Alfalfa growers watch for leafhoppers

(Sioux Falls, SD) Alfalfa growers have faced many challenges this year starting with winter-kill unexpectedly taking many acres out of production.

Fields had to be replanted this spring to make up the loss. Spring weather did not cooperate for planting either, as it was cold and wet.

As alfalfa production moves through to second crop and into the third, producers will have to watch out for another potential challenge – the potato leafhopper. A migratory insect that feeds on alfalfa, the leafhopper is carried into the area by strong southern winds.

The insects aren’t usually a challenge for the first cutting because they don’t reproduce in the Midwest and they haven’t yet shown up from the South, said Fritz Breitenbach, an integrated pest management specialist for the University of Minnesota Extension.

“If they do arrive, the numbers are lower ... and our alfalfa is tall enough that it can tolerate any injury,” he said.

It is the second and third crop where farmers can expect to have the most problems with the leafhopper, especially in seedling alfalfa.

With newly planted alfalfa, the first cutting is not taken for almost 60 days. This gives leafhoppers more time to reproduce and increase in numbers, creating more injury to the plant. Both adult leafhoppers and younger nymphs will feed on alfalfa plants.

Alfalfa fields that are not fully established have a harder time dealing with the injuries from leafhoppers. One way to protect new fields, is to plant alfalfa in the fall.

Challenging growing conditions are another problem when it comes to fending off the leafhopper. Dry conditions make the problem worse.

The control strategy for leafhoppers will depend on the number of insects in the field and the height of the alfalfa. The threshold equates to about a tenth of a leafhopper per inch of alfalfa growth. When looking for nymphs after harvest, it does not take very many, Breitenbach said.

On the other hand, fields that are just a few days or a week from cutting can tolerate a greater number of insects.

It is important for farmers to monitor their fields and watch the populations of potato leafhopper. Running a sweep net through the fields on a weekly basis is a good practice to stay ahead of potential infestations.

Since leafhopper adults can continually move into a field over the season, farmers can expect to see overlapping generations. Populations will steadily increase instead of coming in waves.

Restrictions on post-harvest intervals makes treating old fields with insecticides a challenge. After a field is treated, there is a timeframe when it cannot be harvested, typically one to 14 days, depending on the product used and the rate. The grower must wait, regardless of the maturity or the weather conditions.

The hay may be ready for harvest and the forecast could be showing a pocket of dry weather before potential rain, but the insecticide interval will determine when the field is harvested.

Harvesting will actually kill some insects or cause them to move on. Adults will often fly away, Breitenbach said. Nymphs, which don’t yet have wings, will stay, but the harvesting equipment kills quite a few, he said.

Treating seedling alfalfa with herbicide is a must, in his eyes. “That is the most critical, almost an automatic application in seedling alfalfa,” said Breitenbach.

It is best to consult with an expert to ensure the treated crop remains marketable after application and harvest.