Image 1. Corn rootworm injury on corn roots. (Source: GROWMARK, Inc.)
- Corn rootworms (CRW) are one of the most important insect pests of corn in the Mid-Atlantic with the potential to cause significant yield losses and increase lodging.
- As of mid-June, depending on where you are across the Mid-Atlantic, corn rootworm egg hatching has started, and larval feeding is underway.
- Scouting is key to predict the potential for CRW damage and adjust our future management practices accordingly.
Corn rootworms (CRW) are one of the most important insect pests of corn in the Mid-Atlantic. Root feeding by the larvae can cause significant yield losses and increase lodging. A compilation of field research trials suggests that around 15% yield reduction can occur for each nodal root consumed by CRW larvae (Tinsley et al. 2013).
Female CRW beetles lay eggs in the soil in late summer, generally between August and early September. Eggs overwinter and resume development the following spring when soil temperatures increase. As with other insects, corn rootworm development and peak egg hatch can be measured by calculating degree days or heat accumulation. Peak CRW egg hatch occurs between 684 and 767 accumulated degree days (52°F base soil temperature). As of mid-June, depending on where you are across the Mid-Atlantic, corn rootworm egg hatching has started, and larval feeding is underway . Adults typically start to emerge near corn tasseling, around late June to mid-July. Adult beetles feed on pollen, green silks, and leaves.
There are different types of scouting for CRW: Scouting for larvae and root damage and scouting for adults. Scouting for larvae and damage will help determine which fields may be prone to yield loss and lodging issues during the season, and monitoring adults can help to predict which areas may be at increased risk for CRW issues the following season.
Scouting for larvae
Scouting should start about two weeks after peak hatch but can also be done later in the season to evaluate the extent of root damage. A general rule of thumb is that larval feeding starts around the same time as lightning bugs start to become active. To evaluate the number of larvae present, carefully dig and remove a 7” x 7” cube of soil around the base of one plant in at least ten locations of the field. Sample from across the field, starting at least 50’ from field edges. Place and crumble the soil over a dark background such as a black trash bag or tarp and examine for CRW larvae. Alternatively, you can immerse the sample in a bucket of water and if CRW larvae are present, they will float to the surface. Finding an average of two or more larvae per plant using the soil crumbling method or eight or more using the float method may indicate the potential for economic damage and elevated lodging potential. Larvae are white and thin, approximately 0.5 inches long, with dark heads and black plates at their tail end.
Corn rootworm larvae. Image J. Obermeyer Purdue
To determine the severity of root damage, evaluate root injury around mid-late July. Dig one plant in each of ten randomly selected areas and wash the roots to remove the soil. Using the Iowa State node injury scale, rate the roots from 0-3 where 0 = no damage and 3 = three or more nodes have been eaten. Root tips will appear brown to black and pruned to the plant base. Larvae may also be found tunneling into roots and crowns.
Where to sample first for larvae and damage? High risk fields should be sampled first. These include:
- Fields planted with hybrids without CRW Bt traits
- Fields with a history of CRW Bt trait failures
- Fields on continuous corn
- Fields with high CRW beetle counts the previous year
Scouting for adults:
Optimal timing for scouting the adult beetles is at pollination. To scout a field, start at least 50’ from field edges and scout 10 areas across the field in a “W” pattern. At each site assess five non-consecutive plants for CRW adults. To count adults, reach out and cover the silks in your hand to trap beetles feeding in silks and prevent them from moving to other parts of the plant and/or escaping. While holding the silks, assess the leaves and ear axels for adults. Finally, remove your hand from the silk and count the number of adults in and exiting the silks. If adult beetle counts average more than 0.75 beetles per plant across the field, that field would be at elevated risk for CRW larvae the following season.
Other methods to monitor adults include the use of sticky traps and CRW monitoring networks. Contact your crop specialist for further information on participating in these networks.
Summary
Corn rootworm resistance to Bt traits was first documented in 2009 and cross-resistance was confirmed soon thereafter. Scouting provides information about the performance of CRW Bt traits and can help diagnose resistance development in a field. Regarding CRW adult counts, they can help predict the potential for CRW larval damage in specific areas in the following season. Even though low CRW populations have been observed in recent years, a few hot spots have been consistently identified across surveyed areas. Scouting is key to predict the potential for CRW damage and adjust our future management practices accordingly.
References and Resources:
Tinsley, N.A., R.E. Estes, M.E. Gray. 2013. Validation of a nested error component model to estimate damage caused by corn rootworm larvae. Journal of Applied Entomology. 137(3):161-169
Degree Day Calculators (select a local weather station within the website):
NEWA (Cornell): http://newa.cornell.edu/index.php?page=growing-degree-days