Migration?

[Michael Gochfeld]

The northward migrations driven by limited food resources may make 
sense for the massive migrations of Ladies (Vanessa) or of Snouts 
(Libytheana), or of some Pierids. 

However, we have several species of skippers (hesperids) that migrate 
northward in small numbers, colonizing New Jersey and even southern New 
England beginning in late summer and some that migrate northward only 
sporadically.  It's hard to believe that they have significantly 
depleted the grasses on which their larvae feed. 

These include regulars such as the Sachem (Atalopedes campestris)
and Fiery Skipper (Hylephila phyleus)   and more vagrant species such as 
the Ocola Skipper (Panoquina ocola).

The Fiery Skipper caterpillars, for example, utilize several species of 
grasses including the ubiquitous lawn-weed, Crab Grass (Digitaria). If 
they were able to significantly impact this pest, someone would figure 
out how to exploit them commercially. 

Sachem also uses several species including Crab Grass, and another 
common weed, Bermuda Grass (Cynodon dactylon). 

Ocola likewise uses several grass hosts, including some coastal grasses 
that must be superabundant. 

Maybe dispersal has a more basic function of expanding ones range to 
minimize impact of non-density dependent events such as hurricanes, and 
fire.  Flooding on the coastal plain could have wiped out large segments 
of a population (and is likely to do so in the future if sea level rise 
continues).  Thus the ability not to have all ones eggs in a few baskets 
could be beneficial.  On the other hand,these immigrants get wiped out 
in winter.

If ever there was a winter for them to survive in the north it would 
have been the warm winter of 1997-1998, so if we hear of a rash of 
spring records for these skippers, it might validate this evolutionary 
strategy.  On the other hand, the almost complete lack of snow cover, 
may have worked against some of the species exposing immature stages to 
dessication or rodent predation. 

Mike Gochfeld