Visit St Nicholas Fields on a sunny summer day and your heart will surely be lifted by the attractive sight of the butterflies on the reserve. Up to 20 species appear regularly each year, some of them in abundance, others harder to find but worth the search. But come Autumn and the show is almost over.
Our last butterfly sightings of the year are usually a Speckled Wood, or occasionally a Small Tortoiseshell, both in October. Then we have to wait at least four months before the first Brimstone or Comma appears the following spring. So where do St Nicks butterflies go in the winter months? What are their strategies for surviving the cold? The answer is a complex one. Butterflies have four life-stages: egg, caterpillar, chrysalis and winged adult. Entomologists call these stages ovum, larva, pupa and imago. Each species adopts one (or occasionally more than one) of these stages to get through Britain’s winter chill.
Two of the reserve’s butterflies leave our shores for warmer climes in the winter. These are our continental visitors (summer migrants): Painted Lady and Red Admiral. Occasionally, and increasingly, adult Red Admirals are found overwintering in Britain, but the vast majority of St Nicks newly-emerged Red Admirals fly south in late summer and autumn to winter in Europe and the Mediterranean area. Painted Ladies never overwinter here, and in a ‘good’ Painted Lady year (such as 2019) they are seen in millions crossing the English Channel to spend the winter months often as far as Africa south of the Sahara. Radar has tracked them flying high over the channel even at night.
Small Tortoiseshell and Peacock butterflies sometimes migrate too, but most of them spend the British winter as adults, hibernating in hollow trees, log piles, outbuildings and even houses. Strictly speaking butterflies don’t hibernate – this stage in their lives is more properly called dormancy or diapause. On warm winter days they can wake up prematurely and be seen flying in January or February. These early wakers probably don’t survive more than a day or two as there is little or no nectar for them to feed on. If you find a too-early woken-up butterfly in your house or outbuilding, carefully capture it (perhaps in an open cardboard box) and find it a cool dark place in the hope that it will settle and sleep again. For further advice go to Butterfly Conservation or UK Butterflies websites.
Two other St Nicks species also overwinter as adults – the Comma and the Brimstone. The Comma hibernates in dry woodland areas, usually low down in the leaf litter or tree roots, and on waking can be seen flying as early as February and March. Brimstones hibernate in sheltered places such as ivy, holly trees and bramble patches and wake up to fly from February onwards. Brimstone is our longest-lived butterfly. July’s newly-emerged adults survive the winter to breed the following April and May. Compare that with the Speckled Wood whose adults seldom live longer than a week.
Our other St Nicks butterflies will not survive the winter as adults. They spend the winter months on the reserve as a chrysalis or the hardier ones as caterpillars. Holly Blues, Orange Tips, Large Whites, Small Whites and Green-veined Whites all overwinter in the chrysalis stage, attached to a stem or other safe place. This winter (2019-20) a Green-veined White chrysalis has been observed attached to a small tree trunk on the reserve (see photo). It’s surprising that it hasn’t yet been found by a foraging Blue or Great Tit.
Speckled Wood butterflies can overwinter either as a pupa or a larva. Several generations of this attractive chocolate and cream butterfly occur on the reserve every year. They are seen at St Nicks in every month from March to October, often in some numbers, which is a bonus since this species only re-colonised Yorkshire in the 1990s after a long absence.
Some butterfly species survive the winter because their caterpillars can withstand cold winters. After summer breeding the adults die, but the caterpillars continue to feed until colder temperatures, dying vegetation and reducing day length stimulate them to spin a protective case, crawl inside the stems of plants and grasses or find other protected hiding places. With the coming of spring the caterpillars start feeding again until they are ready to pupate before emerging as adult butterflies. At St Nicks the butterflies that adopt this strategy are Large and Small Skipper, Ringlet, Meadow Brown, Gatekeeper, Small Copper and Common Blue.
There are a few species of British butterfly that spend the winter in the egg stage, for example the hairstreaks and some blues, but we have no records of these at St Nicks. Essex Skipper is our nearest candidate – they have spread north as far as Doncaster and Fairburn Ings, so we must keep our eyes peeled around the reserve!
The butterflies that overwinter as adults have a head start on all other species. They are the first of the year to be seen, nectaring on the spring flowers to get into breeding condition early. Those species that spend the winter as a chrysalis are next to emerge, with summer migrants also arriving, depending on weather conditions. Finally come the species that have spent the winter in the caterpillar stage. They have needed time to feed up and pupate before their winged summer can begin.
Come and enjoy St Nicks ‘butterfly summer’ and be entranced by the multicoloured show put on by these winged beauties, a few of which are pictured on this page with photographs all taken at St Nicks.