FRIENDS OF EDGEWOOD NATURAL PRESERVE
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FRIENDS OF EDGEWOOD NATURAL PRESERVE
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BIOCONTROL REPORT II: THE BATTLE FOR EDGEWOOD By Paul Heiple Last year I wrote about the invasion of Edgewood park by aliens, first plants like Yellow starthistle (YST), Centaurea solstitialis, and then the insect biocontrol agents which can easily escape the notice of the casual observer. This year the battle has intensified. The invasions also continue with a new fly on the YST and a weevil on the Italian Thistle, the first biocontrol agent noted on that weed. An old invader also was found attacking another weed species in Edgewood, tocalote.
Last year, the forces of fire, mowing, hand weeding and biocontrol agents came together in such a way that this year Yellow starthistle appears to be in retreat. If you can remember where and how much YST you saw in years past, I think you will agree there were fewer plants this year and it was less noticeable or missing in some areas it had grown before. Another observation is that the YST plants tend to be larger, a result of decreased competition from other plants of the same species. This year, those forces have (with the exception of fire) continued and biocontrol seems to be at its peak. Even a somewhat casual look at the fields of YST will tell the observer that something is wrong this year. A closer look will show that many flower heads are damaged or dead. The flowers seem to be arranged on the plants differently than in other years, clustered along the stems instead of at the ends of the branches. This is the story of this year’s battle that has lead to reduced seed production. The story begins in April when the first biocontrol agent became active, Chaetorellia succinea, the false peacock fly. In April, YST has not begun to flower, and the false peacock fly is after another plant, tocalote (Centaurea melitensis). I confirmed the usage of this plant by taking some plants and placing them in plastic bags. Within a week, adult flies had begun to emerge from the flower heads. The damage to tocalote appears to be minimal, and it may never be hit hard but it will aid in the buildup of flies in the spring so that they can attack YST. In May while I was inspecting a weedy area in Portola Valley, I noted evidence of insect activity on the flower heads of Italian thistle (Carduus pycnocephalus). This turned out to be a new species of biocontrol that I had not seen in previous years. It was the weevil Rhinocyllus conicus. I soon found that it was widespread with large populations nearly everywhere I looked. Edgewood had a good population and the damage they did to Italian thistle in Edgewood is extensive. While identifying this species of this weevil, I came across some disturbing news that this species attacked some native thistles in the prairie states and had possibly contributed to their decline. This weevil eats many species in the thistle tribe: the genus Cirsium is frequently a food plant. I checked our rare population of C. fontinale at Coyote Ridge south of San Jose where I found R. conicus on the flower heads. The literature stated that R. conicus is active only in the spring and I found this to be the case. C. fontinale is a perennial and flowers over a long season. The weevil will no doubt reduce seed production but its impact on the populations is unknown at this time. The impact on Italian thistle, which is a spring flowering annual, should be much greater.
By June when the YST should have been getting into full flower, another interesting observation was made: many of the first buds had been destroyed before they could open. I attributed this to the bud weevil, Bangasternus orientalis, which I had noted in Edgewood three years earlier but had always found confined to an area on the south end of the Central Ridge. The bud damage this year was park-wide. The hairy starthistle weevil, Eustenopus villosus, also attacks young buds. I have never observed this behavior, in fact I have never seen either weevil eating young buds. The young bud damage three years ago was before I ever observed E. villosus in Edgewood but B. orientalis was observed and eggs of this species were also observed at that time. I suspect B. orientalis is most active at night or that it has a habit of falling off the plant when disturbed, making it difficult in either case to be caught in the act of feeding on YST buds. Further study will be needed to determine the insect responsible for the damage to the buds. The bottom line however is that one of the biocontrol agents delayed the flowering of YST by several weeks. Had the rains of late June not come, some of those plants might have never gotten a chance to flower.
July saw the full complement of biocontrols on YST. The second growth of buds made the YST plants look odd, many axial buds and few terminal buds. Chaetorellia succinea and Eustenopus villosus were abundant. Throughout the park were a few Bangasternus orientalis and a new biocontrol, Urophora sirunaseva, the YST gall fly making occasional appearances. The gall fly forms galls in the seed head, taking up the space where achenes would form and using the resources of the plants to form the galls which the larvae eat. With all four species in Edgewood, very few heads were left unaffected. By late August, many of the YST plants were nearing the end of their lives and fewer than 1% of the flowers had produced any achenes. You can observe this for yourself: look for flower heads that have dried and opened exposing a white fluffy interior between the spiny phyllaries. These are the ones that have produced seeds. Most of the ones you will find will be dried and closed with the dead flower parts a brownish yellow and still attached to the flower head. These have produced nothing but insects. This year’s battle will rage for the next few months, then all will be quiet again. Early next spring we will be able to assess the damage to YST and Italian thistle to determine the strategy for next year’s battle. Perhaps this year was the last big battle with YST for the grasslands of Edgewood. Ed. note: Photographs for this article provided by Kathy Korbholz. |
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