Monarchs / natural factors

[Martha Rosett Lutz]

Hi Paul and others!

Paul wrote:

"the natural biotic or abiotic factors
that could have led to the monarch population crash"


and I am taking this one phrase out of his context in order to ask a
scientific question:  does anyone know if anyone is studying the dynamic
interaction of Monarch populations and parasitic tachinid populations?

I bring this up because I have several species of milkweed on my property
(planted specifically to attract Monarchs so that they will come to me
instead of me and my children having to scout for eggs and larvae) and
usually get a dozen or so larvae each year from these plants.

Last year we had a bumper crop:  about two dozen plus larvae on a
relatively small patch (less than one square meter) of milkweed.  I can't
recall the exact numbers, but about 30-50% of these larvae turned out to be
parasitized, most of them with more than two maggots of a tachinid fly.
This year I have had zero Monarchs in my yard, although plentiful milkweed
bugs feeding on the pods.

This raises a question (or several) that might be interesting--I don't know
enough to judge whether this is a valid topic or not.  The question is:
does a dynamic exist between the tachinids and the Monarchs, such that a
larger population of Monarchs (presumably at a higher density on the food
plants) makes it easier for the flies to find the butterflies and therefore
has a greater effect of suppressing the Monarch population?  Do the flies
leave some marker detectable to Monarchs so that the Monarchs do not return
to the same host plant patch the season following a major infestation of
flies?  If Monarchs are relatively less numerous in a given year or a given
area, do the flies have so much trouble finding them that the fly
population is seriously reduced and there are far fewer of them the
following year(s), thereby making them an insignificant factor (on a life
table) in years following reduced Monarch populations and significant
mortality factors in years following larger than usual Monarch populations?

I hope I have made these questions reasonably coherent.  I have had an
interest in the Monarch/tachinid interaction since about 1986, but have
never had time or opportunity to pursue it in a meaningful way--just making
small observations and thinking about questions.


Thanks (in advance) for any information anyone can provide!

In Stride,
Martha Rosett Lutz



 
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