Possibly Painted Arachnis Caterpillar in Mount Washington

Subject: Mt. Washington caterpillar
Location: Mt. Washington, Los Angeles
September 13, 2013 4:26 pm
Hello again, Bugman. Shortly after Moth Night at Elyria Canyon, I dropped you a note asking about this caterpillar that I had found in my Mt. Washington back yard. I think you were on vacation at the time and it may have slipped through the cracks or got lost in the ether, so I’m re-sending it. Or, maybe it’s just impossible to identify and I’ll have to wait until it decides to spin a cocoon and hatch.
As I mentioned, there were 6-8 of these that would come out every night to feed (on a certain kind of plant that I can’t identify!). I noticed one last year, too, so they seem to like the area. I haven’t seen them since shortly after collecting this guy, who now lives in a terrarium in our living room. I’ve checked caterpillar for some of the common moths and butterflies we have around here, with no success. I thought (hoped) it might be a Giant Leopard Moth caterpillar, but it doesn’t seem to have they red bands associated with that species. Then again, maybe they’re just hard to see, and I’ll get lucky when it decides to pupate.
Thanks for the help!
Signature: Jonathan V

Painted Arachnis Caterpillar
Possibly Painted Arachnis Caterpillar

Dear Jonathan,
It is great to hear from you.  As you surmised, we were away for 2 1/2 weeks, and much of the mail we received during that time has gone unanswered.  This is one of the Woolly Bear Caterpillars in the subfamily Arctiinae and we believe it is most likely the caterpillar of a Painted Arachnis,
Arachnis picta, a common species in Mount Washington.  According to BugGuide:  “Larvae are generalists of low herbacious plants” so they feed on a wide variety of plants, including many weed species.  The adult Painted Arachnis are attracted to our Mount Washington porch light each winter.  We will double check with Arctiid expert Julian Donahue to see if he agrees with our ID.

Julian Donahue offers his perspective and advice on raising to the adult
It’s either Arachnis picta or Spilosoma vestalis–I never succeeded in figuring out how to distinguish them. I began some rearing experiments, but never had time to study the results.
Arachnis adults fly in the fall/early winter, while Spilosoma adults fly in the spring. So this larvae could be the former, getting ready to pupate and emerge soon, or the latter, preparing to overwinter as a larvae and emerge in the spring.
If your contributor has the time or interest, I’d suggest rearing the larva to an adult to be sure. Put it in a screened container with leaf litter, in a shady protected spot outdoors (so it will be exposed to normal light and temperature), and if it’s hungry, offer it dandelions (Taraxacum) or lamb’s quarters (Chenopodium), or cheeseweed (Malva)–larvae of both moths are general feeders, not too picky about what they eat.

Many thanks, Daniel. I will certainly try to rear it, per the advice below. I may need to find a new container, as I’m not sure the large glass bowl that I have it in now would do well outside. I also worry about the heat, but I guess that’s part of the natural cycle it needs to know when it’s time to pupate. It hasn’t been hungry in some time, seems to have stopped eating a while ago. I will keep you posted and let you know if anything (or what) emerges. I’m leaning toward the Arachnis, since I think I’ve seen those around our property before, but never knew what they were. Thanks, again.

Update:  October 3, 2013
Subject: Mt. Washington large black caterpillar – pupating?
Location: Mt. Washington, Los Angeles
October 3, 2013 1:32 pm
Hi there! I wanted to give you a quick update about the large black caterpillar that we talked about a few weeks ago. I noticed that it was moving from one spot to another, and today when I picked up the last leaf that I saw it under, I noticed that it has spun some silk. I think it may be getting ready to pupate, but I don’t want to disturb it any more than I have to. I think I will wait a few days now before checking on it again.
Julian Donahue suggested that it might be either Arachnis picta or Spilosoma vestalis, and now I’m leaning toward the former. A week ago, I noticed that something had laid a large number of miniscule eggs on one of our curtains. I carefully scraped them off and put them in a jar to see what would come out. And they looked pretty close (both the eggs and the larvae) to what I see on this page about rearing the Painted Tiger Moth. I don’t think I fed them correctly, though, and many seem to have died. I’ve since put them out in the yard, hoping a few of them make it.
This page on rearing them (http://www.insectnet.com/photos/tech2/photos_tech2.htm#evans) also notes that some of them spin silk before pupating, and has a picture that looks pretty similar to mine. I’m including a couple of pictures of the new ones that I found as well, although they’re about 2 mm across and it was hard to get a good clear image. More to come, I will keep you posted.
Signature: Jonathan V

Woolly Bear Cocooning
Woolly Bear Cocooning

Hi again Jonathan,
We get numerous Painted Arachnis Moths attracted to our porch light each winter.

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