(Baltic, SD, AgPhD Winter Newsletter)- When Bt corn first came out, I talked about it to my mentor on all things in the bug world (yes, I lead an exciting life). Anyway, he was an insect guru who was well-renowned, and he told me he didn’t think much of Bt. For me, a young agronomist, I was super-excited about Bt corn, and I couldn’t understand why this guy wasn’t happy to see this new technology.
“Brian,” he said, “we used to use that same type of Bt as an insecticide sprayed right over the top of crops. After we used it for a little while, bugs built up a resistance, and the Bt no longer worked.” Ever since that conversation, I’ve been watching for signs of Bt resistance, and honestly, I didn’t see any problems for the first 10 years we had Bt corn. In the last 3 years, though, we’ve seen issues in certain areas with some Bt rootworm corn.
Please don’t misunderstand. Most Bt products are working just fine, and they’re definitely worth the money. However, we have seen some VT3 corn fail to effectively protect the roots from rootworms, so here’s what I would advise you to do:
- If you’re not in a heavy rootworm pressure area (i.e. if less than 35% of the acres in your area are planted to corn), you probably don’t have a lot of worries.
- Use 2 Bt traits for the same insects as much as possible. For example, VT3 has 1 rootworm trait, while SmartStax has 2 rootworm traits. While some bugs may be resistant to 1 trait already, chances are they aren’t yet resistant to 2. While most people were bashing SmartStax last year, the top 3 fields on our farm this year were all SmartStax corns that yielded over 205 bushels per acre… obviously the yield is OK.
- Use a planting-time insecticide. If you want to plant refuge corn or VT3, I’d strongly consider using a full rate of Aztec or Force. That’s what we do on our farm. Keep in mind that even if your Bt works, bugs still have to take a bite or bites out of your crop to ingest the Bt. Your best liquid insecticide options are Force and Capture, although I still prefer the dry if your planter is set up that way. With SmartStax, to be on the safe side, we’re using a one-third rate (or maybe even a half rate this year) of insecticide. Investing $4 to $8 an acre to protect SmartStax is worth it in my opinion, because we have heavy pressure in our area, known issues in the past, and we’re shooting for very high yields.
- If you’ve really got a problem, I’d consider spraying post-emerge around tasseling/silking time to control the adult corn rootworm beetles BEFORE they lay their eggs for next year. If the timing is right, you may be able to combine this with a fungicide if you want to. One thing I want you to think about here, too, is this… spraying for soybean aphids reduces corn rootworm pressure. Many corn rootworm beetles lay their eggs in soybean fields. When you rotate back to corn the next year, those eggs hatch and cause problems in your fields. Fortunately, the same products you use to stop aphids will control rootworm beetles.
- Bugs are a lot like weeds. If you allow their populations to build, you’re in for major issues at some point down the road. Even though it hasn’t always seemed tremendously profitable at the time, we’ve been using insecticide in corn, soybeans, and wheat for many years. Post-emerge, we pull the trigger at fairly low economic thresholds, but this is part of the reason why. I don’t want insects to get ahead of us, increase their populations, and then watch normal insecticide treatments fail in the future simply because of heavy insect pressure.
- Understand that resistance can happen. You’ve seen it with weeds. You’ve seen it with diseases. We’re now seeing it with a host of different insects around the world, too. Follow the refuge requirements when planting your seed, learn more about the best ways to stop insects on your farm, and most importantly, don’t be afraid to pull the trigger when it comes time to control bugs. Resistance shouldn’t mean non-control or yield loss. It should just mean you need to take extra management steps to achieve success. Good luck!