Knysna Skolly (Thestor brachycerus brachycerus)
KNYSNA SKOLLY (Thestor brachycerus brachycerus)
The Knysna Skolly, Thestor brachycerus was discovered at Knysna by Roland Trimen, the discoverer of the Brenton Blue (Trimen 1883). Whilst it was originally very abundant, it has become very rare due to its very narrow habitat specificity, small geographic range (only known from Knysna) and the loss of habitat due to property developments and farming activities within this range. A declining population trend has been witnessed since the 1960’s and this has accelerated since 1990, and Thestor brachycerus is now facing extinction if there is no human intervention. Our ability to execute such an intervention has been considerably improved during the same period from the knowledge gained during successful efforts to save the Brenton Blue butterfly, which have resulted in an increase in its population to sustainable levels. A similar programme of research followed by informed habitat management is proposed for Thestor brachycerus. A by-product of this project will be an improved knowledge of the life histories and habitat requirements of butterflies of the Thestor genus, which is endemic to South Africa.
IUCN RED-LIST STATUS
Originally Red-Listed as INDETERMINATE (Henning & Henning 1989). Has been upgraded to CRITICALLY ENDANGERED in the latest revision of the Red List, following investigation of its taxonomy and distribution over the last 15 years (Henning et al. 2009), and confirmed in the South African Butterfly Atlas (Mecenero et al., 2013), and in the South African Lepidoptera Conservation Assessment (Mecenero et al., 2015).
CURRENT SCIENTIFIC KNOWLEDGE
Discovered at Knysna by Trimen. The only place they have been found in modern times is at, and to the east of, the Eastern Heads, which may have been close to Trimen’s original locality.
The inland populations were found from the Eastern Heads, Knysna, extending eastwards to the Pezula Golf Estate. Also found along the coast close to sea level from Coney Glen beach for about 1km eastwards.
Inland sites The Pezula site is on a north-west facing slope with an inclination varying between 10 and 15 degrees (12.5° average). The substrate is medium grained acidic to neutral regic sands, with occasional rocky outcrops. The vegetation was classified by Mucina and Rutherford (2006:671–2) as Knysna Sand Fynbos (FFd 10), a Critically Endangered vegetation type with almost 70% already transformed and only 5% conserved (Rouget et al. 2006). Important plant taxa recorded on the site by Bazin & Edge (2015) were Seriphium plumosum, Erica canaliculata, Anthospermum aethiopicum, Helichrysum cymosum, Restio leptocladus, Tristachya leucothrix, Restio triticeus, Leucadendron salignum, Searsia lucida and Phylica axillaris. The vegetation had been transformed to some extent by past agricultural practices (e.g. low intensity cattle grazing) and artificial fire regimes (there were too many regular burns to promote grazing in earlier years and then fire was excluded since the mid-1990s) (Edge, 2005). In the absence of megaherbivores and fire the shrub elements of the vegetation were becoming more and more dominant, rendering the habitat at ground level too cool and unsuitable for T. brachycerus and its Anoplolepis custodiens host ant (Bazin & Edge, 2015).
The coastal sites are on fairly level ground with the highest spring tide mark only 5m to the south. The study site is situated on a littoral terrace (the Swartkops horizon) formed during the Pleistocene around 40–45 000 years ago. The extant substrate consists of a jumble of rocky boulders and stones with thin soil cover in places. The abiotic conditions are severe with high salt deposition (exposure to salt spray and seawater splash), extremely high insolation, and high levels of UV radiation. Nutrient input by birds, and constant deposition and erosion of fine soil by relentless winds are ecologically critical. The vegetation was classified by Mucina and Rutherford (2006:685–6) as Cape Seashore Vegetation (AZd 3). The study site is the south coast variant with rocky marine shelves below rocky cliffs. Important taxa recorded by Bazin & Edge (2015) were Stenotaphrum secundatum, Gazania rigens, Delosperma patersoniae, Hypoestes aristata, Asparagus aethiopicus, Eriocephalus species, Tarchonanthus littoralis , Carpobrotus delicosus, Drosanthemum species and Limonium scabrum. The site has been traversed regularly by fishermen and this has caused some littering; otherwise it is in fairly natural condition.
Adults fly from late November to the end of February.
Male territorial behaviour
Male butterflies exhibit territorial behaviour and spend long periods of time perched in one spot, which they returned to over a number of subsequent days. At Pezula the perching sites selected by males were usually low, dead plant material whereas at the coastal sites perches were mostly on Stenotaphrum secundatum or bare rock. Territorial males were observed leaving their perching sites to engage in aerial interactions with conspecific males. These interactions involved the butterflies circling around one another while moving away from the perching site; one butterfly would return to the perch after chasing the other off.
When a female approaches a territorial male a courtship dance ensues, during which the butterflies whirl around each other for a few minutes. The courtship dance is not always followed by mating. During copulation the male and female position themselves alongside each other; and the male curves the tip of his abdomen round to meet the tip of the female’s abdomen and they become joined.
Female oviposition behaviour
Oviposition is very rapid and eggs were laid singly. After a second or two the female then flies a short distance to another plant and lays again. It appears that oviposition sites are selected at random as there is no pattern in plant species choice and the distance between depositing eggs also varies. The majority of eggs are laid on the underside of plant leaves, however sometimes oviposition occurs on the plant floret. Occasionally up to three eggs are laid on the same plant but usually the female moves to another plant after having laid only one egg. The eggs were laid between 10cm and 130cm from the ground at the Pezula colony and between 5cm and 75cm at the coastal sites. This behaviour strongly supports the hypothesis that the larvae of T. brachycerus are aphytophagous (i.e. that they do not feed on plant material).
Early stages and ant associates
Clark & Dickson (1971) described an egg and first instar larva of T. brachycerus, which died fairly soon after hatching. Since there have been no further studies of the life history of this species, one can only draw analogies from knowledge of its congeners. The only ant associate of Thestor larvae so far recorded is the common pugnacious ant Anopolepsis custodiens Smith (Formicinae) (Heath and Claassens 2000; Heath et al.2008; Williams 2014). All Thestor larvae studied by the authors listed above appear to be completely aphytophagous. The early instars of some species have been found in association with, and preying on, coccids and psyllids (Hemiptera: Homoptera) – T. protumnus aridus and T. basutus basutus (Clark and Dickson 1960, 1971) and T. basutus capeneri (Williams and Joannou 1996). The later instar larvae of T. yildizae have been observed being fed by trophallaxis (Claassens and Heath 1997). In further studies on T. b. basutus Claassens and Heath (2003) found that ants were feeding the final instar larvae by trophallaxis, and the larvae were preying on host-ant eggs or larvae and feeding upon organic detritus within the nest.
Bazin & Edge (2015) recorded A. custodiens as the most ubiquitous ants at both the inland and coastal sites, followed by Monomorium australe, Lepisiota capensis, Crematogaster peringueyi and Pachycondyla caffraria.
In the recent revision of Thestor (Heath & Pringle 2004) the Knysna population was recognised as a unique taxon at subspecies level, but more recent DNA work indicates that it should be separated from Thestor dukei, its closely related montane cousin, at species level. T. brachycerus is therefore only known from the Knysna populations.
Colony losses over the last 30 years
The strongest colonies of the Knysna Skolly were close to the Knysna Eastern Heads (Trimen 1883), and have been destroyed by building activities there since the 1980s. The last colony there was lost in 1996, when the farm owner built a house on the exact centre of this colony. A strong colony was found on Sparrebosch Estate, where a golf course development was planned in 1996, but despite the developers being made aware of this in the EIA report, it was exterminated by the building of the access road to the (now Pezula) Golf Estate in 1998. A small colony on the Duthie farm on the hills overlooking the Woodbourne pan was eliminated by the change to sheep farming in the early 2000s. Overgrazing with sheep and regular burning similarly weakened the moderate sized colony on the easternmost portion of the Duthie farm. This colony was further threatened by what became the Fernwood development. The developers were instructed in their approval ROD to put aside an area for the butterfly, which they did, but failure to manage the site has led to the loss of this colony. Another small colony at Pezula (opposite the hotel) was destroyed during the building of the golf course. The strongest colony now remaining on Pezula is below the sixth fairway of the golf course, and the Pezula owners have undertaken not to disturb it, and to manage the site in accordance with the recommendations of butterfly experts. In December 2004 a small colony was discovered during the scoping study for a development proposed in the south west corner of Pezula, just outside the proposed development area, and a few individuals survived there until the site became overgrown in the absence of regular grazing or fire.
Threats to the remaining populations
The inland populations are now all extinct because the landowners refused to allow fire or grazing and regular cutting was deemed to be too expensive. The coastal populations are relatively secure.
The following research on the butterfly’s life history and ecological requirements was completed and the results presented in Bazin & Edge (2015):
- Adult butterfly territorial and mating behaviour.
- Female butterfly oviposition (egg laying) behaviour.
- The vegetation occurring in the habitat of the butterfly, both in terms of species composition, but also structure and age.
- Sampling and identification of ant species assemblages.
- Monitoring of the population of the butterfly at its various locations.
The following research needs to be pursued:
- The behaviour of the larval stages of the butterfly, including its association with ants and Homopterans.
Now that the populations on the Pezula Golf Estate have been rendered extinct by mismanagement, the only hope of restoring an inland (fynbos) population lies further to the east. The north-facing slopes in the north-east corner of the Pezula Private Estate which were covered by plantations have been rehabilitated, and suitable habitat could be created, providing the owners were cooperative. Plantations extend eastwards as far as the Harkerville State Forest and most of the way to Plettenberg Bay. Investigation has not revealed any undisturbed north facing fynbos slopes so far but searches for such habitat should continue.
Another possibility is to search on the north facing slopes of the Brenton peninsula, where the odd individual has been recorded.
Searches for more coastal colonies as far east as the Noetzie River have been conducted, but potential sites exist (but are difficult to access) still further eastwards along the coast towards Robberg.
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Mecenero, S., Ball, J.B., Edge D.A., Hamer, M.L., Henning, G.A., Krüger, M., Pringle, E.L., Terblanche, R.F., Williams, M.C. (2013) Conservation assessment of butterflies of South Africa, Lesotho and Swaziland: Red List and Atlas. Saftronics (Pty.) Ltd., Johannesburg & Animal Demography Unit, Cape Town.
Mecenero, S., Edge, D.A. and Staude, H.S. 2015. Southern African Lepidoptera Conservation Assessment (SALCA). Metamorphosis 26: 116–122.
Trimen, R. 1883. Descriptions of twelve new species of South African Rhopalocera. Transactions of the Entomological Society London 1883: 353.
Williams, M.C. 2014. Afrotropical Butterflies 13th Edn.. Lepidopterists’ Society of Africa, Johannesburg, South Africa.