Last week I was sent a puzzling series of photos by Matt Bertone, who works at the North Carolina State University Plant Disease and Insect Clinic (and also takes fantastic photos of insects and other things). They show some leaves of southern magnolia (Magnolia grandiflora) given to him by a landscaper.
Here’s the upper surface of one leaf–nothing out of the ordinary…
…but some little creatures have been busy on the underside:
At first glance at the above photo, my impression was that the brown patches were the affected areas. But this is what the underside of a normal leaf looks like:
Something has been removing the brown trichomes (“hairs”), leaving the leaf largely green on the underside.
I don’t have much experience with southern magnolia, and the only insect sign I have seen on the undersides of its leaves looked like this:
On this leaf I found in Georgia, a caterpillar has been feeding on the tissue on the lower leaf surface while spinning a protective sheet of webbing, which incorporates the removed trichomes, presumably to protect and camouflage itself. I believe this is the work of Paralobesia liriodendrana (Tortricidae), known as the tuliptree leaftier moth.
But what’s happening on these leaves from North Carolina is exactly the opposite: something is removing the trichomes while leaving the green tissue intact. I can think of three examples of trichome harvesting, and they all involve sycamore. One is this moth, which piles up the trichomes to form a shelter in which it rests when not feeding. (Incidentally, that moth now resides at the Canadian National Collection. Jean-François Landry dissected it and confirmed that it is a Gelechia; he also confirmed that the forewing pattern matches that of a specimen from the US National Museum determined as G. albisparsella by Ron Hodges, but he hadn’t seen the genitalia of that moth to be 100% sure of the match.) The other two I discussed here; both involve a larva incorporating the trichomes into a portable disguise.
What is happening on the North Carolina leaves is something entirely different. Here, something is apparently actually feeding on the trichomes, because all over the lower leaf surface are little piles of frass like this:
According to Matt, the pellets are about 1 mm long and about 0.5 mm wide. When cross-sectioned, they are solid and contain only trichomes.
I’m clueless about what might be responsible for this. Here is a little information about the context, provided by the landscaper: “[The magnolia] is planted in a nice residential lawn where the homeowner takes good care of her plants and it is approximately 12-15 feet tall. It’s pretty much in full sun, or at least mostly sun throughout the day. [The feeding sign was present on] a good amount of leaves. And they were about 4-6 feet high in the tree. Otherwise I would have thought [the frass] was dirt being splashed up on lower leaves.”
The homeowner is no longer worried about what is causing this because there seems to be no real damage to the leaves, but I for one would love to get to the bottom of this!