Issue No. 5, Article 4/April 27, 2007
Effects of Feeding by Alfalfa Weevil Larvae on Freeze-Damaged Alfalfa and Armyworm Larvae on Freeze-Damaged Wheat
A question posed by an agricultural reporter last week spurred me to share my thoughts on the potential effects of alfalfa weevils and armyworms on freeze-damaged alfalfa and wheat, respectively. The article I wrote in last week's issue of the Bulletin (No. 4, April 20, 2007) focused primarily on the effects of freezing temperatures on some insects. (By the way, check out the results from Marlin Rice's experiment regarding freezing temperatures and black cutworm eggs on the Iowa State web site.) However, one wonders what will happen when larvae resume feeding (alfalfa weevils on alfalfa) or begin feeding (armyworms on wheat) in fields with fewer leaves because of freeze damage. Because both of these insect pests are defoliators, it seems logical that with fewer leaves to feed on, large densities of either pest will have a potentially greater impact on crop yields. Consider the armyworm, for example. It is widely known that the primary impact on wheat yield occurs when armyworm larvae feed on the flag leaves. Well, if there are fewer leaves on any given wheat plant, it's safe to assume that armyworm larvae will find their way to the flag leaves (they tend to work from the ground up) sooner in the spring than if the wheat plants had not be damaged by freezing temperatures.
People should be especially watchful for insect activity in fields of freeze-damaged alfalfa and wheat. Published economic thresholds may not apply when plants have been compromised by other factors, e.g., freeze-killed leaves. So use your experience and adjust decisions about control with insecticides accordingly. We do the same thing for corn and soybeans when crop development is affected by other factors (e.g., drought conditions).--Kevin Steffey