Dr Terry Mabbett considers the role of the arborist in establishing and sustaining butterfly populations, and how many plants regarded as weeds can prove useful in such an endeavour.
OPEN green spaces such as parks, commons, golf courses and cemeteries, and especially those in urban, suburban, and peri-urban areas, are important sites and habitats for butterflies. And the arborists, groundsmen and greenkeepers who manage such areas have a crucial part to play in habitat creation. By its very nature, biodiversity is a broadly-based and integrated web of flora and fauna, although few would dispute that butterflies are one of highest-profile components.
So, what are the requirements for establishing and sustaining butterfly populations? First and foremost, it is a case of basic requirements related to a site’s intrinsic capacity to support and sustain a particular species of butterfly, depending on core factors like climate, topography and environment. For instance, a coastal site in north-east Scotland may be unable to support and sustain a specific butterfly simply because the location is beyond the natural northern limit of the species and/or due to the exposed nature of the site.
That said, environmental and ecological endeavour can go a long way to establish butterfly populations through habitat creation and manipulation. And this is where the expertise and efforts of site managers comes into play by encouraging, and planting, if necessary, trees, shrubs and ground plants which the butterflies need for food, shelter and to successfully complete their life cycles.
Butterflies are dependent on wild plants and usually specific ones at that. The main categories of direct plant dependency for butterflies are:
- Oviposition (egg lay) sites on or near to food plant(s) for the larva (caterpillar) stage, and often the site where a resting stage (pupa or chrysalis) hangs on until the adult butterfly emerges. Pupa is a general term applicable to both butterflies and moths while chrysalis is specific to butterflies.
- Nectar plants as food sources for the adult insect stage (the imago or butterfly).
- Overwintering sites for the egg, larva (caterpillar), pupa (chrysalis) or adult (imago) stages depending on the species of butterfly.
And sometimes vegetation to protect a vital (Viola species) as food plants for fritillary butterflies on open and exposed hillside sites. Success with butterflies may require site managers to be tolerant of plants not generally regarded as useful or desirable and even some regarded as invasive weeds.
Biodiversity is all about building tiers. By starting near to the top with butterflies, a rich array of flora falls into place as the plant-associated profiles of the following three frequent, generally ubiquitous, and easy to encourage butterfly species shows.
PEACOCK BUTTERFLY (AGLAIS IO)
Aglais io (peacock) is a prime example to begin with because the adult butterfly’s form and markings, with brownish-red velvety wings and bearing its own unique badge as ‘peacock eyes’, are so distinct from every other one of our native species. The underside of the wings, however, presents a stark contrast to such splendour with streaks of funeral blacks and browns. This contrast affords a most peculiar effect when the butterfly, sitting on a flower head, alternately opens and closes its wings with a fanning motion.
Moreover, the peacock’s countrywide presence from Cornwall to Scotland, ubiquitous larval food plants and a ‘catholic’ taste in nectar plants means Aglais io can be everyone’s friend. The species overwinters as the adult butterfly in cavities offered by old trees, woodpiles, and equipment sheds. So, make a point of keeping those old cavity-filled ancient and veteran trees alive and standing.
Back in the 1800s, a Mr Doubleday wrote about winter hibernation retreats of the peacock butterfly. “Last winter, some large stacks of beech faggots which had been loosely stacked up in Epping Forest the preceding spring, with dead leaves still adhering to them, were taken down and carted away, and among these were many scores of peacock butterflies.” Subsequent research identifies Henry Doubleday (1808–1875) as the son of an Epping grocer, a most eminent naturalist – ornithologist, entomologist, and lepidopterist – of the time with a particular interest in Epping Forest in Essex.
Peacock butterflies emerging from hibernation are already on the wing in March, visiting and feeding on early flowering nectar plants like ground ivy (Glechoma hederacea). Blackthorn (Prunus spinosa) blossom is another early spring favourite but generally not encouraged in public places due to the tree’s wickedly sharp thorns.
This subsequent plant species requirement will be even less welcome than blackthorn, but if you want peacock butterflies you have to be nice to nettles. Urtica dioica (stinging nettle) is where the olive green, ribbed eggs are laid by the adult peacock butterfly during April and May, in batches on the upper part of nettle plants and specifically on the undersides of young leaves.
The larvae are jet black in colour with white-spot markings and coarsely hairy to the extent that some early lepidopterists describe them as thorny. They feed gregariously on nettle leaves before metamorphosing into a greenish-coloured and well-camouflaged chrysalis which hangs on to the nettle plant until the adult butterfly is ready to emerge. This drab larval affair is in stark contrast to the peacock butterfly’s colourful splendour, but all is forgiven in July, August and September when large numbers of beautiful butterflies, having emerged from the chrysalis stage, eagerly drink the copious nectar of summer flowering plants, with bramble (blackberry – Rubus species) and buddleia (Buddleja species) as two clear favourites.
THE GATEKEEPER BUTTERFLY (PYRONIA TITHONUS)
The gatekeeper butterfly presents ideal opportunities for innovative habitat creation and manipulation. The common name of this attractive brown and orange butterfly originates from a favoured habitat of its number one nectar plant, the common fleabane (Pulicaria dysenterica; plant family Asteraceae). Common fleabane was traditionally found growing around farm gates in favourable soil conditions created by herds of dairy cows poaching the wet ground, and the source of the common name ‘gatekeeper’ for Pyronia tithonus.
So, why not install gates or stiles across public rights of way forcing walkers to stir up wet ground and create conditions favoured by common fleabane and therefore the gatekeeper butterfly? Perhaps purchase and broadcast some common fleabane seed to speed things up.
The gatekeeper butterfly has a wide spectrum of nectar plants including wild marjoram (Origanum vulgare) and bramble. Many woodland owners frown on bramble due to invasive tendencies despite it being a native species. However, tree officers responsible for publicly-accessed woodland value the presence of bramble growing at the woodland edge and along rides to minimise footfall into sensitive areas of the site, while at the same time providing nectar sources for gatekeeper and other butterflies. Creeping thistle (Cirsium arvense; plant family Asteraceae), with its attractive blue/mauve flowers, is another plant species on the gatekeeper’s list of nectar plants. Despite the invasive weed potential of this native thistle, site managers should think again before trying to eradicate Cirsium arvense.
Larval food plants for Pyronia tithonus should not present a problem because they are common grasses including Poa annua (annual meadow grass), Dactylis glomerata (cocksfoot), Lolium perenne (perennial ryegrass) and Agropyron repens (couch grass). Pale yellowish eggs are laid on a blade of grass. They hatch into larvae which feed on these grasses to become bright green in colour and clothed with whitish hairs before metamorphosing into the resting chrysalis stage. So, if you want to encourage the gatekeeper butterfly don’t be too rough on your rough grasses. The gatekeeper overwinters at the larva stage.
The gatekeeper can be confused with other closely related brown butterflies such as the meadow brown, but is ultimately distinguished by the black spot towards the tip of the forewings and uniquely enclosing two white dots. Pyronia tithonus is historically common and widespread in southern England and Wales but has been spreading northwards in recent years.
HOLLY BLUE BUTTERFLY (CELASTRINA ARGIOLUS)
The male butterfly is a pretty, lilac-tinged blue with a narrow black edging on the outer margin of the forewings and a narrow black line on the outer margin of the hind wings. The female is of the same shade of blue, or sometimes much paler. Throughout history, Celastrina argiolus has carried a variety of names. Historically called the ‘blue speckt’, this name was changed to ‘azure blue’ in 1775, only to be changed again to the ‘holly blue’.
Even ‘holly blue’ is not entirely accurate because this small, sky-blue coloured butterfly has two larva broods per year, but only one feeds on Ilex aquifolium (English holly). The late summer/early autumn generation of butterflies lays eggs on Hedera helix (common ivy) so that the hatching larvae have a ready source of food in common ivy’s highly unseasonal flower buds.
The spring generation of holly blue butterflies emerging from the overwintering chrysalis stage lay whitish or bluish green-coloured eggs on the undersides of the calyx (collective name for sepals) of holly flower buds. Hatching larvae feed on the flower buds and later the young green berries of this native, evergreen broadleaf tree. The full-grown larva, with its blackish head and yellowish-green body, metamorphoses into a pale brown-coloured chrysalis.
The adult butterflies emerge in late summer and early autumn when most self-respecting wild plants have done with sexual reproduction for another year, while common ivy is just starting to flower. Come September, common ivy is covered with young, compact flower heads and, having selected one of these, the female butterfly affixes an egg to the underside of the flower-head stalk. Larvae hatch, feed through autumn and metamorphose into the chrysalis stage, the form in which Celastrina argiolus passes the winter. Holly blue butterflies favour tall, stand-alone holly trees, ivy-bound hedges and ivy-clad walls. The holly blue, traditionally regarded as a south of England species, has more recently moved north to colonise parts of the Midlands and northern England including Yorkshire.
BUTTERFLIES AND WEEDS
Weeds are plants considered as undesirable in a particular situation, but this definition of weeds as ‘plants in the wrong place’ is essentially a creation of farmers, horticulturalists and gardeners, and of little relevance to managers of open green spaces and especially those with naturalist ambitions. As this snapshot of three common butterfly species shows, a whole panoply of plants regularly regarded as weeds, including blackthorn, bramble, common ivy, stinging nettles, buddleia and couch grass, are clearly not out of place if you want to encourage these butterflies.
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