Record Low Monarch population expected in Mexico this winter.

[Patrick Foley]

Paul makes a good point about the possible importance of egg mortality in deciding
Monarch abundance. Does anyone know of any data on survivorship rates in monarch eggs?

It could be fairly easy (given the immobility of eggs) to estimate survivorship _and_
compare parasitism to predation by either finding or placing eggs on plants, marking
localities and later surveys. I suspect ants for much egg predation, but there are a
host of suspects.

I don't have time to hunt for literature on this today (I am teaching Entomology, Plants
and Soils, Biometry and Botany Seminar this semester), but if anyone has something on
the top of the head, especially recent reviews of egg mortality causes in Leps, I would
like to hear about them.

Some egg parasites (Trichogramma minutum for example) are generalists, but there may be
some monarch specialists in abundance out there in milkweedlandia. Anybody know?

John F. Anderson (1976 Egg Parasitoids of Forest Defoliating Lepidoptera IN Anderson and
H. Kaya 1976, Perspectives in Forest Entomology, Academic Press) states that little work
has been done on the population biology consequences of egg parasitism (largely due to
taxonomic difficulties), but what has been done suggests that rates of parasitism from
50-100% are common in Forest Leps. Judging from his long table, perhaps 25% of the
careful studies have shown such high rates of egg parasitism.

Patrick
patfoley at csus.edu

Paul Cherubini wrote:

> Patrick Foley wrote:
>
> > Is your group keeping enough time series data on
> > Monarchs and natural enemies to see if predator-prey
> > oscillations are going on over time? Or do you have any
> > relevant information? If monarchs are the main hosts of
> > some Tachinids or some other insect, then some coupling
> > may be ccurring. If weather fluctuations are
> > driving most of the Monarch population fluctuations, then
> > such a coupling may be one way. Monarch pops may drive
> > tachinid posp but not vice versa. I am assuming
> > that the ultimate Monarch density regulation comes from either
> > Winter roosting habitat or from Milkweed abundance. DOes
> > anybody have a handle on that?
>
> Pat, in private industry an employee must produce product
> and/or results or be fired. Same deal in the academic community.
>
> Because of this situation, scientists will tend to focus their attention
> on butterfly mortality factors that can be easily observed and quantified
> and therefore will quickly produce publishable results.
>
> So the scientists mainly study mortality factors like weather extremes,
> tachinid fly parasitism, milkweed condition and abundance, overwintering
> mortality, etc.  But they will largely ignore mortality factors like egg
> predation which may be far more significant because it is logistically
> difficult to physically observe and measure this predation (egg predation
> by ants, for example, can occur at night) .
>
> In Minnesota and northern Iowa this past July and early August I observed
> a great abundance of monarch eggs on milkweed plants but no caterpillars.
> Therefore my reasonable conclusion was that a tremendous amount
> of egg predation was occurring that would prevent the normal buildup of migrant
> monarch butterflies in late summer.
>
> As early as Aug. 21 I predicted a record low population of monarchs in
> Mexico this coming winter based on this severe egg predation and lack
> of caterpillar abundance (beyond the first instar).
>
> I would be a hypocrit if I didn't also mention there are other possible reasons
> for the lack of caterpillar abundance beyond the first instar besides egg predation.
> For example, there might be some kind of obscure disease that affects
> periodically affects monarch eggs or first instar caterpillars that has never been
> studied.
>
> Paul Cherubini
>
>
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