Question of the Week – Southern Purple Mint Moth

Where did all the webbing and frass on this rosemary come from?

This is from the Southern purple mint moth caterpillar (Pyrausta laticlavia). This caterpillar has been seen on rosemary throughout the Charleston area for over 10 years. Commonly called “rosemary webworm,” this pest can affect the foliage on over half the plant and likely leads to decline and death, if not managed. Infestation can be seen as a mass of dead leaves held together on branches by webbing with attached frass particles. Damage is seen on the upper portion of branches, sometimes associated with nodes and side branches. The large cultivar ‘Tuscan Blue’ is routinely, but not severely infested, while ‘Chef’s Choice’ has had much less to no damage, depending on the year.

Occasionally, webbing from spiders can be mistaken as an early infestation of this caterpillar. Caterpillars are slender and small and with the help of magnification can easily be identified by rows of black spots on pale brown or green bodies. Moths are also small and visually appealing with pinkish and yellow wings, but are unlikely to be encountered. Caterpillars can be managed by spraying “Bt” (Bacillus thuringiensis) products labelled for caterpillars, or pruning away infested branches. These Bt products selectively kill caterpillars but may need to be reapplied several times under heavy infestations. More information and pictures can be found here.

This week’s question is from Tom Bilbo, Extension Vegetable Entomologist, and Tony Keinath, Extension Plant Pathologist.