Source: http://www.omafra.gov.on.ca/IPM/english/sweet-corn/insects/corn-rootworm... (read entire article here)
Corn rootworm larvae are cylindrical worms with a white body, brown head and 6 small legs behind the head. They are 3 mm to 1.5 cm (1/8- 3/5 in.) long when fully grown. Adults have hard shells and are roughly 6 mm (1/4 in.) long. The Western corn rootworm is yellow with black stripes on the wing pad. The Northern corn rootworm is pale green-yellow.
Both the adults and the larvae attack sweet corn. Larvae feed on root hairs and tunnel into the roots resulting in poor water and nutrient uptake, as well as a loss of structural support to the stalk. Indications of infested fields include poor root formation, plants with curved stalks (goose necking) or a high proportion of lodged plants. Corn rootworm damage affects both yield and harvestability.
Later in the season, adult rootworm feed on fresh corn silks. Severe feeding may affect pollination of sweet corn if the silks are clipped prior to seed set, resulting in a barren cob.
The corn rootworm over-winters in the soil as an egg. Adults lay their eggs in the fall in corn fields. Eggs must go through diapause (winter chilling) before they hatch. Development begins once the soil temperatures reach 10°C (50°F). The larvae emerge and feed on corn roots for 3- 4 weeks. Pupation takes 1- 2 days.
Corn rootworm complete only one generation per year. After emerging, adults will look for corn fields on which to feed and lay the overwintering eggs.
Sweet corn fields planted after sweet corn or field corn have the highest risk of infestation. A new variant of the corn rootworm is present in Ontario and across the U.S. mid-west. This variant lays its eggs in soybean crops. While this variant is currently present at very low levels in Ontario, it is important to scout all sweet corn fields for the presence of corn rootworm. Control may be necessary in the future should this variant establish itself in Ontario soybean fields.
Period of Activity
Corn rootworm larvae emerge in late May to mid-June. Feeding continues through mid-July. Damage symptoms may not become noticeable until the crop reaches the early-tassel stage. First generation adults emerge in late July through early August.
It is difficult to scout for this pest prior to the onset of symptoms. To diagnose symptomatic plants, dig around the plant and remove its root mass. Place it on a dark surface (garbage bag) and break apart the soil looking for the white larvae. Also inspect roots for feeding injury. Rootworms may occasionally be found under the leaf collar. Inspect plants in several different areas, as populations tend to be variable across the field.