Corn rootworm adult emergence is underway in Iowa. The three species of rootworm found in Iowa include the northern corn rootworm (NCR), southern corn rootworm (SCR), and western corn rootworm (WCR). Adults of all three species can be found until the first frost. Knowing how to distinguish the three species is important for making management decisions for future growing seasons.
Southern corn rootworm (left), western corn rootworm (middle), and northern corn rootworm (right). Photo courtesy of Adam J. Varenhorst.
Adult NCR adults are typically one solid color, but their coloration can range from light green to dark green. NCR are approximately 1/4 inch long and do not have markings on their hardened forewings.
Northern corn rootworm. Photo courtesy of Adam J. Varenhorst.
Though mated females lay eggs in cornfields, adult NCR will readily move to other plants. There is one generation of northern corn rootworm per year; however, some NCR populations survive as eggs in the soil for multiple years. This characteristic is referred to as extended diapause, and is an adaptation to crop rotation. Injury to first-year corn by extended-diapause NCR occurs most commonly in the western two thirds of Iowa, but extended diapause may be found elsewhere Iowa.
Adult WCR are typically slightly larger than NCR and are yellow in color with three dark stripes running lengthwise on their hardened forewings. These stripes can vary from three distinct lines to one large stripe covering most of the forewing.
Western corn rootworm (the three on the right are males). Photo courtesy of Adam J. Varenhorst.
WCR females primarily lay eggs in cornfields, however there are instances where eggs are deposited in fields containing other crops. This is a resistance mechanism to crop rotation that is commonly referred to as the “soybean variant.” However, the rotation-resistant WCR can lay eggs in corn, soybean, oats, alfalfa, and winter wheat. Populations of rotation-resistant WCR are found in the eastern Corn Belt but remain rare in Iowa.
Adult SCR adults are bright yellow with 12 black spots on their hardened forewings. Sometimes this species is also known as twelve-spotted cucumber beetle. Their heads, legs, and antennae are always black. Adult SCR are the largest of the CRW species found in Iowa.
Southern corn rootworm. Photo courtesy of Adam J. Varenhorst.
SCR overwinter as adults in states to the south of Iowa, and then establish in Iowa each spring. Adults become active in the spring when temperatures exceed 70°F. After emerging, adult females lay their eggs near the base of corn stalks and larvae will feed on the corn roots. Both adult and larval SCR consume a wide variety of plants. Though SCR are commonly found in cornfields, they rarely cause economic injury to corn.