APPROVED BIOLOGICAL CONTROL AGENTS
Bangasternus orientalis, or the yellow starthistle bud weevil, reduces seed production of yellow starthistle plants by 50 to 60%. Adults cause minor defoliation, with the majority of the damage caused by larval feeding.
Adults overwinter outside the host plant, leaving their pupal chambers in the seed head in late summer. Females may produce up to 470 eggs, laying them singly on or near scale leaves beneath the immature buds of flowering shoots in late spring to early summer. Larvae hatch and tunnel through the scale leaf, the flower stalk, and into the seed head where they feed on bracts, receptacle tissue, and developing seeds. The weevil pupates within the seed head in chambers formed from damaged and undamaged seeds. This weevil is very widespread and can be collected via sweep net or hand picking in the spring or early summer.
Eustenopus villosus, or the yellow starthistle hairy weevil, attacks the seed heads of yellow starthistle reducing seed production by 90 to 100%. Adult weevils emerge in late May and persist through August.
Females lay eggs inside mature, closed buds in early to mid summer. Females chew a hole in the side of the seed head for oviposition, sometimes causing the flower head to become distorted. Larvae hatch in three days and develop in 16 days, feeding on the receptacle and developing seeds. The pupal stage takes eight to 13 days and occurs within the seed head in a chamber composed of chewed seeds and pappus hairs.
In addition to the damage caused by the larval feeding in the seed head, the adults feed on small seedhead buds, usually destroying a high percentage, and on mature buds. The weevil is widespread throughout most of the west and can be collected with a sweep net from host plants in early summer.
Larinus curtus, or the yellow starthistle flower weevil, can reduce seed production up to 100%. Adults occur from June to early August and feed on flowers and pollen. The weevil may be covered in pollen and they can typically be found with their heads buried deep into flowers leaving their posteriors exposed.
Adult females lay their eggs at the base of flowers and in heads with open flowers. Larvae hatch in four days and begin feeding on developing seeds, completing development in 17 to 20 days. Pupation occurs in four to five days within chambers formed in the seed heads. The larval stage is the destructive stage, although adult feeding on flowers and pollen may have a small impact on the plant. Adults can be collected by hand-picking or sweep nets when flowering plants are at 10% bloom.
Approved Biological Control Agents for release in Idaho: