No. 3 Article 3/April 14, 2006

Early Spring Insects and Weeds--Some Things Just Don't Work Together

With the warmer temperatures across Illinois last week, the landscape has begun to change colors. Unfortunately for some, the new bright colors are not limited to yards and gardens. Traveling to St. Louis last week, we saw not only the damage left from recent storms but also carpets of green, purple, and yellow in many fields (and some of these aren't flowers).

These lush carpets of weeds are made up of many winter annual, biennial, and perennial species common throughout most of Illinois. At this time of year, the vegetative growth of these weeds in no-till fields becomes very noticeable, especially where fall-applied or burndown herbicides have not been used. Weeds that most commonly make up these carpets include purple deadnettle (hence the purple fields), henbit, field pennycress, Virginia pepperweed, shepherd's-purse, chickweed, dandelion, horseweed, and butterweed (which will produce bright yellow flowers in a few weeks). Occurrences of poison hemlock, downy brome, and wild carrot also can be found along field borders. Many resources are available for identifying these weeds, as mentioned in previous articles in the Bulletin (e.g., April 8, 2005, no. 3, article 7).

For quite some time, entomologists have used degree-days to predict insect presence and development. We can use information from growing degree-day (GDDs; base 48°F) as an estimate of heat units required to reach 10% of the total emergence for a weed species. While heat units are easy to characterize, they are not the only factor that influences weed emergence; others include soil type, soil moisture, crop residue, and nitrogen.

To help determine when to scout and what weeds to scout for, along with whether to implement control tactics, midwestern weed scientists have developed a fact sheet to provide guidance on emergence timing and duration for summer annuals and certain perennial weeds. The fact sheet can be downloaded as a PDF file from the Web (Adobe PDF) or you can request it by e-mailing us.

The current degree-day accumulations for Illinois (Figure 1) show that the southern half of Illinois has accumulated enough heat units (150 to 300 GDDs) that folks should begin seeing weeds identified as Group 2 (Figure 2) emerge. The northern half of Illinois is beginning to see emergence of lambsquarters, Pennsylvania smartweed, and giant ragweed from Group 1.

Figure 1. Current degree-days (base 48F) from January 1 to April 10, 2006.

Figure 2. Relative emergence of weeds based on GDDs (base 48F).

These early-season weed species will begin to grow rapidly with an increase in air and soil temperatures; with increasing size, they are more difficult to control and they serve as food and oviposition sites for many early-season insects. In areas where weed problems are greater in the spring, the risk of early-season insect problems is also high. Several insects also utilize weeds as hosts on which to overwinter. The survival of the weeds well into the growing season gives these insect populations ample time to build before moving to other hosts, such as corn or soybean plants.

Black cutworm flights are just beginning in most of the state. Moths have been captured in several southern and central Illinois counties, with Pulaski County recording the first, and only, intense capture of black cutworm moths in 2006 (refer to "Black Cutworm Flights Getting into Full Swing" in this issue). As black cutworm moths migrate into our region, they are attracted to fields with significant weed cover. Winter annuals, such as chickweed and henbit, provide a welcome oviposition site for black cutworm females at the time of peak egg laying. Cutworm larvae survive on these weeds before moving to corn stands. Another migratory moth, the armyworm, also lays eggs in weedy fields. Severe armyworm infestations are generally associated with no-till corn planted into grassy areas. If herbicides are used to control these grasses, larvae will move from the dead grasses to the corn. With the concern of armyworm flights (refer to "True Armyworm Moth Captures Reach Impressive Levels in Kentucky" in this issue), armyworm moths could benefit from an increased weed presence in area fields as they are searching for suitable egg-laying sites.

Other insects overwinter on or near many weed hosts. Corn flea beetles overwinter in the soil in fencerows, roadsides, and edges of woodlands. They become active when soil temperatures near 65°F. Though corn is their primary host, you will often find corn flea beetles feeding on secondary hosts (such as orchardgrass, crabgrass, fall panicum, red top, Kentucky bluegrass, yellow foxtail, and giant foxtail) until corn becomes available. Though mainly a late-spring and early-summer pest, the stalk borer is very dependent on weed populations. Stalk borers overwinter as eggs on grasses (smooth brome, quackgrass, woolly cupgrass, and wirestem muhly) and giant ragweed. Larvae hatch and will feed on weed hosts and continue moving to larger-stemmed hosts, including corn, as they continue to grow.

Twospotted spider mites mostly overwinter in sheltered areas of field margins and generally cause economic damage during years of drought. Infestations usually begin at field edges as the mites move from the weeds on which they overwintered into soybean fields. However, during the summer of 2005, we learned that infestations of spider mites in many areas began within fields because the females had overwintered on weeds such as henbit and chickweed. The presence of these weeds throughout the winter and early spring gave the spider mite populations a host to survive on during dry conditions. In fields that were sprayed with glyphosate just as soybean seedlings were emerging, spider mite populations moved from the dying weeds to healthy soybean seedlings.

We can utilize degree-day accumulations to predict both insect activity and emergence of summer annuals. The presence of weeds in early spring can be easy to control with the use of burndown herbicides (see the Bulletin April 15, 2005, no. 4, article 9, "Considerations for Controlling Existing Vegetation Before Planting"). It is important to remember that these weeds are a suitable host for many early-season insects. This may allow for the buildup of these insect populations before the crops emerge this spring, leading to potential problems later in the season.--Kelly Cook and Dawn Nordby

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