Nest Architecture and Nesting Site Preference of Tetragonula iridipennis Smith in North-Western Plains of India

The study area, the Punjab Agricultural University, Ludhiana, is situated at 30°–56′N and 75°–52′E at an elevation of 247 m above mean sea level in the central plain agro-climatic zone of the Punjab state. Most of the area is cultivated or under permanent structures. The main agricultural crops are rapeseed and mustard, cotton, maize, sunflower, linseed, sesamum and fodder crops as clover and pearl millet, and pulses as pigeon pea, chickpea and green gram. Vegetable crops grown in the area are chiefly cucurbits (cucumber, bitter gourd, squashes, sponge gourd, ridge gourd etc.), crucifers (cauliflower, cabbage, radish, turnip etc.), solanaceous crops (tomato, brinjal, capsicum and chillies) and umbelliferous (carrots and fennel). Horticultural plantations like pears, peaches, plums, guavas, gooseberries, kinnows and mangoes, and landscape plantations like mountain-ebony, golden shower, flamboyant tree, silver oak and dinner plate tree.

The mean maximum and minimum temperatures of the area fluctuate considerably during summer and winter seasons. The long term weekly mean maximum and minimum temperatures of Ludhiana were 39.7 and 5.4°C, respectively. However, on certain days, the maximum temperature reached 45°C and the minimum 0°C. The average rainfall was 400–1300 mm with approximately 80% of the rainfall received during July to September (Kaur et al., 2016).

T. iridipennis colonies of were searched in and around the PAU, Ludhiana campus between 2013 and 2015. Search operations were conducted in various localities following an all-out search method through ocular vision (Sheetal & Basavarajappa, 2009). Data on various variables as enlisted by Roubik (2006) for the study of nesting biology of stingless bees were recorded.

Regular surveys were done in the study area and nests of T. iridipennis were searched visually around human-made structures including residential buildings, educational buildings and roadsides and natural vegetation including trees and shrubs. The data was also recorded on nesting in either brick or stone walls.

Data was recorded visually on twenty-eight different sites, walls of residential buildings, accommodating 48 live colonies of stingless bees to know the aggregation of T. iridipennis colonies.

The height of a colony was recorded with the help of a measuring tape, but when a colony was too high to observe, black markings were recorded to locate the colony (Fig. 1). These black markings, usually present on brick walls, were associated with the presence of a colony. After that, the location was ascertained through the observation of bee movement. The height of such colonies was measured by counting the number of brick lines and multiplying it by the average height of a brick line. Observations were also taken on the occurrence of T. iridipennis colony with the ant or termite nests.

Fig. 1

Nest orientation was recorded using a standard compass. The direction of the opening of the entrance tube of each colony was recorded.

The observations of an entrance structure’s, shape, colour and material were recorded. The measurements of the entrance tube’s length and orifice were also recorded. A total of twenty-six colonies (only those which were in reach) were observed. Observations were also recorded of the number of entrances per tube and the presence of any defensive structure deployed at the entrance.

The number of guard bees present at the entrance gate during different time intervals of a day was also recorded and then averaged.

The nest architecture of only those colonies present in the trees was studied because it was not possible to break open brick or stone wall with a nest inside. Four colonies were retrieved from the mulberry trees. The hollow stems containing T. iridipennis colonies were cut and opened carefully with the help of chisel and saw. Such observed aspects of nest architecture as the placement of brood and food stores, size and shape of brood and food cells, pillars size and shape were recorded. Afterwards the whole content of a nest was transferred in a mating nucleus of Apis mellifera Linnaeus.

A layout was made depicting the placement of the colonies. Monthly observations were recorded for a year to know the overwintering success. This was ascertained only through the recording of bee activity at the entrance.

The collected data were compiled for various variables and wherever necessary. A general descriptive analysis was performed while the Z-test was employed to determine variations in colony characteristics and dimensions.

Nests were usually concealed inside the hollows of tree trunks or cracks in the cemented brick/stone walls of residential and educational buildings (Fig. 2). The nesting habits of sixty stingless bee colonies were observed in Ludhiana. Of these, eight colonies (13.33%) were found in the hollows of tree trunks.

Fig. 2

Hollow Morus sp. tree trunks. (Fig. 3) were most preferred (07 colonies observed) while only one was observed in a Cassia sp. tree trunk (Fig. 4). A statistically higher number of colonies (Z-test: P=0.0001), fifty-two colonies (86.67%), were observed nesting in walls, and of these ten colonies (19.23%) were observed in stone walls and the remaining (80.77%) in brick walls.

Fig. 3

Fig. 4

Data revealed that at the majority of sites, there was a single bee colony (13 sites; 46.44%), followed by two colonies per site recorded at nine different sites (32.14%). A few sites had three colonies (03 sites; 10.71%), whereas two sites had four colonies, each (7.14%). At one site, there were more than four colonies (3.57%). Significantly higher per cent aggregation (78.57%; Z-test P=0.0008) was recorded for combined aggregation of one and two colonies as compared to others. Hence, mostly one or two bee colonies were established per site. The minimum and maximum distances the between two nearest colonies were 12.5 cm and 900 cm, respectively.

No bee nest was observed associated with termite or ant or wasp colonies. The lowest height of the bee nest observed from the ground was 0.75 m. The modal value for this parameter was 3.50 which showed that most of the T. iridipennis colonies were located at 3.50 m height, though these were spotted from 0.75 m to 6.32 m in height from the ground. The average height of the colonies inhabited in natural cavities in tree trunks was 1.85 m (0.75–4.80 m), whereas colonies inhabited in walls were observed at the mean height of 3.60 m (1.20–6.32 m).

Data on the orientation of the bee colonies revealed that significantly the higher number of colonies (34; 54.67%; Z-test, P=0.02) preferred making a nest entrance facing east followed by those who made an entrance facing west (13; 21.67%). The rest of pairs showed the values at par with each other. South-facing and north-facing entrances were favoured respectively by only 15.00 and 6.67% of colonies.

Only a single distinct entrance tube per colony was observed in all the colonies but its size and shape varied among the colonies. Observations of twenty-six easily assessable colonies revealed that the colony entrance was made up of resin. Although the newly built entrance was soft and light brown in colour, it retained its shape in various seasons. The colour of the entrance tube turned darker and became rigid with the passage of time. The entrance tube orifice was somewhat circular in only a few cases, and elliptical in the majority of them. Nocturnal closure of the entrance, mentioned by Roubik (2006) as a defensive character prevalent in Tetragonisca sp. in which the entrance was closed at night by a network of fine thread, was not observed.

Data showed that seven colonies (26.92%) nested in brick/stone wall structures lacked a distinct entrance tube which appeared as a rim of less than 2 mm height. The remaining colonies were noticed to have a distinct entrance tube with the average entrance tube dimensions (longer side diameter × shorter side diameter × protruding tube length) of 11.62 × 11.73 × 14.04 mm. Four colonies were observed to inhabit hollow tree trunks, and the average dimensions of the entrance tube (longer side diameter × shorter side diameter × protruding tube length) were 17.00 × 13.00 × 35.80 mm.

Concealment of the nests inside tree trunk or wall cracks, nest aggregation and a sufficient height above ground may provide defence against robbers and enemies. In addition, guard bees (average 7.5 bees) were also observed at the entrance gate. No defensive biting behaviour toward the observer was observed, as the bees were timid and retreated within the entrance upon mild disturbance. However, when a colony of T. iridipennis was cut open, these bees showed defensive behaviour by grappling the robber A. mellifera bees and biting the soft tissues of other intruders.

The architecture of the colonies nested in hollow tree trunks (mulberry) were studied when the hollow mulberry tree trunks were carefully opened longitudinally with the help of a large-toothed saw and chisel (Fig. 5). The hollow tree trunks were found to be lined with a thin-layer of batumen. Brood cells, honey and pollen pots did not follow a set sequence inside the cavity. In one of the colonies, brood cells were recorded just behind the entrance gate, whereas in another colony, pollen pots were present next to the entrance. However, the brood area in all the colonies was surrounded largely by pollen pots and a few honey pots. Colony length varied between 17.9 and 89.0 cm corresponding to colonies IV and I, respectively (Tab. 2), while colony IV had the smallest size as there was no queen bee or brood there. Pollen pots in these colonies occupied 10.0 to 42.0 cm of cavity length, but the thickness of the rind surrounding the pollen area was 3.0–6.5 cm. Honey pots were present in a cavity of 4.4 to 47.0 cm length. The cavity diameter ranged between 3.0 and 7.2 cm and rind thickness surrounding the cavity between 3.5 and 7.5 cm. The shortest brood length in cavity was 12.5 and the longest 22.0 cm; the diameter of the latter ranged between 4.0 and 10.0 cm. The plant rind surrounding the brood area was between 6.0 and 8.0 cm thick.

Fig. 5

The mean dimensions (length × width) of brood cells, honey pots and pollen pots all differ significantly from one another- 2.79 × 2.62 mm, 5.33 × 4.52 mm and 6.93 × 5.19 mm, respectively (Tab. 3). The lower most layer of brood cells was supported by the pillars made of soft cerumen, which measured 2.83, 0.91 and 1.46 mm in height, width and base, respectively. On these pillars, the cells were joined together with spaces in between (voids) as a free passage for the workers. The average distance between two distant cells separated by void was 1.96 mm (1.01–3.14 mm).

The sixty colonies were observed weekly for bee activity at the entrance gate during the winter season prior to the onset of autumn and spring seasons, starting in November and up to March. All the colonies, whether in brick/stone walls or in hollow tree trunks, overwintered successfully. Foraging by the colonies ceased during the second week of December and resumed during the second week of February when mustard (Brassica juncea and Brassica campestris) was in bloom. During the winter period from the end of December to the first week of February, the colonies appeared to be deserted and almost no foraging activity was recorded. However, on some sunny days in the last week of January bees were observed at the gates of some of the south-facing colonies exposed to sunlight. Previously, stingless bees had been thought to be generally accustomed to only a tropical climate, but their existence in the Punjab subtropical conditions with extreme summers and winters revealed that this species adapts to extreme climatic conditions and cease foraging only in extreme winters.