Spraying for alfalfa weevils can improve relative feed value (RFV) by 10 points, plus there likely will be small gains in crude protein content and yield.

That’s according to Mike Catangui, a South Dakota State University extension entomologist. Carefully analyzing these potential improvements can help growers decide whether or not to spend money on insecticides, says Catangui.

He’s been studying the real and perceived impacts of alfalfa weevils on South Dakota alfalfa since 1993. His data for 1999 through 2000 show that, on average, spraying weevils when the initial population was 200-300 larvae per 10 insect-net sweeps increased first-cutting RFV by about 10 points. But the RFV of unsprayed alfalfa was still quite good, averaging 132.

Based on historical hay prices and a 1.66 ton/acre dry matter yield, he calculated the value of RFV gains at $1.13/point/acre.

"Thus, the 10-point improvement in RFV resulting from spraying with an insecticide can translate to $11.29/acre in benefit," says Catangui.

In addition, spraying was found to improve dry matter tonnage by an average of 0.06 ton/acre, or $3.90, assuming an alfalfa market value of $65/ton.

Crude protein was improved by 0.65 percentage point or $4.32/acre. That assumes that crude protein lost to alfalfa weevils will be replenished using soybean meal that costs $180/ton.

Tonnage and crude protein in untreated alfalfa were still good at 1.61 tons/acre and 18.35%, respectively.

The total benefit of spraying alfalfa weevils was $19.51/acre ($11.29 + $3.90 + $4.32). Alfalfa growers who get these full benefits must keep spraying cost below $19/acre (chemical plus application).

"Growers who do not sell alfalfa for cash, and only need 132 or less RFV, must keep spraying costs below $8/acre," says Catangui. "Obviously, the less money one spends on chemical plus application cost, the more benefit goes to the bottom line."

Spraying is recommended only if three out of 10 growing points show signs of severe weevil feeding, and if live larvae are still feeding on the leaves. An alternative economic threshold would be three larvae per stem (assuming a spraying cost of $8/acre and an alfalfa market value of $65/ton).