Sanet Post, Guy Ames
Plant Resistance to Herbivory
June 25, 1997

Guy Ames wrote:

I'd like to make at least four points here. Two are basically in support of the hypothesis that less stressed plants are more resistant to pests. The other two are contrary to that argument. And I'd like to conclude with a comment about the anthropocentric and teleological fallacies inherent in the wishful thinking of some in the "organic movement."

First, in support of the stronger plant-less pests theory: Fruit trees that are in good vigor can apparently cast out or drown invading borers with sap. So, good nutrition and plentiful water, especially plentiful water, are important components in protecting fruit trees from borers. However, borers can still successfully attack apparently healthy trees; but even then such trees can better tolerate borer damage than a similar, drought stressed tree.

Grasshoppers (at least western species) prefer drought stressed plants as a rule. The theory proposed by at least one entomologist is that the grasshoppers' digestive system is better able to handle the "concentrated," less watery, drought stressed herbage. It takes more energy for the hoppers to digest water-laden herbage.

Now, the "contrary" arguments don't necessarily DIRECTLY counter the notion that stronger plants are more resistant to pests, but it takes a closer, more detailed look as well as a larger systems look.

First the detailed look: ...at plant parts. As a fruit grower I know that my TREES are at least more tolerant of pests if they're healthy (see the above borer example), but the FRUIT is not the same at all. The whole of natural and human selection for fruit is to make the fruit delectable to a wide range of organisms. The main goal of such an evolutionary strategy is probably dispersal, but there is also some evidence that certain fruit rots at the right time can increase seed germination. I'm not saying that all "pests" of the fruit are beneficial to the ongoing survival of the plant species, but I am suggesting that the plant's overall evolutionary strategy does not protect the fruit the same way that it might the parent plant. For instance, an organically grown apple tree in Massachusetts might be a thriving, healthy example of its type, resisting borers, aphids, and mites, BUT perhaps something near 100% of its fruit will be rendered commercially worthless by the ravages of the codling moth, oriental fruit moth, and the plum curculio. I'm not making this up. Just ask an organic fruit grower in the East.

Okay, for the larger systems look: there is now good evidence that organically fertilized corn is comparitively more resistant to certain corn pests than coventionally-fertilized corn. However, the way I read the study, my understanding is that this is based on feeding preferences trials. The corn borer has a choice between the conventionally fertilized stuff and the organically fertilized stuff. The borer prefers to attack the conventionally fertilized stuff. BUT what happens when the borer has no choice? If you put the borer in with only organically fertilized corn, it attacks that corn. So, if you had a large solid block of organically fertilized corn, you may end up with a lot of damage still. Maybe less than if you had fertilized conventionally, and maybe a lot less if your neighbor has a big field of conventionally fertilized corn that pests might prefer. It's good, then, from our point of view, to organically fertilize that corn, but it's no panacea.

(By the way, if it matters, I'm not a "certified organic" grower, because I use some insecticides, but I do fertilize my trees "organically.")

In concluding, I want to briefly critique the antropocentrism that seems to pervade the thinking of some (often beginning) organic growers. It is only a smaller reflection of the anthropocentrism and teleological thinking that underlies much of western (especially Christian) thought. Humans were not PUT here to rule the planet. We have no more "right" to the planet's resources than the lowliest cockroach. We want to eat, they want to eat. If there is a nice, big, lush, organic field of tomatoes out there, no amount of organic certification is going to keep a hungry horde of blister beetles out of them. No amount of microbially-balanced compost is going to protect those tomatoes from birds or raccoons. There is a notion that if our crops were in "perfect balance" nutritionally, they would have no "pests." Who are the pests? Perhaps we are the pests? To the birds who want my grapes, I'm the pest. If we produce the most nutritionally perfect crops possible, why should they only be suited to OUR consumption? Why wouldn't other organisms want them?

Hey, it's all relative! Sometimes, in some situations the organically grown stuff is gonna be more pest resistant or more pest tolerant. Hell, it may almost always be more resistant compared to conventionally grown stuff. BUT beware of casting a big net, of making the generalizations. We are going to have "pests," whether we're organic or not.