunity to evaluate forage stands several days to a week after the freeze event to determine if, and how much damage has occurred. Sulc gives the following recommendations and guidelines for scouting and evaluating your stand health:

"If a third or less of the tops are damaged, do nothing as the remaining undamaged stems will provide enough growth and yield. There may be some yield reduction, depending on the stand vigor and health. There will likely be some delay in growth resulting in a later first cutting which will help the stand recover more fully. If most, but not all, of the stem tops are damaged and the stand is less than 10 inches tall, it should recover in time.

"New existing buds in the axils of leaves in the lower canopy will grow and new crown buds might be initiated and grow as well. Mowing existing top growth will not improve the recovery. If most of the stem tops are damaged and the stand is more than 12 inches tall, harvest the forage and allow it to regrow. If all stems are frozen back with severe plant necrosis, the plant is probably dead.

"If a large portion of the plants in the field exhibit these symptoms, it would be best to plant an emergency forage or interseed the stand with an annual grass forage crop. This scenario is most likely for stands that were abused, older, were not healthy and vigorous, were cut in the fall, or have inadequate soil pH and fertility."

By being proactive and evaluating your stand and assessing any damages, you can prepare yourself to make timely and effective management decisions.

Frank Becker

Frank Becker is an OSU Extension Agriculture and Natural Resources Program assistant and IPM Program coordinator.