FRIENDS OF EDGEWOOD NATURAL PRESERVE
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FRIENDS OF EDGEWOOD NATURAL PRESERVE
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BIOCONTROL REPORT V: YST PERSISTS By Paul Heiple The weevils and flies that attack Yellow star thistle (YST) have now been in Edgewood Park for five years. They have had time to build up their numbers to the levels of maximum control. Is it enough to make Yellow star an uncommon plant? No, it appears that even with six biocontrol agents released in the state and five species now established in Edgewood, Yellow star, if left alone, would remain a common weed. However, in Edgewood, YST is not being left alone and the combined efforts of the volunteer weeders and the biocontrol agents have begun to make a big difference in the area covered and the number of plants remaining. Two large areas on both sides of the central ridge are now down to very low numbers and some areas had no plants at all this year. With new biocontrol agents on the horizon and continued weed removal, we may in the next five years be able to say that YST is an uncommon plant in Edgewood. The past year's weather has once again been good for Yellow star thistle; plenty of rain fell in the period before March to recharge the subsoil, an important factor for YST growth and survival. The rains ended in March with unseasonable warm dry weather. This early end to the rainy season has brought the plants to flower earlier than normal. YST seemed to flower a bit too early for the weevils and flies, as more of the early flowers survived than in previous years. An additional factor to help YST was a cool summer which is not the favored weather for the biocontrol agents. The combination made this year one of the best years for seed production since the first biocontrol agents entered the park. I expect in untreated areas the number of plants could increase next year. Control efforts however still benefited from the biocontrol agents in two important ways: a reduction and a delay in seed production. Even a good seed production year like this is way below the seed production before the insects arrived. No plants can be found that do not have some damage. With fewer seeds produced there are fewer plants to pull. More surface area can be treated and the delayed seed production gives the weeders more time to pull the plants before any seed is produced. In some areas of poor soil, YST seed production reduced by the biocontrol agents can not keep up with normal seed and plant mortality. These areas now have far fewer plants even without any treatment; some areas have no plants at all.
It was noted this year that where YST has grown in very thick stands in the past, there were no new plants among the skeletons. This may be the result of the hairy starthistle weevil, Eustenopus villosus, which does not like to travel very far if it can find food. These insects can build up locally to very high numbers if they encounter plenty of food. The adults’ habit of feeding on young buds can prevent a plant from flowering for months. The adults then deposit their eggs in the few flowers that do bloom and the larvae eat all or most of the seeds in those heads. So it would seem that the thicker the stand of YST, the more likely it is to die out if the weevils are present.
A new biocontrol agent was released in California this year. It is a plant pathogen, the first plant pathogen approved for release in the continental United States. The pathogen is a rust, Puccinia jaceae var. solstitalis. Since 1978, This pathogen has been tested under domestic quarantine. Approval for field trials came in 2003. This pathogen was found to be highly species specific during the testing so there is little concern that it will impact any non-target species. So far it has been released in twenty-five locations in twenty counties. I do not know if it was released in San Mateo County and have seen no evidence of this pathogen in Edgewood so far. It is expected to cause damage to the leaves of young plants and weaken or kill those infected. Since it acts on the plant at a different part of the life cycle, it will enhance the current biocontrol agents. Some other insects are also under consideration for release, one of which is a crown weevil that will also attack young plants. The current state of biocontrol is that they slow the spread of YST, slow the recovery of treated stands, and remove YST from marginal areas. They delay the seed production and make the window for successful removal much larger. With the new control agents, the table may tip to make all locations marginal. |
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