Deck the halls with boughs of holly - and maybe some leaves with the native holly leafminer, Phytomyza ilicicola

 

Oh no, these less-than-pretty snakelike galleries are the handiwork of the native holly leafminer.

 

Holly trees play a significant role in the beliefs and traditions of the season. To the Romans, hollies were the trees of the god Saturn and wreaths of holly were given as gifts during his holiday, Saturnalia. In Celtic legends, the evergreen hollies with their beautiful red berries announced the triumph of the Holly King during winter over the Oak King, who ruled the forest with his green leaves in summer. For Christians, the pointed leaves of holly are associated with the crown of thorns worn by Jesus and bright red holly berries symbolize drops of his blood. For a bug guy, this is the season to marvel at a pair of fascinating flies whose lives are intimately tied to our stunning native holly.

Adult holly leaf miners resemble small houseflies. The male is on the left and the female is on the right, a cute couple. Credit: John Davidson

On my neighbor's beautiful American holly, red fruits intermingle with deep green leaves. Some leaves have unusual sinuous trails on their surface. Within the trails are the larvae of small flies, the young of the native holly leafminer. Back in the warmth of spring the adult stage of native holly leafminer, a small black fly, pierced the holly leaf with a structure on the tip of its abdomen called an ovipositor. This egg-laying appendage was then used to deposit an egg through the tough leaf surface into the soft tissue beneath. Upon hatching, the tiny larva consumed nutritious cells of the holly leaf and snaked its way through the leaf, enlarging the trail as it grew. In the dead of winter the larva rests, but on warm days during winter and early spring, the leafminer will continue to feed until it completes development and forms a pupa in spring. Before the larva changes into a pupa, it cuts a small window in the surface of the leaf to enable the adult fly to escape.

See the tiny yellow leafminer larva near the tip of the forceps on the inside layer of leaf epidermis. As it feeds within the leaf, it creates a snake-like serpentine mine.

Feeding punctures on the leaf surface made by adult flies may cause leaves to curl.

The emergence of the adult fly is timed to coincide with the appearance of tender new holly leaves in spring. 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. Leaves with many feeding holes are often curled or puckered. A horticulturalist once told me that these holes were caused by holly leaves with sharp spines bumping into one another, but now we know differently. As you gather holly sprigs to decorate your home, don’t worry if a leaf or two bears the serpentine signature of the native holly leafminer. They will not emerge in your home.

Spoiler Alert: Get ready for one more story about a fascinating fly found on holly in next week’s episode. 

Acknowledgements

We thank John Davidson for providing the inspiration and images for this week’s episode. The interesting references “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 the book “Managing Insects and Mites on Woody Landscape Plants by John Davidson and Michael Raupp” were used to prepare this episode.