Corn rootworms (Diabrotica spp.) are part of the leaf beetle family (Chrysomelidae). The western and northern species are a significant pest on corn crops in North America. All corn rootworm larvae are ½ inch long with slender white bodies and dark heads. The northern corn rootworm beetle changes from tan to light green as it ages. The western corn rootworm beetle has black and yellow stripes lengthwise on its body. The southern corn rootworm, or cucumber beetle, has black spots over a yellow to green body. All beetles are around ¼ inch in length.
Target Crops
The larvae attack corn and soybean crops while the adults will also damage squash, melons, pumpkins, cucumbers, dahlias, roses, and other flowers.
Life Cycle
Northern and western corn rootworm beetles lay very small (less than 0.1 mm long), white eggs that are shaped like a football. Eggs overwinter in the top 6 inches of soil and begin to develop when spring soil temperature reach 50 to 52°F. Larvae emerge and begin invading corn roots by mid-June, with the largest number of larvae usually found in early to mid-July. Corn rootworms go through three larval stages (instars) and can move as much as 20 inches through the soil during this time.
Different-sized larvae (instars) can be found at any given time. This is due to eggs hatching over a long period of time and larvae growing at different rates. In fact, larvae, pupae, and adults can be found simultaneously in July and August.
After feeding for about 3 weeks, the larva leaves the roots, forms a small earthen cell, and transforms to the pupal stage. Transformation to the adult takes 1 to 2 days. Pupae are white, delicate, resemble the beetle and are often found next to the plant base. The beetles in corn fields feed primarily on pollen, green silks, or leaves.
Rootworm beetles are very mobile and can readily move between fields and may cause damage in locations other than where they emerged. A late maturing corn field can attract large numbers of beetles since neighboring corn may have stopped producing pollen. Beetles may also move into fields which have an abundance of pollen-producing weeds, as well as nearby flowering crops.
Beneficial Insect Control
Steinernema feltiae—a beneficial nematode species. These parasites can be applied to the soil to infect and kill soil dwelling pests. Since corn rootworm larvae are found in the soil, the nematodes are watered in and can help control larval populations. Reapply nematodes every year. For Sierra Biological, release 1 million nematodes per 1000 sq feet of soil medium. For each square foot you will need .66 gallons of water as a carrier to saturate to a depth of 2”. Unless using your nematodes immediately, store in a refrigerator. Do not freeze! Use cool water for application, and do not apply during hot dry conditions.
Dalotia coriaria—rove beetles can prey on eggs, larvae or pupae in the soil. Release 100-500 per every 1,000 sq. ft.
Insecticide Options
For adults on foliage
- Azadiractin or neem oils act as insect growth regulators, antifeedants and ovipositional deterrents. These include AzaGuard and Molt-X.
- For a quick knockdown, use Pyganic with Pyrethrin for an organic adulticide.
- Surround WP is another option. This contains kaolin clay which acts as a mechanical barrier, irritant and disrupts the beetle’s ability to find host plants.
For larvae/grubs below ground
- Mycoinsecticides containing entomopathogenic fungal spores. These include Botanigard, Mycotrol, NoFly and PFR 97. LalGuard M-52 OD used as a drench is especially good for the soil dwelling grub stage.
Cultural Control Tips
- Crop rotation is the most effective way to manage corn rootworms. Western and northern corn rootworm adults have strong fidelity to corn, meaning they almost exclusively feed on and lay their eggs in cornfields. Rootworm larvae are not highly mobile and are generally unable to complete development on other plants. Crop rotation disrupts the corn rootworm life cycle. This is not effective if the rotated crop is in the Cucurbit family, to which adult corn rootworm beetles are attracted.
- Planting corn early can result in plants that have larger root systems when eggs hatch and larvae begin to feed. This can reduce the stress placed on the plant from larval feeding and prevent or reduce lodging.
- Managing weeds eliminates other pollen sources for adult corn rootworms later in the growing season and may force adults to find other areas with food resources for laying eggs. Any volunteer corn should be managed to reduce the chance of non-variant egg-laying in soybean fields that will be rotated to corn the following year.
- Use yellow sticky traps to catch adult beetles and help with decisions regarding best management options for the following season.
