This week while admiring my neighbor’s beautiful American holly, amidst the dark green leaves and crimson berries I noticed several leaves adorned with unusual serpentine trails. After carefully peeling back the epidermis of the leaf just above the discolored trail, I discovered bright yellow fly larvae just beneath the leaf’s surface. What strange hibernal surprise was this? These were no ordinary maggots, like houseflies or blowflies. These were the larvae of the native holly leafminer.
Back in the warmth of spring, the adult stage of the holly leafminer, a small black fly, pierced the surface of the holly leaf with a structure on the tip of its abdomen called an ovipositor. The fly deposited an egg into the soft tissue beneath the tough surface of the leaf. From the egg hatched a tiny larva that consumed nutritious cells deep within the holly leaf. As it snaked its way through the leaf, the gallery, called a leaf mine, expanded and grew with each passing molt of the insect. On warm days during the winter and early spring, the leafminer will continue to feed until it completes development and forms a pupa in spring. Just before the larva changes into a pupa, it cuts a small window in the leaf’s surface to provide an escape hatch from the leaf when it becomes an adult.
The emergence of the holly leafminer in spring coincides precisely with the appearance of tender new leaves. If the fly emerges too late, after leaves have expanded and become tough, the female fly will be unable to pierce the leaf with her ovipositor and her legacy will end. I noticed several holly leaves bearing small punctures so numerous that their margins curled. One urban legend has it that small holes in holly leaves result when sharp spines of adjacent leaves bump into one another, but this is not the case. In addition to laying eggs, the female holly leafminer uses her sharp ovipositor to poke holes in the leaf’s surface. These small holes exude droplets of sap, thereby providing a source of food for the hungry female.
All this may lead one to believe that hollies are completely at the mercy of these treacherous small flies, but this is not the case. If leafminers become too abundant on a particular branch, the holly simply drops the infested leaves prematurely, thereby sending the undeveloped leafminers to a premature death as the leaf withers beneath the tree. So as you deck your halls this holiday season, with a bit of luck you may have an added surprise of holly-mining flies.
See the tiny yellow leafminer larva near the tip of the forceps on the peeled back layer of leaf epidermis.
Two fascinating articles “Population regulation of the native holly leafminer, Phytomyza ilicicola Loew (Diptera: Agromyzidae), on American holly” by Daniel A. Potter, and “Seasonal allocation of defense investment in Ilex opaca Ation and constraints on a specialist leafminer “by D.A. Potter and T.W. Kimmerer, and “Managing Insects and Mites on Woody Landscape Plants” by John Davidson and Michael Raupp were used to prepare this episode.