Participants at this week’s S.W. Ohio BYGLive! Diagnostic Walk-About held at the Cincinnati Zoo and Botanical Garden observed the highly visible handiwork of the Magnolia Serpentine Leafmining Caterpillar (Phyllocnistis magnoliella). The moth belongs to the leafmining family Gracillariidae. The tiny caterpillars of this aptly named moth feed close to the upper leaf epidermis, producing long, thin, serpentine mines that appear as silvery tracks snaking across the leaf surface.
Reported hosts for this leafminer focus on members of the Magnolia genus including bigleaf, cucumber, southern, star, sweet bay, and umbrella magnolias. However, I’ve often observed very similar leafmining damage on the leaves of another member of the magnolia family (Magnoliaceae): tuliptree (Liriodendron tulipifera). I cannot say that this is the same leafmining caterpillar, but it’s certainly suspicious.
Large numbers of mines on a single magnolia leaf can cause the leaf to turn brown and drop from the tree. Little is known of the life cycle of this moth making the effective timing of insecticide applications to control the caterpillars problematic. Indeed, efforts to control this leafminer in nurseries in Ohio and in the southern U.S. are marked by reports of high insecticide failure rates. Fortunately, heavy populations involving multiple leaves appear to be a rare occurrence in landscapes and nursery plantings. Thus, populations may be managed by removing and destroying infested leaves when mines first appear.