Late planting, no planting, flooding, cool weather, weed pressure, compaction and nitrogen loss are just a few things that spring to mind when we think about the 2019 season so far.
What about corn rootworm pressure? Will CRW be another challenge to add to our already challenging year? Are we at greater risk or lower risk?
The cliché agronomist response once again applies: It depends. Eggs are much more likely to survive flooding than hatched larvae, so the timing of rains plays a key role. Also, late-planted corn may be a good or bad thing when it comes to survival of larvae.
If no corn roots were present when egg hatch occurred, larvae most likely starved. If roots were very small due to late planting, then damage could be more dramatic.
It’s a fine line and really does depend on individual circumstances. Here’s some background information to help you assess your risk.
Egg hatch typically occurs throughout the Midwest in late May and early June. This is dependent on soil temperature and quantified by growing degree-days.
Historical research has shown that between 684 and 767 GDDs, 50% of egg hatch occurs. A simpler, albeit less accurate way to estimate the start of egg hatch is to watch for the appearance of lightning bugs.
Once hatched, larvae sense CO2 released by corn roots and travel to them to begin feeding. Initially, larvae are too small to be detected easily.
However, they will pass through three instars and grow to about half an inch prior to pupating and emerging as beetles. Beetles can usually be found emerging by late June or early July.
When scouting for CRW larvae, dig several roots and wash them in a bucket of water. Larvae will float to the top.
This will provide you with a rough count may help gauge populations early. Sampling multiple areas of the field is always a good idea since pest pressure often occurs in pockets.
Carefully examining roots and using the Iowa State node-injury scale is the best way to determine if CRW is causing issues in your fields. Mid-July to early August is the prime time for evaluation.
Roots will appear brown and pruned. Consequently, this reduces water and nutrient uptake and can lead to additional issues such as poor grain fill and lodged plants.
A meta-analysis evaluating the economic impact of CRW damage shows a 15% yield loss for each node removed by CRW feeding. If you see root damage now, make plans for handling a difficult-to-harvest crop and adjust your yield goals.
It’s not just CRW larvae that can cause issues in fields. CRW beetles prefer to feed on corn silks and pollen. If you suspect high pressure, consider using sticky traps to obtain accurate beetle counts and properly time insecticide applications, if economic thresholds warrant. Refer to your nearest land-grant university for threshold information.
Controlling adults also will reduce egg laying and, therefore, lessen populations next year.
Best Management Practices
Because corn rootworms can adapt to control strategies, it’s important to proactively manage these pests for 2020.
Options for a management plan include using pyramided Bt traits, crop rotation, soil insecticides and aerial application of insecticides.
Additional scouting and threshold information is available at extension.entm.purdue.edu/fieldcropsipm/insects/corn-rootworms.php and mycogen.com/blog/details/proactively-manage-corn-rootworm.
Contact your local seed adviser or agronomist for help selecting the best hybrids for corn rootworm protection.