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Wasp larvae parasite of squirrels?

Jack J. O'Brien jobrien at JAGUAR1.USOUTHAL.EDU
Thu Aug 24 10:39:53 EST 1995

Dear Amy:

This is a reply to your question about "wasps" that lay eggs in squirrels. 
Although there are quite of lot of wasps that lay eggs inside and
consequently feed upon the tissues of other insects, the organisms to
which your friend was referring are actually flies, members of the family
Oestridae, subfamily Cuterebrinae, common name "botfly," "warblefly," or
here in southern Alabama "wolves."  The species name is Cuterebra
emasculator.  The female flies lay eggs near the nest or on the fur of the
host (cats and dogs are sometimes infected too). After hatching the
grub-like larvae occupy cavities in the subcutaneous connective tissue of
the host. These cavities possess an opening to the outside which provides
oxygen to the developing fly larva.  The larvae become more visible on
squirrels in the Fall here in Alabama. To complete development, larvae
emerge from the warble, drop into leaf litter at the base of trees and
pupate. Although our campus is crawling with squirrels, I have never seen
any on campus with the tell-tale "wolves" on their backs. I suspect that
this is because the grounds are cleared of leaf-litter and thus the flies
are denied the microhabitat necessary for pupation.  Squirrels in forests
are commonly infected. Adult flies are very fast and rarely caught in the
field by collectors. The best way to obtain an adult is to maintain a pupa
on moist sawdust in a glass jar until the adult emerges. Although the
warbles look disgusting, they apparently do little harm to the squirrel
(other than form unsightful scars--the warbles rarely become infected with
bacteria) if they are located on the shoulder region. The species name,
however, comes from the effect caused by warbles forming near the
testicles of male squirrels. The subcutaneous swelling caused by the
warbles somehow prevents full development of the squirrel's testicles; the
reproductive capabilities of female squirrels are apparently unaffected. 
I know, I know. This doesn't seem fair, but Mother Nature is sometimes
like that. 

Happy hunting.

Jack O'Brien 
Assist. Prof. 
University of South Alabama

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