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Re: plant resistance to herbivory

> Date:          Wed, 25 Jun 1997 00:05:01 -0400 (EDT)
> From:          joel b gruver <jgruv@wam.umd.edu>
> To:            sanet <sanet-mg@ces.ncsu.edu>
> Subject:       plant resistance to herbivory

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 

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.  


> Hello to all...
> A general school of thought amongst "sustainable" growers is that
> plants that are under significant stress are more often the recipient of
> herbivory than less stressed plants. I have observed that the
> scientific literature substantiates this theory to some extent.
> My question for the group is... what is the ecological logic behind
> pests consuming weak plants preferentially to strong plants ? and what are
> the specific mechanisms by which herbivores discriminate
> between
> stressed and less stressed plants ?
> In animal ecology it seems very logical that predators have an
> easier time catching and killing weak/young/injured prey...
> How well does this analogy hold true for plants ? Do strong/less stressed  
> plants tend to have more active defense mechanisms than weaker/stressed
> plants ? 
> ...but some stressed plants have elevated levels of alkaloids and
> other anti-feedant compounds... 
> Any insights or recommended reading on this subject ?
> On another subject:
> I have read about the insecticidal compounds in Assimina triloba (Pawpaw)
> foliage... What is the current status of research on these bioactive
> compounds and can anyone provide any recommendations for how one would
> prepare pawpaw leaf extract for pest control use ?
> Joel Gruver
> U of MD, Agronomy