We recently had a customer report rootworm problems in a Bt corn hybrid, which suggested rootworm resistance to the trait.
Rootworm resistance to the transgenic Cry Protein (Cry3Bb1) is the main component of Bt resistance and has been observed in Illinois, Iowa, Minnesota, Nebraska, and South Dakota.
Farms that grow continuous corn year after year, or have short rotations with corn, are becoming key candidates for setting up rootworm resistance scenarios. This provides a good reminder that relying on agronomic fundamentals may be just as effective, or more so, than traits.
Continuous corn and short rotations with corn (even corn after beans) can greatly increase rootworm and other pest problems.
The two main types of rootworm to be aware of are Northern and Western Rootworm. Western rootworm usually hatches earlier, and has shown resistance to the Bt trait in the Midwest.
Seed treatments help, but they, too, are influenced by basic agronomic practices. Most insecticide seed treatments are effective for 6-8 weeks, so early-planted corn whose emergence and early growth may be slowed by cool weather will likely be more susceptible. Rootworms hatch in late May to early June, at which point the seed treatment will be losing effectiveness in an early-planted corn.
To complicate things further, hatching is related to soil temperature. A cooler, wetter year like 2014 probably delayed hatching, reducing the effectiveness of seed treatments more. The hatching period may have fallen even more behind the time frame of the seed treatment’s effectiveness.
Be aware that seed treatments containing Cruiser 250 are not labeled to control rootworm; only Cruiser 1250 is said to be effective against them. Now I have seen that even the C-1250 is also not working as effectively at some locations, and it seems the rootworms are building up a tolerance for it.
No-till versus tillage is also a major influence. Tillage buries eggs and larvae and greatly disrupts and reduces pest populations. No-till practices leave residue on the soil surface, which is excellent for building organic matter and soil life, but the lack of tillage also means less burying of eggs, and no-till soils provide stable shelter for the rootworm population. Residue management can be critical to your success with pest management, and there are both benefits and challenges, whether you cultivate, moldboard plow, or no-till.
Other management questions to ask if there is rootworm damage –
- Were in-row insecticides used?
- Was any scouting done to look for rootworm presence or abundance?
- Was corn planted previously on adjacent fields? Adult corn rootworm beetles can be very mobile
Western and Northern Corn Rootworm have only one generation per year. Eggs are deposited in the soil (typically around the base of the corn plants) during mid-summer through autumn, they overwinter, and hatch in late May through early June the following year. Eggs hatch over the course of 3-4 weeks. Then the larvae pass through the instar stages. Most feeding is done in the later instar stages, then pupating and egg-laying takes place again from mid-July to mid-August.
Adult beetles feed on pollen, silk and leaves, and can move among fields more readily. They can also feed on soybean flower pollen or late-flowering weed pollen. This can complicate matters even when one is trying to avoid a corn-after-corn rotation, since the adults can lay eggs in a soybean crop in time for them to hatch in and take advantage of a corn crop the following year.
Another challenge when trying to extend and diversify a rotation for pest management is a phenomenon known as extended diapause, which is when eggs can remain dormant in the soil over two winters, and can then do damage even in first year corn. (This currently applies only to Northern Rootworm, not Western.)
Control late weeds in soybeans to help prevent the adults from laying eggs in fields that will be rotated to corn.
However, larvae can’t survive on the roots of broadleaf plants like soybeans or alfalfa, so those in fields that are rotated to legume or other broadleaf crops for another year aren’t likely to survive.
Rotating out of corn for more than one year can greatly reduce rootworm problems and this is still my PRIMARY recommendation.
Controlling adult beetles is one tactic for preventing egg-laying, and is an alternative to using soil insecticides at planting the following year.
Scout for rootworm damage as well as adult feeding activities in fields that are planned for corn the following year. Scout for emerged beetles in late May to early July, and keep records of any insect pressure.
Corn plants that lodge or fall over easily are a key indicator of possible larval feeding of roots. Dig and check the roots of fallen plants for signs of chewing.
Watch for adult beetles feeding on pollen, silk and ear tips.
In problem areas or corn after corn areas, you can also –
- Look at alternatives or additions to the Bt trait
- Use in-row insecticides at planting
- Rotate insecticide chemistry
- For very early planted corn consider applying insecticide at cultivation or side-dressing time to better line up the application with egg hatching.
One lesson to be learned from any pest problem is that most pests can adapt to most management practices that are used year after year – even traits. Mix it up with a lengthy rotation and use as much diversity as possible. This applies to crops, varieties, chemicals, and traits, and even tillage/no-till practices.