Crop Sense

Importance of Non-Bt Corn Refuge

May 24, 2021 NC State Extension
Crop Sense
Importance of Non-Bt Corn Refuge
Show Notes Transcript

Dr. Dominic Reisig, N.C. State Extension Entomologist stopped by to discuss non-Bt corn refuge.  Why we need it, and other common questions regarding the topic.  

Welcome to crop sense presented by North Carolina Cooperative Extension.  I’m Jacob Morgan a field crops agent with NC Cooperative Extension.  Today we have Dr Dominic Reisig,  North Carolina State University Professor and Extension Specialist in Entomology. Good morning Dr. Reisig, could you tell us a little bit about yourself. Hey Jacob uh yep that's my title where i work at the university I’m an entomologist and  I have a majority extension appointment and because of that I’m based at the Vernon James Center which is a couple hours east of Raleigh and I have field crop responsibilities and work across the state and I share those with the counterpart on campus uh Anders Huseth.  All right so today we're here to talk about bt traits in crops so anyone who grows corn or cotton has probably heard of bt or at the very least is paying for this trait so let's start off by answering what does bt mean and how does it work in the plant? Yeah that's a good question you kind of wonder if some folks know what it stands for.  Bt stands for bacillus thuringiensis and that's actually a type of bacterium that lives in the soil.  They're naturally occurring and these bacteria make proteins that are toxic to insects. Humans have taken advantage of that for a long time and actually been able to grow these bacteria and harvest some of these proteins and they were used as an organic foliar insecticide for a long time.  But within the past I would say 40 years folks have figured out how they can genetically modify plants to express these proteins to kill certain insects.  So a Bt crop is just defined as one that's been genetically modified to produce Bt proteins that are active against certain insects.  So how does this Bt trait actually control or repel insects in corn and cotton. So the plant that's genetically modified its cell machinery makes the protein and the proteins are pretty specific they you know only kill certain insects and there's ways to limit where on the plant the proteins are made but to the best of my knowledge like the Bt corn and cotton we have express it throughout the plant and so the insect consumes the plant which has the protein in it the protein will get activated by their gut bind to certain receptors on their gut. We think it kind of pokes holes in their gut and then allows infections to happen inside the insect and that's what kills it.  So it's basically poisoning them so they have to bite the plant and ingest the protein or the toxin and it basically kills them. That's right.  What specific insects are we targeting with this trait?  Well here in North Carolina the Bt that we're using is really for above ground insect pests and caterpillar pests.  That will change next year, Bayer is introducing ThryvOn cotton and ThryvOn cotton targets some piercing sucking insect pests so it's going to have activity on thrips it's going to have some lygus activity but right now the bt corn and cotton we have is active against caterpillar pests.  There's also some types of Bt they have for example in the Midwest targets below ground pests they have a lot of trouble with western corn rootworm there we actually have some dairy farmers here that  do a lot of continuous corn just for transportation issues. They struggle with western corn rootworm as well so there's Bt traits that target those below-ground beetles but really here in North Carolina most of what we're targeting is these above-ground caterpillar pests. All right so anybody that plants Bt crops is supposed to plant a refuge is my understanding so kind of explain what a refuge is and why is the refuge important when we're planting these bt crops.  Yeah so refuge, it can just be a place where insects are able to shelter away from harmless conditions but it's kind of specific when we're talking about insecticides.  You know Bt is an insecticide, we're not spraying it, when we're working with a genetically modified crop but specifically I would guess for Bt crops you might define refuge as an area with a plant host and a plant host is something that the insect eats where Bt is not used. So that provides a way to make susceptible insects in the system. 

All right so why is the refuge important and how is maybe how big does it need to be, how best to implement it? Yeah, so I mentioned that the refuge is important to make susceptible insects it's because we can't keep putting this selection pressure across the environment for an extended period of time.  I mean I think folks are pretty familiar with the roundup situation what happens when you overuse a single mode of action.  The same thing is true for Bt traits if we were to just use Bt entirely across the environment we'd have resistance pretty quickly. So that the refuge reduces that selection pressure provides a source for those pests that are not selected with Bt and because they're not selected with Bt we can have these insects out there that have susceptibility to Bt that when we do get resistant insects in the Bt they're able to mate with those resistant insects and sort of drown it out and uh we're able to maintain the use of Bt for a longer period of time.  There's actually different types of refuge we have what's called a natural refuge and so that can be hosts that the insect uses that are maybe crops that don't express Bt or things like weeds. So an example in our system is something like bolworm actually can feed on corn which is bt it can feed on cotton which is bt but soybeans are not bt. So in this case soybeans are the natural refuge for those insects.  You could also have blended refuge or refuge in the bag so a lot of folks are familiar with these rib hybrids and that's where you just blend in non-bt and bt seeds together so they're planted in the same field.  This is a permitted strategy in the Midwest.  Here in the southern us where we grow cotton we still need to plant um what's called a block or structured refuge.  So even if we're growing rib hybrids we still have to plant a block refuge and that's just an area of corn that's dedicated to non-bt in addition to the to the bt area.  So in reference to if we're going to plant a block refuge does it need to be within so many feet of Bt corn or how does that need to be placed amongst your crop we've got a lot of growers in east north Carolina have small fields you know they may be they may have 50 corn fields can they plant one field in bt and then another field two miles down the road in non-bt well how does that need to work.  Yeah it's a great question I don't want to get too far into the weeds on it because it there's  a lot of options so I would encourage folks to actually uh if they look on the back tag of their seed it'll talk about what refuge options they can use. But in brief 20 percent of the total corn acres that includes non-bt and bt corn acres have to be planted to non-Bt and then the non-bt has to be within a half a mile of the bt so there's different ways they can lay it out it can be a it can be a separate field it can be a border in the field it can be strips within the field.  Again I'd refer folks to the to the bag tag and lots of good resources online about it.  I guess let's just address the elephant in the room. Are there any short-term or long-term yield penalties for using a non-Bt refuge? Yeah that's a great question and I think the answer is it depends you know like everything in nature right. So um one of the things that Bt has done is it's been really really effective for some formerly yield limiting pests so the above ground Bt corn traits were really designed for a pest called European corn borer. Now this was pretty prevalent from what i understand in northeastern north Carolina I’ve seen it once on non-bt corn since i've been here in 12 years. We actually have pretty good evidence that Bt corn has driven their populations really really low and so your risk of getting that pest is really low. As a result some other pests that maybe weren't a big deal in the past kind of worry growers. We think about some of those ear feeding pests things like fall armyworm and corn earworm they tend to be more prevalent later on in the year and generally what our research has found is that when you plant corn within the planting window those pests do not reach yield limiting levels. I mean it's pretty rare when that happens I think the yield penalty would probably be greater if you planted corn later. So in most cases what we found is that if growers plant a corn hybrid that's non-Bt with good genetic yielding potential and they plant it on time they're not going to receive a yield penalty. We're still testing this hypothesis we've had replicated trials for a number of years across the state and this year we're pairing up bt and non-bt hybrids on grower fields. A lot of agents are helping with this a lot of growers are helping with this we're going to take stand counts we're going to take insect measurements we're going to measure all the inputs we're going to hand it over to the economists and even measure things like what's the hassle of planting non-bt if you've got to switch out seed. You know there's a cost too that so we're actually going to put pen to paper and try and come up with a an exact answer to that but right now we really don't think there's a yield penalty if you plant it on time. So just for my own benefit so we have bt and non-bt varieties they be different varieties or how does that play out as far as when you're talking to your dealer? yeah that's a great question so something like sweet corn you could probably get the same variety that's bt or non-bt because field corn is a hybrid you're not able to get that exact same variety it's pretty close genetically related but they're not the same thing so what i would encourage growers to do is to to talk to their dealer to tell them they want you know a hybrid that's in a similar relative maturity. Buy it from the same company and and tell them you want it with the same sort of genetic background if you can get it and i think that'll go a long way to helping growers fit those bt hybrids in with their non-bt hybrids.  Is there anything else that you think the listeners need to know regarding non-bt and refuge uh before we wrap this podcast up?  Yeah maybe a couple things that i maybe should have touched on i didn't answer your question very well on you know how big the refuge should be for example and it's actually not entirely known scientifically how big we have to have refuges i mean the answer would be don't grow Bt at all if you don't want resistance to it right.  But we do want Bt traits um a lot of the refuge strategies were designed for these former yield limiting pests things like European corn borer you know we're pretty concerned with boll worm or corn earworm right now especially since we've had some bt resistance and some big yield problems in cotton.  We we know that bigger refuges are better we're pretty confident that if growers plant the 20 percent required refuge we can probably get quite a bit of life out of our existing traits.  So I’m pretty comfortable with the 20 percent number but again bigger is better.  The other thing i want to mention is to a common question i get is well can i just count my neighbors non-bt corn is my refuge and the answer is no.  You know you have to have your own Bt refuge i mean what if what if everybody said that about their neighbor who would plant non-bt corn.  But it is a common question i get.  So just correct if I’m wrong but it seems like we're using the refuge in corn to help with resistance against the bt trait in corn ear worms and cotton bollworm which is the same insect mainly for the cotton side of things and maybe some other some other crops because those cornear worms may not have a lot of damage to corn that comes into the system am i understanding that correctly?  Well you're understanding that because that's the impression i gave you.  I should say that non-Bt refuge in corn is important for cotton we don't require a refuge we rely on the help from non-Bt corn and then the natural refuge i mentioned earlier things like weeds and soybeans that don't express Bt.  However we do need a refuge in corn because even though we don't have a lot of European corn borer around we still could get some resistance and this has actually happened so a couple years ago up in nova scotia. They actually got European corn borers that were Bt resistant and it.  So if we if we don't plant a refuge corn we could run into the same situation here much like nova scotia we have fields that are smaller we have lots of natural areas where that pest can harbor and the area where i've seen european corn borer is actually on a wildlife refuge.  I think it was harboring in some weeds there and just jumped all over some non-Bt corn so if we didn't plant refuge corn we could probably have resistance to some of these other pests as well.  So it's still important in corn.  So we've driven down the European corn borer but we don't want it to come back with a vengeance once the possible resistance builds up in that and then we got a big problem in corn as well. Exactly, yep! Well thank you Dr Reisig for your time today to discuss this important topic of non-Bt refuge in corn.  If you like this podcast please subscribe and leave a 5 star review and as always thanks for listening to crop sense because if it isn't making money it isn't making cents!