Pesticides and Prediction
Without pesticides1, farmers risk not growing enough to recoup their costs or growing crops that are too blemished to sell at market2. But by using pesticides, farmers take on other risks: many are hazardous to those applying them, they can kill beneficial “non-target” bugs or plants3, runoff can harm ecosystems4 and long-term over-use of pesticides can make convenient and safe methods of control less effective.
So how do farmers balance these competing risks? One method is to predict whether or when a pest will be a problem. Farmers already do this for many crops (and various practices under the name “integrated pest management” usually require trying to predict pressure before using pesticides). But several common ways farmers control pests in crops like corn require up-front decisions because the pesticide is in the plant. This post was intended to jot down a sci-fi idea I’ve had stewing with no story to go with it. It turns out that idea isn’t as science fiction as I thought!
How do farmers predict pests?
If a field is not going to have enough pest activity to cause economic losses, there’s usually no reason to risk using a pesticide. But a farmer is going to want to be sure before making that decision. As an example, strawberries are often affected by molds that (as you can imagine) make them unsellable. Farmers usually control this by applying fungicides (pesticides that kill fungi usually yeasts and molds) repeatedly during the growing season. It’s a tricky balance. Apply too often and a farmer is wasting money. Don’t apply at the right time and she might lose a lot of berries (also, most shouldn’t be applied too close to harvest). Kevin Folta posted an example in Florida of using prediction to decrease application of fungicides. The mold is only likely to cause problems under certain environmental conditions, so they’ve built a network that keeps track of weather and other data and predicts when the fungicides are actually needed. The result is less pesticide being applied — good for workers, the environment and consumers.
In the case of crops like corn5, weather isn’t necessarily how they decide whether to use a pesticide. Western Corn Rootworm (Diabrotica virgifera virgifera) is one of the most economically important pests on corn crops in the United States and Europe. This insect (like many others) goes through several phases of life. It starts in the ground as an egg laid the previous summer which hatches in a larval form the following spring or early summer, feeding on roots of young corn plants (hence the name). It becomes an adult after it pupates in late summer at which point it mates and the females lay eggs. The adult beetles are usually not very damaging to corn crops. The larvae however feed on the roots. Smaller amounts of damage make it hard for the corn to take up nutrients and decreases yield. Greater amounts of root damage and the corn plant might fall over (or “lodge”) — and yields no corn at all. For most corn growers, they need a certain amount of yield to be economically viable so some pests are tolerable, but above a certain amount and large swaths of the field will be damaged to the point of economic loss. But the lifecycle of corn rootworms allows a farmer to predict when pest pressure will be high! Integrated pest management (IPM) practices recommend capturing the adult beetles the previous season to estimate the likely numbers of larvae the next season6.
Currently, the most popular way to control these rootworms are transgenic Bt varieties. There are several types of Bt toxins (called “Cry” proteins) that affect rootworms and different varieties of corn have genetic traits to express one or more of them. Under current EPA requirements, a farmer has to plant at least part of his field in non-Bt corn. The part of the field planted in non-Bt corn is called a refuge. The idea is that if any larvae survive eating the Bt toxins (and maybe have the ability to survive those toxins) they will breed with ones that lived off the non-Bt corn in the refuge. Hopefully this interbreeding makes sure that the pest insects as a group don’t have resistance. But there’s growing evidence that Bt resistance is developing7 and also that refuge compliance is lower than expected8. Moreover, some farmers are choosing to plant Bt varieties every season, regardless of whether or not they are likely to have a lot of rootworms that season8! The ag scientists are working on better ways to manage Bt crops including the idea of “refuge in a bag” which intermixes a percentage of non-Bt seed with Bt seed. The hope is farmers will find it easier to comply with refuge requirements (though it’s not clear how well it will hold off resistance — I have a post coming up on this). But it still doesn’t fix the problem of using a control method (in this case, Bt toxins created by the plant) when it’s not needed. Farmers rightly see this as a form of low cost insurance since if they chose not to use it and don’t really need it, it doesn’t cost much.
Triggered Expression: Science Fiction … or Not?
I had an idea a long time ago for a science fiction story that we could make seeds that have multiple useful traits. In the story idea (I never came up with a decent plot) a farmer would be able to “dial-up” his desired seed traits for the season. Punch some buttons and sometime later she has seed ready for planting, customized to her field conditions. Awesome, right? Of course, as I learned more about how seed breeding works, I realized that any particular combination of traits requires someone to have bred them into that seed at least the year before! Moreover, any one seed might require a complicated set of different parents9. So my story idea just wouldn’t work. Further, at least one of the traits I was imagining farmers “dialing up” were pest control ones like Bt toxins. Since Bt is still a pesticide, ideally farmers wouldn’t have to use it every year if there aren’t enough pests to bother, so obviously it shouldn’t be in the seed a farmer dials-up every year. Curses! There’s just no way to rescue this science fiction idea for increased farmer control of seed traits, is there?
The other day I realized there was an even cooler sci-fi idea (and for about five minutes I thought I was thinking of something new): what if a farmer bought seed with all the traits she might want to use but then used some harmless chemical to trigger the trait working? For Bt trait, the plants would always have the genes for Bt but they would only produce the toxins if triggered by a signal. If we could figure out different signals for different traits, a farmer could plant seeds with all kinds of traits, but decide later when to use them. Science fiction idea rescued! But this idea isn’t as science fiction as I thought: about five minutes after I mused about this on twitter, I was pointed to some research on using plants as chemical signals to detect chemical warfare attacks. The idea is we could grow genetically engineered plants around a city or other sensitive area that would react to common chemical warfare agents and change color or appearance as a warning. This seems awfully expensive as a chemical warfare defense method, but in crop plants it seems like it might be worth it. Give farmers the insurance that they can use a particular method of control, but allow them to make the decision very late. Unsurprisingly there’s already research on this idea10 but it’s no where near commercialization.
There’s a short post on science fiction ideas in agriculture that aren’t as sci-fi as I thought. The future is always coming sooner than I expect.
Contrary to popular misconception, pesticides are used in all forms of agriculture, including organic which uses a restricted list (a not necessarily rational list). Even biological forms of control such as releasing eggs of a pest’s predators would legally count as a pesticide! Pesticides can be used to control or kill insects, plants, birds, mammals, fungi and even bacteria and viruses (in plant agriculture, these last two must often be controlled by controlling the host that carries them to the plant).
Grain farmers might still have something to sell if their crop is damaged, but it might be at a lower price.
“Non-target” actually refers to any life that isn’t intended to be harmed by a treatment: insects, birds, mammals, etc or plants. Often we want more of them of them in fields like predatory insects that eat pests or insects that pollinate plants.
Atrazine is an herbicide commonly used on corn fields because corn itself is naturally resistant to it. It’s usually applied on competing weeds are killed, leaving the corn to grow tall. There’s some evidence that atrazine runoffs are damaging to vertebrates, including amphibians and fish, though evidence is (unsurprisingly) inconclusive. Notably, atrazine is banned in the EU.
The following explanation is a long for a reason — I’m experimenting with giving more background.
Obviously it’s more complicated with recommendations on where and how often to sample and then formulas to figure out likely larvae density. But that’s the gist. I don’t really understand it, but there’s some great agricultural extension programs out there.
Some surveys of Illinois corn and soy farmers found around 20-25% of farmers weren’t complying with refuge requirements and further that 75-80% would plant Bt corn even if they thought they would have low pest problems. Note while this post focuses on western corn rootworm, there seem to be similar questions for other pests controlled by other Bt toxins.
Corn is a bit special. The best corn seeds are hybrids that have high yields and other desirable characteristics. Transgenic traits are usually bred into one parent line which is bred with another line to produce the hybrid seed farmers actually plant. The resulting seed from that seed just don’t perform as well if re-planted. This complexity in seed production is one reason most corn farmers buy seed every year: maintaining the seed lines necessary to create those hybrids would be space- and time-consuming. It’s easier to out-source. Look up hybrid vigor for more.
Right now, most Bt plants express the toxin throughout the plant. I can’t find the references but I’ve heard of attempts to have expression only be in relevant tissues. For Western Corn Rootworm, we might only want the toxin to be produced in the roots. Other researchers have looked at expression of Bt toxins only at places being attacked by pests. I’m sure there are others but I haven’t done a full literature search. But this sci-fi idea is clearly not as sci-fi as I thought it might be!