Illinois Natural History Survey - University of Illinois

"Clean Sets in Clean Ground" -- Successful Management of the Imported Crucifer

Weevil on Horseradish

 Illinois leads the nation in the production of horseradish, a spicy cruciferous root used as a condiment. Although a minor crop grown primarily in the American Bottom region of southwest Illinois, horseradish contributes nearly $4 million to the state's agricultural economy. Of the over 50 insect species identified from horseradish in 1955, most were only occasional pests and none produced serious injury directly to the roots. A new insect found in the late 1970s, however, threatened that situation. In May 1977, a Survey entomologist working in the East St. Louis area was called by a grower to examine insect larvae and severe tunneling in harvested roots. Once reared and field-collected adults were available, Survey taxonomists narrowed the identification to the genus Baris and ruled out native species, suspecting that the invader might be Baris lepidii Germar, a small bluish weevil sometimes intercepted in U.S. ports of entry. When the U.S. Department of Agriculture specialists confirmed this identification, Illinois growers were faced with an exotic insect--previously known only on cruciferous crops in eastern and central Europe--that threatened the horseradish crop through direct root damage and as a potential contaminant in the prepared product.

The Tri-County Vegetable Growers Association asked the Survey to determine the incidence and severity of this weevil problem. With state support, a team of entomologists and horticulturists from the Section of Economic Entomology and the University of Illinois set out in August 1977 to determine the biology of the weevil and its pest management on horseradish. The urgent, sensitive nature of the project was soon evident when researchers learned that the first published report of the weevil's occurrence in the Western Hemisphere was being used by some processors to downgrade the price offered for purchase of Illinois horseradish, sight unseen.

Surveys of the growing area found infestations in over half the fields, indicating that the insect now known as the imported crucifer weevil undoubtedly had been present but undetected for several years. An exhaustive bibliography developed from available literature revealed only that B. lepidii was an occasional minor pest in its region of origin and provided little useful information to aid the Illinois studies.


The crucifer weevil, Baris lepidii Germar.

Key to the success of the crucifer weevil project were the multidisciplinary-team approach, grower cooperation, and knowledge of horseradish production practices. Horseradish is produced from secondary roots ("sets") kept in cold storage or earthen pits for spring planting. Harvested late fall or the following spring, primary roots are sold for processing and sets are saved as planting stock. Root pieces remaining after harvest produce extensive patches of volunteer horseradish the following year in rotation crops of corn and soybean. Thus, horseradish is available to harbor the weevil year-round. Within that time frame, however, there were weak links that provided opportunities for population management once studies on the weevil's life history, temperature-related development, and best sampling methods were completed. Feeding injury by weevil adults was minimal; tunneling by larvae caused the greatest damage. The weevil was found to overwinter primarily as adults and eggs in unharvested or volunteer horseradish and as eggs in stored sets; few larvae survived the winter. Eggs laid in roots began development once temperatures exceeded 42deg.F. Because few adults have fully developed wings, dispersal was mainly by ground movement of adults from unharvested or volunteer horseradish and by planting of infested sets.

Destruction of eggs in sets and control of volunteer horseradish became the framework for the pest management program. Seasonal field surveys were conducted to alert growers to possible infestations. Evaluation of several methods indicated that soaking sets in 0.1% permethrin killed eggs without harming sets. The same product could be used as a foliar spray in August, if needed, to kill egg-laying adults. Discing fields after rotation crop harvest followed by glyphosate application four to six weeks later proved to be very effective for control of volunteer horseradish. Growers now had an environmentally friendly program--clean sets in clean ground--with which they could control their own weevil problems.

B. lepidii thankfully is now viewed only as an occasional minor pest of horseradish. We will never know for sure how it came to Illinois. Perhaps it was introduced in sets from out of state; there are unconfirmed reports of the weevil in two other growing regions. Nevertheless, the project provided the Survey with a valuable opportunity to spearhead a cooperative effort with growers, other researchers, and educators to combat an "unwelcome guest" and keep its appetite under mannerly control.

Cathy Eastman, Center for Economic Entomology, with reference to the research and contributions of these individuals from the Illinois Natural History Survey and University of Illinois: Dan Sherrod, John Bouseman, Rick Foster, Bill Luckmann, Mike Burke, Chris Doll, Bill Ruesink, Roscoe Randell, Herb Hopen, Clarence White, and Bonnie Irwin.

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