Corn rootworms (Diabrotica sp.) are a devastating pest of cultivated corn in North America, with losses and management expenditures totaling over 1 billion dollars. In Eastern Canada, the northern corn rootworm is the most common species found on corn. While adult Diabrotica barberi can feed on a variety of plants, their larvae survive exclusively on the roots of corn. This allows producers to control infestation, for the most part, by rotating their corn crop yearly with another plant (typically soybean).
Life cycle and appearance of Northern Corn Rootworm
Adult Diabrotica barberi beetles are between 5–6 mm long and uniformly pale green or yellowish green in colour. They appear in late July and remain in corn fields until first frost. Females lay their tiny, white eggs in the soil at the base of corn plants in late summer/early fall and each can lay up to 300 eggs.
Northern corn rootworm eggs remain dormant in the soil through fall and winter until late spring/early summer (univoltine diapause). The white, threadlike larvae then emerge to begin feeding on corn roots. Certain populations of this species have evolved a resistance to crop rotation through prolonged diapause; the eggs of “semivoltine” Diabrotica barberi remain dormant in the soil for two winters. This allows the larvae to effectively skip the field’s soybean rotation (when they would starve for lack of their corn host) and hatch after corn has been replanted. Univoltine and semivoltine rootworms can interbreed, and they coexist in population patches across corn-producing regions, depending on the method of crop cultivation used in each field. Semivoltine insects are at a disadvantage under continuous corn cultivation because they have effectively half the reproductive rate of their univoltine counterparts. Because of this, semivoltine Diabrotica barberi populations are most common in fields under crop rotation, while univoltine insects predominate in fields where corn is replanted every year.
The rootworms undergo three larval instars then pupate in the soil near the corn roots in late June or early July. After 5–10 days the adults begin to emerge, with males appearing about one week before the first emergence of female beetles.
Adult northern corn rootwormbeetles feed on the foliage, pollen, silks and developing kernels of corn, with the following consequences:
- When corn silks are cut all the way back to the ear and when beetle populations are high, the insects can interfere with pollination and germination to the point that ears are completely barren or only bear a few kernels.
Larval feeding disrupts root system function, reducing the amount of water and nutrients available to developing corn plants, affecting grain yield. Feeding damage can result in the following consequences:
- Plants develop secondary infections of root and stalk fungi.
- Extensive root injury makes plants more likely fall over (lodging), which reduces crop yield due to increased difficulty harvesting.
- Stalks take on a bowed appearance (goose-necked) as the plants try to straighten themselves.