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In peak years in Silwood Park, Ragwort can reach densities of more than 10 stems per square metre (left). This year, the plant has almost disappeared (right). I’d be interested to hear about this year’s ragwort density where you live.
One of the reasons that the dynamics of ragwort are so interesting is that its herbivore, the Cinnabar Moth, reduces the death rate of the plant (rather than increases it as you might guess).
The thing that kills Ragwort is setting seed. All the reserves are removed from underground and invested in seeds. When the plant is prevented from setting seed, it keeps its reserves and perennates and lives for another year. It is big flowering that mean lots of deaths
The twist in this story is that Ragwort is a biennial, and the life cycle goes: seed to rosette to flowering plant to death. So if all the flowering plants die in one year, there should be a big cohort of rosettes ready to flower in the following year.
So why has the Silwood population disappeared in 2020? The non-mysterious part is that there was mass flowering and very few cinnabar moths in 2019, so we expected mass mortality. But why were there no rosettes ?
We need to go back another year to the summer of 2018 for the answer. That year there was very low seed production by Ragwort because Cinnabar numbers were exceptionally high. This meant very little seedling recruitment and hence low rosette densities in 2019.
Any seed germinating in early 2020 would have produced seedlings that were exposed to the most protracted spring drought we have ever documented. I can't find any surviving seedlings this month.
So what will happen to this year's Cinnabar Moth population? In Silwood, the prospects are bleak. Adult females have had great trouble locating plants on which to lay eggs, and the few caterpillars that have hatched are wandering about hopelessly searching for food.
Most of Silwood's caterpillars will starve to death before they are big enough to pupate. But there will be places where Ragwort plants and Cinnabar Moths survive, and these will re-populate Silwood in due course. And so it goes.
