50 Years of Invasive Insect Pests: Managing Western Corn Rootworms, an Old Foe
|Michael E. Gray
Professor & Extension Coordinator
Department of Crop Sciences
It has been suggested that western corn rootworms became an economic pest in the New World following the Spanish introduction of more modern corn production practices that followed European standards (e.g., plowing fields and using large monocultures). Western corn rootworms were first recorded in Kansas in 1868 and were initially described as the Colorado corn rootworm based upon specimens that had been collected in Colorado. Using Colorado as the “staging ground,” western corn rootworms began migrating eastward across Nebraska and were first observed in that state in 1929 and 1930. The most significant infestations of western corn rootworms were found in irrigated fields. Looking back, there seems to be little doubt that irrigation systems used throughout Nebraska facilitated the dispersal of western corn rootworms into less arid states of the Corn Belt. Prior to 1955, the distribution of western corn rootworms was confined to Nebraska, eastern Colorado and Kansas, and smaller more remote locations in South Dakota and Iowa. In 1964, the western corn rootworm was reported for the first time in Illinois. Specimens were collected from Rock Island, County. By the mid-1980s, the migration of this species culminated in its establishment in several coastal states of the eastern United States. However, the eastward dispersal of the western corn rootworm was not yet complete.
Figure 1. Farmers in Illinois become acquainted with the western corn rootworm invasion (circa early 1970s).
Since their invasion of Europe in the early 1990s, western corn rootworms have spread throughout many countries (estimated 15 countries, 2005) on the other side of the Atlantic Ocean. The precise manner in which western corn rootworms gained entry into Europe is not known. In mid-July of 1992, an infestation of western corn rootworms was found in a small cornfield near the Belgrade airport. Early speculation suggested that this infestation may have served as the focal point from which dispersal of this invasive species began across Europe. In November of 2005, a paper was published in Science (Volume 310) in which the authors examined the genetic variation among European and American western corn rootworm populations (eight microsatellite loci) and based upon their statistical analyses, suggested western corn rootworms had most likely entered Europe several times, not just once. Those who are charged with developing and implementing management tactics for western corn rootworms in Europe should be able to learn from the mistakes (e.g., development of insecticide resistance) made in the United States regarding our attempts to control this formidable insect adversary.
Figure 2. Don Kuhlman, Extension Entomologist, University of Illinois, evaluates western corn rootworm damage (circa early 1970s).
Following the discovery of western corn rootworms in Illinois, Extension entomologists recommended the rotation of corn and soybeans as the most effective management tactic for this pest that thrives in continuous corn production systems. The application of granular soil insecticides, at planting, became the primary tactic used in fields where corn was not rotated. Soil insecticide use soared across the northern one-third of Illinois, an area of the state dominated by continuous corn production practices. Although aerial applications of insecticides were applied to some continuous cornfields to reduce egg laying by female beetles, this practice never became as widespread as in western Corn Belt states. Despite the fact that soil insecticides were commonly used to prevent economic losses by western corn rootworms, resistance to these products never developed, primarily because they were applied in-furrow or in a narrow band. Producers unwittingly were employing a refuge strategy (untreated area between rows) before this concept was formally introduced in the mid-1990s when transgenic (Bt) corn became commercially available for the European corn borer. In 1995, the use of crop rotation as an effective management practice for western corn rootworms unraveled across east central Illinois and some areas of northern Indiana. During the past 10 years, we’ve documented extensively that a variant of the western corn rootworm no longer restricts its egg laying to cornfields. Egg-laying targets now include other crops including soybeans, oat stubble, alfalfa, wheat, and wheat double-cropped with soybeans. We believe the exaggerated phenological asynchrony between corn and soybeans (in part caused by the long-term trend to plant corn in early April), in part, may have helped to facilitate the selection for a variant western corn rootworm that has expanded its egg-laying range. In recent years, we have just begun to learn more about the utility of transgenic (Bt) hybrids and their role in an overall corn rootworm management program that should include multiple tactics. Because of the relative convenience and effectiveness, the use of transgenic (Bt) hybrids is becoming the dominant corn rootworm management strategy. In addition, the use of seed treated with systemic neonicotinoid insecticides is rampant. These inputs (seed treatments and Bt hybrids) are utilized with little or no scouting inputs and instead, used almost exclusively on a prophylactic basis. It can be argued that some of the fundamental underpinnings of IPM are being ignored. Will this invasive insect pest and old foe once again teach us some fundamental lessons that we have failed to learn? Time will tell.