A year ago this month in this magazine, I told you corn rootworm pressure was on the rise due to more conventional corn and less insecticide use. In many areas, 2021 turned out to be the worst year we’ve seen in a long time in terms of rootworm issues, but the level of rootworm damage was even worse than we expected. I’ll tell you why we believe that happened and what our forecast is for 2022.
The life cycle of a corn rootworm is roughly as follows. I’ll give you approximate dates for my area (southeast South Dakota). While the dates may be slightly different for your region, the life cycle is still the same. Once you get enough heat in the spring, corn rootworms (larvae stage) hatch in mid to late May. They feed on corn roots for just over a month. They pupate and become adult beetles in early July. The adults feed on corn tassels, mate, and lay eggs over the next month. Many western (yellow with black spots) corn rootworm beetles have been found to lay their eggs in soybean fields, while many northern (green) corn rootworm beetles have delayed diapause, meaning they hatch almost two years later. In other words, both the westerns and northerns have methods to beat the standard corn-soybean rotation.
Since the root feeding occurs early in the year, that means your plant is damaged way before the root system and the plant can get fully developed. This makes your crop more vulnerable to all other stresses, including disease, competition from other plants, wind (lodging), and drought.
In my opinion, here are the two biggest reasons why rootworm damage appeared to be so severe this year.
1. DROUGHT
2. LACK OF POTASSIUM
These two issues really go hand in hand. When the weather is dry, fewer nutrients get into the plant since fertility goes in with water. That’s part of the reason we push so hard all the time on the base saturation potassium numbers.
Don’t buy into the myth that just because you have 200 parts per million of potassium in your soil you have enough K to properly feed a good corn crop. In addition to checking parts per million (or pounds per acre), you need your base saturation potassium levels to be at least 4%, and on our farm we are working on building K levels to 7% so the effects of drought can be minimized.
Consider this. Is the lack of rain really hurting your crop or is it primarily a lack of fertility? By boosting the concentration of potassium in your soil, every drop of water your crop’s roots bring in will have a higher level of plant food. Since potassium is by far the number one nutrient your crop needs (way more than nitrogen), it should be your top priority.
Notice how earlier I said, “why rootworm damage APPEARED to be so severe”? Since almost no one scouts for rootworms by digging up plants, the only reason the problem is typically noticed is because of lodging. Since plants didn’t get the potassium they should have due to the drought, we saw far more lodging than normal. Yes, part of the problem was corn rootworms, but in my opinion we didn’t see any more rootworm feeding this season than we normally do.
That brings me to the second question I asked in the title of this article…Will rootworm damage be worse in 2022?
Maybe.
Rootworm damage is always bad, but most years you don’t see that 5-bushel to 30-bushel loss because your corn doesn’t fall over. So whether it’s worse or just equal to last year, the problem should still be addressed.
The big questions then are how much does it cost to stop rootworms and can you afford the investment? It will be rare for me to tell you to plant corn with no insect control strategy because it can be done so inexpensively. I’ll describe below how I’d encourage you to look at it on the next page.
HEAVY ROOTWORM PRESSURE:
⊲ Plant a Bt rootworm corn, preferably SmartStax. On average, this costs about $30 per bag more, so you’re probably talking $10 to $12 per acre.
⊲ Plus, use a full rate of insecticide for another $10 to $25, approximately.
⊲ I know this adds up to $20 to $37 per acre, but at $5 corn that’s only 4 to 8 bushels per acre, and in heavy pressure fields a loss of 20 to 40 bushels per acre untreated would not be surprising, not to mention the lodging and harvestability issues.
MODERATE ROOTWORM PRESSURE:
⊲ Use an insecticide. The nice thing with insecticides is they stop a whole host of pests that Bt rootworm corns will not control, including white grubs, seed corn maggots, seed corn beetles, wireworms, and many more.
⊲ The best insecticides are the dry products, including Force & Aztec. Those are around $25 per acre for the full rates. Liquid Capture LFR is around $10, and generic bifenthrin is about $6. Keep in mind that generic bifenthrin needs to be applied separately from liquid fertilizer, where Capture LFR formulations are designed to mix with fertilizer.
LOW ROOTWORM PRESSURE:
⊲ If nothing else, I would still recommend the use of at least a low rate of insecticide in corn at all times. By investing $3 to $5 per acre, you only need one bushel of corn back to pay for that. Plus, the occasional use of insecticide helps keep insect pressure down so you can often avoid disasters at some point down the road.
Here’s one last thing I’ll leave you with. How many adult corn rootworm beetles have you seen when you’ve been out scouting your fields the last couple of years? If you see the adults, that means you had rootworm larvae feeding on your corn roots, damaging your yield. With corn prices still pretty high, you may have a great chance for a good return on investment by stopping this tough pest in 2022.