Plant absorption of ammonium instead of nitrate influences the degree of insect susceptibility. Excessive nitrogen in any form results in excessive soluble amino acids within the plant that enhances insect development and population growth.

From the podcast interview with Larry Phelan.

John: I’d also like to understand the insect attraction piece a bit better. You mentioned that when you have this surge of nitrogen supply on the non-organic farms from the nutrient application period, you have an increased attraction because you have a nutrient-rich food source. Why can insects not utilize plants that don’t contain high levels of amino acids as a food source? They would also contain some level of amino acids, wouldn’t they?

Larry: Yes, all plants are going to have some levels of free amino acids, and they can also digest protein—let’s keep that in mind too. But it’s going to take energy for that insect to digest that protein.

Furthermore, a lot of plants have defenses that involve the inhibition of enzymes that break down protein in the insect. These are called proteinase inhibitors. And in terms of what we call inducible defenses, this is one of the responses of many plants to insect attack. They start the production of these proteinase inhibitors in order to reduce the insect’s ability to digest that protein. It basically knocks out those proteinases in the insect. The insect doesn’t necessarily starve, but it slows its development way down.

Well, in that situation, if you’ve also given that plant these high levels of nitrogen and it’s accumulating these free amino acids, that now short-circuits that plant defense system. So, in other words, the insect doesn’t need those proteinases as much because it’s getting these free amino acids that it doesn’t have to break down.

John: Got it. That was a piece that I had always not fully understood. I’ve heard it described that insects don’t have the capacity to digest complete proteins and that they’re dependent on soluble amino acids as a food source. And that never quite made sense to me. It made sense from a theoretical perspective, but it didn’t make sense to me that plants might have no amino acids versus one plant having very high levels of amino acids.

Larry: Yeah, it’s a matter of degree. And even proteins vary in terms of their digestibility for insects too. And, of course, insects evolve enzymes that are going to be most effective in digesting the proteins they’re likely to encounter, depending on what host plant they feed on.