Farmers who produce alfalfa need to manage it with a row crop mindset instead of an old fescue pasture mentality.
Pat Miller, University of Missouri Extension agronomy specialist, says more attention to pest management offers higher alfalfa yields for farmers.
“Alfalfa is the queen of the forages,” she says. “It has some high standards to earn that name, and we're going to have to do some things with it that we don't typically do with our other hayfields. And one of those is control the insects.”
There are two main pests alfalfa growers should look for — alfalfa weevil in spring and potato leafhopper in summer. Since spring scouting for weevil is complete, during a recent MU Forage & Livestock Town Hall, Miller encouraged farmers to focus their attention on the potato leafhopper.
Inflicting damage on alfalfa plants
Potato leafhoppers are a small insect about a quarter-inch long appearing yellowish green. The adult has wings and can fly long distances. They do not overwinter in Missouri, but travel via jet streams from the south, which makes their estimated time of arrival unpredictable most years.
“We're never quite sure when we're going to get them,” Miller says. “It may be late May, maybe early June.” Once in the state, they start feeding on alfalfa plants, causing what is known as “hopperburn.”
Hopperburn is a V-shaped yellowing of the leaves because of the leafhopper sucking the plant juices from the leaves and stems from the underside leaf of the plant. They also inject an enzyme that stops the vascular flow, making the leaf turn yellow on the tips. Miller says some alfalfa leaves even turn brown and fall off.
Unfortunately, farmers who wait until seeing yellowing of leaves have already lost yield and have possible plant damage beyond repair. Miller says scouting early is key to controlling potato leafhoppers.
Scouting and control options
Farmers should sweep a field with nets to determine infestation population. “If you're going to grow alfalfa, you need to buy a sweep net,” Miller says. “There's no substitute for having a sweep net to get these samples because [leafhoppers] hop or fly away out in front of you.”
Below are the recommended treatment thresholds at varying plant heights for alfalfa fields with nonresistant potato leafhopper.
Miller explains that resistant potato leafhopper alfalfa varieties have small hairs on the plant, so adults do not crawl and lay eggs on them. These resistant varieties do not require treatment unless a threshold reaches about three times the above numbers. Spraying insecticides during a light infestation may not be beneficial on these types of alfalfa varieties. However, she notes that heavy infestations warrant chemical applications.
There are few other options to mitigate potato leafhopper damages. Early harvest is one option, but Miller warns it typically doesn’t work with high leafhopper populations.
“If they're starting to build up, and it's about time to make the second cutting,” she explains, “you can go ahead and harvest early. You would mechanically kill some of the nymphs that can't fly, but just doing that the adults tend to fly off to the trees nearby. As soon as that alfalfa starts to regrow, you're going to have them back out there, so early harvest might not be the best option if you've got high numbers.”
Grazing may help reduce numbers if done at the right timing. Miller also notes that mixed stands of alfalfa with other pasture grasses can cut down on the number of leafhoppers in a field.