BIOLOGICAL CONTROL Unintentionally Released Chaetorellia succinea (Diptera: Tephritidae): Is This Natural Enemy of Yellow Starthistle a Threat to Safflower Growers?

In mid-1996, we detected an unintentionally introduced seed-head ßy, Chaetorellia succinea (Costa), destroying seeds of yellow starthistle, Centaurea solstitialis L., one of the worst weeds in the western United States. In overseas studies, Chaetorellia succinea had been considered as a potential biological control agent for yellow starthistle, but had been rejected because of fears that itmight become a pest of safßower,Carthamus tinctoriusL., in theUnited States. Frommid-1996 through early 2000, we conducted both laboratory and Þeld evaluations to determine whether this ßy could cause signiÞcant damage to safßower, a widely planted crop in California. In laboratory no-choice host range evaluations, adult females would oviposit, and the larvae completed development, on all Þve varieties of safßower thatwe tested.However, in choice tests, only one head each of two varieties of safßower was attacked. No safßower was attacked at three sites in California and Oregon, with large populations of Chaetorellia succinea, where we grew Þve varieties of safßower as Ôtrap plants.Õ Our monitoring of possible Chaetorellia succinea attack on safßower growing in 47 Þelds in California detected a small, but persistent population of this ßy infesting an uncommon safßower variety at one Þeld.We feel that our results indicate aminimal risk to commercial safßower growers, and this ßy continues to show promise in assisting toward the eventual biological control of yellow starthistle.