Q. Can you help me identify this caterpillar and tell me how to control it? There are lots of them eating my baptisia, and I am afraid it is going to die.
A. The insect sample submitted with the question is the genista broom moth caterpillar (Uresiphita reversalis), also known as the sophora worm because it is a major pest of mescal bean (Sophora secundiflora) in the Southwestern United States. It is known to exist from Nova Scotia to Florida, west to California, and north to the Midwest.
This caterpillar has been damaging false indigo (Baptisia australis) in Western Pennsylvania this summer, including specimens in our demonstration garden in North Park. We also received another sample of this insect that was damaging a golden chain tree (Laburnum x watereri). What do these plants have in common? They all belong to the pea family, Fabaceae. Quite honestly, this is the first time in my professional life that I have seen anything (except deer in South Park) damage baptisia. Apparently, there are periodic outbreaks of this pest across the country.
Adult female moths lay cream-colored to yellow eggs in masses on the foliage of host plants. Newly hatched larvae construct webs and feed together until they mature a bit, then move off and feed separately. Mature larvae are about an inch long and are green or orange with rows of clustered white hairs surrounded by black rings. They usually move from their larval food plants to adjacent plants or objects where they spin cocoons and pupate.
These caterpillars are voracious eaters and can reduce baptisia to stems almost overnight, so they do warrant control. Small infestations can be picked off by hand and crushed. Larger infestations may call for the use of Bt, Bacillus thuringiensis, sold under trade names such as Dipel or Thuricide. This naturally occurring bacterium is specific to caterpillars, and they must consume treated foliage for it to work, so it has no impact on other insects. Bt only works on small caterpillars and would not provide effective control for the mature larvae submitted by the writer. Another option is to cut the baptisia back to a few inches, securing the cuttings and caterpillars in a plastic bag. While it is not ideal to cut perennials back before their time, most baptisias should grow back with no lasting ill effects.
Many species of Genista, also known as broom, have been introduced to the United States as ornamental plants, and some have become terrible weeds in certain parts of the country. In fact, genista broom moth caterpillars are considered a control method where these plants have escaped cultivation to wreak havoc on natural areas.
Send questions to Sandy Feather by email at firstname.lastname@example.org or by regular mail c/o Penn State Extension, 400 N. Lexington Ave., Pittsburgh 15208.