Adult Corn Rootworm Emergence Underway In South Dakota

 

Northern and western corn rootworm are the most damaging insect pests of corn in South Dakota. Both species have a single generation per year; eggs overwinter in the soil, larvae hatch in the spring and feed on corn roots, and adult beetles emerge and are present within cornfields from mid-July through the first frost. Corn yield losses are primarily associated with corn rootworm larvae feeding on corn roots, although adults infrequently interfere with ear fill. As August approaches, adult northern and western corn rootworm populations will reach their peak and scouting these populations can help assess the risk of injury to corn that may be planted during the following year.

 

 

 

Adult corn rootworms are approximately 1/4 of an inch in length, with the western corn rootworms being slightly larger than northern corn rootworms (Figure 1). Western corn rootworm adults are yellow with three black stripes running lengthwise along their hardened forewings, although these stripes sometimes appear as one single band (Figure 2). Northern corn rootworms are one solid color, which can vary from dark to light green (Figure 3).

 

 

 

Yield loss attributed to corn rootworm is primarily caused by larvae feeding on corn roots. Injured roots take up less water and nutrients, leading to reductions in corn yield. Corn yields are reduced by 15 to 17% for every node of the corn root that is injured by corn rootworm larvae. Smaller root systems also lead to goose-necked and/or lodged plants that can complicate harvest. Adult corn rootworm beetles may also reduce ear fill by eating silks and pollen, however significant yield loss caused by adult beetles is infrequent and requires large populations of beetles to be present within cornfields when silks are still green.

 

 

 

There are several options for management of corn rootworm. The simplest option is to evaluate a field’s cropping history. Fields that are planted to corn continuously for several years generally have an increased risk for injury from corn rootworms. Crop rotation is effective against corn rootworm because any larvae hatching within a field that is rotated to a non-corn crop will starve. However, there are rotation-resistant northern corn rootworm (sometimes referred to as extended-diapause northerns) which can still threaten first-year corn. Extended-diapause northern corn rootworm lay eggs exclusively in cornfields, however their eggs can survive for multiple years in the soil.

 

Genetically-modified corn hybrids that produce Bt toxins are widely used for management of corn rootworm. However, Bt-resistant populations of western corn rootworm have been reported in Iowa, Nebraska, and Illinois. To date, there are no official reports of Bt-resistant corn rootworm in South Dakota. Soil applied insecticides are also effective at protecting corn yields, and a list of insecticides that are currently labeled for management of corn rootworm can be found in the current edition of the South Dakota Pest Management Guide: Corn.

 

Some farmers may be tempted to use both soil-applied insecticide and a Bt corn hybrids. Research has shown that combining these two management practices can increase the selection pressure for corn rootworm to adapt to Bt corn. Additionally, using both soil-applied insecticide and planting a Bt corn hybrid can result in unnecessary input costs, and lower economic returns.

 

Digging corn roots and rating them for larval feeding injury can help assess how effectively your current rootworm management practices are working. To analyze corn rootworm larval feeding, dig ten roots from throughout the cornfield using a spade. We recommend digging approximately 8-12 inches from the stalk to ensure that roots are not accidentally removed while digging. After roots are dug, soak them in water for 24 hours and then remove the remaining dirt with a high-pressure hose. Allow the roots to dry prior to rating. When rating the roots, examine nodes 4, 5, and 6 (the brace roots are considered node 7). Count and record the number of roots present at each node (Figure 4). Then repeat the process, but count the number of roots that are pruned to within ½ of an inch of the stalk. Divide the pruned root number by the total root number, repeat this process for each node and add the scores together. Research has shown that for each node injured by corn rootworm, a rating of 1 (all roots on a node pruned), there is a 15-17% yield loss. Unfortunately, there are no therapeutic management treatments once root injury has occurred.