Fwd: Re: Record Low Monarch population expected in Mexico this winter.

[Chip Taylor]

>
>>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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