North Dakota State University
NDSU Extension Service

Leaf-feeding Weevil in Sugarbeet

E-1273, September 2004

Mohamed F. R. Khan, Extension Sugarbeet Specialist
North Dakota State University and University of Minnesota

Mark A. Boetel, Research and Extension Entomologist
North Dakota State University

Lonny M. Buss, Agriculturist,
Southern Minnesota Beet Sugar Cooperative

Click here for an Adobe Acrobat PDF file suitable for printing. (86KB)

A leaf-feeding weevil, Tanymecus confusus, was first observed feeding on sugarbeet in the southern portion of the Red River Valley (Richland County, ND, and Wilkin County, MN) in 1975. The weevil has since caused problems for growers in southern Minnesota (Big Stone, Chippewa, Lac Qui Parle and Swift counties) in 1992 and 1997, and again in Richland County of North Dakota in 2004. A related species, Tanymecus palliatus, is an occasional pest of sugarbeet in Europe. Feeding injury by adult Tanymecus confusus typically occurs in small areas within sugarbeet fields. Damage occasionally is severe enough to require replanting.

Description and Biology

Adults of the leaf-feeding weevil are about ½-inch long and their head narrows slightly toward its tip into a broad snout. This is a typical feature of the weevil group of insects (a.k.a. "snout beetles"). Leaf-feeding weevils are mottled brown to brownish-gray in color (Figure 1) and their bodies are covered with minute, gray hairlike structures. Since this weevil is a rare sugarbeet pest and it has not been reported on other crops grown in the area, little is known about its biology or its control. It is believed that the insect overwinters as an adult in the soil or plant residues. Adult weevils emerge in the spring and mate, and females then deposit eggs in host plant tissues. Known host plants include pigweed, cocklebur, wild sunflower and ragweed species. Larvae (immature weevils) are believed to feed on these plants until they develop into adults. The adults then re-infest these hosts or, on occasion, infest neighboring sugarbeet plants. No larvae have been observed feeding on sugarbeet.

Figure 1. Adult leaf-feeding weevil on damaged sugarbeet leaf. (23KB color photo)

Damage

Damaging infestations of leaf-feeding weevils in sugarbeet fields have usually occurred during the first three weeks in May, although weevils also have been observed feeding on sugar-beet as late as June. Adult weevils use chewing mouthparts to feed on cotyledons and leaves of young sugarbeet plants (Figure 2). Field damage usually appears as circular or oval-shaped patches in affected areas (Figure 3). The weevils generally feed on leaf edges and interveinal areas, but usually leave the midrib intact. Significant feeding damage can result in seedling death. Younger plants are especially vulnerable to attack. Weevils are difficult to find on or near damaged plants because they fall to the ground and "play dead" by remaining motionless after being disturbed. Also, their dark mottled coloring makes them difficult to detect on the ground. Careful inspection will reveal the insects in plant debris or the soil immediately surrounding damaged plants.

Figure 2. Leaf-feeding weevil damage on young sugarbeet plant. (34KB color photo)

Figure 3. Bare patch in sugarbeet field caused by leaf-feeding weevils. (15KB color photo)

Management

Control of early season broadleaf weeds should be an effective cultural strategy for minimizing the risk of weevil feeding injury to sugarbeet. Field scouting for adult weevils and timely application of insecticides will be necessary to control damaging populations. Insecticides that are labeled for use on sugarbeet and have activity as stomach poisons in other beetles, especially weevils, should control the leaf-feeding weevil.

Photo credits

Figures provided by M. F. R. Khan, L. M. Buss and R. S. Nelson.

Acknowledgements

Partial printing costs of this publication were provided by the Sugarbeet Research and Education Board of Minnesota and North Dakota, which had no editorial input into the content.

Selected References

Armstrong, J. S., A. Cattanach and L. Buss. 1998. Defoliating weevil identified and damage described from the southern Minnesota Beet Sugar Coop area. In: 1997 Sugarbeet Res. Ext. Rep. 28: 256-257.

For more information on this and other topics, see: www.ag.ndsu.nodak.edu

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North Dakota State University
NDSU Extension Service