I’ve just arrived on Nantucket, where this weekend I’ll be presenting at the 5th Biennial Nantucket Biodiversity Research Conference about my multi-year survey of gall-making and leaf-mining insects on the island. So for this month’s mystery, I thought I’d feature a leaf mine I’ve found here about which I’m completely clueless. I first noticed it last June in Maine’s Penobscot Bay, but when I encountered the host plant here a year later, it didn’t take me long to find the mines here too.
The plant is sea lavender (Plumbaginaceae: Limonium carolinianum), and it can be found all along the Atlantic and Gulf coasts. In June in New England, it looks like this:
(That photo was obviously taken in Maine; rocks are virtually nonexistent on Nantucket.) By August, it is clear how it got its name:
When I first saw discolored patches on the leaves, I wasn’t sure I was looking at leaf mines…
…but as I continued to investigate, I found many that were distinctly linear, with a hole at one or both ends.
One thing that is clear is that the organism responsible for these does not complete its development within a single mine as many leafminers do. Many of these mines clearly have two small holes–an entrance and an exit–whereas a larva that makes just one mine would start feeding where it hatched from its egg, only leaving a hole when it departs the leaf. The four groups that include all known leafminers–true flies, moths, beetles, and sawflies–all include some species that have the ability to exit and reenter leaves multiple times, so this feature doesn’t help much in narrowing down the possibilities. Plus, the fact that these plants grow in the intertidal zone leaves open the possibility that the mines are not even made by insects, but by tiny crustaceans, marine worms, or…?
Since I am largely ignorant of marine life, it’s entirely possible that these sea lavender leafminers are well known to someone out there. Then again, since I also found on sea lavender (both in Maine and on Nantucket) an insect belonging to a genus not previously known to occur in North America, it’s clear that not enough people have been paying attention to what feeds on this plant around here.