Gatekeeper

The Gatekeeper, Pyronia tithonus

Gatekeepers galore! After the tension of “the June gap”, the Gatekeeper appears in numbers in early- to mid-July to help signal the “Butterfly Bonanza” of late summer in woodlands, hedges and larger gardens all around the parish. It is a medium-small sized orange-brown butterfly generally seen fluttering over hedges or nectaring on brambles. It is most easily confused with the Meadow Brown, but it is brighter orange by contrast, slightly smaller and more commonly seen in hedgerows, more rarely in open grassy areas.

There is just one generation and their flight period is virtually over in 6 – 9 weeks, by the beginning of September. On average the males emerge before the females. Each individual has a lifespan of only about 3.5 to 8 days.

Description

When viewed from the upper-side, the orange patches on the brownish wings are fairly obvious. The next point to look out for, if you can get close enough, are the two white spots in the dark eye, where the similar Meadow Brown usually (not invariably) only has one white spot.

This is a fairly typical upper fore wing, and this is probably a male, with a sex-brand in this species of a dark smudge in the centre of each fore wing, which the females do not have. On this specific insect there is a greenish wear mark as well, indicating this butterfly has probably been around for a few days already. Males are smaller and darker on average than the females. Both sexes have the clear brown bands around the outer edges of both of the wings, with a neat tiny fringe of hairs at the wing edge, nice and even when fairly fresh.

When viewed from the underside the fore wing is largely orange, with the (generally) two white spots in a dark eye, while the hind wing is a subtler pattern of browns and greys with a characteristic sharply “eared” pattern to the boundary between the darker front and the lighter back, a whitish patch about midway along the top edge of the wing (this sometimes looks like Bart), and several (2 – 5 normally) small white flecks in a semi-circle in the two darker areas somewhat to the rear of the hind wing, all of which helps to distinguish this butterfly from the somewhat larger, browner, duller, but otherwise quite similar Meadow Brown, which has black flecks. In my limited experience the lowest white fleck, if present, tends to show through onto the upper side of the wing as well as the lower, but the upper ones do not, however clearly they appear on the lower side.

Here are several Gatekeeper “faces” – this butterfly often has a “surprised” look on its face:

Distribution

Good places to find Gatekeepers in Hadlow are many of the woodland or shaw edges, often where footpaths enter the woods (- so-called gateways – how the butterfly got its current name of Gatekeeper) and also most of the larger hedgerows rich in bramble or other flowers – the open flowers of brambles if available are particularly popular with Gatekeepers which (for butterflies) have a rather short proboscis! As the Gatekeeper season progresses through August bramble flowers become harder and harder to find, and the butterfly moves on to other flowers. The Gatekeepers share their flowers with other species of butterfly such as Meadow Browns, but only the Gatekeeper carries such relatively bright orange markings overall on an otherwise brown medium-to-small-sized butterfly! The Gatekeeper is absent from the northern half of Britain and most of Ireland, but is very common in the southern half of England, perhaps being the commonest butterfly in Devon and Dorset.

The male Gatekeepers have been found to search, inspect, engage potential mates, and perch (bask and rest) on shrubs or shrub edges significantly more frequently compared with male Meadow Browns, but do move off the shrub edges if they need nectar for example. Similarly, female Gatekeepers perch on shrubs and oviposit at shrub edges more frequently than do female Meadow Browns. This fits in with ecological theory, which predicts that the butterflies with the brighter “eyes” (the Gatekeepers) will tend to be more greatly associated with scrub and woodland.

The ideal combination is a large hedgerow, woodland edge or track with tall wild grasses (the larval food plants) flowering along the base of the shrubs. One such bountiful hedgerow is to the East of the farm reservoir beyond Malt Cottage, reached by footpath from Victoria Road or by footpath via Style Place from Court Lane, but hedgerows of reasonable size along one of the many public footpaths may often have anywhere between one and a hundred Gatekeepers sunning themselves, only to be startled into flight by our approach. Other good footpaths are those between the College’s Equine unit and along the River Bourne, the footpath along the north bank of the River Medway between Hartlake and Tonbridge, the southern half of the Access Trail, the footpaths through the Oxenhoath Estate, and those around Pittswood. The best hedges are of reasonable size, not tightly trimmed every year, have large and diverse headlands with alternate nectar sources, and are not directly neighbouring arable crops subject to spraying, or intensive grassland.

However not all apparently suitable habitat does actually hold Gatekeepers. Research In Belgium confirms this, particularly in comparison with the virtually ubiquitous Meadow Brown. I think that the same is true on a micro-scale in Hadlow. Some hedge sections contain several individuals (small colonies) but others apparently just don’t, despite appearing quite suitable. It is possible that this is due to a relatively slower establishment rate in this species, so that local accidental extinctions are “re-filled” at a slower rate in the complex agricultural matrix, leaving some habitat opportunities temporarily unfulfilled. Alternatively, it may just be that habitat is not always suitable at any one time, and we simply do not understand why not, or indeed the factors that are required for good habitat. Shelter may be more important for many butterfly species than we have realised to date, and we still know little of the requirements for nectar sources, larval food-plant requirements, temperature and rainfall factors, to mention just a few.

In and around woodlands Gatekeepers are also often found feeding on the brambles, knapweeds, thistles, fleabanes, ragworts and other fairly open flowers close to the shrub edges where they have their territories. a good example is along the sunnier tracks of Dene Park which is completely Public Access. (From the Puttenden Road car park take the main track and either turn left along the track into Knight’s Wood (best), or check out the first section of the track straight ahead of you). They are also common along the sunnier footpaths through and around the edges of Clearhedges Wood in West Peckham Parish, which you can reach by crossing the road CAREFULLY. When flowers are scarce Gatekeepers may feed on the honeydew available higher up in the canopy in woodlands;

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The Gatekeeper is thought to be quite a “static” and “home-loving” butterfly despite being a good strong flyer, and apparently spending more time on the wing than other species such as the Meadow Brown. The colonies are fairly self-sufficient, and do not appear to intermingle much in comparison to other species. When artificially moved or displaced the individual butterflies have a preference for returning to their familiar haunts, even tending to cross railway lines to return to their “own” side! However on occasion short and long distance movement does occur, so colonisation of new sites clearly can occur on occasion.

Each hot-spot therefore depends on its own habitat quality being maintained, although Gatekeepers will often briefly also use resources outside their home-range and these may indeed be vital to them! The butterflies found in gardens generally come from hedgerow colonies nearby. However there are large patches of the parish where there is no habitat close by, and the butterfly is almost never seen – for example it is only rarely seen in my garden although my house backs onto the hedgerows along School Lane and there is a small colony located in a sunnier section about 200 m away. It is also extremely rare in London, even in the parks, again perhaps because it finds it difficult to spread where suitable habitat is rare and well-separated by inhospitable areas. 

Life cycle

The Gatekeeper has good years and bad years. Good years tend to be when the previous August was generally hot and dry, and 2014 was an exceptionally good year after the great summer of 2013! In addition, following a mild and wet winter (just like the recent 2013/2014 winter – an example of a “positive North Atlantic Oscillation”) emergence tends to be earlier because of the warmer weather. 

Over July and August the Gatekeepers mate and the females lay their eggs at the base of shrubs close to, or occasionally on, the sheltered grasses on which the caterpillars will eventually feed. The caterpillars first of all eat their own (protein) eggshell, then start eating the tender sheltered grass tips. Once the weather gets colder, the caterpillars hibernate (after the first of four moults) in the bases of the grass plants, resume feeding in March or April of the following year, and eventually form a chrysalis in June-July.

The white specks on the underside vary

It would be very interesting to find out if the white speck pattern variability is linked in any way to the individual colonies. E.B. Ford did some ground breaking work on the dark specks on the underside of Meadow Browns, on islands of different size versus the mainland which founded some fundamental ecological theory as a result.

This is an individual from the hedgerow to the east of the reservoir with nearly three white specks, one at the top and one half way down the wing, with perhaps a third below it.

This is an individual from the hedgerow colony to the northeast of Hope Cottage. It only has two white underside specks, this time both very high on the wing:

This is a  much more worn individual, with the start of a third speck lower on the underwing.

This is another individual, this time more clearly with three white flecks.

This is an individual from a hedgerow on the other side of the Bourne, between the college’s equine unit and The Mill, with four white specks:

This individual from the nearby Tudeley Woods car park has four white specks, but unusually with three on the higher side and only one on the lower. As in most individuals, each speck seems to generate a brownish surround sometimes with a very dark immediate rim. Interesting developmental genetics!

Photographed in Tudeley Woods, Capel, the 17th of July, 2014.

Oddities

The scales of a butterfly or moth are extremely varied in form and function. Some are specialised to be upright and fringed, the better to release the scent pheromones, such as those found in the male “sex-brands” mentioned in this and previous blogs, or in tufts in some other species, but some other scales are developed into extremely long hairs, as seen on the body and in these patches on the wings in this Gatekeeper. This perhaps explains why where on the insect you see the hairs you get fewer coloured scales (whether genuine pigment colours or refractive colours). The function of hairs is presumably primarily insulation – but I wonder how the presence of the hairs affects other things such as flight, and drag for example.

The hairs on the under-wing appear more vertically orientated away from the wing – is this gravity or not? It doesn’t look like it. If it’s a deliberate difference, then could it be for aerodynamic purposes?

Parasites

Like other species of butterflies, the Gatekeeper may be infested by bright red mite larvae of the species Trombidium breei or perhaps as originally identified Trombidium poriceps. About two thirds of individual butterflies are likely to be temporarily infested with mites at some stage in their short lives, perhaps for a day or so, males slightly more than females.  The mites come off the butterflies onto new flowers about as easily as they hitch rides, and although they do suck their host butterfly’s blood, they don’t seem to have a significant effect on their host’s behaviour or survival.

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