Mountain laurel is a small tree that usually grows between 10 and 20 feet tall, although it can grow more than 30 feet and is also grown as a shrub. It has dark green, glossy leaves and clusters of lavender-coloured flowers. With the exception of one pest, mountain laurels don't suffer from many insect problems. The one exception is the caterpillar of the Genista moth.
Genista moth caterpillars, or Sophora worms, are orange or green with dark spots. Clusters of white hairs grow out of the spots. The caterpillars spin loose webs over the leaves they're feeding on and, when they're young, feed in groups. Mature caterpillars, which reach a length of 1 inch, are solitary, and feed throughout the plant. Several generations of caterpillars are produced every season, and generations can overlap, resulting in caterpillars of different sizes appearing on the same plant.
Genista caterpillars cause most of their damage to young, tender leaves, although they will sometimes eat older leaves and stems. Symptoms of feeding include pinholes and small tracks on young leaves and signs of feeding on the edges of leaves from older larvae. In addition, the leaves may curl and become misshapen, although they will still be able to photosynthesise. Occasionally, heavy leaf loss results from large numbers of caterpillars feeding. Other damage, such as the appearance of webs on the leaves, is mainly aesthetic.
There's no point in treating your tree until eggs or larvae appear in the spring or early summer. The eggs can be hard to see. Start checking after your tree has blossomed. Look for small overlapping clusters on the bottom sides of young leaves.
Caterpillars don't usually eat enough leaves off mature trees to make them a problem, but young trees need to be protected. Picking the caterpillars off by hand, or using a high-pressure water spray is effective, if you only have a few small trees. You can also spray your tree with Bacillus thuringiensis (BT), a bacteria that naturally produces chemicals that are poisonous to this caterpillar. BT should be sprayed on young leaves, covering the top and the bottom sides of leaves where eggs are laid and young caterpillars feed.
Mountain laurel leaves contain chemicals to protect them from plant pests, but Genista caterpillars are immune to them. Instead, the caterpillars store these poisonous chemicals inside their own bodies, making themselves poisonous to most creatures that feed on caterpillars. However, some spiders and assassin bugs feed on Genista caterpillars, which helps to control them.
- University of Texas Native Plant Database: Sophora Secundiflora
- University of Texas Ladybird Johnson Wildflower Center: Mr. Smarty Plants
- University of Arizona Cooperative Extension: Sophora Worms
- Texas AgriLife Extension: Genista Caterpillar on Texas Mountain Laurel; Bastiaan M. Drees, et. al.
- The Plant Guy: Articles and Questions and Answers