Pest Alert: Genista Broom Moth
Posted: September 11, 2012
Although this insect is native in much of the United States, it is more common in the southwest where it typically has two generations a year. Since we have a shorter growing season than Texas, we may only see a single generation. Genista broom moth is known to exist from Nova Scotia to Florida, west to California, and north to the Midwest, and is also found in Mexico. Apparently there are periodic outbreaks of this pest across the country such as the one we are experiencing in 2012. Reports from Connecticut, Maryland, Ohio, Tennessee, and many other states indicate the widespread nature of the outbreak.
Host Plants and Insect Life Cycle
Although damage to Baptisia has generated the most calls and questions, we have also received samples of golden chain tree (Laburnum x watereri) and photos of maackia (Maackia amurensis) being damaged by the same caterpillar. What do these plants have in common? They all belong to the pea family, Fabaceae.
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, newly-planted trees in the legume family might also warrant protection, while larger, more mature trees should be fine without intervention.
There are a number of ways to control genista broom moth caterpillars. Small infestations can be pruned out or picked off by hand and crushed. Perennials such as baptisia can be cut back close to the ground and allowed to re-grow. While it is not ideal to cut perennials back before their time, most baptisias should grow back with no lasting ill effects.
Larger infestations or those out of reach 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 species of insects. Bt only works on small caterpillars and would not provide effective control for caterpillars over an inch long. Products containing spinosad (Conserve) and permethrin (Astro) provide more effective control of mature larvae.
Many species of Genista, also known as broom, have been introduced to the United States as ornamental plants, and some of them have become terrible weeds in California and the Pacific Northwest. In fact, genista broom moth caterpillars are considered valuable control agents where these plants have escaped cultivation to wreak havoc on natural areas.