FRIENDS OF EDGEWOOD NATURAL PRESERVE
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FRIENDS OF EDGEWOOD NATURAL PRESERVE
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BIOCONTROL REPORT VI: YST MAKES A COMEBACK By Paul Heiple As I wrote last year, the biocontrol agents we have in Edgewood Park are not enough to make Yellow star-thistle (YST) an uncommon plant. This year shows that they are not even enough to hold the gains we have made if the weather favors the YST. Weather Favors YST This year had many favorable features to help YST gain ground. First there were the early rains, then the rains spaced themselves out so that there were no dry periods or dry soil at depth. The amount of rain was also above normal so that the deep soil moisture that YST depends on during the summer was well recharged. The result was a germination and survival rate for YST that was above the normal level and exuberant growth because of all the rain. Weather Discourages Biocontrol Agents As if that were not enough, the cool June weather kept the biocontrol agent inactive during the early flowering stage. Normally, cool weather delays the flower formation on YST, but that was not enough to delay the flowers until the insects came out this year. The hairy starthistle weevil (Eustenopus villosus) often damages a great many flower heads before they open, but this cool year kept that from happening. In fact, looking back at my writing on this subject in previous years, this is the third year in a row with plenty of winter rain, an early start to the rainy season and a cool spring, all helping YST and holding back the biocontrol. I did think that I was finding more hairy starthistle weevils with dark bodies this year; perhaps the beetles are beginning to adapt to the cooler climate by absorbing more heat from the sun. It would be nice to check this observation against local collections of the weevils made over the last five years. New Agent Not Released Last year I reported a new biocontrol that was about to be released, Puccinia jaceae var. solstitialis, a rust that damages the plants in the rosette stage of development before any seed production has begun. The release never took place in San Mateo County. Puccinia jaceae var. solstitialis does not attack only YST; it also attacks Centurea cyanus or corn flowers which are grown in the county for the florist market. Since that would have a negative impact on the grower, this county did not have any release sites. Release sites in other counties are reporting a very slow spread of this biocontrol so I am not expecting to see this in Edgewood any time soon. Weeders Holding their Own So now to the good news: the weeders got out and held the line. Although no new areas were cleared, the weeding crews cleared all the areas worked on in previous years. Since this year probably germinated much of the seed bank, we can hope that we have reduced the number of plants we will face next year in most areas. In areas that have had years of treatment, we swept the areas several times to get almost every plant before seed was produced. Such constant pressure on YST will yield areas that are free of plants in a few more years, moving us to the goal of a YST-free Edgewood. YST Adaptability I made an observation and tested a theory this year, that YST has seed heads that can open and close depending on conditions. The mechanism for this movement is moisture dependent. When the air or the seed head is moist, the seed head is closed; when the air is dry, the seed head opens. The result is a seed head that is only open and releasing seeds on sunny warm days. What drove this to evolve is unknown; one can speculate that the plant is protecting its seeds from some night time seed eater or that releasing seeds during wet weather is not to the plants’ advantage. The answer to this problem is in Europe. For weeders, it is best to pull ripe seed heads in the morning before they open. |
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