Ohio alfalfa growers should start monitoring for alfalfa weevils now that the weather has warmed, warn Ron Hammond, Andy Michel and Bruce Eisley, Ohio State University entomologists.

When heat unit (HU) accumulations, beginning Jan. 1, reach between 250 and 300, weevil eggs hatch and growers will be able to see feeding. In southern Ohio, Piketon is showing 243 HU and Jackson, 234 HU. The Western Branch, near South Charleston, is only at 174. Central and northern Ohio should reach the 250-300 level in the coming week or so, the entomologists estimate.

Fields with south-facing slopes tend to warm sooner and need to be checked for weevils earlier. For heat unit accumulation updates, watch for Ohio State’s C.O.R.N. newsletter in coming weeks.

To scout for alfalfa weevils, collect a series of three 10-stem samples from random field locations. Place stem tips down in a bucket. After 10 stems have been collected, vigorously shake stems in the bucket and count the number of larvae in it. Shaking dislodges late third and fourth instar larvae that cause most of the foliar injury. Closely inspect stem tips to detect early first and second instar larvae. Also record alfalfa height at this time.

Economic threshold is based on the number of larvae per stem, the size of the larvae and the height of the alfalfa. Rescue treatment may be needed if one or more large larvae per stem are found on alfalfa 12” tall or less. Where alfalfa is between 12 and 16” tall, the action threshold should be increased to two to four larvae per stem depending on the vigor of alfalfa growth. If alfalfa is 16” tall and more than four larvae are found per stem, harvest early, they recommend.

See the OSU Alfalfa Weevil Fact Sheet for more on alfalfa weevil scouting and thresholds. For insecticides labeled for alfalfa weevils, see Alfalfa Bulletin 545 – 2010.

Ohio growers don’t have to worry about potato leafhoppers yet, the experts say, because these pests don’t move into the state until May.