The western corn rootworm, Diabrotica virgifera virgifera, is one of the most destructive insect pests impacting corn agriculture across North America. Often dubbed the “billion-dollar bug,” this pest causes significant economic losses annually, including reduced crop yields and substantial management expenses. Understanding its biology and the damage it inflicts is important for addressing its persistent threat to agricultural productivity.
The Pest’s Life Cycle
The life cycle of the western corn rootworm unfolds through four distinct stages, primarily occurring within the cornfield environment. Adult female beetles lay eggs in cornfield soil during late summer and early fall, typically from August into September. These eggs then overwinter in the soil, remaining dormant until the following spring.
As soil temperatures rise in late May or early June, the eggs hatch into larvae. These larvae feed on corn roots, and this is the most damaging stage of the insect’s life. After feeding for several weeks, usually through June and early July, the larvae transform into a pupal stage in the soil.
The pupae develop into adult beetles, which emerge from the soil in mid-to-late summer, typically from July through August. Adult western corn rootworms are yellowish-green with three distinct black stripes running lengthwise down their wing covers. These adults will then feed on corn plants, mate, and lay eggs, completing the cycle.
How Rootworms Damage Corn Plants
The western corn rootworm causes damage to corn plants through both its larval and adult stages, with larval feeding being far more detrimental to crop yield. Newly hatched larvae feed on fine root hairs and burrow into root tips. As the larvae mature, they tunnel into and prune the larger primary roots, impairing water and nutrient absorption. This extensive root damage can lead to stunted plant growth and significant reductions in grain production.
When root pruning is severe, plants may exhibit a symptom known as “goosenecking,” where the weakened root system causes the corn stalks to lodge or fall over. This not only reduces the plant’s photosynthetic efficiency but also makes mechanical harvesting difficult, leading to further yield losses. While less severe, adult beetles also contribute to damage by feeding on corn silks. This silk clipping can interfere with pollination, resulting in poorly filled kernels and reduced grain quality.
Control and Management Strategies
Farmers employ various strategies to manage western corn rootworm populations and mitigate crop damage. Crop rotation is an effective method, as it disrupts the pest’s life cycle. By planting a non-host crop, such as soybeans or alfalfa, in a former cornfield, eggs hatch but larvae perish without a suitable food source. This breaks the continuous cycle of corn production that the rootworm relies upon.
Transgenic corn hybrids, often called Bt corn, are another strategy. These corn varieties produce proteins from Bacillus thuringiensis (Bt). When western corn rootworm larvae feed on the roots of these Bt corn plants, the proteins disrupt their digestive system, causing mortality. This internal defense mechanism reduces the need for external insecticide applications.
Insecticides also remain a tool in rootworm management, applied in various ways to target different life stages.
Types of Insecticides
Soil-applied insecticides can be incorporated into the soil at planting to protect young roots from larval feeding.
Insecticidal seed treatments provide a protective coating on the corn seed, offering early-season control of newly hatched larvae.
Foliar sprays may be applied to control adult beetle populations, particularly when their numbers are high and silk feeding is a concern.
The Challenge of Resistance
Despite the array of control methods, the western corn rootworm remains a persistent and evolving challenge for corn production due to its ability to develop resistance. One significant challenge is “rotation resistance,” where rootworm populations have adapted to traditional crop rotation practices. Instead of hatching after a single winter, eggs from these resistant populations can remain dormant for two or more winters, a phenomenon called extended diapause, hatching only when corn is planted in that field again.
A more widespread concern is the evolution of resistance to Bt corn hybrids. Initially, Bt corn provided effective control, but rootworm populations have developed the ability to survive feeding on corn expressing certain Bt proteins. This has rendered some of the earlier Bt traits less effective over time. In response, agricultural companies have developed corn hybrids with “pyramided traits,” which incorporate multiple Bt proteins with distinct modes of action. However, even with pyramided traits, monitoring for resistance evolution continues to be an ongoing effort to ensure long-term effectiveness.