Bugs only bug unhealthy plants? (fwd)

Allyn Weaks allyn at u.washington.edu
Fri Aug 8 18:43:20 EST 1997


In article <33EB8906.7B1 at foxinternet.net>, Neason
<Rebecca.Neason at foxinternet.net> wrote:

> Rather than call it necessarily a counter-example, could one not argue that 
> periodic swarms of insects serve the dual purpose of "cleaning up"
overcrowded 
> vegetation

I'm not at all fond of this interpretation, because it implies that nature
has an intent, and/or that the insects 'care' about the plant community as
opposed to just wanting to survive and reproduce.  Much too teleological
for my taste...  Besides, on the prairies, fire was pretty common, wasn't
it?  And that would take care of cleaning out overcrowded vegetation just
as thoroughly (maybe a little more thoroughly!) 

> while at the same time protecting the insect from predators?  (Because 
> the insect is not available each season but only several seasons apart, the 
> predator population stays lower.)

This makes much more sense, and is very common.  Mast years for nuts,
acorns and other seeds, periodical cicadas (with periods that are prime
numbers, which makes it harder for the predators to synchronise :-)).  But
this explanation is unconnected from plant health...when the 17 year
cicadas come out in droves, it isn't because tho plants are more stressed
that year (though they will be by the time they lose much of their foliage
:)).  I gather that the locust swarming is more complicated: they change
into a different body form and change their behavior dramatically when
their population density increases beyond a certain point, so it seems more
a strategy for dispersing the gene pool.  But much variation is just
random.  Perhaps winter weather conditions are better some years for the
herbivore bugs than the predator bugs, in which case more annual plants
(which should be less sensitive to winter conditions, so I can try to rule
out that the winter affected the plants too :-)) will get eaten, even if
they aren't as 'tasty' due to stress.  

So on the whole, in a 'normal' year, in 'normal' conditions, healthier
plants will be much more resistant to insect and disease attack, but I
think it's oversimplified to extropolate that to 'healthy plants will
_never_ be seriously in danger from bugs or disease'.  Even in as near
perfect a garden as possible, some years the gardener will have to squish
bugs if she wants to eat dinner.  In commercial sized plots, even properly
managed strictly organic plots with very healthy plants, there will
sometimes be a year where you either spray or lose the crop.  If you _only_
spray under these circumstances, choose which spray to use carefully, and
apply it properly, it's probably not going to cause problems.  It's the
routine use of pesticides that breeds stronger pests and puts the system
thoroughly out of wack.

>  Thus, the insects are "attacking" the "stressed" 
> plants, as the original post suggested.  

Well, I still don't think so.  Over an area of hundreds of square miles,
which includes at least several kinds of habitat (prairie and riparian at
least) certainly not all of the plants are going to be stressed, but every
single one of them gets eaten.  And from the reports of the observers, the
'hoppers aimed at whatever was closest to get their mandibles on, not the
'sick' plants first (though it could be hard to tell when everything in
sight vanishes in a couple of hours!).  It's hard to even imagine the
magnitude.  The swarms were so huge and so dense that they blocked the sky
for miles around, for days and weeks at a time.  One of the Little House on
the Prairies books has a good account, and I've seen others that matched it
pretty well, so I don't think she was exagerating.  Too bad they didn't
realize that they could just harvest all those 'hoppers and eat them
instead--grasshopper meal is very nutritious :-)
-- 
Allyn Weaks  allyn at u.washington.edu
PNW Native Wildlife Gardening:  http://chemwww.chem.washington.edu/natives/
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