Subject: Re: Wasp statistics and Re: generalizations
From: wtmorgan@pilot.msu.edu (Bill Morgan)
Organization: Michigan State University
Date: Sun, 28 Sep 1997 07:14:52 -0400
Message-ID: <MOD$970928.9709@rec.gardens.ecosystems>
References: <MOD$970922.22377@rec.gardens.ecosystems> <MOD$970922.26760@rec.gardens.ecosystems> <MOD$970926.24426@rec.gardens.ecosystems> <MOD$970926.13246@rec.gardens.ecosystems> <MOD$970926.20144@rec.gardens.ecosystems> <MOD$970927.28329@rec.gardens.ecosystems>
In article <MOD$970927.28329@rec.gardens.ecosystems>,
wmcclain@salamander.com (Bill McClain) wrote:
> I have for some time wondered if encouraging predators for insect
> control isn't a bit of a myth. As in: bats and martins etc eat xxx
> mosquitos every day. Isn't there a nearly infinite supply of backyard
> insect life, and wouldn't the predator population suffer if they
> actually did reduce the number of prey?
A very good point. It all depends on what you mean by "control". If you
mean complete annihilation of a (local) population, you are likely to be
disappointed. If you can tolerate a certain amount of damage, then you
might consider the control to have worked. In the case of mosquitoes, their
predators (dragonflies, birds, fish, etc. etc.) kill a lot in both the
larval and pupal (aquatic) stages and in the adult phase. But people tend
to very sensitive to the skeets, so they generally would not recognize
these things as being effective controls. OTOH, if weevil is eating your
alfalfa, but the predators and parasites are holding the loss down to a few
percent of the foliage, you might consider that to be effective control,
considering the expense of doing anything interventionist.
When people go out of their way to initiate pest control (whether by
"organic" or other means), they often fail to ask the first question, which
is "Do I need to do anything at all?" In many cases, it does not make
economic sense to do anything. It costs money to buy and apply pesticides
(or to bring in predators), and if that cost exceeds the cost of the
damage, there's no reason to do anything. (BTW, the point at which
treatment becomes cost-effective is called the "economic threshold", or
sometimes the "action threshold".
In the old days, it wasn't uncommon for people to spray pesticides on a
schedule without even bothering to look closely at the situation to see if
any control was needed. While this practice is not dead, it is much less
common these days.
(Except of course, for lawn care services, some of which still spray first
and ask questions later.)
More recent trends have been to assess the need first, taking it down to a
sort of "ask questions first, spray later" paradigm. This was done
initially because of the expense of pesticides, but environmental concerns
have accelerated that trend.
Still more recently, simple assessment of the problems in the field have
lead to the development of computer models which take into account a large
number of factors in order to determine if treatment is necessary. Aside
from reproductive biology of the pest, some of the factors are: last year's
ending population size, the severity of the winter, abundance of the 3 P's
(pathogens, predators and parasites), and so on. This has been an important
development in IPM (Integrated Pest Management), and where implemented
fully, the practice can greatly reduce the use of pesticides.
Of course, the models are still imperfect in many cases. While some pests
have been worked with for a long time, and the models have become fairly
refined, that's not the case for every pest you might encounter.
Of course, Jack is looking to do something different here by increasing the
number of predators (paper wasps). That will help him out some, but will
probably never provide total control. I venture that most people on this
group would agree that is a step in the right direction, though. I think
with a bit of diversification, he can get more predators and parasites to
help him out.
> Boom and bust predator-prey cycles do occur with larger animals, but how
> likely is such a relationship when insects are the prey? I know of one
> supporting case: the population of a bird (Yellow-Billed Cuckoo?) is
> said to be correlated with outbreaks of tent caterpillars. Although I'm
> not sure even then if the birds exhaust the number of caterpillars.
Exhaust? No. Never more than locally, if that. I think that it is a
fundamental error to think that natural controls are going to virtually
eliminate a pest in the way that pesticides sometimes do (at least
temporarily). Rather, to keep the 3 P's in place, you need to keep the pest
around in low numbers so its enemies can stay in business. Or the enemies
need an alternate host/prey item to survive the timew when the pest is
absent. (Easy enough with generalist predators, much harder the more
specific they are...).
And it goes almost without saying: having a suite of predators, parasites
and pathogens around is more likely to be effective than trying to rely on
just one.
Regards,
Bill
--
Bill Morgan
wtmorgan@pilot.msu.edu
Center for Room Temperature Confusion