The Vapourer caterpillar and moth, Orgyia antigua
The Vapourer moth is a member of the Lymantriidae family and has a fascinating life cycle.
It is found in a wide range of habitats including open woodlands, moorland, valleys and urban gardens throughout most of the British Isles ranging from northern Scotland to the extreme south west of Cornwall.
The adult moths fly in a protracted single generation flight period between July and October. Much earlier records and variable peaks and troughs in numbers have led to some debate over its univoltine status.
The males are attracted to light in small numbers but are most frequently seen flying in Autumnal afternoon sunshine.
For most people the Vapourer moth is best known for its spectacular caterpillars which are commonly seen feeding on a wide variety of plants, shrubs and trees in gardens and the countryside.
The on going Cornish Vapourer Study suggests there may be a uniquely coloured Cornish yellow form of caterpillar and questions whether the Vapourer Moths flight period may be more restricted and later in Cornwall than in many other parts of the British Isles. A full update on the Cornish Vapourer study will appear shortly.
The Male Vapourer Moth
The male Vapourer has a forewing of between 25-30mm and is most frequently seen flying in sunshine when it can appear a rich orange brown.
It has a weak, fluttering flight that can be mistaken for a small butterfly.
Despite being diurnal the males are recorded sufficiently often at light traps to suggest they may also be partially active at night.
Overall it is a dull chestnut brown with a white spot towards the outer edge of the forewing. At rest it may either take on a flat triangular posture or retract its wings into a tighter scroll.
The males pectinate antennae are used to detect pheromones given off by the females.
Having no size or weight limitations associated with flight her large rounded abdomen can contain hundreds of eggs.
She attracts males by giving off pheromones.
More information can be seen in the update of the ongoing Cornish Vapourer moth study shortly to be added.
Life Cycle of the Vapourer Moth
The adult moths emerge in a protracted single generation generally between May and October but earlier records and variable peaks and troughs in numbers have led to some debate over its univoltine status. There also appears some regional variations to main flight periods.
On emerging the flightless female remains on her cocoon and attracts males by giving off pheromones.
Following fertilisation she lays a large batch of eggs on the side of her cocoon and adjoining leaves.
Females that have already laid a batch of eggs following mating have been observed attracting males several days later, although it is uncertain whether this ever results in a second batch of fertilised eggs.
The eggs over winter before starting to hatch in May. One of the unusual feature about the Vapourer moth is that eggs from a single batch do not all hatch within a few days of one another, instead periodic hatching occurs over many weeks. In captivity the time span between the first and last egg hatching during the Cornish study was 8 weeks. (Cornish study information being added shortly)
The Vapourer Moth caterpillar
The hatching caterpillars feed on their egg before seeking out the leaves of nearby food plants.
Dispersal has also been recorded by the caterpillars being blown on silk threads.
In captivity as part of the Cornish study the first caterpillars were fully grown and pupated within 37-44 days. Pupation takes place in a cocoon spun to either the food plant or on other stable structures such as tree trunks, wall of a buildings or fence posts.
Adult moths in the Cornish study emerged within 8-15 days.
The spectacularly colourful, fully grown Vapourer Moth caterpillar is one of the most instantly recognizable caterpillar in the British Isles.
It is also one of the most commonly sighted caterpillars in gardens where it can sometimes become so numerous to be considered a pest.
The caterpillar reaches 40 mm in length, has two ‘horn-like’ projections of black hair from the first segment and a further black ‘tail like’ projection from the rear eleventh segment.
One of its most distinctive features are the bristly, dorsal tufts on the fourth to seventh segment which can vary in colour from white, brown or yellow.
The ground colour is a grey/dark blue with red pinaculars from which sprout clusters of light/grey hair. In Cornwall there is a ‘yellow form’ which may be unique to Cornwall see ongoing study.
Thanks to Martin Frankham for use of the image showing yellow tufted form above right.
Foodplants of the Vapourer Moth caterpillar
The caterpillars feed on a wide range of deciduous trees such as Blackthorn, Hawthorn, apple and Oak as well as many shrubs.
In gardens they are frequently reported early in the year on Wisteria climbing up the sides of houses as well as many other shrubs such as Roses and Blueberry.
In Cornwall it’s suggested that ling may prove to be an important larval foodplant.
Rearing the Vapourer Moth
The caterpillars are very easy to rear and can be collected by sweeping. Late instar caterpillars found wandering prior to pupating are often parasitised.
Finding the eggs can be a laborious exercise and the simplest way to see the whole life cycle is to collect caterpillars and rear them through hoping a female will emerge.
She can then be taken outside on a sunny afternoon into an area known to have a population of Vapourer Moths to attract a male. Once mated she will readily lay eggs.
Overwintering eggs should be kept in a cool outhouse to experience outside winter temperatures – ( I’ve often put part of an egg batch in the fridge for several weeks during unusually mild Cornish winters)
Occasional fine spray of water prevents drying out.
Pest Status of Vapourer Moth caterpillars
In the majority of cases the Vapourer moth causes little damage to shrubs and trees other than slight defoliation. However, in some urban areas and in gardens they can occasionally become so numerous they become a pest, causing severe defoliation of shrubs and specimen trees – although, most trees will fully recover the following year.