June 5, 2003

 

Watch Alfalfa Now for Weevils, NDSU Specialists Urge

Last season, alfalfa growers in southwestern North Dakota reported significant infestations of alfalfa weevil, according to Phillip Glogoza, North Dakota State University Extension Service entomologist. This year, NDSU specialists are encouraging producers to manage the pest using temperature-based predictions.

Glogoza says the key to effective control is early and persistent scouting. Temperature, or degree day, models indicate that producers need to be on the lookout now for hatching larvae. The degree day system allows entomologists to predict larval hatching within about two to three days.

"For calculating degree days (DD), we recognize that alfalfa weevil development occurs within a range of temperatures. There is a minimum temperature below which no development occurs. From there, the rate of development increases with higher temperatures up to a maximum threshold. Thresholds are determined experimentally and are different for each species of insect," Glogoza says. Egg hatch for alfalfa weevil begins after 300 degree days have accumulated in the spring.

Alfalfa weevil larvae are yellow to light green and legless. When mature, larvae are plump, 3/8 inch long and green with a white stripe down the back.

Once weevils are found, Glogoza recommends monitoring alfalfa tip feeding injury to assess infestation levels and damage. He suggests the following technique:

• Select 50-100 alfalfa stems (10 to 20 randomly selected stems from each of 5 locations) and examine for signs of feeding damage in the leafbuds and growing tip leaves.
• Divide the number of stems with recent tip injury by the total stems collected, convert to a percent, and compare with the threshold.

Thresholds are dependent upon weather conditions. Growers suffer the most evident weevil damage during times of drought. In contrast, rapidly growing plants that receive sufficient moisture have the ability to tolerate greater larval infestations.

The alfalfa weevil threshold before the first cutting is 35 percent (weak stand) of the plants with feeding damage or 40 percent (vigorous stand) of the plants with feeding damage and/or two live larvae per stem.

When the threshold is exceeded, treatment options include early cutting and insecticides. Insecticide application, however, delays the cutting process due to the post treatment interval and can actually lead to economic loss if quality of the hay declines with delayed harvest. Fortunately, Glogoza notes that alfalfa weevil generally do the most damage around normal cutting time, thus immediate cutting can prevent substantial damage.

Roger Ashley, area extension specialist at the Dickinson Research Extension Center, adds that cutting prior to full bloom is advantageous, as alfalfa quality and yield suitable for beef cattle peaks when just one in ten stems is in bloom. Thus, cutting early to combat weevil can also be beneficial in terms of tonnage and value.

Weevil can survive the first cutting. Thus, it is essential to assess the need for management after cutting. Glogoza suggests that producers:

• Monitor regrowth for potential stubble infestations, particularly beneath windrows.
• After the hay has been picked up, sample the stubble and early regrowth in 20 one square foot samples, 4 chosen randomly from 5 locations.
• When regrowth after harvest is sufficiently tall, go back to monitoring tip injury.

After the first cutting, the threshold in stubble is 8 or more larvae per square foot (6 per square foot on sandy soil). If the larvae suppress regrowth, this also indicates that the threshold has been exceeded.

Insecticides are more cost effective at this time, because producers may be able to restrict treatment to the areas where the swaths were laying, as this is where larvae concentrate.

The following insecticides are labeled for alfalfa weevil: Baythroid, carbaryl (Sevin), Furadan, Imidan, Lorsban, malathion, methyl parathion (including Penncap M), Mustang, permethrin (Ambush and Pounce), and Warrior.

For more information about alfalfa weevil control, contact your local extension agent.

###

Source: Phillip Glogoza, (701) 231-7581, pglogoza@ndsuext.nodak.edu
Roger Ashley, (701) 483-2349, roger.ashley@ndsu.nodak.edu
Writer: Teresa Oe, (701) 879-6270, teresa.oe@ndsu.nodak.edu
Editor: Tom Jirik, (701) 231-9629, tjirik@ndsuext.nodak.edu