For the last few weeks I’ve been asked what’s causing the webbing on the branches of rosemary plants brought to the diagnostic lab. The answer is the caterpillar or larval stage of the southern purple mint moth (Pyrausta laticlavia). The tiny larvae produce conspicuous amounts of webbing on leaves and branches.
The small moths are purple and yellow, and even though they are only about a third of an inch long, their bright coloration makes them relatively easy to see. When you see them flying around rosemary it’s likely the ugly webbing and caterpillars will appear soon.
The long-term damage to rosemary is typically minimal, like that shown in the first two pictures. Pruning out the damaged portions of the plants should quickly improve the plants appearance. Insecticides containing spinosad (sold as Bonide Captain Jack’s Deadbug Brew, Monterey Garden Insect Spray and Natural Guard Brand By Ferti-lome Spinosad Bagworm, Tent Caterpillar and Chewing Insect Control) will control the caterpillar stage. All of these spinosad products are labeled for use on rosemary and are suitable for organic gardening. We recommend spraying when you first see the moths or the first signs of damage. Look for the southern purple mint moth feeding on rosemary anytime between mid-June and October.
By Jim Jacobi, Extension Plant Pathology Specialist, Alabama Cooperative Extension System. Contact Jim at firstname.lastname@example.org.
The Alabama Cooperative Extension System (ACES) operates as the primary outreach organization for the land-grant functions of Alabama A&M and Auburn Universities. ACES is committed to affirmative action, equal opportunity and the diversity of its workforce. Educational programs of ACES serve all people regardless of race, color, age, sex, religion, disability, sexual orientation, or national origin.