Nutritional differences in insect susceptible plants

Healthy plants are completely resistant to insects.

We have observed this to be true in the field on many occasions. Over time, more connections come to light describing the scientific reasons for how this occurs.

Larry describes their pioneering research seeking to identify the nutritional differences between insect resistant and insect susceptible crops in our podcast episode here.

So, if you have an imbalance—if you have too much nitrogen relative to potassium—what happens is you get the buildup of free amino acids in those plants. And insects love free amino acids. They’re a very digestible source of nutrients for them because they’re also highly limited by nitrogen.

 

John: With some of the original research that you did—comparing the insect pressure on the organic farms versus the nonorganic farms—this is actually something that I often hear from organic growers, but I haven’t uncovered a lot of research where people have actually tried to do a comparison and evaluate the differences. What were the differences that you were seeing? What really stood out for you?

Larry: Again, keep in mind that this was twenty to twenty-five years ago. There’s been a lot more research in organic systems now. But at the time, organic farming was considered very unscientific, and more New Age, and not amenable to large-scale production. And so I was working with a lot of organic farmers who were basically doing their own research—not necessarily replicated research as we would do at the university, but trying different things and seeing what worked on their farm. This is the context in which we got started.

I actually received a lot of criticism at the time—that what I was doing was not very productive, or not the direction I should be going as a non-tenured faculty member. So, we went to these organic corn and soybean farmers, and the ones we worked with had animals integrated into their system as well. First of all, we just did a census of their corn, looking at the levels of European corn borer damage in their fields versus their conventional neighbors. And, as I’m sure you and many of your listeners have often heard, I kept hearing this idea that if we have healthy soil, then we’re going to have a healthy plant and that insects don’t like healthy plants—that’s why they don’t see the damage.

Although that in itself is not a very scientific statement, we could reformulate it as a hypothesis that we could test empirically. And so we collected soils from these organic farms and then went right across the street to a conventional neighbor, collected their soils, and brought these soils into the greenhouse and planted them to corn. And we fertilized each of them either with an organic fertilizer, like manure or compost, or with a chemical fertilizer.

What we were interested in figuring out was, first of all, whether the insects could tell a difference in these plants. And if they could tell a difference, was it associated with that short-term effect of the type of fertilizer we used or that long-term effect of that history of management and how that impacted the soil community. And so, after setting these plants up and letting them grow to a certain stage, we released European corn borer females that had been mated into the greenhouse. All of this was replicated and randomized.

And we just let them loose to see where they would lay their eggs. And what we found very consistently—we actually repeated this experiment, I think, with four or five different pairs of soils from farms—was that if the plants were growing in a soil from an organic farm, irrespective of the fertilizer we applied, they received relatively few eggs. Whereas if the plants were growing in a soil from a conventional farm, sometimes they would receive a lot of eggs and sometimes they would receive only a few eggs.

This gave rise to this concept that I call biological buffering. The way we envision this—and this was our working hypothesis—was that in those organic systems, where you have a recurring influx of organic matter into those soils, either in terms of cover crops or plant manures or animal manures, you create this soil community that is beneficial to the plant. And when nutrients then go into that system, they get absorbed by this microbial community and then they release those nutrients very slowly over time. As a result, we hypothesize that those plants are in better mineral balance than when you’re putting down high levels of nutrients.

Why this would be important is because plants are almost always limited by nitrogen levels. They don’t have mechanisms for dampening the levels of nitrogen that they take up. They’re going to take up whatever they can get. And in this context of putting down inorganic, highly soluble nitrogen sources, often these plants are taking up much higher levels of nitrogen than they are really set up to deal with.

We then hypothesized that in this situation, those plants would tend to accumulate the simple compounds. When you have an imbalance of nutrients—let’s say nitrogen and potassium—if you think about those two elements, nitrogen is of course very important in terms of protein synthesis and potassium is important in terms of converting amino acids into protein. So, if you have an imbalance—if you have too much nitrogen relative to potassium—what happens is you get the buildup of free amino acids in those plants. And insects love free amino acids. They’re a very digestible source of nutrients for them because they’re also highly limited by nitrogen.

In situations where you have these imbalances, that plant becomes very nutritious for the insect; whereas in the plant that is in better mineral balance—one that’s getting its nutrients relatively slowly—that metabolic machinery of the plant is able to act more efficiently. As amino acids and sugars are produced in the plant, they are more immediately converted to the less digestible and more complex building blocks of the plant, like proteins and starches and cellulose and that sort of thing.