Alfalfa weevil control guidelines differ among states.
When Oklahoma Ag Experiment Station entomologist Richard Berberet tried to introduce an alfalfa weevil control recommendation table designed for Illinois in his home state back in 1978, he received a nasty surprise.
“It was a disaster,” he recalls. “The farmer's field where I was testing the recommendations was nearly destroyed. The larval counts from Illinois were far higher than the thresholds we have since established in Oklahoma.”
Berberet admits he received a hard and lasting lesson on weevil demographics.
“Growers should always look at weevils based on local conditions,” he says. “What is recommended in one part of the country isn't necessarily true in another.”
Why did recommendations that worked for Illinois fail in Oklahoma? Berberet believes it has a lot to do with climate and the fact that alfalfa weevil larvae often infest the crop during late winter in Oklahoma when plants are 1-2" tall, as compared to much later in Northern states.
Also, introduced weevil parasites are much better adapted to the climate of Northern states.
“There are some weevil parasites that are effective in the North that have done very little for us here,” says Berberet.
Ted Ratcliffe, University of Minnesota entomologist and leader of a biological control program that targeted the weevil in his state, agrees. Before the introduction of Microctonus aethiopoites, a parasitic wasp that lays its eggs in the adult weevil, spraying for alfalfa weevil control was a common practice in southeastern Minnesota.
Now, 20 years later, with the wasp well-established, he says spraying for alfalfa weevils is the exception in Minnesota.
When one compares the economic thresholds for weevil larvae in the northern Great Lakes states with that of Oklahoma, a clear picture emerges. In Wisconsin, the economic threshold is between three or four larvae per stem, depending on the value of the crop and the application cost. In Oklahoma, the average drops to one or two larvae per stem.
The primary reason for the difference is that the weevil attack begins much earlier in Oklahoma, when the plants can tolerate less feeding. Also important is the contribution of the parasites.
“We're seeing a higher mortality rate in weevils where the wasps are established,” says Dan Undersander, University of Wisconsin forage agronomist.
But that doesn't mean growers should assume that weevils are no longer a threat in Wisconsin or neighboring states. In years that favor an increase in the weevil population, the damage can be significant, says Undersander.
“Because we had a long warm spring last year, quite a few alfalfa growers had to spray for weevils,” he says. “It's unusual in Wisconsin but it does happen.”
Even in years that don't favor weevils, growers should continue to scout and be willing to take their first cuttings earlier than normal to reduce weevil damage, he adds.
Berberet notes that in central states like Missouri and Kansas, where temperature variation from year to year is the norm, alfalfa growers should understand that weevil populations can fluctuate dramatically from one year to the next.
“These states have a great deal of variation in populations of damaging larvae,” says Berberet.
He believes growers in those states and farther south should treat weevils as a serious threat. Parasite populations are low, plus the climate gives weevils ample opportunity to lay their eggs.
“Unlike the Northern states where all weevil eggs are laid over a short period in spring, weevils in Oklahoma lay eggs from November through April,” he says. “The peak period is at the end of January.”
Berberet recommends an aggressive scouting regimen beginning when the number of degree days accumulated from Jan. 1 reaches 150. A degree day is computed for each 24-hour period in which the temperature rises above 50°.
Once 150 degree days have been reached, collect a random sample of 30 stems per 20-30 acres of alfalfa, avoiding the edges of the field. Try to cover as much of the field as possible, picking entire stems without dislodging any larvae.
Place the stems in a 2- to 3-gallon plastic container and shake vigorously. Transfer larvae to a flat pan and count them. Record the larvae number and average stem length. In large fields where more than one sample is needed, average the results of the field. Then check your local treatment recommendation chart or ask your forage extension specialist if spraying is necessary, Berberet advises.