1. Introduction
Managed bees are considered the most efficient pollinators throughout the world, but wild bees have also received considerable attention for the last few decades due to their high pollination efficiency and because they provide equivalent services to those of managed bees [
1,
2]. Sometimes wild bees also enhance the pollination services provided by managed bees through their behaviors [
3,
4]. Among 20,000 species of bees, the majority of the species are wild [
5]. The population of bees is declining throughout the world, ultimately threatening the productivity of major crops, vegetables, and fruits [
6,
7]. Several factors have been attributed to bee decline, e.g., habitat degradation, climate change, intensive use of pesticides, predators, and parasites [
8,
9,
10]. The availability of suitable foraging resources and nesting habitats helps minimize the decline [
11,
12,
13,
14].
The family Megachilidae, including more than 4000 species, occurs throughout the world [
15,
16]. Within Megachilidae,
Megachile Latreille, 1802 is the most diverse genus, comprised of 32 subgenera and 431 species only in the neotropical region, along with several unidentified species [
16,
17,
18,
19,
20]. Numerous species of
Megachile are efficient pollinators [
21,
22,
23,
24,
25,
26,
27] and only
M. rotundata could be artificially reared and conserved on a commercial basis [
28,
29]. In Pakistan, 18 species of the genus
Megachile are known [
15], yet no information is available on the nesting biology and ecology of megachilid bees.
Megachile species are solitary and highly adaptive and build their nests in pre-existing cavities, e.g., wooden logs, hollow stems of bamboo and roses, burrows in the soil, cracks and crevices, and slits in rocks or manmade structures [
16,
30,
31,
32,
33]. For the construction of their brood cells, female
Megachile spp. use a variety of materials, e.g., leaf pieces, flower petals, mud, pebbles, and a combination of resin and salivary material [
16,
30,
34,
35,
36,
37,
38,
39,
40]. A recent study reported the use of plastics from wrappings, flags, and bags [
41]. This behavior applies to some species, but it is not a general behavior of all
Megachile species. Poor knowledge about nesting biology is the main barrier to utilizing the diverse megachilid bees as a management tool for pollination services.
Each nest consists of a linear series of brood cells with pollen provisions (nectar and pollen mixture) packed by the female [
42]. The female lays eggs on pollen provisions and finally closes the cell with pieces of leaves [
43]. Once the first cell is fully completed, the females repeat this process several times from the closed to the open end of the nesting cavity. When the nest is filled with cells, they close the nest from the outside with masticated leaves or mud to protect their offspring [
42]. Depending on their resources,
Megachile bees can be oligolectic [
44] or polylectic [
16,
45]. Most research has been conducted on the nesting biology of leafcutter bees [
46,
47,
48,
49,
50,
51,
52,
53], while resin bees are poorly understood [
42,
54,
55,
56,
57].
By using bee hotels or trap nests, the nesting biology and ecology of bees are studied for various purposes, i.e., the study of nest architecture [
50,
58,
59], natural history [
60,
61,
62,
63], evolution [
16], crop pollinators [
64], population monitoring and community structure [
13,
65], bioindication and recording changes in habitat type [
66,
67,
68] and as a tool in conservation [
59,
69] and quantitative ecology [
70,
71,
72]. The latter includes the quantification of multiple trophic interactions between bees, wasps, their food objects, and natural enemies [
73].
Megachile (
Callomegachile)
cephalotes is a solitary bee that is widely distributed in Sindh and Punjab, Pakistan [
15]. From Pakistan, it has been reported as an effective pollinator of
Grewia asiatica [
25,
26]. The genus
Grewia has 140 to 150 species, of which only
G. asiatica is of commercial importance as a fruit crop in subtropical and tropical regions [
74,
75]. Little is known about the nesting biology and ecology of this bee [
54]. The aim of this study was to provide information about the nesting biology (nest architecture and pollen types used) and ecology (seasonality and plant species providing pollen and nectar) of
M. cephalotes for the first time in Pakistan. This nesting behavior differs from that observed in other parts of its range and can provide the basis for comparative studies.
3. Results
A total of 242 nests (bamboo sticks = 184, wooden blocks = 58) of
M. cephalotes were collected from January 2020 to December 2020 at three locations: CIDS, ARF, and FC. The maximum number of nests was collected from CIDS, followed by the FC and ARF (
Figure 2).
The first occupied nest was seen on 26 March 2020. The males of
M. cephalotes emerged earlier than the females. The emergence started in the last week of April with a sharp increase until July and then a gradual decline until September. We did not observe any emergence from October 2020 to February 2021. These five months represented the hibernation period of
M. cephalotes. The emergence of these hibernated populations started again in the first week of March 2021. This showed that
M. cephalotes remained active from March to September, comprising the spring and summer seasons (
Figure 3 and
Figure 4).
For the study of nest architecture, five completely occupied wooden block nests were collected in May 2020. These nests were similar in length and diameter. The number of cells ranged from 7 to 8 with minimum and maximum cell lengths of 1.2 and 2.3 cm, respectively (
Table 1). In each nest, all the cells were constructed in a linear series and aligned horizontally. The construction of the first cell started at the base of the nest (
Figure 5). Before constructing the first cell, female
M. cephalotes collected resin and deposited it at the base of the nest, making a thick layer. The brood cells were slightly rounded at the base and elongated. The female made several trips to collect pollen and nectar to provision the brood cells in the form of pollen lobes at the cell base. After provisioning the first cell, the female laid a single egg on the provision mass. After oviposition, the female made more trips to collect resin, which was used for cell closure. They made seven to eight such cells in a single nest. They also left a space (a vestibular cell) ranging from 1.4 to 2.5 cm long between the outermost brood cell and the nest entrance (
Figure 1). The nests were then plugged with mud or animal dung at the entrance (
Figure 5).
There was a statistically significant difference in the development period of male and female
M. cephalotes. The males became adults earlier than the females, i.e., after 65.30 and 74.78 days, respectively (
Table 2). There was no impact of the three locations on the sex ratio of
M. cephalotes. The sex ratio of
M. cephalotes was significantly biased toward females at all three locations (
Table 3).
Grewia asiatica was the predominant pollen grain species found in the brood cells of
M. cephalotes, followed by
Alhagi graecorum and
Parkinsonia aculeata. Among all plant species, the minimum number of pollen grains was found for
Rosa indica (
Table 4).
The floral host plants of
M. cephalotes are presented in
Table 5. Females of
M. cephalotes visited six plant species. The maximum visits were recorded on
G. asiatica, followed by
A. graecorum,
P. aculeata, and
Cajanus cajan (
Table 5). Resin-producing plants visited by
M. cephalotes are presented in
Table 6.
Parasitoids
Adult parasitoids of three species emerged from 242 nests of
M. cephalotes. Two belonged to the order Hymenoptera, i.e.,
Euaspis carbonaria (Megachilidae) (
Figure 6a) and
Coelioxys sp. (Megachilidae) (
Figure 6b), and one belonged to Diptera, i.e.,
Anthrax sp. (Bombyliidae) (
Figure 6c).
4. Discussion
We reported the nesting biology and ecology of
Megachile cephalotes for the first time in Pakistan. In the present study, the maximum number of nests was collected from CIDS, followed by the FC and ARF. CIDS consists of a natural semi-desert landscape with abundant nesting cavities (cracks or holes in mud walls, hollow tree branches, and bamboo or reed sheds) and floral resources. The FC is mostly covered with
G. asiatica fields that provide adequate nectar and pollen for bees. High species richness and an abundance of flowers usually favor the species richness and abundance of bees [
80,
81]. Apparently, there are factors other than floral availability that can limit wild bees. Empirical evidence shows that nesting resources affect the abundance of bees. There is a need to study how the availability of natural nesting resources affects solitary bee populations [
82,
83,
84] since this resource is also essential for bee existence.
The results of the present study showed that
M. cephalotes remained active from March to September, comprising the spring and summer seasons. The data over several years suggest that solitary bees exhibit marked spatiotemporal fluctuations in their abundance and diversity [
16]. The species of the Megachilini tribe are reported to have two generations a year in northwestern India, and emergence occurs from late February or early March until May and again at the start of August until November [
85]. Kunjwal et al. [
86] reported that in India,
M. cephalotes is multivoltine by nature and remains active from March to December. Moreover, they recorded the peak activity of
Megachile spp. two times throughout the year: from March to May and from October to November. Rauf et al. [
14] also found that
M. cephalotes remained active from March to November in Punjab, Pakistan. Kumari and Kumar [
85] reported gregarious nesting and foraging behavior in
M. cephalotes that could be helpful for pollination. Depending on the locality and resources, Megachile bees remain active during the hottest months of the year [
16]. Several seasonal and regional factors might affect the voltinism in Megachile bees. For example,
M. rotundata is univoltine in its native range in Eurasia but bi- or multivoltine in North America, where it was accidentally introduced in the late 1940s [
87,
88,
89,
90]. Hence, there is a need to study these factors thoroughly in order to determine their effects on voltinism.
In the present study, female
M. cephalotes construct their nests in bamboo sticks and wooden blocks with lengths and diameters of 15–16 cm and 7 mm, respectively. This species preferred to construct its nests in bamboo sticks with diameters ranging from 8 to 10 mm [
14] and lengths from 8.2 to 18 cm [
54]. In our study, females of
M. cephalotes constructed 7 to 8 brood cells with minimum and maximum cell lengths of 1.2 and 2.3 cm, respectively. Previously, the nesting biology of this species had been described in India [
54]. They reported that females construct 7 to 12 brood cells in castor sticks with minimum and maximum cell lengths of 1.0 and 1.8 cm, respectively. The number of brood cells constructed can vary depending on the sex ratio, nest length, and age of the female provisioner. Cavity nesters typically provision multiple nests. As a female ages and approaches mortality, cavity nesters tend to build fewer cells per nest since if she dies before completing the nest closure, the cells will be unprotected.
In the current study, the vestibular cell was recorded in each nest with lengths ranging from 1.4 to 2.5 cm. Many studies have reported that the majority of
Megachile species made a vestibular cell in each nest [
50,
53,
57,
91,
92]. In the present study, female
M. cephalotes constructed their brood cells exclusively using plant resin. Contrarily, Gupta et al. [
54] found that this bee solely used mud for its nest construction. Species of the subgenus
Callomegachile mostly collect plant resin but sometimes also collect mud for their nest construction, hence the name resin bees [
93]. Plant resin is a versatile material that can easily be shaped when fresh and structurally rigid when hardened, and it can be used as a nest substrate or to bind loose structural or camouflaging materials for the construction of brood cells. Resin is also waterproof, which permits moisture regulation in the nest, and some resins display potent anti-microbial properties [
94,
95,
96,
97,
98,
99].
The sex ratio of
M. cephalotes was significantly biased toward females at all three locations. Our results are in agreement with those of Torretta et al. [
91], who found that the sex ratio in
M. gomphrenoides was female-biased. Contrarily, few studies have found that the sex ratio of
Megachile spp. is male-biased [
51,
100]. Nest length, availability of floral resources, and flight distance from the nest to floral resources are the major factors that affect the sex ratio of bees. In short nesting cavities, the sex ratio shifted toward the sex whose brood cells were closest to the nest opening [
101]. Gruber et al. [
102] found that lengths shorter than 15 cm favored male production, which acted as a shield for females from parasites [
103]. A female-biased sex ratio is the outcome of rich floral resources, whereas a male-biased sex ratio is the outcome of low floral resources [
104]. Peterson and Roitberg [
105] found that more sons were produced with an increase in the flight distance between the nest and floral resources. Furthermore, females provide fewer resources, which also results in fewer offspring.
Grewia asiatica was the predominant pollen grain species found in the brood cells of
M. cephalotes, followed by
Alhagi graecorum and
Parkinsonia aculeata.
Megachile cephalotes exhibited a wider range of plant interactions [
106]. The yellow flowers of
G. asiatica are zygomorphic with a good “side on” advertisement [
107]. Bees tend to forage on food resources near their nest site. Bees exhibit floral constancy as a strategy that targets rewards and balances energy expenditure, i.e., they tend to feed on the most dominant species in the landscape [
108]. Megachilid bees prefer zygomorphic and yellow-colored flowers, i.e., flowers with high UV reflection and pigmentation patterns and with a “side on” advertisement [
109,
110,
111,
112].
In the present study, adult parasitoids of three species,
Euaspis carbonaria,
Coelioxys sp., and
Anthrax sp. emerged from the nests of
M. cephalotes. Similarly, Rauf et al. [
14] found that the nests of
M. cephalotes were parasitized by
E. carbonaria and
Anthrax sp. All three species have already been reported as cleptoparasites of
Megachile spp. in different regions [
113,
114,
115,
116,
117,
118].