FRIENDS OF EDGEWOOD NATURAL PRESERVE
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FRIENDS OF EDGEWOOD NATURAL PRESERVE
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BIOCONTROL REPORT VII: WAITING FOR THE NEW AGENTS By Paul Heiple As I predicted last year, Yellow Starthistle (YST, Centaurea solstitialis) is still a common weed in Edgewood and throughout the state of California. The weevils and flies do put enough pressure on YST to decrease the number of plants, but with the lower densities of the stands, the plants grow larger and the area of cover remains the same. This change has made it easier to weed; there are fewer plants to pull. Weeding can proceed faster while pulling plants; however, dealing with the larger plants can slow down the process, as they are more difficult to bag. Overall, I think we are covering more ground in less time. The new rust pathogen, Puccinia jaceae var. solstitialis, released last year is not yet in Edgewood. This biocontrol was not released in San Mateo County because the rust also affects an agricultural crop in the county, Bachelor’s button or Centaurea cyanus, grown in San Mateo County as a cut flower crop. Oddly enough, this plant is also considered a weed in some areas of California. On the horizon are many other agents that may tip the balance for YST. Testing has been completed on the root crown weevil Ceratapion basicorne and permission for field release has been requested. Should the approval be given, this insect will be released as early as next year. The root crown weevil feeds on the plant during the vegetative period long before seeds are forming. The larva eats the inside of the root just below the rosette, cutting off the leaves from the root and killing the plant. The history of successful insect releases for YST in not good however. Of six species approved for release, only two insects are common and doing extensive damage: the hairy starthistle weevil (Eustenopus villosus) and the false peacock fly (Chaetorellia succinea), and the latter was not one of the insects approved for release. To cover this difficult gap in successful insects, four more insects and a mite are being tested along with four more fungal diseases. If one of these is as effective as the two already successful controls, maybe YST will finally become uncommon. Insect photos taken from http://www.cal-ipc.org/symposia/archive/pdf/2002_symposium_proceedings2390.pdf.
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