Cooperative
Extension
Service


University of Illinois
at
Urbana-Champaign


No. 5/April 24, 1998

Update on Alfalfa Weevil Activity

Because of a glitch in the system this week, we are unable to provide mapswith accumulated and projected heat units above a base temperature of 48degrees F. However, since the last maps were generated (issue no. 4, April 17,of this Bulletin), the temperatures have been quite cool, so very few heatunits have accumulated. We will provide updated maps next week.

Nevertheless, some folks have reported some alfalfa weevil activity, so I'dlike to share that information with you. On April 20, Omar Koester, Extensioneducator in Randolph County, and Robert Bellm, Extension crop systems educatorin Edwardsville, observed one field in Randolph County in which 75 percent ofthe leaf tips were skeletonized and there were six alfalfa weevil larvae perstem. Another field in the vicinity had 15 to 20 percent skeletonization ofleaf tips and two larvae per stem. Other fields had injury and weevildensities that were at or near our suggested thresholds of 25 to 40 percentskeletonization and two to three larvae per stem. Most weevil larvae in thesefields were first or second instars. Obviously, a few fields in southernIllinois probably have enough damage that treatment with an insecticide couldbe warranted. Refer to last week's Bulletin (issue no. 4, April 17) for a listof suggested insecticides for control of alfalfa weevils.

Matt Montgomery, Extension crop systems educator in Sangamon County, observedsome pinhole feeding caused by very small alfalfa weevil larvae in MenardCounty on April 20. As the weather warms up throughout central Illinois,symptoms of feeding injury caused by alfalfa weevil larvae will become moreevident.

Again, we remind you to watch alfalfa fields carefully for signs of injurycaused by alfalfa weevils. As higher temperatures accelerate theirdevelopment, we should be able to get a better handle on the extent andfrequency of economic problems around the state.

Someone has asked whether our static thresholds should be the same for dairyquality hay as for beef-quality hay. That is an excellent question because thevalues of the two types of hay likely are quite different. As most of youknow, as the value of a commodity increases, the economic threshold usuallydeclines. A few years ago, entomologists at the University of Nebraskadeveloped economic thresholds for alfalfa weevil larvae at the early bud stageof development of alfalfa based on increments of forage value and managementcosts. The thresholds are based upon value of the hay ($ per ton) and controlcost ($ per acre) and are provided in Table 2. These thresholds should allowalfalfa producers some flexibility when they are trying to make decisionsabout whether or not control of alfalfa weevils is economic. I hope thisimproves some of the decisions you folks need to make.

Table 2. Economic thresholds for alfalfa weevil larvae at the early budstage of development, based on increments of forage value andmanagement costs.

Control cost
($ per acre)
Forage value ($ per ton)
455565758595105115
No. of alfalfa weevil larvae per stem
74.03.32.82.42.21.91.81.6
84.63.63.22.72.42.22.01.8
95.24.23.63.12.72.52.22.0
105.84.74.03.43.02.72.52.2
116.35.24.43.83.43.02.72.5
126.95.64.84.23.73.33.02.7
137.46.15.24.53.93.53.22.9

Kevin Steffey (ksteffey@uiuc.edu), Extension Entomology, (217)333-6652