What's That Bug? does not endorse extermination

Subject:  Luna Moth Caterpillar
Geographic location of the bug:  Eagle River, Wisconsin
Date: 10/01/2019
Time: 10:19 AM EDT
Your letter to the bugman:  We found this caterpillar on a nearby wooded pathway yesterday, and didn’t know what it was or where it was going–end of September can usher in very cold temperatures here.  So, at home we identified it as a Luna Moth Caterpillar.  We want to properly release it back into the wild.  It would be lovely to have seen it develop into the moth, but we don’t feel confident that we can keep it healthy.  Will it over-winter here in the North?  or Will it still be able to mate yet this autumn?  It was found under a soft Maple tree quite close to a lake and alder bushes near the lake and surrounding wetland.  I was even wondering if it could drown?  Thank you for information so that we can release it soon and get it on its way to the right environment.
How you want your letter signed:  The Rasmussens

Polyphemus Caterpillar

Dear Rasmussens,
Luna Moth Caterpillars and Polyphemus Moth Caterpillars can be difficult to distinguish from one another.  We believe your caterpillar is a Polyphemus Caterpillar.  The identifying feature is a pale yellow band that runs through the spiracles or breathing holes on the Polyphemus Caterpillar.  It is described on BugGuide as:  “Larva: body large, bright green, with red and silvery spots below setae, and oblique yellow lines running through spiracles on abdomen; diagonal streak of black and silver on ninth abdominal segment; head and true legs brown; base of primary setae red, subdorsal and lateral setae have silver shading below; end of prolegs with yellow ring, and tipped in black.”  At this time of year in your location, we speculate this individual is preparing to pupate and it will overwinter in the cocoon.  Caterpillars are not aquatic.  They can drown.

Dear Daniel:?? Thank you for the information.?? It is nice to know what it is– Polyphemus, not Luna, and that it will overwinter.?? It started spinning yesterday between two leaves in the leaf litter at the bottom of the container, currently in our garage.?? So now, we will have to decide the next step:?? possibly to get info on overwintering it in our refrigerator with a constant temperature or it will be subjected to?? subzero temperatures for much of our Northern Wisconsin winter.?? If you had thoughts and time on this, don’t hesitate to drop a line.?? We appreciate and feel fortunate to have had your communication.?? Much of the information we were finding is not specific in details or confusing.???? –Patty & Eric Rasmussen

Dear Patty and Eric,
We do not raise caterpillars, but in captivity, one needs to be cognizant of temperature and humidity.  Too warm and the moth will emerge prematurely.  Too damp or too dry it might not survive.  We would recommend keeping it outdoors in a protected location where it will benefit from precipitation, but not get too wet.

What's That Bug? does not endorse extermination
Location: Eagle River, Wisconsin

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