Scouting for alfalfa weevil and potato leafhopper

By Renae B. Vander Schaff
agripen@live.com

Date Modified: 06/04/2012 2:08 PM

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LUCAN, Minn. — When John Deprez of Lucan checked his alfalfa field he saw lots of insects.

The infestation prompted him to call Extension IPM specialist Bruce Potter at the University of Minnesota Southwest Research and Outreach Center in Lamberton.

It was decided to hold an impromptu scouting clinic on May 15 on 37 acres owned by Dale Mossey, of Clearwater.

"When the idea came up to make a field day of it, I was definitely in favor of it," Mossey said. "I have been the beneficiary of other field days and have often relied on the Extension's good advice."

"It's a pretty good looking field of alfalfa," Potter said. "Insects are indeed plentiful here, some are harmful, others are beneficial. Just because a field has insects, spraying may or not be necessary. If it is, timing of that spray application is important."

If sprayed too quickly, the field might require another application because newly laid insect eggs will have hatched. If sprayed too late the damage is done.

Potter, Deprez and Mossey scouted the field just prior to the clinic. They had a few insect samples to show to the dozen people who attended.

The first insect shown was a spotted alfalfa aphid, which is usually not a problem, Potter said. Natural enemies generally control them and the more common pea aphids. The insects lure predators in, and they dine on more harmful pests. On the other hand, removing natural enemies with unnecessary sprays can lead to an explosion of aphids.

"We did see the alfalfa weevil, though," said Potter, "In both its larval form and as an adult. That is a bit troubling as the adult is still laying eggs and populations will increase."

The weevil in the larvae stage is green with a white stripe down the back with a black head. He explained that adult weevils found here over wintered in grassy and wooded areas along the edges of the field. The weevil is a problem at first cutting and in re-growth. Small holes were found in alfalfa leaves, indicating that there was injury. With the alfalfa in bud stage, Deprez can harvest it instead of spraying. Insecticide shouldn't be used because bees could be harmed.

Deprez and Mossey will scout the field after the hay is harvested for the weevil. Its larvae can get concentrated under windrows. Any larvae surviving the harvest will feed on new leaves and buds, possibly preventing fields from greening up. If regrowth is slow and eight or more weevils are found in a square foot, spraying will be necessary.

If moisture is plentiful and regrowth rapid, weevils are less likely to be a problem. Weevils will thrive if regrowth is hampered by poor soil fertility or drought.

More of a menace during the second and third cuttings of alfalfa is potato leafhopper. They just showed up last week, said Potter. So they haven't been here long. He hasn't found any nymphs yet.

Crop height and number of insects found using the sweep method determines whether spraying is necessary. New seeding alfalfa is the most susceptible. Newly cut fields should be scouted for potato leafhopper as soon as the alfalfa starts to green up.

"Potato leafhopper damage causes stunting and hopperburn, a wedge shaped yellowing of leafet tips," explained Potter. "Alfalfa that has crinkled leaf tips that appeared to have been sprayed is evidence of two other sap feeding insects, tarnished and alfalfa plant bugs."

Mossey wasn't disappointed with the knowledge he took home.

"I was taught how to net sweep the field," said Mossey. "I was taught to find harmful pests and to make the correct decision as to whether to spray or not."