Insects and Mites

Corn Rootworm

Scientific name: Diabrotica virgifera (Western) and Diabrotica barberi (Northern)
Order: Coleoptera
Family: Chrysomelidae

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Info

Biological Description
  • Adults: Beetles are about ¼ inch long. Northerns are tan when they first emerge but then turn pale green. Westerns are yellow with three black lines down the back. The lines on the male western blur together.
  • Larvae: Fully grown larvae are approximately ½ inch long and the diameter of medium pencil led. Heads are brown to black and there is a dark plate on the dorsal side of the last abdominal segment.
  • Pupae: White in color.
  • Eggs: Eggs are oval, creamy, white, and measure about 0.1 by 0.02 inches (0.3 by 0.5 mm).
Economic Importance
  • Corn rootworms are some of the most destructive insect pests of corn in the southern two-thirds of Wisconsin.
  • Outbreaks:
    • Late-planted corn is more susceptible to adult feeding injury on leaves and silks because beetles are attracted to fresh pollen and silk. Late-planted fields will attract beetles from surrounding, more advanced fields.
  • Susceptible crops:
    • Corn (the only plant that larvae can feed upon)
    • Ornamental flowers (pollen feeding by adults)
    • Vegetables (pollen feeding by adults)
Life Cycle
  • Northern and western corn rootworms overwinter in Wisconsin as eggs in the upper soil layers.
  • In the late May and early June eggs complete development and larvae emerge. Larvae may be present throughout the summer, but commonly damage peaks mid-July.
  • After three weeks larvae pupate. Pupation lasts about two weeks.
  • Adults typically appear between July 16-24.
  • Females begin laying eggs in corn about two weeks after emerging from pupation. In Wisconsin, this starts in early to mid-August and continues well into September. Females can lay up to 1,000 eggs, but 300-500 eggs is most common.
  • Generations:
    • There is one generation per year.
Damage/Symptoms
  • Types of feeding:
    • Adults: feed on silks (reducing pollination, which can result in incomplete kernel fill), pollen, and leaves.
    • Larvae: tunnel into and feed upon roots, which can cause plants to lodge and gooseneck.
      • First instar larvae feed on smaller branching corn roots.
      • Larger larvae migrate toward roots at the base of the plant.
  • Evidence of larval rootworm feeding consists of brown, elongated scars on the root surface, tunnels within the roots, and varying degrees of root pruning.
  • Larval damage peaks mid-July.
  • Lodging caused by root pruning is common after storms with heavy winds and rains. Slight to moderate lodging can result in reduced ear weight and a goose-necked stem.
  • Adult beetles lay more eggs in moist soil than in dry soil.
Scouting Procedure and Economic Threshold
  • Scouting is used to determine the potential damage to corn planted the following year.
  • When to scout: Beginning late July through mid-September, scout three times in 7-10 day intervals.
  • Examine 10 random plants in five separate areas (total of 50 plants) for each variety and planting date.
    • Move quietly through the field to not disturb the beetles.
    • When approaching a plant, hold the ear tip tightly in your hand so beetles do not escape as you look for beetles on the rest of the plant first. When you are done with the rest of the plant open your hand slowly and count the number of beetles that come out of the silks as you strip the husk away from the ear tip.
    • Pull leaves away from the stalk to adequately examine leaf axils.
  • Record the number of northern and western corn rootworms per plant.
  • Economic threshold: If an average of 0.75 beetles per plant is found during any of the three field samplings, a rootworm soil insecticide, crop rotation, or a Bt CRW hybrid is recommended for the following year.
Integrated Control
  • Natural control: Low soil temperatures in the winter as a result of little snow cover may contribute to high egg mortality of the western corn rootworm.
  • Cultural control:
    • Crop rotation is an excellent way to control rootworm populations, though problems with extended diapause (northern) and egg laying in soybeans (western) have been noted in areas outside of Wisconsin. In 2005 and 2006 variant western corn rootworm was found in Wisconsin in high enough numbers to recommend treatment for particular first year corn fields. (More information can be found here.) Larvae cannot survive if a crop other than corn is planted in an infested field.
    • Planting early season corn will reduce the severity of injury caused by adult silk clipping because most of the corn will be finished pollinating by the time beetle emergence peaks.
    • Late-planted corn usually avoids the worst root damage because roots are too small to feed the rootworm larvae. Corn planted after mid-June usually does not require a rootworm soil insecticide. Late-planted corn will, however, attract adults to the silks.
    • Early harvest in August (e.g. for silage) can reduce the amount of eggs that are laid in that field.
    • A semiochemical-based bait can be used to attract and kill adult corn rootworms. This type of bait pairs a small amount of insecticide with a natural feeding stimulant specific for rootworms. Used during oviposition, these baits can suppress adult population densities to below-threshold levels.
  • Biological Control: Ground beetles and predacious mites may control rootworm populations by feeding on eggs, larvae, and pupae. Adults and larvae are essentially free of parasites.
  • Chemical Control: For current Wisconsin recommendations, consult University of Wisconsin-Extension Bulletin #A3646, Pest Management in Wisconsin Field Crops.
  • Insecticide Resistance: Resistance has developed to carbaryl and methyl parathion in areas of Nebraska where there has been a history of adult control.

Excerpts from Integrated Pest Management Program – University of Wisconsin-Extension, Cooperative Extension Service. Field Crop, Crop Scouting Manual. 2001

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