(Steve Harris) wrote in message ...
In article ,
(Neil Jones) wrote:

In fact I can think of reasons why there
might be less damage to the cabbages if more eggs were laid.

Do go on! :-)

Steve Harris - Cheltenham - Real address steve AT netservs DOT com

Oh boy, this is complicated larger populations may( I stress may)
attract more predators.
For example cabbage whites are attacked by a parasite called Cotesia
glomerata
(formerly Apanteles glomeratus) this is a tiny tiny wasp whose larvae
eat the caterpillars alive. There are other wasps too and they can
emerge from early stages of the caterpillar killing them while quite
small.
Without digging into journals I don't know what is known about the
interactions here and what "density dependent" (to use jargon) effects
have been observed.

I do know that in two other butterflies The Marsh Fritillary which has
a specific parasite called Cotesia Bignelii and the Holly Blue which
has a parasite called Listrodomus nyctherimus the populations have
been observed to be markedly affected by parasite density.
Only the Holly Blue would be really expected in a garden. The Marsh
Fritillary is a listed protected species not usually found in gardens.
(Having said that it used to come into my old garden 20 years ago but
that is like getting a peregrine falcon on your bird table!)

--
Neil Jones- http://www.butterflyguy.com/
"At some point I had to stand up and be counted. Who speaks for the
butterflies?" Andrew Lees - The quotation on his memorial at Crymlyn
Bog National Nature Reserve