Immature stages and the larval food plant of Nacaduba pactolus ceylonica Fruhstorfer, 1916 (Lepidoptera: Lycaenidae) in Sri Lanka

1 Department of Natural Resources, Faculty of Applied Sciences, Sabaragamuwa University of Sri Lanka, Belihuloya, 70140, Sri Lanka 2 Sri Lanka School of Agriculture, Department of Agriculture, Karapincha, Kuruwita, Sri Lanka. 1,2 Nature Explorations and Education Team, No: B-1 / G-6, De Soysapura, Moratuwa 10400, Sri Lanka 3 University of Mumbai, CST Road, Kalina, Santacruz East, Mumbai, Maharashtra 400098, India 4 Postgraduate Institute of Science, University of Peradeniya, Sri Lanka 1 tharakas.priyadarshana@gmail.com, 2 ishararulzz777@gmail.com, 3 sagarsarang53@gmail.com, 4 nayana.wijayathilaka@gmail.com (corresponding author)

The genus Nacaduba Moore, 1881 (Lepidoptera: Lycaenidae: Polyommatinae) is distributed throughout the Indo-Australian region. Its range extends from Sri Lanka to the Fiji Islands (Brower 2008). It represents eight species in Sri Lanka of which two are endemic to the country (MOE 2012a). Although Nacaduba pactolus is wide spread in the tropics of its range, it is an uncommon butterfly in Sri Lanka. The subspecies, Nacaduba pactolus ceylonica, Fruhstorfer 1916 is endemic to the island (MOE 2012b) and listed as near threatened (MOE 2012a). It is the largest species of the genus in the island and the only member which has a characteristic whitetipped antennae (Woodhouse 1949;d'Abrera 1998). The larval food plants of N. pactolus have been recorded from India; Entada pursaetha (Fabaceae) (Bean 1964;Bean 1988), Taiwan; Entada rheedii (Fabaceae) (Hsu et al. 2004)  . We opened the rearing containers twice a day to refresh the air inside. Larval excrement as well as the post-feeding remnants of young leaves, stems and tendrils in the rearing containers were cleaned routinely. They were then wiped clean with a dry cloth and the larvae placed back in their respective containers with a fresh supply of food plant when the plant materials were consumed or no longer suitable.
All observations were made when the rearing containers were cleaned. Larval stages were identified, measured and photographed. Eggs and early instars were measured using Image J, image analysis program (Abràmoff et al. 2004) and a Carrera Precision digital vernier caliper was used to take measurements of later instars and pupae. A Canon EOS 60D digital SLR camera fitted with a Canon EF-S 18-55 mm lens was used to take pictures. The larval food plant was identified using the Revised Handbook to the Flora of Ceylon volume 1 (Kostermans 1980) and by following the Plant Name database, the Plant List (2013) for the updated Latin names. Results: Egg-laying of Nacaduba pactolus ceylonica was observed in Katepola, on 03 April 2015 around 09:40hr and they were laid singly on young leaves Image 1. Early stages of N. pactolus ceylonica. a -Egg; b -First-instar; c -Young second-instar; d -Mature second-instar; e -Yong third-instar; f-k -Mature third-instar; l -Mature fourth-instar. © Authors and young shoot of a woody climber, Entada rheedii (Fabaceae). Also we observed some immature stages (eggs and larvae) near the entrance to the Samanala nature reserve, "Kuruwita-erathna" foot path, in Ratnapura District, Sabaragamuwa Province, on 18 April 2015.
Egg: 0.52 ± 0.02 mm in diameter (n=4), flattened, button-shaped, white to pale yellow coloured, crisscrossed with a network of ridges on the surface (Image 1a). The eggs hatched in three days after they were laid. Upon emergence, larvae consumed the top part of the egg shell.
1 st instar: pale yellowish-green with whitish primary setae occurring dorsally and sub-spiracularly. Anal plate and pro-thoracic shield are diamond shaped and light green (Image 1b); length 1.50 ± 0.01 mm (n=3); molt in three days. 2 nd instar: setae absent; early second instar was purplish-brown with a yellowish light green tinge (Image 1c) and gradually the colour changed to yellowish light green (Image 1d); dorsum of the thorax slightly depressed; length 3.55 ± 0.06 mm (n=3); molted in two days.
Final instar: little or no change in colour (Image 2c -g); length 15.92 ± 0.07 mm (n=5), development time was five days. Parasitoid larvae were observed in a final instar larva (Image 2h) which was collected from the field while the parasitoid larvae were not reared and identified.
Only the 1 st and 2 nd instar larvae fed on leaf buds while other instars fed on young leaves, stems and tendrils. None of the larval stages consumed the green color leaves and they always fed on pinkish-brown color young leaves. Larvae usually preferred stems, tendrils and rachis for resting. Pupation occurred on top of a mature leaf blade. Final instar larvae secreted a silkpad on top of leaf blade, and created a silk girdle across the thorax which attached to the silk-pad. Pupae was a typical lycaenid shape (Image 2i-j) and 10.31 mm ± 0.03 mm in length (n=3). In the laboratory, it took 28 days from oviposition to adult emergence.
During the survey we recorded N. pactolus ceylonica from 10 locations from wet and intermediate zones of the island (Fig. 1) The sea bean, Entada rheedii (previously recognized as Entada pursaetha) is the only representative member of the genus in Sri Lanka (Kostermans 1980) which is distributed throughout the wet zone of the island including river banks and estuaries. Previous studies have identified E. rheedii as the larval food plant of two subspecies of N. pactolus in India and Taiwan (Bean 1964(Bean , 1988Hsu et al. 2004). In Singapore the butterfly larvae feed on a different species belonging to the same genus, Entada spiralis (Tan 2009). So in its known range of distribution, N. pactolus is found to use only one plant genus. So this study confirms the fact that N. pactolus is a stenoligophagous species. Exploring Entada sp. in its range would help to identify the larval food plant in other countries where the butterfly occurs.
In Sri Lanka six other butterfly species also depend on the same plant as their host for larvae, Eurema blanda citrine (Three Spot Grass Yellow), Charaxes psaphon psaphon (Tawny Rajah), Curetis thetis (Indian sunbeam), Rathinda amor (Monkey-puzzle), Jamides celeno tissama (Common Cerulean) and Cheritra freja pseudojafra (Common Imperial) (Jayasinghe et al. 2014) which may compete for resources. But, as E. rheedii is a common wide-spread species, it may not be a limiting factor for this rare subspecies. Not only as a larval host plant but also as a nectaring plant Entada rheedi is an important species for the island butterfly fauna.
The stenoligophagous nature of N. pactolus ceylonica across its natural range will make it vulnerable to extinction if the host plant population declines. This highlights the need for proper conservation planning of rare butterflies that takes into consideration the larval stages and larval food plants.