Mining for Gold
Leaf miners are often overlooked and don't get no respect. Yet they are everywhere and offer a fascinating look at life inside a leaf and the subtle communication between the miner and the host plant.
Leaf miner inside apple leaf
Where do you find leaf miners?
As with many insect signs—that is, evidence of their presence—leaf mines can go unnoticed despite being everywhere. Neither trees, grasses, flowering plants nor vegetables are spared from their actions. Some mines are obvious while others require a closer inspection of a leaf; certain mines are more readily seen on leaves backlit by the sun.
Because the larvae develop and grow during warmer months, the mines tend to be larger and more noticeable in late summer or fall.
Leaf miners can cause unsightly blemishes on leaves, not that plants concern themselves with appearances. However, if the miners are few in number, damage to the plant may be negligible and have no significant effect on the plant’s health.
Nevertheless, extensive leaf mining can reduce a plant’s photosynthesis, predispose the plant to other infectious organisms, interfere with the plant’s water balance, and even cause the leaf to drop from the plant prematurely.
For example, in Alaska the Aspen Leaf Miner periodically affects hundreds of thousands of acres of quaking aspen. Keeping the long-term view in mind, however, the Aspen Leaf Miner is a native moth and there may be a dynamic balance between those factors that favor population growth of the moth—e.g. the aspens—and those that put the brakes on growth—e.g. moth predators.
My home is my castle
Why live in a leaf? Clearly, there must be benefits since at least 10,000 species in multiple insect orders have independently converged upon the leaf mining lifestyle. Yes, the sugars and proteins provide food for the growing larvae, but the structure of the leaf provides shelter and protection from predators as well. Because the leaf’s waxy cuticle is still largely intact, it protects the larvae from excess sunlight, dehydration, wind, rain, other pathogens, and may conceal the larvae’s feces from the watchful eyes of predators (or perhaps they just want a little privacy).
Nevertheless, certain parasitoids4 and predators that target specific leaf miners have evolved efficient hunting, detecting, and capturing strategies tailored to particular prey. In these cases, the confined and limited space within a leaf mine may become a disadvantage when attempting escape or avoidance. Estimates of mortality by parasitoids or predators range as high as 75%.
So, life in the castle sometimes amounts to a life under siege.
Attempts to control leaf miners with insecticides on a large scale is neither safe ecologically nor practical. Even localized pesticide usage, say in a garden, may kill the predators or parasitoids that naturally keep the leaf miners in check, but not kill the miners because of their protected refuge within the leaf. Unchecked, the miner population may increase to the point of causing severe and damaging miner infestations.
“You have to grow from the inside out” – Swami Vivekananda
A leaf miner’s one-year lifecycle generally follows a pattern of complete metamorphosis—egg, larva, pupa, and adult. However, the details of specific miners can be quite varied. For example, in order to survive from year to year at least one of the developmental stages must overwinter, and that stage can be the larva, pupa, or adult, depending upon the species.
Just as pencil marks on a door jamb might track the growth of a child, the changing size of a leaf miner’s tunnel reflects the growth of the larva. Serpentine or trumpet-shaped tracks are better at demonstrating this growth than are blotches.
The top arrow represents the beginning of the larva’s life. The black line is frass (aka poop) that accumulates as the larva grows.
Free will or where do we go from here?
The lead photograph in this article shows “green islands” in aspen5 leaves associated with larvae of leaf miner moths that tunnel at the base of the leaves. Normally, as a leaf ages and turns the colors of fall —red, yellow, orange—the photosynthetic machinery, including the green pigment chlorophyll, is dismantled and recycled by the plant. This is good for the plant – “waste not, want not” – but is not good for a leaf miner who relies on those nutrient-rich materials for growth.
In order to avoid the diminishing returns from an aging leaf, the moths strike a bargain with a tiny bacterium, Wolbachia,6 that lives in the larvae and serves as a “universal translator”7 between the moth and the plant. In essence it asks, “I know you’re getting old, but please make more nutritious food for me.” Resistance being futile, the plant obeys and makes a green island for the moth larvae that successfully postpones the inevitable. The measured sugar and protein content of the green islands is no different than that of a normal green leaf, while the yellow remainder of the leaf continues to age.
The fact that such different organisms like a tiny bacterium and a tiny insect larva can form a pact to “communicate” a meaningful message to a third, different organism—the plant— is extraordinary.8 Who knew that those small blotches we see on leaves held such a fascinating story of intrigue and collaboration?
1 An excellent source of information about leaf miners is Charley Eiseman’s website which is the last link in the reference list.
2 Chlorophyll is the green pigment that powers photosynthesis to make the plant’s food.
3 Leaves have several cell layers between the waxy cuticle that forms protective barriers on the top and bottom leaf surfaces. Different miners target different layers.
4 A parasitoid is a parasite that ultimately kills its host organism, like the parasitoid wasps whose larvae grow within and eat their host caterpillars.
5 Not the same Aspen Leaf Miner from Alaska.
6 Wolbachia “infections” are fascinating and are widespread among insects and other invertebrates. They influence many aspects of invertebrate lifecycles and survival.
7 A universal translator is a common device in science fiction, like Star Trek, that allows alien species to communicate with one another.
8 In terms of the hierarchy of life, bacteria, insects, and plants are not even in the same Kingdoms.