monarchs:Australia and North America

[Paul Cherubini]

John Lane wrote:
> 
> When monarch butterflies reached Australia (aided by humans), they found numerous native
> Asclepiadaceae, upon which they could become established. Although monarchs may prefer
> introduced milkweeds, native genera include: Leichardtia, Ischnostemma, Pentatropis,
> Cynanchum, Secamone, Hoya, Gymnanthera, and Cryptostegia (all known hosts of native Australian
> Danaines). Source: Common & Waterhouse's Butterflies of Australia.

Technically correct, but the species that supports the monarch migration in the temperate 
zone latitudes in Australia and New Zealand is the introduced species, i.e. Asclepias 
fruticosa. Therefore the monarch migration in Australia is a man made phemonenon. 
Asclepias fruticosa  flourishes on ground where the native Australian vegetation has been 
cleared for agricultural, road building and human settlement purposes.

> In North America, overwintering monarchs
> are reported to all be in reproductive diapause.  But at Sydney, it is my understanding that
> (at some locations, at least) where monarchs overwinter, and hostplants grow adjacent, part of
> the population is reproductively active, with oviposition and immatures present through the
> "winter" period.

Exactly the same phenomenon occurs in California. Along the Malibu coast between Los 
Angeles and Ventura, a native milkweed, Asclepias fascicularis, is abundant right next to 
several overwintering sites. Egg laying and larval development occurs throughout the 
winter. This part of the California coast is at the same relative latitude as Sydney, 
Australia.

> At Santa Cruz, CA, there is a milkweed garden maintained adjacent to the very large
> overwintering monarch colony at Natural Bridges Beach State Park, and reproduction clearly
> ceases each fall: oviposition falls drastically to zero and latest-season immatures suffer
> greatly increased mortality.

In cold winters, true. In warm winters, such as 1997-98 egg laying and larval 
development occurred throughout the winter at milkweed (Asclepias currassavica and 
Asclepias fruticosa) patches plantedby park employees at the Ardenwood Historic Farm 
and Coyote Hills overwintering colonies in Fremont, California (San Francisco Bay 
Area). 

Paul Cherubini, Placerville, California