Did you Know?Leaf miners produce two main types of mines: serpentine and blotch. Serpentine mines look snake like as they trail back and forth through the leaf whereas blotch mines bleed together and form a irregularly-shaped spot.
Leaf Miner- Tunnels in Leaves
Leaf miners describe a group of insects that range across multiple insect orders including moths, sawflies, flies, and beetles. Typically only a nuisance, leafminers are more aesthetically displeasing to humans than harmful to the trees. The most common trees affected by these pests are Siberian Elm, Bur Oak, and Hawthorn.
From a distance, the tree canopy will appear to be browning out. Upon closer investigation, the individual leaves will look clear or almost translucent. When the leaf is examined in the hand it will be noted that it is actually hollowed out in the middle like an envelope. When held to the light, it is noticeable that the top and bottom of the leaf will be intact, but clear, and the center will be filled with little black granules and worms if it is June or early July. A very common statement from our customers is that, “Worms are raining down on us!” This can also happen with Brown Headed Ash Sawfly. However, those larvae are generally fatter and greener and only attack the Fraxinus (ash) species of trees.
Females lay eggs on leaves of host plants. The larvae hatch and immediately begin tunneling under the epidermis of the leaf. They feed until just before the pupal stage when they exit the leaf and drop to the ground. After the pupal stage, they emerge as flying adults and begin the cycle again.
Typically, the damage is minor. Because the larvae consume the photosynthetic machinery of the plant (chloroplasts), severe infestations can be stressful on trees since food production for the tree is diminished. If left untreated for several years of severe attacks, trees can become vulnerable to other pests and diseases.
Identification of the leafminer pest is crucial for most control options. A systemic treatment with imidacloprid is the most effective treatment on most leafminer pests (except for moths). Because the larvae are inside the leaves they are well protected from most methods of control. Systemic treatments act through the vascular system of the plants themselves and become available to feeding insects in the cells where they feast.
Moth leafminers are not affected by applications of imidacloprid. A systemic application of acephate, chlorantraniliprole, or acetamiprid can be made. However, insecticidal intervention is typically not necessary as natural enemies tend to catch back up with the high populations.
Proper watering and annual soil treatments can reduce the likelihood of outbreaks.