This 6-mm moth is a typical representative of the genus Cremastobombycia (Gracillariidae).
Larvae form “underside tentiform” mines on leaves of plants in the aster family (Asteraceae). The mine starts out as a flat blotch on the lower leaf surface, then becomes wrinkled and tentlike as the larva spins silk inside and consumes more of the leaf tissue.
On the upper surface, the mine is visible as a discolored yellowish patch.
The question is, what exactly is the host plant of this moth? The plant was growing in chaparral habitat in San Diego County, California, and it looked like this on March 12:
I posted photos of the plant to the Facebook Plant ID group, and the only suggestion I got was Dieteria asteroides (=Machaeranthera asteroides). The leaf shape is certainly similar, but photos of that species show an open, branching inflorescence quite different from this one. Looking for other possibilities, I checked out every mention of Cremastobombycia in my book manuscript and looked at photos of all the recorded host plants that were unfamiliar to me. I found one promising lead: Powell (2002)* mentions an undescribed species that feeds on Hazardia and Isocoma in California, and my mystery plant looks a lot like Harzardia squarrosa.
However, I’m not sure the inflorescence of that plant is quite right either, and apparently the variety of H. squarrosa that occurs in San Diego County is var. grindelioides, which looks like a pretty different plant. The Flora of North America key to Hazardia squarrosa varieties indicates that var. grindelioides is distinguished by having non-resinous leaves; I don’t remember this plant being sticky, but my close-ups of the leaves clearly show that they are covered with stipitate (stalked) glands; it seems to key to var. squarrosa, if it is in fact Hazardia.
So, I’m hoping that someone out there who knows southern California plants well can confidently identify this plant from my photos. Here are the other ones I took:
The stipitate glands are visible in this shot of the moth’s pupal skin protruding from the moldering leaf:
* Powell, Jerry A. 2002. Lepidopteran caterpillars feeding on California native plants. Fremontia 30(3-4):5-14.
Edit, 10/15/2017 – Here is what Barry Prigge, former curator of the the UCLA herbarium, had to say about this plant:
Hazardia squarrosa var. grindeloides. Hazardia squarrosa is a variable species over its range of distribution, and identification, especially to variety, is often rather tentative. Flowering material would be helpful, but the vegetative, fruiting inflorescences, and distribution match that of var. grindeloides. Inflorescences can appear quite different from flowering to fruiting specimens. Check out our description and Tony Valois’ photos of this species at:and click “ANF Description” at end of images. Our plants are obscurely glandular, but evidently there is more variation in the glands than what we described for plants from the Sta Monica Mtns.