Scouting for Corn Rootworm

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There are two common species of Corn Rootworm present in Eastern Canada; Northern Corn Rootworm and Western Corn Rootworm.  Although Western is most predominant, Northern does exist as a pest.  As their name implies, these pests target corn.

Description

The Larvae of both species looks exactly the same, 10 to 18 mm long and white with a brown head and dark distinctive plate at the tip of the abdomen.

Western Corn Rootworm adults are 4 to 7 mm in length and are yellow to green with three wavy black stripes on the wings which may blend together.  The abdomen of the Western Corn Rootworm is yellow, which differentiates it from the Striped Cucumber Beetle, which looks similar but has three parallel stripes and a very black abdomen.  Northern Corn Rootworm adults are 4 to 7 mm long with a green or yellow body without stripes or markings on the wings. 

Life Cycle

Each species has one generation per year.  The eggs overwinter in the soil and hatch when soil temperatures reach 10°C and the larvae begin feeding on root hairs.  This typically takes place around mid to late June.  The larvae go through three instars over a three week period.  The first instars feed on small root hairs, the second and third feed on the main roots, and the oldest larvae may also feed on the brace roots.  Pupation occurs in the soil and adults emerge in mid-July.  Adults prefer to feed on tassels, pollen and silk but will also feed on leaves in late planted corn.

Mating and egg laying typically occurs in the same corn field but adults will eventually move to later flowering fields as food sources deteriorate.  Most eggs are laid mid-August; Northern Corn Rootworm can lay up to 300 eggs per female and Western Corn Rootworm can lay up to 1,000.

Damage

Corn planted on heavy soils is at the highest risk but crop rotation can eliminate risk because adults often lay their eggs in the same field they fed in as larvae. However, volunteer corn in crop the next year allows for Corn Rootworm to continue to feed.

Larvae are primarily a below-ground pest; they leave brown scars on the root surface and small holes and tunnels along the roots which can encourage the onset of fungal diseases.  The affected roots will appear pruned and ragged. Corn plants may lodge at ground level due to pruned roots; this is often called goose-necking.

Adult feeding creates clipped silks, which may reduce pollination; but once pollination is complete, silk feeding is no longer a threat to yield.  Adults may also feed on leaf tissue, leaving window-paned strips parallel with the leaf veins.  Leaf feeding rarely causes economic injury.

Scouting & Thresholds

Scouting for larvae is very difficult and time consuming, but if you think you have a problem and would like to know how to scout, information can be found on DEKALB.ca under the Agronomic Information section and Summer Corn Insects. 

Scout for adults from mid-July to the end of August, before 70% of plants reach the silking growth stage (R1).  Inspect 20 plants in five different areas of the field, do this weekly from the time adults emerge to the end of August.  Look for clipped silks and count the number of adults per plant at ear height. 

If fresh silks are being clipped within 1.25 cm of the ear tip, control is recommended.  Field corn can withstand heavy adult activity, usually requiring at least 10 adults per ear before control is recommended.  However, seed corn may require control if adult populations are causing extensive silk clipping.

To determine whether preventative control is needed for next year’s crop (if corn is going to be planted again), monitor adult numbers during the month of August.  One Western Corn Rootworm adult per plant or two Northern Corn Rootworm adults per plant require control.

Management Strategies

Crop rotation is the most effective method of Corn Rootworm control.  Early planting may also reduce crop susceptibly to larval feeding.  If crop rotation is not an option and adult populations were high in the previous crop, effective control may be achieved using Bt Corn Rootworm hybrids, high rate insecticide seed treatment or soil insecticides.  DEKALB® Genuity® SmartStax® RIB Complete® corn hybrids combine DEKALB genetics with Genuity SmartStax trait technology for the most advanced insect and weed control system available.  They provide the broadest spectrum of above ground and below ground insect protection with multiple modes of action against Corn Rootworm.

     
/_uploads/images/agronomic-info/Corn20Rootworm20Larva.jpg   /_uploads/images/agronomic-info/Western20Corn20Rootworm.jpg /_uploads/images/agronomic-info/Northern20Corn20Rootworm.jpg  
 Corn Rootworm Larvae Western Corn Rootworm Northern Corn Rootworm

 

Source: Grain Farmers of Ontario. Guide to Early Season Field Crop Pests. (verified May/15).