Many moths and butterflies have boom and bust years. It’s one of the things that makes assessing population trends so difficult as it can take decades to assess the long term trend in some species. For example, Marsh Fritillaries – although now a rarity, and highly protected due to habitat loss, back in 1928 a field in Fermanagh was described as being ‘black with caterpillars’.
In a boom year, it can get pretty spectacular; who can forget the invasion of the Clouded Yellows in 1992 (well, everyone but me apparently: am I the only person who remembers these things?).
On a smaller scale, this ghostly coating of the hedge in Wickes car park, by the larval webs of the Bird Cherry Ermine Moth in 2016.
Sometimes a boom is less than welcome, by humans anyway. While reading my local paper, The Alloa Advertiser, I came across one of Valerie Forsyth’s “Walk in the past” articles (my favourite bit of the paper). The article was about the “The Great Caterpillar Plague” of 1937.
This is when millions of Antler Moth Caterpillars destroyed thousands of hectares of summer pasture. The caterpillars were so numerous that a reservoir supplying Denny had to be shut down because of the amount of caterpillar bodies floating around in the water, polluting supplies.
The Antler Moth, so known because of the Antler pattern on the wings, is normally a fairly unassuming medium sized moth which usually goes unnoticed. Antler Moth booms are not particularly unusual, occurring only once or twice a decade. Generally they’re localised to small areas. What made the outbreak of 1937 notable was its size. The caterpillars went on the rampage all over southern and central Scotland, from Dumfries-shire to East Perthshire. Stampeding? Swarming down from the hills, travelling up to a mile day and attacking arable crops. The plague was so severe, it even came to the attention of the House of Commons (in between debates on the Spanish Civil War and the Gold Standard) .
Provoking this exchange:
“Mr. T. Johnston
(by Private Notice) asked the Secretary of State for Scotland, whether he is aware that a destructive plague of caterpillars has swept from the Ochil Hills, in Perthshire, to the Campsie Hills, in Stirlingshire, and that some 10,000 acres of sheep pasturage have already been affected; whether outbreaks have been reported from other parts of Scotland: and whether he can make any statement as to the origins and causes of the plague and as to the steps which may be usefully taken by agriculturists and the Department of Agriculture for Scotland for the speedy arrestment of the plague.
The Secretary of State for Scotland (Dr. Elliot)
I am aware that extensive damage has been done during recent weeks by caterpillars of the Antler Moth in the areas referred to in the Question and in other upland areas in South and Mid Scotland. The position has been investigated by the authorities of the Edinburgh and East of Scotland College of Agriculture, and it is stated that the rare periodic outbreaks of this pest have always followed upon a season in which much snow has lain on the hills until late spring or early summer protecting the young caterpillars. I understand that control by artificial methods at this juncture is regarded as out of the question. It appears that in the opinion of expert entomologists the outbreak is past its zenith, that the pest is not likely to spread to the low ground, and that the damage done is not permanent, and may very quickly be repaired by growth of new grass. The Department are in communication with the College as to whether any further action is required by way of advice to farmers on the subject.
Can the right hon. Gentleman say whether this pest is also attacking the fronds of the bracken, and whether investigations are being made as to whether some permanent advantage might not be secured in that direction?
My information is that it is rather an underground pest than a leaf pest; that it attacks the roots. I will call for a report as to whether it is attacking bracken.
Does the right hon. Gentleman not think that this pest, which is ruining small agriculturists, is completing the work which has been done in this part of the country by the Milk Board?
I do not think the Milk Board is concerned with these particular areas”
Hansard proceedings July 1937
The esteemed Secretary of State: Dr Walter Elliot; wasn’t that wide of the mark in his replies.
Late snow protecting caterpillars from weather and predators is thought to be a factor in Antler Moth booms, but more recent research has; low viral and parasitic loads, also contributing to population explosions.
The caterpillars are quite wasteful. Their food plants are grass; preferably Mat-grass and Fescues; they start munching at the base of the stem and work their way down. Although they don’t kill grass; the result is a sort of extreme mowing, with the uneaten stems left to rot.
Fortunately these population booms are temporary, and caterpillar eating birds; have (quite literally) a field day.