Sometimes, to prove a point, you have to starve a larva. It’s all too easy to be bewitched by the beauty of a passing butterfly but not so easy to admire the foraging of a voracious caterpillar. We should, though: it turns out that the two are linked.
Researchers in the US have been putting monarch butterfly larvae on strict diets and observing the consequences for the butterflies they become. This kind of butterfly is in grave trouble. Every year, the insects perform an extraordinary 3,000-mile migration from Canada and the northern US down to central Mexico, where they gather on a few mountaintops for the winter.
In 2012, about 60 million monarchs arrived in the Mexican mountains. That was a record low until the 2013 figures came in – the number arriving was almost half that of the previous year. It was the lowest figure since records began in 1993.
A big problem is the widespread disappearance of the milkweed plant. Monarch larvae eat only milkweed and urbanisation and industrial farming practices have made it scarce.
Hence the research, which was published last April. The researchers found that restricting the monarch larva’s milkweed intake reduces the size of its wings when it emerges from the chrysalis. It is likely that this stunted growth makes the migration much harder work.
Monarchs also seem to need food to maintain their looks. The colours of the orange-and-black wing patterns are deeper, with more striking contrast, in monarchs covering the most distance. It may be that feeding up the larvae to produce better fliers will also produce more beautiful butterflies.
The aesthetic argument is one of very few available to conservationists. Researchers can’t point to any specific benefit that monarchs bring: they don’t seem to play a crucial role in any ecosystem and their removal from specific ecosystems “would probably not have lasting repercussions”, as one report put it. The monarchs don’t contribute anything apart from a beautiful spectacle when they migrate.
British butterflies are even worse off – they don’t have a spectacular migration. In March, the UK government issued a draft of its “pollinator strategy” document, which outlines what might be done about the severe decline in numbers of pollinating insects such as bees, butterflies and moths. The charity Butterfly Conservation said that it “strongly welcomes” the proposed strategy (though it dismisses as “ludicrous” the idea that pesticide companies should self-regulate).
Yet there is a slightly dejected tone to the charity’s chief executive’s observations. He points out that while bees are acknowledged to have economic value as pollinators of agricultural crops, butterflies are not and are therefore less likely to receive government help. In the age of market forces, it is not enough to be beautiful.
However, as the Intergovernmental Panel on Climate Change noted in March, butterflies’ relatively short life cycle and “high dispersal capacity” make them very useful markers of climate change.
That is certainly true of the British butterfly population, which, apart from a few cold-loving species, is thriving in our warming climate. In the past 20 years, for instance, the orange-and-black comma butterfly has spread 137 miles northwards.
If you lived in Edinburgh in the early 1990s, you would not have seen one. Now they’re not an uncommon sight: an effect of global warming that’s hard to complain about.