FRIENDS OF EDGEWOOD NATURAL PRESERVE

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It was another good year for Yellow star-thistle, as heavy rains began the wet season in November and were followed by good rains in December. This rainfall pattern gave the plants an excellent start and insured deep root development. Unusual heavy April rains gave the plants an extra shot of growth before bolting.

A negative factor from the April rains may have been the germination of some of the seed bank which had only two months to grow before they were forced to flower. This resulted in a large number of rather small plants during the summer flowering period. The cool, rainy April weather also seemed to set back the biocontrol agents which showed up in numbers a month later than in previous years and in numbers which seemed lower than I expected to find. So this year YST grew well and was infested later, both positive for the weed.

The good news for Edgewood is that even though it was a good year for YST, the weed does not seam to have increased its area of coverage or its density. Like the Red Queen in “Through the Looking Glass,” YST must run faster to stay in the same place.

False peacock fly, Chaetorellia succinea http://www.countyofsb.org/agcomm/3226_28.htm

Hairy star-thistle weevil, Eustenopus villosus Forestry Images, http://www.forestryimages.org/

The weeders of Edgewood are not stationary however. With the biocontrols slowing the production of seeds, any YST pulled before the first of August is not going to have produced seed. This was also true this year; once the biocontrol agents showed up, they were hungry and damaged all the early flowers. Four agents were common this year, the false peacock fly and the hairy star-thistle weevil remained common with an increase in the bud weevils and a big increase in the gall fly. These abundant insects kept the seed production very low through the end of August.

An incident occurred in July which shows how abundant the insects have become. One of our weeders working in the Clarkia trail area put some bags of pulled YST in her car for later disposal. Some days later, it was noticed that the area below the rear window was covered with what looked at first like mouse dropping. They turned out to be biocontrol agents that emerged from the bags and tried to escape the car by going out the back window.

It will soon be five years since the biocontrol insects became abundant in Edgewood. So far the greatest effects remain:

1. YST is disappearing from marginal soils,
2. Plants are fewer in good soils but the individual plants are larger so that ground cover remains great, and
3. Seed production is delayed by at least a month.

Bud weevil, Bangasternus orientalis

Gall fly, Urophora sirunaseva Forestry Images, http://www.forestryimages.org/

The last one is the most important to us, since it give the weeders and mowers more time to clear the plants out before they can produce seeds. Because of this, we are increasing the area cleared and pushing the YST back. If the trend continues, YST could become uncommon in Edgewood in three or four years.

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