Wooly Bears

[Dale Hoyt]

I've been interested in what influences the width of the orange band in
Wooly Bear larvae (_Pyrrharctia_isabella_) and have some casual
observations to offer in the hope that others might have additional
information, data or references to contribute. A few years ago I did a
casual literature search and couldn't find much of anything on the factors
that influence band width. Just general statements to the effect that the
width of the band varys with instar -- nothing quantified or very specific,
just remarks that suggested it might be "common knowledge" or unattributed
observations passed on from one entomological generation to another.

I decided to see if there was any genetic influence on band width and
thought I'd raise some Wooly Bears and, if there was any variation, attempt
to establish lines of narrow-banded and broad-banded moths by inbreeding.
I've been frustrated in accomplishing that goal, but I've made some
observations that might be of interest to the list.

First, the extent of the orange band does vary with instar. I started out
with over 300 larvae from a single female and soon couldn't manage all of
them. I reduced them to two dozen by selecting 12 with the narrowest and 12
with the broadest bands. I released the others into the wilds of my
backyard. Now the strange thing happened. As the remaining 24 grew and
molted the extent of the orange band increased in both groups until most of
them were just one big orange band. Some had a little tuft of black on the
front, but most were completely orange. Unfortunately I also had shifted
the larvae to a new food plant at this time, so I don't know if the change
was due the age or the new food. (I ran out of the broad-leaf plantain I
was using and started feeding them on wild blackberry. I actually tried a
number of species, but they seemed to prefer the blackberry in a
taste-choice test.) Later instars regained their black ends and started
looking more like standard wooly bears, but the completely orange phenotype
persisted throught a couple of molts.

I lost this group of larvae when I attempted to overwinter them. 

The next year I got another batch of w.b. larvae started earlier in the
summer and they pupated in July. These had also been all orange. I eagerly
awaited their emergence so I could mate them and get a sibship line going,
but all the moths were female! Twice I have selected the larvae that have
the most orange in one of the earlier instars and the resulting moths have
been all female.

I wish now that I had kept more detailed notes -- I know better, but this
was a casual project that I hadn't devoted a lot of thought or attention
to. I hope to pursue it again this summer more carefully.

Again, if any of you have references or know of studies on the factors that
influence the width of the band in these caterpillars I'd appreciate
hearing from you.

Cheers!

Dale Hoyt

 
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