Evaluating implementation success for seven seed head insects on Centaurea solstitialis in California, USA.

A stage-based invasion model proposed by Colautti and MacIsaac (2004) is used to evaluate the success of implementation efforts for seven seed head insects introduced on Centaurea solstitialis L. (Asteraceae) in California, USA. Six insects were introduced intentionally; one was introduced unintentionally. Establishment of initial foreign material (stage III invasion) at field nursery sites was very successful (86%) with six of seven species establishing (five of six intentional, one of one unintentional). For the intentional species, initial releases occurred at multiple locations, and establishment success among locations was 100% for four of the five species that established. Statewide distribution of seed head insects introduced intentionally (stage IV invasion) was performed through the network of County Agricultural Commissioners. Training workshops provided knowledge and insects to county biologists for release in their county. Hundreds of releases occurred in this way, and follow-up monitoring estimated establishment success above 87% for four of six species. Regional spread and population increase to high density (stage V invasion) was examined in an independent statewide survey of C. solstitialis seed heads in 2001 and 2002. Results showed that only one of the intentional species, Eustenopus villosus (Boheman) and the unintentional species, Chaetorellia succinea (Costa), appear to have achieved high densities over large areas. The fly, C. succinea, appears to be the most successful seed head insect, while E. villosus was second in abundance statewide. The other seed head insects were recovered in low numbers statewide and appear to contribute little to the herbivore pressure on this weed.

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