What's That Bug? does not endorse extermination

Subject:  black caterpillar
Geographic location of the bug:  Noth Umpqua area of Oregon
Date: 06/13/2018
Time: 04:44 PM EDT
Your letter to the bugman:  Hi I’ve been having trouble identifying this critter. They showed up June 2, 10 days ago by the thousands. At first they were about 1″ long , now they are around 2″. I’ve been watching their progress and today I noticed some pupae forming. I had thought they were Ceanothus Silk Moth caterpillars but now I don’t thinks so.
How you want your letter signed:  Thanks Bill

Tortoiseshell Caterpillar

Dear Bill,
We immediately wrote back to see if you could provide the name of the plant upon which these Tortoiseshell Caterpillars are feeding, because we are certain the genus is
Nymphalis, but we are not sure of the species.  Our likeliest candidate because they are often found in great numbers is that they are Mourning Cloak Caterpillars, but the caterpillar lacks the red spots found on Mourning Cloak Caterpillars and the chrysalis does not really look right.  According to BugGuide, Mourning Cloak Caterpillars feed on ” primarily willow (Salix spp.) but also other trees and shrubs including Cottonwood (Populus deltoides), Trembling Aspen (P. tremuloides), American Elm (Ulmus americana), Paper Birch (Betula papyrifera), and Hackberry (Celtis occidentalis).”  Based on images posted to BugGuide, we believe they are probably California Tortoiseshells, and Oregon is well within the range, and since BugGuide states “Larva feeds on various species of wild lilac (Ceanothus),”  knowing the food plant would greatly assist in the identification.  Our least likely candidate due to your location is the Compton Tortoiseshell because most caterpillars are green, but BugGuide does picture this black individual and BugGuide indicates the range as being “southeastern Alaska and across Canada south of the tundra, south in the west to Montana and Wyoming, south in the east to North Carolina and Missouri known to wander; has been recorded as far south as California and Florida, and as far north as Baker Lake, north of treeline in Nunavut” and also states “larvae feed in groups on willow (Salix spp.), birch (Betula spp.), and poplar (Populus spp.).”  In recapping, we are leaning toward California Tortoiseshells, and knowing they were feeding on Ceanothus would seal the deal for us.

Metamorphosis of a Tortoiseshell Caterpillar

Hi Thanks for the response They were feeding on Buckbrush (Ceanothus) and Schoolers Willow. There were people clearing trees and brush from under a major powerline up the hill behind me and they said that the caterpillars completely defoliated over 2 acres of buckbrush. Yesterday I was still seeing them coming towards our home and on the driveway but it is slowing down and they are attaching and forming  the chrysalis. I’m sure looking forward to seeing them when they emerge. Thanks Bill

Please try to send us images of any adults that emerge.  That will surely verify the species.

Tortoiseshell Chrysalis

How long will they be in the Crysalis? It’s warming up to the 90’s next week.

Two weeks is a good average, but temperature and humidity may affect eclosion time.

What's That Bug? does not endorse extermination
Location: Umpqua, Oregon
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