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Issue No. 4, Article 2/April 20, 2007

Effects of Cold Temperatures on Some of Our Early-Season Pests (Alfalfa Weevil, Black Cutworm, Subterranean Insects)

Much has been written about the effects of last week's low temperatures on crops (alfalfa, wheat), but some have wondered about the effects on some of our early-season insect pests, particularly alfalfa weevils, black cutworm, and white grubs. Mortality of all insects is caused by temperatures both above and below designated lower and upper lethal temperatures, which vary among insects and fluctuate (to a certain extent) according to many factors. Although we often have considerable information about a given insect's development at different temperatures, upper and lower lethal temperatures are not known for most insects. However, some generalities may apply to our conjectures about the effects of last week's low temperatures on some insect pests.

In general, when temperatures are below freezing, most insects die as a result of their tissues freezing. Insects can be categorized broadly into freeze-tolerant and freeze-intolerant types. Most overwintering insects and most temperate species are thought to be freeze-intolerant. Without delving deeply into insect physiology, suffice it to say that even in freeze-intolerant species, several factors affect mortality or survival. One of the primary factors is length of exposure to below-freezing temperatures. In other words, will a dip below freezing on any given day cause instantaneous mortality, or is sustained exposure to freezing temperatures required before significant mortality occurs? The scientific literature indicates that mortality of western and northern corn rootworm eggs overwintering in the soil increases with the length of exposure to freezing temperatures (<20°F). Eileen Cullen, extension entomologist at the University of Wisconsin in Madison, wrote an excellent article about this in the March 22, 2007, issue of Wisconsin Crop Manager. However, we often can only speculate about the effects of freezing temperatures on insect pests that become active early in the season.

It is safe to assume that early instars of black cutworms are freeze-intolerant, so any larvae that may have hatched from eggs laid in early to mid-March likely perished if they encountered freezing temperatures. Black cutworm eggs probably are more freeze-tolerant than early instars, but it's possible that some black cutworm eggs froze, too, possibly negating degree-day accumulations that began with any intense captures in March (9 or more moths captured in pheromone traps in one or two days). Freezing or very cold temperatures also might cause indirect death in black cutworm larvae if the larvae stopped feeding and starved to death.

The scenario for alfalfa weevil larvae and freezing temperatures may be the same as for black cutworm larvae. Young instars (first and second) probably are more freeze-intolerant than older instars (third and fourth instars), but early instars often are protected within folded leaves. If the leaves froze, however, it's likely that the larvae did not survive. Kelli Bassett, natural resources management extension educator in Hillsboro, found many dead (brown and shriveled) third instar alfalfa weevils in an alfalfa field in Montgomery County, and it's likely that freezing temperatures caused the mortality.

So, what about our friends the white grubs, especially those widely despised Japanese beetle grubs? Throughout the winter meeting season, I shared with many people the biological fact that third instar Japanese beetle grubs (the last instar--the instar that overwinters and feeds in the spring) are susceptible to freezing below 20°F. However, we are not certain about the effects of length of exposure to such low temperatures on Japanese beetle grub mortality. To that point, it is worth sharing an observation by David Faulkner, crop testing technician in the Department of Crop Sciences at the Dixon Springs Agricultural Center. He examined 1-cubic-foot samples from several plots for white grubs and found plenty of seemingly healthy specimens, many within 1 to 2 inches of the soil surface. About 90% of the average 8 to 10 grubs per cubic foot of soil were Japanese beetle grubs.

Japanese beetle grubs 1 inch beneath the soil surface in plots at the Dixon Springs Agricultural Center, April 12, 2007. (Photo courtesy of David Faulkner, University of Illinois).

Although the cold temperatures last week had some devastating effects on some crops, it's likely that populations of some of our most worrisome early-season insect pests suffered, too. Unfortunately, insects somewhat insulated from the freezing temperatures (e.g., in the soil or under considerable vegetative growth or residue) probably were not affected much. So as an early heads-up (and not a novel statement), we should prepare ourselves for Japanese beetles that will begin emerging in June. In the meantime, keep watching crop fields for the presence and activities of pests, the only guaranteed way to determine their potential threat.--Kevin Steffey

Kevin Steffey

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