Peppered Moth - Tumblr Posts

Moth of the Week

Peppered Moth

Biston betularia

The peppered moth is a part of the family geometridae. It was first described in 1758 by Charles Linnaeus. This moth gains its name from its speckled coloration, which has been studied as an example of natural selection and population evolution.

Description This species has a short body with narrow forewings. The body and wings are the same white base peppered with black dots and irregular black lines. This speckled pattern may vary with some moths having very few spots and others having so many that they look as if they are black with white spots as opposed to white with black. In rare cases, the black on the wings and body is replaced with gray or brown and in even rarer cases the spots are a combination of brown and black/gray. These spots help the moth camouflage against lichen on trees.

The evolution of this moth had been studied extensively during the last two hundred years, which created the term “industrial melanism.” During the Industrial Revolution, air pollution killed off lichen and covered trees in soot. This caused moths with a black spots on white base (typica) coloration to lose their camouflage and die off due to predators. This caused a spike in population for moths with a darker coloration (carbonaria) because they had the camouflage advantage. Once environmental conditions improved, the lighter colored moths once again became the dominant coloration.

The male’s antennae are bipectinate, meaning it has two rows of rami going down either side of a singular flagellum.

Wingspan Range: 45 - 62 mm (≈1.77 - 2.44 in)

Diet and Habitat The caterpillar of this moth eats many trees, shrubs, and small plants such as Blackthorn (Prunus spinosa), Hawthorn (Crataegus monogyna), Downy (Betula pubescens) and Silver Birch (Betula pendula), limes, sallows, poplars, oaks, Sweet Chestnut (Castanae sativa), Beech (Fagus sylvatica), Bramble (Rubus fruiticosus), Broom (Cytisus scoparius), Black Currant (Ribes nigrum) and Hop (Humulus lupulus).

They have a wide range, being found in China (Heilongjiang, Jilin, Inner Mongolia, Beijing, Hebei, Shanxi, Shandong, Henan, Shaanxi, Ningxia, Gansu, Qinghai, Xinjiang, Fujian, Sichuan, Yunnan, Tibet), Russia, Mongolia, Japan, North Korea, South Korea, Nepal, Kazakhstan, Kyrgyzstan, Turkmenistan, Georgia, Azerbaijan, Armenia, Europe and North America. They prefer habitats of woodland, scrub, hedgerows, parks and gardens.

Mating Depending on its location, this moth can have one or two generations per year. In Great Britain and Ireland, the peppered moth has one generation per year, whilst in south-eastern North America it has two generations per year. They emerge from the pupea in late May to August.

The females attract males with pheromones, which are carried by the wind. Males follow the concentration gradient to find the female. The male guards the female from other males until she lays the eggs. The female lays about 2,000 pale-green ovoid eggs about 1 mm in length into crevices in bark with her ovipositor.

Predators This species is a night-flying moth, making the vulnerable to bats. The males in particular fly every night to search for a female while females fly only the first night.

To protect themselves from birds during the day, this species rests on lichen covered trees to camouflage themselves.

The day time resting positions of this moth have been recorded and studied. This study shows that the peppered moth prefers resting spots that are covered such as below where the trunk and a branch meet, the underside of branches, and leafy twigs.

Additionally, the study found peppered moths with a lighter coloration (typica) blend in better against crustose lichens rather than foliose lichens because birds can see ultraviolet light. The peppered moth reflects UV light while crustose lichens don’t, making them easier to pick out.

Fun Fact The caterpillars of the peppered moth resemble things in both color and size. An experiment published in 2019 done on the caterpillars of the peppered moth showed that the larva (even when blindfolded) could sense the color of the tree they live on and change their body color to match and/or would move to a different twig that was closest in color to their own body.

(Source: Wikipedia, Butterfly Conservation, Max Planck Institute)

Natural Selection and the Case of the Peppered Moth

If you're anything like me and you spend a lot of time talking about Interesting Facts and Things to anyone who will listen, you may have heard a slightly warped account of the story of the peppered moth. Maybe someone said something like: "Oh yeah and in England there was this moth that turned black during the industrial revolution due to all the coal dust in the air". Which is... not exactly true.

Let me start the story from the beginning. Before the industrial revolution, peppered moths (Biston betularia) were distributed across England, Europe and North America. They existed in 3 morphs: typica (mostly white), carbonaria (mostly black) and insularia (inbetween). Note: I'm mainly going to talk about the typica and the carbonaria morph here.

Before the industrial revolution, the typica morph was the predominant morph. Peppered moths lived in forests filled with light trees and lots of lichen, a good place for a typica morph to blend in. The carbonaria morphs were living life on hard mode, though. They did not blend in, and were more easily picked out by predators like birds. Below you can see how well a typica morph blends into lichen.

Then the fire nation attacked. Jk, then came the industrial revolution. Factories were pumping out coal fumes into the air, covering forests nearby with coal dust and killing the lichen that grew on them. In these forests, the carbonaria morph blended in better. Predators began to pick out typica morphs more often. And so, carbonaria morphs became the predominant morph in forests near industrialized areas.

Naturalists, noticing this, wanted an adequate answer on why this was happening. In the 1950s Bernard Davis Kettlewell used various methods to test the hypothesis that it was natural selection. And indeed it was. Typica morphs were 2x more likely to be eaten in a polluted forest than carbonaria morphs, and vice versa.

This phenomenon was so common, and observed in many other moth species across industrialized areas, that it was even given a name: industrial melanism.

So there you go. The full story of the peppered moth :)

Oh and I should add that in areas where the effects of industrialization were reversed, the typica morph once again becomes more common.

Sources:

Peppered moths: moth life cycle

Peppered moths: natural selection

Peppered moths: dr ketllewell