Fwd: Re: Record Low Monarch population expected in Mexico thiswinter.

[Patrick Foley]

Chip,

 Thanks for your detailed response.

Patrick
patfoley at csus.edu

Chip Taylor wrote:

> >
> >>Responses to Patrick Foley
> >>
> >>Is your group keeping enough time series data on Monarchs and
> >>natural enemies to
> >>see if predator-prey oscillations are going on over time?
> >
> >Karen Oberhauser and a large number of volunteers are engaged in a
> >monarch larval monitoring project at a relatively large number of
> >nature centers distributed throughout the eastern US. They are
> >gathering information on predation and parasitism. Although some
> >larval monitoring has been underway for the last 5 yrs or so, this
> >is the first yr of a larger project. If there are oscillations, they
> >should detect them.
> >
> >
> >>Or do you have any
> >>relevant information?
> >
> >My IMPRESSION is that monarchs are largely weather driven. Droughts
> >and other extremes have an  immediate and sometimes longer impact on
> >the population. Predators and parasites, particularly fire ants in
> >the south and tachinids in the lower midwest, are significant
> >factors in some yrs and some locations. In eastern KS we usually
> >lose 80-98% of the 5th instar larvae to tachnids. If we keep in mind
> >that only a few % of the eggs/larvae reach the fifth instar, this
> >level of parasitism can keep the population from increasing
> >significantly over the summer. The tachnid population can vary and
> >seems to drop following long cold winters.  The same applies to
> >paper wasps and yellow jackets but the latter are also influenced by
> >prey availability of the previous summer. The general trend is lower
> >rates of tachnid parasitism as one goes further north and east
> >although there can be episodes of pop increases of this species in
> >some areas - however, they seldom exceed 30-40% infestation.
> >
> >
> >
> >>  If monarchs are the main hosts
> >
> >The tachinid that parasitizes most monarch larvae (there are
> >reportedly five such parasites) is Lespesia archipivora, a species
> >that parasitizes a large number of leps. The abundance of this
> >species is therefore likely to be governed by the general
> >availability of lep larvae and other factors.
> >
> >>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?
> >
> >Monarchs have a population growth phase -March -September -
> >depending on latitude and a death phase which occurs from mid August
> >in the north (50 degrees N) to the following spg when the monarch
> >move north through MX. So, the question becomes what keeps the
> >population from growing and what influences the rate of mortality
> >during the migratory (death phase) of the annual cycle. Droughts
> >decrease birth rates in the growth phase and increase deaths in both
> >reproductive and migratory phases. This applies to most butterflies
> >and lots of other insects. The number of monarchs returning to the
> >US seems to be less important than the conditions they encounter in
> >the spg. but obviously large overwintering pops can disperse in such
> >a way that they overfly regional droughts. So, the size of the
> >overwintering population and the proportions surviving the winter
> >can be important. Conservation of the core overwintering areas in
> >Mexcio is obviously essential to the survival of migration as we
> >know it.
> >
> >Milkweed distribution and abundance does not appear to be a
> >significant factor at this time but it could be in the future. The
> >adoption of herbicide resistant crops, particularly Roundup Ready
> >corn and soybeans will reduce milkweeds in croplands. Our estimate
> >is that 90 % of the monarchs originate in the agricultural landscape
> >and most of these from croplands. Roadsides only account for about
> >2% of the milkweed/monarch habitat in the northern breeding range
> >and do not appear to be a major source of monarchs since losses due
> >to predation and other factors are higher in these habitats than in
> >croplands in late summer.
> >
> >In spite of my statement that monarchs are largely weather driven,
> >their populations fluctuate less than those of many other insects.
> >The largest monarch overwintering population known to date was less
> >than 7 times larger than the smallest population.
>
> --
>
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> e: monarch at ku.edu
> w: http://www.MonarchWatch.org/
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