Yes, I know the holidays for most people are now officially over, but better late than never right? Besides, it is still winter (where I am anyway), and what looks prettier that holly leaves during the winter :) Probably not the crazy loopy patterns etched into the holly leaves, but I’m going to talk about them anyway, lol.
I love America Holly trees (species: Ilex opaca). You know - the kind you see pictured on holiday greeting cards all the time :) We had a lot of them around the area I group up in. At least two in our yard that I’m sure my parents didn’t plant, and a number more just growing wild in the woods nearby. So pretty. I’m not sure why, but apparently American Hollies just don’t grow naturally in Michigan. According to the distribution map on Wikipedia they are much more of a southeastern species with parts of Maryland (where I grew up) just making the cut. * sigh * Maybe I can buy some from a local nursery for my yard – if they have any that is…
Anyway, I was home for Christmas and enjoying all the lovely holly trees near my parent’s place, when I noticed it. A bunch of the holly leaves had crazy squiggly patterns carved in the leaves! What luck! Even when the ground is covered in snow, if you look hard enough, you can find an insect somewhere. Or at least some evidence that they were there. These crazy serpentine patterns on the holly leaves were made by the Holly Leaf Miner, Phytomyza ilicicola, (family= Agromyzidae)!
Leaf miners, or insects that “mine leaves”, are a type of insect that eats, grows, and develops between the outer layers of a leaf. It’s the larvae (aka baby insect) that makes the mines or tunnels in the leaf as it has to keep moving to eat a bit more. It’s usually an excellent strategy for the insect - the leaf provides constant food and a great hiding spot for the soft defenseless larvae. After they eat enough, the leaf miner baby turns into an adult, which usually flies away to mate and make the next round of babies.
Leaf miners aren’t just one type of insect though. There are leaf miners that turn into moths (check a different leaf miner I found earlier this year), beetles, herbivorous wasps (“sawflies”), and even flies. The leaf miner on my holly leaves is a fly! Or at least it will be. Right now it’s just a little yellow maggot. The maggots are supposed to overwinter (spend the winter months) as larvae inside the leaf before turning into a fly in the spring/summer. I tried opening a couple of the mines up to take a picture of one, but didn’t have any luck finding the maggot. Maybe I didn’t get an active mine. A knife and tweezers probably would have helped though. I’m guessing my fingernails may not have been sharp enough to adequately tease apart the leaf layers, lol. In hindsight maybe I should have brought some home with me where I have an assortment of tools, but I generally try not to move pests across state lines… You’re welcome USDA.
The Holly Leaf Miner is a pest native to North America and can be found wherever American Hollies are (there are other species of holly leaf miner, but they feed on other species of holly). Because it’s a native pest, the American Holly tree has had plenty of time to adjust to its native leaf-mining pest and develop several effective defense mechanisms. Firstly, the American Holly tree has a very tough and spiky leaf (as anyone who’s accidentally brushed up against one of these trees knows!). This super tough leaf means the adult leaf miner fly can only penetrate the leaves to lay eggs during a short window of time when the holly leaves are young and not fully hardened. The second line of defense comes from other insects that live in the area. The Holly Leaf Miner has seven different parasitoid wasps! There is one Braconidae wasp species, four Eulophididae, and two Pteromalidae wasp species that actively seek out the Holly Leaf miner for their wasp babies to feed on.
The Holly tree’s third line of defense against the leaf miner, and what I think might be the most interesting, is that it just drops its leaves. Apparently, if too many leaf miners start feeding on the tree, the tree will just shed the leaves that the miners are feeding on! Without being attached to the tree the leaf dies and so does the leaf miner. Maybe that’s why I only found a couple leaf miners on each tree I looked at. The adult leaf miners have adapted to the tree’s ability to do this and they purposely space out where and on what tree they lay their eggs so the holly trees they pick won’t drop its leaves when their babies start feeding. Cool, right?!
Who would have thought such a pretty, Christmas card worthy, icon iconic tree had so much going on? All this effort to defend itself against native bugs and stay pretty… Well, healthy anyway. I’m sure the tree doesn’t care if we think it’s pretty or not, lol.
Want to learn more about the Holly Leaf Miner? Check out:
- Missouri botanical Gardens
- Bug of the Week
- Scientific paper (written by one of my grad school professors!)
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