INSECTindex
Holly Leaf Miner By Carol Allen
I have a deep love for American holly (Ilex opaca). It is tolerant of a wide range of environmental conditions and found growing in flood plains, as well as upland areas. In our region, it can be found from the coastal plain to the mountains and all through the piedmont. Associates can include the pines and sweetgum of the coast. Holly has some salt tolerance and can be found in the beech-oak-hickory woods of the piedmont. In the landscape, holly will tolerate some shade and do well in sun. It has high wildlife value in its berries and dense evergreen boughs for shelter.
Pollinators include bees, ants, wasps, and night-flying moths. Deer do not generally eat them. Holly is fairly pestresistant, although it does have one very curious pest: the holly leaf miner. You may never notice the small, house-fly-like adults, but they emerge at the same time as the new holly leaves unfurl in the spring. What you may notice on the new holly leaves are scattered, pin-prick-like damage where the female fly has used her ovipositor to pierce the leaf surface and allow the sap to well up in droplets. The hungry flies slurp up that liquid. That pricking damage is generally misidentified as
holly spine damage having poked the other leaves in a wind storm or similar disturbance. It is with this new leaf emergence that the female lays her eggs on the underside of the leaf. The eggs hatch in about four days. The developing larvae or maggots burrow and feed between the upper and lower leaf surfaces, leaving serpentine trails at first and then blotches. They molt twice during the summer and overwinter as larvae. After pupation, they emerge just in time to start the cycle again with the new holly leaves. Males live two days and females three. They do not have sharp mouthparts so the pin-prickderived welling sap is the only food they get during their short lives. There are several species of holly leaf miner and each is specific to a species of holly. Our native Phytomyza ilicicola and Phytomyza opacae only feed on American holly. If you have English holly (Ilex aquifolium), the leaf-mining damage would be from Phytomyza ilicis, which is native to Europe. Inkberry (Ilex glabra) has its own leaf miner, Phytomyza glabricola, also a native species. Generally, no control is necessary. Often the holly will drop infected leaves before the flies can mature. Beneficial insects also effect natural control. Heavy populations seem to occur only in stressed trees found in the managed landscape. Although this pest is widespread throughout its range, severe damage is not commonly found in nature. o Carol Allen describes herself as a committable plant-a-holic. She has more than 25 years’ experience in the horticulture industry, with a special interest in plant pests and diseases; is a Licensed Pesticide Applicator in the state of Maryland; and is an ISACertified Arborist. She can be contacted at carolallen@erols.com. Shown at top, left: American Holly Leaf Miner (Phytomyza opacae) in Mason Neck, VA. Photo by Judy Gallagher, CC BY 2.0 <https://creativecommons.org/licenses/ by/2.0>, via Wikimedia Commons.
MARCH 2022
WASHINGTON GARDENER
21
