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Western corn rootworm can hinder pollination
By Daniel Davidson
DTN Agronomist
OMAHA (DTN)--Western corn rootworm (WCR) is a pest whose larvae feed on root hairs and small roots, larger rootworms feed on primary roots and adults feed on leaves and silks, all of which can reduce pollination.
Eggs are deposited in the soil in mid- to late-summer by adult females. The WCR, which produces only one generation a year, overwinters in the egg stage. Eggs start to hatch in late spring.
After hatching, the small rootworm larvae move to nearby corn roots and begin feeding on root hairs and small roots. Larger rootworms feed on and tunnel in primary roots.
Under heavy rootworm pressure, root systems can be partially or completely destroyed by larvae. Economic losses occur after one or more primary roots are chewed to within 1 to 2 inches of the plant. Adults feed on newly formed corn silks and may clip them, resulting in ears with few kernels.
Injured roots result in growth retardation and weak roots, and the plant may fall over after a heavy rain or wind. Rootworm damage also can result in "gooseneck" lodging and harvest difficulties.
The WCR goes through six stages after hatching--first instar, second instar, third instar, pupa, adult and oviposition.
An instar is a stage between successive molts. For example, the first instar stage happens between emergence from the egg and the first molt. The oviposition is the egg-laying stage.
By mid-July, most larvae have finished feeding and transformed into adult beetles. They emerge from the soil to feed, mate and lay eggs. These eggs produce the larvae that will attack the following year's crop.
Females start laying eggs when they are about two weeks old. They prefer moist areas, so eggs usually are concentrated in the soil between the rows in irrigated corn. Heavier soils with high organic matter content are preferred.
DTN Ag Services is launching a WCR model to track develop of the rootworm from egg to adult to oviposition. The model is a growing-degree-day (GDD) model that provides predictions of life-stage development to a six-mile resolution.
The model is run by GDD accumulation, which begins Jan. 1 and is calculated using hourly soil temperatures.
GDDs are measured by taking the average daily temperature and subtracting 50. If temperatures are below 50, then it's not a GDD because corn will not grow below 50 degrees Fahrenheit. If the average daily temperature is above 86, the average will be 86 because corn does not grow at temperatures above 86 degrees Fahrenheit.
For example, if the average temperature for today was 70 degrees, one would subtract 50, resulting in 20 GDDs for the day.
A color map displays the projected life stage--first instar or egg hatch, second and third instars, pupa, adult and oviposition--for a single generation across the U.S. Corn Belt. Since the insect develops based on temperature, development occurs faster in the south than the north.
In the map key, each life stage is depicted by one color. The first shade represents the lead 5 percent of a population entering that stage. Each progressive shade represents an additional 10 percent of the population entering that stage.
I will be following up with another article on scouting and managing the corn rootworm and using the model to trigger when to scout and what to look for.
Date: 5/20/04
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