Re: monarch butterflies
Loren Russell wrote:
>
> There was a fascinating line of research by Lincoln Brouwer and his
> associates a few years ago untangling the milkweed/insect web. Basically,
> their results go like this: the amount of milkweed toxins ["cardiac
> glucosides"= CG] in a monarch's body is directly related to the
> concentration of these toxins in the larval food plant. And this amount
> varies widely between species of mikweeds, between populations of the same
> species of milkweed, and between individual plants in a population.
> Although this thread wasn't followed out in the literature that I read, it
> would appear that something like mimicry is going on at the plant end:
> that some milkweeds free-load [by not producing toxins] on their neigbors.
>
> The Brouwers used bluejays as their test predator. And as you
> say, a "naive" jay happily eats monarchs, and anything that looks like a
> monarch. Up to the point that the butterflies have a certain level of the
> cardiac glucosides. And that point is not lethal [though these toxins
> could kill a vertebrate]. Nor is it just tasting yukky: birds, unlike
> mammals don't seem to depend much on their sense of taste.
> Rather, it seems that many birds and mammals are protected from
> ever poisoning themselves by this class of toxins by an automatic emetic
> response.
> The Brouwers described a "bluejay emetic unit" [BEU] equivalent to
> a
> specified amount of the milkweed toxins. Less than a BEU, and bluejays
> munched away. More than a BEU and the jays lost their lunch, and then and
> only then began to avoid the butterflies, and anything that looked like
> them. Presumably the African mikweeds you describe have fairly low levels
> of CGs, and the jays have learned that monarchs [in that locale at least]
> are pretty good food.
> Another thing to point out is that the monarchs, significantly,
> don't absolutely rely on their chemical protection -- as would be expected
> for a uniformly toxic insect. They have bright and distinctive colors,
> but they retain active defense -- they are fast, erratic flyers, etc.
> Really, the monarch is an amazing species, and much remains to be learned
> from it.
> The work I'm describing was done in the 1970s, and there were some
> Scientific American articles on it. Brouwer published popular book, too,
> if I remember correctly, though I've never read it.
>
loren
The beautiful Monarch is one of our most welcome immigrants in NZ. As
far as I know it got here on its own steam island-hopping across the
Pacific and has been established here for many years. It can only
survive here thanks to gardeners, as there are no native milkweeds. A
great many people plant Asclepiads solely to raise the caterpillars -
mostly the Swan Plant (A. fruticosus) and sometimes
A. tuberosa as well. From late spring each year most garden centres
carry huge stocks of potted plants of these two species for gardeners
"caught short" by the arrival of a female Monarch looking for somewhere
to lay. I was one who this year had such a visitor exceptionally early -
a poor ragged thing, but, my, DID she lay. In the end I was forced to
wipe off some of the eggs so as to have enough food for the remaining
offspring. Anyway, finally the pupae are in place and we look forward to
the hatching soon. Monarchs are very welcome here, as there are only a
small handful of butteflies in the country though a huge variety of
moths (some big bright and handsome). Our native butterfly species are
very few and mostly pretty insignificant and the more showy ones
occasionally seen are nearly all blown across from Australia. some of
these (who presumably arrive in fair numbers) have established
themselves locally, but mostly only to a limited extent.
The monarch butterflies are common though much of the country, but
presumably mostly overwinter in districts which experience little or no
frost. In our area I don't think they suffer badly from predators, at
least at the caterpillar and pupal stages. I have never heard of a local
bird trying to eat them, though European wasps possible take some.
Further north however I do know that a lot of caterpillars fall victim
to the Asian Paper Wasp which has established itself around Auckland.
When staying with a friend one year we found caterpillars were steadily
disappearing and came to the conclusion these wasps were to blame. To
save the rest of the brood we picked branches off the Swan plant and
took them indoors, but I don't know how things went after that as we
came home.
The Auckland butterflies also suffered a manmade disaster one year when
the entire city was sprayed with Bt several times to wipe out a tussock
moth (an immigrant also, but a MUCH less desirable one). After the
spraying finished people in many parts of the country offered
caterpillars and pupae to re-establish the Monarchs in the area.
Moira
--
Tony & Moira Ryan <theryans@xtra.co.nz>
Wainuiomata, New Zealand. (on the "Ring of Fire" in the SW Pacific).
Lat. 41:16S Long. 174:58E. Climate: Mediterranean/Temperate
