ProCrop

Spraying For Alfalfa Weevils Increases 1^st Cutting RFV

Significant increases in relative feed value (RFV) of 1^st cutting alfalfa by spraying for the larvae of alfalfa weevils were documented in studies conducted by SDSU Extension Entomology at Lucas, SD in 1999 and 2000.

On the average, spraying alfalfa weevils (initial population: 200 larvae per  10-sweeps-of-an-insect-net) with an insecticide increased 1^st cutting RFV by about 10 points. Surprisingly, the RFV of the "damaged" or unsprayed alfalfa was still good at 132 points as determined using near infrared reflectance spectroscopy (NIRS).

Based on historical RFV market value of $0.68 per ton, and 1^st cutting dry matter tonnage of 1.66 tons per acre, each point increase in RFV may be worth about $1.13 per acre in benefit. Thus, the 10-point improvement in RFV resulting from spraying with an insecticide can translate to $11.29 per acre in benefit.

In addition to RFV, spraying was also found to improve dry matter tonnage by an average of 0.06 ton per acre, or $3.90 per acre assuming an alfalfa market value of $65 per ton. Crude protein (CP) was improved by 0.65 percentage point or $4.19 per acre. This assumes that CP lost to alfalfa weevils will be replenished using soybean meal (44% CP) that costs $180 per ton. Tonnage and CP in the "damaged" or untreated alfalfa were still good at 1.61 tons per acre and 18.35%, respectively. 

The total benefit of spraying alfalfa weevils (at initial field population of 200 larvae per 10-sweeps-of-an-insect-net) in terms of RFV, CP, and tonnage was $20.09 per acre ($11.29 + $3.90 + $4.90 per acre). Alfalfa growers who get these full benefits must keep spraying cost below $20 per acre (chemical plus application). Growers who do not sell alfalfa for cash, and only need 132 or less RFV must keep spraying costs below $8.80 per acre. Obviously, the less money one spends on chemical-plus-application cost, the more benefit goes to the bottom line. Increases in tonnage or CP alone may not pay for the cost of spraying.

Spraying is recommended only if 3 out of 10 plant tips show signs of weevil feeding, and if (and only if) live larvae are still feeding on the leaves. An alternative economic threshold would be 4 larvae per stem (assuming a chemical-plus-application cost of $9.00 per acre, and an alfalfa market value of $65 per ton).

For more details, see the publication entitled "ALFALFA WEEVIL: EVALUATION OF CONTROL PRACTICES IN SOUTH DAKOTA" available for free at any SD Cooperative Extension county offices. Or visit us on the Internet at:

www.abs.sdstate.edu/plantsci/ext/ent "Alfalfa Weevil in South Dakota"

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