On page 302 of Tracks & Sign of Insects, there is a photo of a fleshy yucca leaf riddled with holes, next to a close-up of the eggs Noah found in one of the holes. As explained in the text, after considering all the various insects that are known to insert their eggs in vegetation, we felt that these most likely belonged to western tree crickets (Oecanthus californicus). I have still found no reason to doubt this, though I have yet to come across any literature confirming that tree crickets oviposit in yucca leaves. Noah took those photos in southern Utah on January 3, 2008, so these eggs could be found at this time of year if anyone feels like investigating.
When Noah and I traveled around the US in the summer of 2008, we found superficially similar holes in agave leaves in southern Arizona and southwestern Texas. But whereas the wounds associated with the yucca punctures were brown and about 3 mm across, the ones on agave were bright white and generally around 5 mm across. It’s conceivable that these differences merely reflect the responses of different plant species to the same stimulus, but the agave wounds also often had a bilobed shape not seen in the yucca wounds, and this seemed unlikely to have been produced by a simple puncture from a cricket’s ovipositor.
Dissecting the holes yielded no clues. When Julia and I found similar holes in abundance last November in Madera Canyon, the first one we cut open had two of these inside:
We briefly entertained the possibility that these were the insects responsible for the holes, but on closer inspection they were thrips. Specifically, they were thrips in the genus Compsothrips, which feed on fungal spores and would have no reason to create holes like this even if they were capable of doing so. Close inspection of any little mystery is likely to turn up a thrips or two, as seen here and here. We cut open several more of the wounds and didn’t find anything else.
These agave leaf holes occur in such densities that it seems like someone must have caught their makers in the act of making them.
The photo above shows another type of blemish that may or may not be related (though I suspect not): a short linear trail with a hole at one end. I don’t think Noah and I saw any examples of this, but when Julia and I were looking more intently at agave leaves, we encountered quite a few.
It would seem that whatever is making these little trails would qualify as a leaf miner. The only insects I know of that mine or bore in agave leaves are caterpillars of giant skippers in the genus Agathymus. Some examples of their feeding signs are shown here, and it seems plausible that these are the same thing, but I’m not sure. We didn’t find anything when we tried excavating a few of them, and we never found any more extensive feeding channels. The much more common round blemishes certainly have nothing to do with giant skipper eggs, which are deposited on the leaf surface as shown here.
It’s hard for me to make much headway in investigating mysteries such as these, since I only seem to get to the Southwest every four years or so. I encourage those of you who have more regular access to agave plants to keep an eye out for these insect signs and see what you can figure out (unless, of course, you already know what’s going on, in which case please fill me in!).
Added 1/2/2014: Henry Hespenheide suggested that some of the round wounds might be feeding lesions of the conoderine weevil Peltophorus polymitus seminiveus (LeConte). He mentioned that Margarethe Brummermann has some nice photos of the weevil, and on searching her Flickr site I found this video of one making feeding punctures. She also has a shot here of a weevil on a leaf with these same distinctly bilobed scars, but those are clearly old and can’t be definitively linked to the weevil. It does seem like a reasonable possibility, though, and confirming this would just be a matter of monitoring/revisiting a leaf on which a weevil has been found feeding.