The caterpillars of most moths do relatively little damage to their food plants. However, some tussock moths increase in numbers to such an extent that their caterpillars can defoliate entire trees.
Tussock moths are named because their brightly colored, conspicuous caterpillars have tufts of hair arranged in tussocks along their bodies. These caterpillars are unpalatable to most predators, including birds, and their coloration is of the warning type. In Britain, the cuckoo is the only bird able to cope with hairy tussock caterpillars.
There are about 2,500 species of tussock moth worldwide belonging to the Lymantriidae family. The gypsy moth Lymantria dispar. the vapourer Orgyia antiqua and brown-tail Euproctis chrysorrhoea moths can all produce caterpillar plagues, as seen in Britain. Europe, and the United States.
The vapourer moth can be seen around cities and also in the countryside and is often seen in gardens.
The red-brown male vapourer, which has a distinctive white spot at the base of each of its forewings, flies by day in late summer and autumn and could easily be mistaken for a butterfly when in flight. It usually flies high in the air, sometimes above the rooftops.
However, it has feathery antennae characteristic of tussock moths, and closer examination reveals many typical moth features. The female vapourer is a rather helpless-looking creature. She is fatter than the male, can hardly move, and, possessing only the vestiges of wings, is incapable of flight.
After hatching from the pupa, which is encased in a silken cocoon attached to a stem or tree trunk, the female waits for a passing male and, after mating (which usually occurs on the day she emerges), lays a cluster of a hundred or more dirty white eggs on her cocoon.
Once the eggs are laid, the female dies. Males can detect the scent of a virgin female from long distances. It is not uncommon to see several males fluttering around a female as she scrambles from the pupa.
When the eggs hatch, the vapourer caterpillars climb to reach the leaves of the food plant. They are remarkably active and, although easily blown off the tree, their hairiness makes them buoyant in air currents.
They can easily be distinguished by the four yellow tufts of hair on the back and by the dotted line of raised red spots along each side.
The leaves of many different species of both native and introduced trees are eaten by these caterpillars. In 1974, there was a plague of vapourer caterpillars in London’s Berkeley Square. The magnificent large plane trees were defoliated, and the Square seemed much brighter and more open than usual.
Many caterpillars dropped to the pavements below the trees and alarmed passers-by and people sitting on seats eating their lunch.
Since the hairs of the caterpillars can cause an irritating rash on the skin, there was even more alarm. By 1975, numbers were back to normal.
Vapourer plagues have been reported in London before, but 1974 was the first time that plane trees had been devastated.
When fully fed, the caterpillars spin cocoons in crevices in bark, under the eaves of outhouses, or on fences, mixing their hairs with the silk of the cocoon as they spin.
The adults hatch in late summer, and the female lays her eggs in her own cocoon, where they overwinter in comparative safety.
The brown-tail moth is mainly confined to low-lying coastal areas. These silvery-white moths fly at night in July and August but can easily be found by day lurking in bushes. The moth gets its name from the large tuft of dark brown hairs situated at the tip of the abdomen. Female brown-tails lay clusters of eggs on twigs and leave them covered with hairs detached from this tuft.
The eggs hatch in autumn, and the young caterpillars, which are blackish with brownish warts, nibble the leaves of brambles, blackthorn, and hawthorn.
In some places, and certain years, brown-tail caterpillars defoliate their food plants. They build extensive silken retreats, often connected by silken walkways along the twigs and branches.
Many wander in search of new food sources, and many starve when they find nothing. Bramble bushes can be found leafless and flowerless. Once they stop eating and change into pupae, the leaves and flowers sprout late and produce a late crop of blackberries.
Like the vapourer, the brown-tail caterpillars can cause a painful skin rash and should not be handled. Caterpillars can become a public nuisance. Not only are the hairs picked up by people walking through bushes but can also be blown by the wind onto domestic washing and cause skin rashes when the washed clothing is worn.
In late June, the caterpillars pupate in a silken cocoon, often in a large mass. The moths emerge in July, but the females don’t fly when they are ready to lay eggs so future infestations tend to be larger in certain sections of a hedge.
We do not know why the brown-tail is so abundant in certain years, nor do we know why the species does not occur in much of Britain where its food plants are common. All that can be said is that in certain places, and in certain years, the population-regulating mechanisms fail, and large numbers of caterpillars result. The brown-tail has been introduced to the United States where it can be a serious pest of fruit trees.
Is The Gypsy Moth A Pest?
The brown male gypsy moths fly by day in August and look like over-sized vapourers. The whitish females do not often fly and when they do it is at night. The caterpillars hatch in April and feed on the foliage of a wide variety of trees and, as in the brown-tail, are sometimes locally very abundant, and scarce at other times.
They are grey and covered with black dots. The moth was introduced into the United States and is now a pest, defoliating vast areas of forest in some years, and requiring extensive and expensive control measures.
Caterpillar plagues may be unusual, but when they occur we need to think about the natural factors that control the numbers of the many species that never become plague moths. There are two explanations for this.
First, predators and parasites take a vast toll of caterpillars and adult moths and so never allow their numbers to develop to the extent that significant damage is done to vegetation. Secondly, the food plants themselves, and the weather, regulate the numbers of moths.
The leaves of the food plants may be rendered unpalatable through toughness or chemical content, and cold weather may kill off both leaves and insects.
How Populations are Regulated
A population can be said to be regulated if it repeatedly returns to a certain size after any departures from that size. A report has been published on the population regulation of brown-tail and gypsy moths.
First, it was thought that they might be limited by unusual weather which kills individuals irrespective of population density. A second proposition considered the biological changes such as predators, food shortages, parasites and diseases, whose effect is large when populations are high and small when they are low.
The likelihood of death for an individual is greater if there are a great many other individuals around at the same time.