No. 12 Article 8/June 11, 2004

What's in Your Weeds? Part I

The past three years have seen increased incidences of stem-boring insects. In various fields, weeds such as giant ragweed, Pennsylvania smartweed, horseweed (marestail), common lambsquarters, and common waterhemp have not been effectively controlled by glyphosate. Because glyphosate is a translocated herbicide, insect injury to plant vascular tissue may reduce glyphosate translocation throughout the plant, compromising performance. Glyphosate is also the only herbicide that allows applications to be delayed until weeds are large enough that insects can tunnel within the stems and roots.

Tops of affected plants become necrotic, but the lower portions seem uninjured and rapidly produce new growth. Upon dissecting the stems of these injured weeds, we have found tunneling throughout the vascular tissue ranging from 1 millimeter wide to 90% loss of the conductive tissue. Various reports have also found tunneling in the root systems of a variety of weed species.

In the summer of 2003, a statewide survey was conducted to determine the distribution in Illinois of this insect-weed interaction. Results identified regions with high occurrences of reduced herbicide efficacy due to stem-boring insects, including western and northeastern Illinois. Table 3 lists the insects we identified and their weedy hosts.

Two main culprits seemed to be quite common throughout the fields surveyed. The first was stalk borer larvae (Papaipema nebris), which is commonly found in giant ragweed. Stalk borer larvae hatch from overwintering eggs in the spring and tunnel into available host plants, usually grasses. The larvae continue feeding in the host plants 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 larger-stemmed host plants nearby, 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. Stalk borer have one generation per year, and more than 100 plant species serve as hosts, including common burdock, curly dock, pigweed species, grasses, and giant ragweed.

Stalk borer larva.

The second insect species found tunneling in weed stems was Dectes stem borer (soybean stem borer), Dectes texanus. Dectes stem borer larvae are about 1 inch long, with an orange-red head capsule and white body. These larvae were found in stems of various weed species. Adult Dectes stem borers, long-horned beetles, are gray and have somewhat flattened bodies. Within a week of emergence, female Dectes stem borers mate; they begin depositing eggs in plant petioles the following week. When the insect reaches the 4th instar (late summer to early fall), it tunnels to the base of the host plant (usually soybean or weed hosts). Tunneling may be so severe that the plant lodges.

Dectes stem borer.

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.

A study was also conducted at the Northern Illinois Agronomy Research Farm in DeKalb to investigate the role of herbicide application timing in insect-weed interactions. Conclusions from DeKalb indicate that the time of application plays a large role in insect infestation (see Table 4 and Figure 1). The time of application affected the number of weeds with tunneling and the number of insects in the weeds. Early postemergence applications and a combination of early postemergence and late postemergence treatments were the most effective in reducing the occurrence of insect-weed interactions. Insects had the highest frequency in the giant ragweed treated with preemergence and late postemergence herbicides. The size of giant ragweed present in the plots tended to be the greatest influence on the presence of insects.

Figure 1. Effect of herbicide application timing on percent of giant ragweed plants with insect tunneling. Herbicide applications were made based on giant ragweed height.

We are just beginning to understand the effects of insect tunneling in weeds. Little is known about the habits and life cycles of many of the insects or about the role of weeds as insect hosts. Additional research is being done to examine the effect of insect tunneling and injury on herbicide rate and uptake effects. This summer we will once again be conducting a survey to determine the distribution of stem-boring insects in Illinois. If you experience this problem in your fields, feel free to contact Kelly Cook (Extension entomologist, 217-333-6652, or Dawn Nordby (Extension weed scientist, 217-244-7497, --Kelly Cook and Dawn Nordby

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