J.M. DiTomaso, The Regents of the University of California

J.M. DiTomaso, The Regents of the University of California

Scotch Broom

APPROVED BIOLOGICAL CONTROL AGENTS


Bruchidius villosus

Bruchidius villosus, or the Scotch broom bruchid, overwinters as adults away from the host plant. Adults become active when the plants begin to flower. Adults mate and the females oviposit between two and 12 eggs onto seed pods where they hatch in one to two weeks.

The larval stage is the destructive stage with larvae hatching and tunneling into the pod until they reach a seed, where they bore in and complete their development. After four larval instars, pupation occurs within the seed coat. The pupal period is temperature-dependent and takes between 10 and 20 days.

As the seed pods mature, they split open and free the adults. Seed destruction by the larvae has been documented at more than 80% in North Carolina, but locally the reduction rate has been reported at 10-25%.

To redistribute, collect adults after they have mated with heavy sweep nets with releases of 100 to 250 adults.

Exapion fuscirostre

Exapion fuscirostre, or the Scotch broom seed weevil, overwinters as an adult in the duff near the host plant, however, adults can be found throughout the year feeding on terminal twigs. Females feed on flowers in the spring to stimulate egg production. Eggs are inserted into seed pods in groups of five to 10 where they hatch in five to 15 days. Larvae feed on the seeds in the seed pods, completing development in 20 to 40 days. The larvae then pupate within the seed pod in 10 to 20 days, with typically one pupa per seed.

Adults emerge when the seed pods dry and split open, flying to the yellow flowers of surrounding Scotch broom plants. This agent is destructive in the larval stage, attacking seeds, and as adults, feeding on terminal stems. Seed reduction in attacked seed pods averages 85%, although only 40 to 60% of seed pods are attacked in the field.

This agent prefers meadows and hillsides with southern exposures and is capable of dispersing up to 2 km per year once established. Collection can be done with a heavy-duty sweep net after the adults have mated (typically mid to late flower). Releases of 100 to 250 adults are recommended to establish the agents at a site.