The 2002 growing season presented some "interesting" weed management challenges for growers around the state. In various fields, weeds such as giant ragweed, Pennsylvania smartweed, horseweed (marestail), common lambsquarters, and common water-hemp were not effectively controlled by glyphosate. Often the tops of these plants would become necrotic, but the lower portions seemed uninjured and rapidly produced new growth. Splitting the stems of these weeds revealed tunneling throughout the vascular tissue that ranged anywhere from a 1/4-inch-wide to 90% loss of the conductive tissue.
The culprit in many of these fields with "poor control" was not herbicide resistance or poor herbicide performance but insects feeding within the stems of these plants. This tunneling throughout the stems seemed to compromise herbicide effectiveness. Because glyphosate is a translocated herbicide, insect injury to the vascular tissue may have reduced its translocation throughout the plant, compromising herbicide performance.
Stem-boring insects. Several different insects were found in giant ragweed, Pennsylvania smartweed, horseweed (marestail), common lambsquarters, and common waterhemp in 2002. Of these insects, three species appeared most frequently in fields that experienced reduced weed control with glyphosate.
The first insect species found was common stalk borer larvae, Papaipema nebris, in giant ragweed. Last year was not the first year common stalk borer has reduced herbicide activity on giant ragweed; however, it seemed to have occurred on a larger scale. Common stalk borer larvae hatch from overwintering eggs in the spring and immediately burrow into available host plants (Ratcliffe et al. 2001). More than 100 plant species may serve as suitable hosts for common stalk borer larvae (Wright et al. 2000). These larvae continue feeding in the host plant until they are fully mature, until the girth of available stem material becomes too small, or until host plants are killed. As larvae outgrow their initial host plants, they move to nearby larger-stemmed host plants, usually in June. After they finish feeding, the larvae pupate, and adult moths emerge in late summer and early fall. Eggs are then deposited on weed hosts and overwinter (Ratcliffe et al. 2001). Common stalk borer have one generation per year and acceptable weed hosts include common burdock, curly dock, pigweed species, grass species, and giant ragweed (Steffey 2002).
The second insect observed tunneling in several weed species is yet to be identified to the species level. Both larvae and adults of this insect were observed feeding in weed stems in 2002. In October 2002, John Bouse-man (Illinois Natural History Survey) identified the larvae as belonging to the genus Lixus. The larvae were legless and white, with orange heads; and the adults were slender and brown. Both larvae and adults could be found within weeds that had small-diameter stems. Some Lixus species are noted for their stem-feeding habits on weeds in the Polygonaceae family (Medland and Fewless 2002) and the Amaranthaceae family (Vrablova et al. 1997).
The third insect species found tunneling in weed stems was tentatively identified as Dectes stem borer (soybean stem borer), Dectes texanus. The Dectes stem borer larvae are about 1 inch in length, with an orange-red head capsule and a white body. These larvae were found in stems of various weed species. Adult Dectes stem borers are gray, with very long antennae, and have somewhat flattened bodies. Within a week of emergence, female Dectes stem borers mate and begin depositing eggs in plant petioles the following week. When the insect reaches the fourth instar (late summer to early fall), it tunnels to the base of the host plant (that is, giant ragweed and cocklebur) (North Carolina Agricultural Extension Service). Tunneling may be so severe that the plant lodges.
Implications of insect-weed interactions. The effectiveness of certain translocated herbicides was compromised in 2002 in certain areas because of insect-infested weeds. Although this interaction has happened in the past, it seemed to be on a much larger scale last year. These interactions raise a number of questions on how we will approach postemergence herbicide applications: (1) Will we continue to see these insect-weed interactions occurring that reduce effectiveness of translocated postemergence herbicides? (2) If these interactions occur, what are the implications of not controlling these insect-infested weeds? (3) What may be some of the approaches to control these insect-infested weeds?
At this time, we don't know whether these interactions were rare occurrences in 2002 or whether they will continue to be a problem in the future. Implications of not controlling these weeds on a larger scale may relate to competition for yield and future weed problems because of seed production. For example, Lixus species such as L. cardui have been reported to reduce plant vigor and size but not seed production (Woodburn and Briese 1996). Although glyphosate continues to be a convenient, efficacious, and economical herbicide, it seems that most of the insect-infested weed escapes occurred on larger plants. If these weeds are targeted for applications at smaller stages, they may be more easily controlled and less attractive to stem-boring insects. Many questions still remain related to these insect-weed interactions that need to be answered.--Christy Sprague, Matt Montgomery, and Aaron Hager
Medland, V., and G. Fewless. 2002. Cofrin Arboretum Center for Biodiversity: Photo of the Week: Week of July 14, 2002. Biodiversity Center Home: Photo Archives: Week of July 14, 2002. University of Wisconsin-Green Bay. Green Bay, WI. Available at http://www.uwgb.edu (posted July 2002; verified September 2002).
NCAES. Soybean Stem Borer. North Carolina IPM Network. North Carolina State University. Raleigh, NC. Available at http://ipm.ncsu.edu (verified September 2002).
NebGuide. University of Nebraska. Lincoln, NE. Available at http://www.ianr.unl.edu (posted April 2000, verified September 2002).
Ratcliffe, S. T., M. E. Gray, and K. L. Steffey. Common Stalk Borer: Papaipema nebris. Insect Information. no. 21. Available at http://www.ipm.uiuc.edu (posted 2001; verified September 2002).
Steffey, K. 2002. Stalk borers could be moving into cornfields in some areas. Pest Management & Crop Development Bulletin. May 24, 2002, vol. 2002, no. 9, p. 98.
Vrablova, M., P. Toth, and L. Cagan. 1997. Lixus species (Coleoptera: Curculionidae) damaging Amaranthus plants in Slovakia. Biological Control of Weeds, p. 22 (poster).
Woodburn, T. L., and D. T. Briese. 1996. The contribution of biological control to the management of thistles. Thistle Management, p. 250. (Reprinted from Plant Protection Quarterly, vol. 11, supplement 2, 1996).
Wright, R. J., T. Hunt, and K. Jarvi. 2000. Common Stalk Borer in Corn.