Minor miners

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NikonUser
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Minor miners

Post by NikonUser »

Today, July 25 2015, I noticed a lot of Leaf Blotch Miner Moth damage on some local aspen.
Damage is caused by the larvae (caterpillar) of a Common Aspen Leaf Miner Moth (Phyllocnistis populiella) in the Family: Gracillariidae.

The leaf shown has larvae mining both surfaces, S=start of the mine where egg was laid, f=end of the mine where the fully-fed larva can be seen.
Each has excavated a trail, seen as a thin black line where it deposits its wastes (frass), and eats the leaf tissue either side of the trail.

If you think of how thin is an aspen leaf you can appreciate how small is the larva and get some idea of how small the ensuing moth will be; truly minor miners.
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NU.
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rjlittlefield
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Post by rjlittlefield »

Very interesting! I had always assumed that when I saw a trail on both sides of a leaf, it was the same trail. But clearly not, as you've shown here.

Wikipedia writes that
The larvae feed on the contents of epidermal cells on both top and bottom surfaces of quaking aspen leaves, leaving the photosynthetic tissue of the mesophyll intact.
http://bugguide.net/node/view/812703/bgimage shows a picture of an adult, which even at 3 mm long seems to have a pretty convincing false head! :shock:

If that is the function of those distinctive markings, then given the critter's size I'd guess the corresponding predator is probably not a bird.

--Rik

Chris S.
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Post by Chris S. »

Fascinating post, NU!
rjlittlefield wrote:Wikipedia writes that
The larvae feed on the contents of epidermal cells on both top and bottom surfaces of quaking aspen leaves, leaving the photosynthetic tissue of the mesophyll intact.
I at first wondered if this were one of the frequent occasions where Wikipedia gets science wrong, because to the eye, the photosynthetic tissue (which should be green, obviously), seems absent from the miner's tracks. But scholarly articles concur with the Wikipedia quote.

On the other hand, the purpose of some of these articles is to report studies on the significant loss of photosynthetic capability in the affected trees. So something is happening to the photosynthetic tissue of the mesophyll.

I could not find an explanation for this during my brief foray into the literature, but will venture a conjecture: Once the miner eats the protective epidermal cells (which, among other things, protect the cells under them from water loss), the cells in the mesophyll rapidly dessicate and die.

Again, excellent post, NU! :D

--Chris

rjlittlefield
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Post by rjlittlefield »

But notice that from each side, we can barely see the other side's trail. If the photosynthetic layer were very disturbed, the other side's trail would be obvious also.

Another possibility is that the photosynthetic layer is simply hidden by reflections from the now-disturbed tissue outside it. That would cause a substantial loss in photosynthetic activity too.

A carefully cut cross-section at high magnification might help to resolve the question.

--Rik

Chris S.
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Post by Chris S. »

rjlittlefield wrote:A carefully cut cross-section at high magnification might help to resolve the question.
Yes, I find myself wishing we had quaking aspen (Populus tremuloides) trees in my area so that I could have a look at a cross section.

There are tons of diagrams of leaf cross-sections on the Net, but none I've found specific to the quaking aspen. I wonder if there is something different about its leaves, such as an additional cuticular layer in the center of the mesophyll. If my conjecture has any validity, this would explain why the mesophyll on the opposite side appears undamaged.

It might also explain why this miner prefers the quaking aspen as a host plant (benefiting from a central cuticle for support). Quaking aspen is a bit odd, possessing a thin stem that allows the leaves to vibrate in a breeze, exposing both sides of the leaf to sunlight. So the notion that it might have an unusual cross section isn't inconceivable.

Perhaps it is simply surface damage, as you posit. But the only appropriate surface I can think of is the lower side of the epidermal cells. We see through them when they are healthy. It would be interesting indeed if they become opaque when damaged.

--Chris

BugEZ
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Post by BugEZ »

Chris S. wrote:
I find myself wishing we had quaking aspen (Populus tremuloides) trees in my area so that I could have a look at a cross section.
The range map for quaking aspen extends south to northern Ohio. The cottonwood tree is a relative (same genus) and those extend throughout Ohio (and Illinois). Cottonwood leaves also "quake" in the wind. The cottonwood "fluff" season is just ending in Northern Illinois and we will be able to vacuum the screens on our porch and have them stay clear for more than a few days. Hurray!

I must say that this interesting post by NikonUser has me inspecting the damaged leaves on my trees in Northern Illinois much more carefully. I have not paid attention to leaf miners before this.

Chris S.
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Post by Chris S. »

BugEZ wrote:The range map for quaking aspen extends south to northern Ohio.
For some reason, the quaking aspens seem not to have read the range maps enough. It's easy to find quaking aspen an hour or so north of here, but in my region and southward, it grows only in isolated pockets, none of whose locations I recall.

However, I'll eventually visit northern Michigan in the company of a retired botany professor and his family, and I'll ask him to bring his microscope. Unfortunately, his microscope is not set up for photography, and the Bratcam will not be going. (Besides, the Bratcam is not set up for oil immersion, and I suspect that anything less would tell rather little.)
The cottonwood tree is a relative (same genus) and those extend throughout Ohio (and Illinois).
Good idea! I looked, today, at specimens of the eastern cottonwood (Populus deltoides) and bigtooth aspen (Populus grandidentata). While I saw plenty of evidence of insect grazing, none of it resembled NU's leaf miner studies in quaking aspen.

--Chris

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