Corn beetles, primarily the Western and Northern Corn Rootworms and the Corn Flea Beetle, pose a consistent threat to corn crops. These insects cause significant damage by attacking the plant at different growth stages, potentially leading to substantial yield loss. Managing these pests requires a proactive strategy that integrates cultural practices, biological controls, and, when necessary, targeted chemical interventions.
Identifying Corn Beetles and Damage Signs
The most damaging corn beetles are small, measuring about one-quarter to five-sixteenths of an inch in length. The Western Corn Rootworm adult is yellow-green with three distinct black stripes along its wing covers, while the Northern Corn Rootworm adult is uniformly pale green or tan when newly emerged. The Corn Flea Beetle is much smaller, appearing shiny black or dark brown, and is known for its ability to jump like a flea when disturbed.
The damage caused by these pests varies depending on the life stage of the insect. Rootworm larvae, which hatch from eggs laid in the soil, feed exclusively on the corn roots, causing pruning that hinders water and nutrient uptake. This feeding leads to plant lodging, often called “goosenecking,” where the stalk bends over due to a compromised root system.
Adult beetles target the above-ground plant parts, particularly during the pollination period. Western and Northern Rootworm adults feed on pollen and clip the silks, which can prevent fertilization and result in poorly filled ears. Corn Flea Beetles create small, circular holes in the leaves, referred to as “shot-hole” damage, and are problematic because they can transmit the bacterial disease Stewart’s Wilt to young corn plants.
Cultural and Preventative Management Strategies
Cultural practices focus on disrupting the beetle’s life cycle to provide the foundation for long-term control. The most effective strategy against the Western and Northern Corn Rootworms is crop rotation. Since the larvae can only survive by feeding on corn roots, planting a non-host crop like soybeans or a small grain eliminates their food source, causing the newly hatched larvae to starve.
Altering the timing of planting can also help the corn avoid peak beetle activity periods. Planting corn earlier in the spring allows the root system to become more developed and robust before the rootworm larvae hatch in late spring. This established root structure can better tolerate minor feeding damage, reducing the overall impact on the plant.
Planting corn hybrids that are genetically engineered for resistance, often referred to as Bt corn, is a preventative measure. These varieties express proteins that are toxic to rootworm larvae, significantly reducing root damage. Specific tillage practices can further aid in prevention by disrupting the overwintering stage of the insect. Fall moldboard plowing, for instance, can expose rootworm eggs to harsh winter conditions and predation, reducing the population that hatches the following spring.
Active Non-Chemical Removal Methods
When beetles are present, non-chemical methods offer immediate control without resorting to synthetic insecticides. Introducing biological controls involves releasing natural enemies that specifically target the pests. Beneficial nematodes, microscopic roundworms like Steinernema feltiae, can be applied as a soil drench to hunt down and parasitize rootworm larvae and pupae in the soil.
Physical removal is a practical option for smaller gardens or localized infestations. Adult beetles, such as the larger Western and Northern Rootworms, can be handpicked from the plants and dropped into a container of soapy water for disposal. For very young seedlings, applying a physical barrier like a floating row cover immediately after planting can prevent the small Corn Flea Beetles from accessing the tender foliage.
Trapping techniques are highly effective for monitoring and reducing adult populations, particularly in the mid-to-late season. Yellow sticky traps, such as the Pherocon AM/NB type, are designed to attract and capture adult corn rootworms. These traps should be placed at ear height in the corn rows and checked weekly to gauge the population density and inform further management decisions.
Organic sprays provide a contact-based solution for immediate knockdown of active beetles. Neem oil, derived from the neem tree, works as an insect repellent, feeding deterrent, and growth disruptor against corn beetles. Insecticidal soaps or horticultural oils can also be applied, working by suffocating the insects; these sprays must directly contact the pest and should be applied in the cool hours of the evening to protect beneficial insects.
Chemical Control Options for Severe Infestations
Chemical control is reserved for situations where cultural and non-chemical strategies have failed to keep pest populations below damaging levels. The application timing of insecticides depends on the target life stage. At-planting treatments, often using granular or liquid soil-applied insecticides, are designed to kill rootworm larvae as they hatch from the soil, protecting the developing roots.
For adult beetles, a foliar insecticide spray may be necessary to prevent silk clipping during pollination. The decision to spray is guided by economic thresholds, which are precise measurements of the pest population. A common threshold for adult corn rootworms is reached when five or more beetles are present per ear tip, and the silks have been clipped back to less than one-half inch while pollination is less than fifty percent complete.
Insecticides available to the public include pyrethroids or neonicotinoids, applied as broadcast sprays or granules. You must read and strictly follow the product label directions, as these provide specific safety precautions, application rates, and pre-harvest intervals. Using chemicals with different modes of action (MOA) is also a recommended practice to prevent the beetles from developing resistance to a single type of insecticide over time.