Worms In My Sweet Broom - Knowledgebase Question

North Hills, CA
Avatar for lgerns
Question by lgerns
October 3, 2001
I planted a lovely yellow sweet broom plant in the middle of my flower bed and it has developed an ugly worm infestation.
They appear to be yellow and black and seem to spin a web of some kind. I've tried picking them off, but they seem to come back. The worms have only attacked that one plant.

Answer from NGA
October 3, 2001
Photo by growitall
It's possible your broom is being attacked by Fall Webworms. These inch-long green or yellow caterpillars, bristling with silky hairs, are actually moth larvae. In summer and early autumn they weave expansive webs that bind together the ends of branches. They eat every leaf within reach, continually building larger webs until late autumn, when they pupate. Resembling dirty rags and filled with black droppings and wriggling worms, the webs ruin the aesthetics of any garden.

Hyphantria cunea attack many kinds of deciduous trees, shrubs and vines. To plan an effective schedule of prevention and control, it helps to understand the life cycle of Hyphantria cunea. In early spring, moths come fluttering up from cocoons carefully hidden in the bark of tree trunks and in ground debris. The adult moths are about 2 inches from wingtip to wingtip and are white spotted with brown. They lay eggs in clusters on the leaves of suitable host plants.

The eggs hatch within a week. Out come tiny caterpillars, the first generation of the growing season, spinning webs and eating leaves. This first infestation is so mild that sometimes the webs go unnoticed.

The first-generation caterpillars have eaten their fill by early summer. They form cocoons and pupate, usually in the bark, of a tree or underneath leaves or other debris on the ground. By midsummer they re-emerge as moths. A second generation follows -- this time larger and more destructive.

Because the webworms -- in one form or another -- are present year-around, it's possible to devise a year-round strategy for dealing with this pest. Here are some tactics that have worked for me:

In winter or early spring, remove fallen leaves, ground debris and mulch, which may harbor overwintering webworm pupae. Replace the debris with fresh, pest-free mulch.

Inspect susceptible plants for the greenish egg masses, which are typically laid on the undersides of leaves and are protected by a woolly or scaly covering. Eggs are deposited from late spring through fall. Remove any affected leaves and destroy them. This strategy requires time and sharp eyes and is obviously impractical for tall trees.

When you see webs, clip the infested branches and burn them, or drown the larvae in a bucket of soapy water.

As a last resort, Bacillus thuringiensis (Bt) is a microbial pesticide that can be applied as a dust or spray. It kills many types of caterpillars but has no effect on warm-blooded animals or bees.

With fall webworms, Bt is effective only if its application is properly timed. Once the caterpillars have enshrouded themselves in webs, they are more difficult to kill. Check on susceptible plants frequently, beginning in late spring, and apply Bt at the first sign of hatching webworms. Bt loses effectiveness after about two days, so it must be reapplied as long as more larvae are hatching.

Always use Bt with care, because it also can kill the larvae of non-pest moths and butterflies.

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