Introduction

The Poaceae or grass family comprises 12 subfamilies, 771 genera and more than 12,074 described species (Soreng et al. 2015). One of those subfamilies is Bambusoideae (bamboos) with 1481 described species in 119 genera (BPG 2012; Clark et al. 2015). The bamboos are classified into three monophyletic tribes: Arundinarieae, Bambuseae (the woody bamboos), and Olyreae (the herbaceous bamboos) (Sungkaew et al. 2009; Kelchner and BPG 2013; Wysocki et al. 2015; Zhang et al. 2016). Arundinarieae species are distributed in Africa, Asia, and North America. Most of the Bambuseae species are distributed in the tropics and are divided into Paleotropical and Neotropical woody bamboo lineages. Almost all of the Olyreae species are distributed in the Americas (Kelchner and BPG 2013).

The flowers of grasses are very small and packaged somewhat differently than in most groups of flowering plants (Clark and Pohl 1996; Kellogg 2015). Grass flowers consist of a gynoecium, androecium, lodicules, palea, and a lemma with the exception of Anomochloa Brongn., and Streptochaeta Schard. ex Nees (Kellogg 2015). The flowers of grasses are arranged on a short axis (rachilla) and follow a distichous pattern forming the spikelet (Kellogg 2015). The flowers of bamboos follow this same pattern of small flowers, pendant position of anthers, and the lack of nectar, all of which are characters of the wind pollination syndrome (Judziewicz et al. 1999; Kellogg 2015). However, there are studies that have demonstrated insect pollination is possible, at least in Eremochloa ophiuroides (Munro) Hack., and Paspalum dilatatum Poir., grass species (Adams et al. 1981; Jones 2011).

Woody bamboo flowers are always bisexual, while unisexual flowers are found in the herbaceous bamboos (Judziewicz et al. 1999). The woody bamboos possess two types of reproductive structures, conventional spikelets, and pseudospikelets. Each of these can have one or more flowers; however, the pseudospikelets are more complex in structure and development due to the presence of gemmiparous bracts. These bracts can develop buds in their axils and rebranch to produce successive orders of pseudospikelets (Judziewicz et al. 1999; Kellogg 2015). In several bamboo genera, such as Dendrocalamus Nees, Ochlandra Thwaites, etc., the flowers are dichogamous and mostly protogynous (i.e. the stigma is exserted first followed by the stamens) (Venkatesh 1984; Nadgauda et al. 1993; Koshy and Harikumar 2001; Koshy et al. 2001). But in some Bambusa Schreb., species and Merostachys riedeliana Rupr. ex Döll both sexes mature together (homogamous) (Koshy et al. 2001; Guilherme and Ressel 2001), though protandrous (i.e. the stamens are exserted first followed by the stigma) flowers have been observed in Melocanna baccifera (Roxb.) Kurz and Aulonemia aristulata (Döll) McClure (Ramanayake and Weerawarde 2003; Grombone-Guartini et al. 2011). Woody bamboos typically exhibit gregarious flowering cycles, followed by the death of the parental plants (monocarpy) (Judziewicz et al. 1999).

Soderstrom and Calderón (1971) documented insect visits to the flowers of some herbaceous bamboo species. Nadgauda et al. (1993) did a floral biology study with Dendrocalamus strictus (Roxb.) Nees, a Paleotropical woody bamboo species, and tested for insect pollination in this species, but found no evidence of it. The insects fed on the pollen but were not pollen vectors. Grombone-Guartini et al. (2011) studied the floral biology of A. aristulata, a Neotropical woody bamboo species, with results similar to those of Nadgauda et al. (1993); i.e., the plant is wind pollinated and the insects only feed on the pollen grains.

Documenting the reproductive biology of woody bamboos is a complex task because of the long periods of time between flowering cycles. There are reports in the literature of flowering cycles that last 3–120 years (Janzen 1976; Guerreiro 2014; Veller et al. 2015). Franklin (2004) described four spatiotemporal patterns of flowering in addition to the gregarious monocarpy detected among semelparous bamboos: (1) a small percentage of clumps flower the year (or two) before and after the main flowering event, (2) gregarious flowering occurs in patches in successive years, (3) variation in periodicity among populations leads to diffuse temporality within species, and (4) sporadic flowering may imply a random or other nongregarious pattern of flowering (Troup 1921; Janzen 1976; Franklin 2004). However, there are documented cases (Table 1) of insects visiting flowers in woody and herbaceous bamboos in all three tribes (Soderstrom and Calderón 1971; Nadgauda et al. 1993; Guilherme and Ressel 2001; Koshy et al. 2001; Koshy and Harikumar 2001; Huang et al. 2002; Grombone-Guartini et al. 2011). Although they are not specific pollinators of bamboo, these insects are known to visit other species in the grass family.

Table 1 Bamboo species and the insect visitors recorded on their flowers
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In this paper, we report insects visiting the flowers of Guadua paniculata Munro and G. inermis Rupr. ex E.Fourn. Finally, based on the literature we wanted to know what role is played by the insects that visit bamboo flowers.

Materials and methods

Field work sampling

A flowering population of G. paniculata was found in the state of Oaxaca, Mexico, in the locality near Pochutla, Km 215 Oaxaca-Pochutla, at 15°51′46″N, 96°28′29″W, 316 m a.s.l. (voucher E. Ruiz-Sanchez and L. Cervantes 447, IEB). Specimens were collected on June 1, 2013, between 1200 and 1500 h. We observed several bee species, along with other insects, foraging for pollen during these hours. Only part of the bamboo population was flowering, and both insects and flowers were collected from a small patch of approximately 5 square meters. For Guadua inermis, we made observations of bees visiting flowers for two consecutive days (July 8–9, 2015) between 1200 and 1600 h. We observed only a couple of bees foraging for pollen during these hours. A small population was found where only one clump of bamboo was flowering, the rest was sterile. The locality was close to Aguilera, Veracruz, on the highway from Acayucán to Matías Romero at 17°52′2.3″N, 94°58′22.9″W, 7 m a.s.l. (voucher E. Ruiz-Sanchez and A. Ortíz 530, IEB). The bamboo specimens were deposited in the herbarium (IEB) and insect specimens in the Colección Entomológica (IEXA) of the Instituto de Ecología, A.C. Xalapa, Veracruz, Mexico, and in the Colección Entomológica Estación Chamela, Instituto de Biología, UNAM, Jalisco, Mexico (IBUNAM).

Scanning electron microscopy (SEM)

The SEM images were taken of bee and fly specimens with a JEOL JSM-5600LV Scanning Electron Microscope. The SEM samples were glued to aluminum stubs and coated with gold–palladium (1:1). These images of bee and fly specimens are necessary to better see the part of the insect’s body on which the pollen grains are being carried. We also used a Leica Camera DFC290HD adapted to a dissecting microscope (Leica Microscope M50) to obtain images of the insect specimens.

Results

Insects visiting Guadua paniculata and G. inermis flowers

Four bee species were observed visiting the flowers of G. paniculata (Table 1; Fig. 1). Three were from tribe Meliponini (family Apidae): Geotrigona acapulconis (Strand, 1919), Plebeia frontalis (Friese, 1911), and Trigona fulviventris Guérin-Méneville, 1844; one from tribe Apini, Apis mellifera Linnaeus, 1758; there was also a syrphid fly, Toxomerus teligera (Fluke, 1953). Meliponini bees and A. mellifera were also observed visiting the flowers of Guadua inermis (Table 1; Fig. 1). Pollen was detected on the legs or bodies of bees, but not on flies. The bees of these species have specialized hairs on their legs for carrying pollen, as shown in Fig. 2. Some Hemiptera were also found feeding on the flowers, the Lygaeidae Neortholomus jamaicensis (Dallas, 1852), or preying on the flower visitors, the Reduviidae Apiomerus pictipes Herrich-Schaeffer, 1846 (Fig. 3).

Fig. 1
figure 1

Bee species included in this study. a Apis mellifera, b Geotrigona acapulconis, c Plebeia frontalis, and d Trigona fulviventris. White arrows indicate attached pollen grains (Photos by Luis Cervantes)

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Fig. 2
figure 2

SEM bee images. a Leg of Apis mellifera with pollen grains. b Close-up of A. mellifera’s leg showing pollen grains. c Leg of Geotrigona acapulconis with pollen grains. d Close-up of leg of G. acapulconis showing pollen grains. White arrows indicate attached pollen grains. e Leg of Trigona fulviventris with pollen grains. f Close-up of T. fulviventris’s legs showing pollen grains (Images taken by Tiburcio Laez)

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Fig. 3
figure 3

Insect visitors to Guadua paniculata flowers. a Apis mellifera, b Meliponini bee removing pollen grains. c Apiomerus pictipes, preying on an A. mellifera bee, and d Neortholomus jamaicensis feeding on G. paniculata flowers. e, f Meliponini bee visiting Guadua inermis flowers (Photos by E. Ruiz-Sanchez)

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We observed that the flowers of G. paniculata and G. inermis are dichogamous, with male and female organs exposed at different times and for different lengths of time and protandrous, i.e. the stamens exsert first.

Discussion

Insects visiting Guadua flowers

Santana-Michel (1992) reported several populations of G. paniculata flowering in the Sierra de Manantlán mountain range, Jalisco, Mexico. He mentioned that this species has flowering cycles of 25–26 years and flowered continuously from 1989 to 1991, but he did not report insects visiting the flowers in these Guadua populations. The beginning of the bloom in a population of G. paniculata in Oaxaca was recorded in 2009, with the last bloom event recorded in 2013 and the first record of insects visiting the flowers of this bamboo species. Our records and those of Santana-Michel (1992) coincide with Franklin’s second pattern of spatiotemporal flowering events (Franklin 2004); i.e., gregarious flowering that occurs in patches in successive years. During fieldwork in the summer of 2015, four different populations of G. inermis were collected in the state of Veracruz, where one clump with flowers was observed in one of those populations. Two populations of G. inermis had mature flowers, one starting to grow pseudospikelets and only one with flowers in anthesis. This flowering behavior coincides with Franklin’s fourth pattern of sporadic flowering and could imply a random pattern or other nongregarious pattern of flowering (Franklin 2004). We observed both species of Guadua have dichogamous and protandrous flowers. Ramanayake and Weerawarde (2003) mention that protandrous flowers are uncommon in bamboos.

Four species of bees from four genera and one fly species were recorded foraging for pollen on G. paniculata and G. inermis, and species of the genera Geotrigona and Plebeia were recorded for first time foraging for bamboo pollen (Figs. 1, 2). The bee species are generalists and typically forage for pollen on many different plant species. The genus Geotrigona is widely distributed throughout the Neotropics, occurring from Mexico to Argentina. It has 22 species and is related to the Trigona and Tetragona genera (Gonzalez and Engel 2012). Geotrigona acapulconis is only known from Mexico (Ayala 1999). The genus Plebeia is the second largest genus of stingless bees in the Neotropics, with 39 species (Camargo and Pedro 2007; Melo and Costa 2009); this genus is likely to be more diverse when fully revised. Plebeia has 11 known species in Mexico (Ayala 1999). Trigona fulviventris is the most widespread Neotropical stingless bee species and occurs from northern Mexico to southern Brazil. Its highly plastic behavior confers flexibility on this species, which opportunistically obtains a variety of resources, such as fungi, dead animals, feces, and many types of pollen and nectar (Hernández et al. 2007). Apis mellifera, the honeybee, is the most widely distributed bee species in the world.

Species of the genus Toxomerus are the most abundant in the New World. In North America, the most common and widespread species is Toxomerus marginatus (Say, 1823). In the Neotropics, Toxomerus dispar (Fabricius, 1794) is the most common and widespread species, but in some areas, species such as T. pulchellus (Macquart, 1846) are more abundant. The common and widespread species T. politus (Say, 1823) is found in close association with corn (Zea mays L.) and sorghum (Sorghum bicolor L.), with adults and presumably the larvae feeding on the pollen of these plants (Nunes-Silva et al. 2010). All other species for which data are available are predaceous as larvae, feeding on aphids (Heiss 1938). The larvae of T. apegiensis (Harbach, 1974) feed on the pollen of Olyra obliquifolia Steudel, an herbaceous bamboo (Reemer and Rotheray 2009). Toxomerus apegiensis and T. politus, which feed on the pollen of corn (Zea mays), are the only known phytophages within the otherwise predatory genus Toxomerus (Reemer and Rotheray 2009). The species Toxomerus teliger (Fluke, 1953) is distributed from the southern United States to Brazil (Metz and Thompson 2001).

Additionally, we found A. pictipes Herrich-Schaeffer, the assassin bug, on flowers preying on bees. In the genus Apiomerus (Hemiptera: Reduviidae), also known as bee killers, there are more than a hundred described species distributed from the northern USA to Argentina. Apiomerus pictipes is distributed from Mexico to Venezuela and Colombia (Berniker et al. 2011). Neortholomus is a Neotropical genus of Hemiptera (Lygaeidae) with nine recognized species (Hamilton 1983); N. jamaicensis has a wide distribution from California to Brazil and the Caribbean islands and feeds on different plant species (Hamilton 1983).

Insects visiting bamboo flowers have been recorded on species of herbaceous (Olyreae), woody tropical (Bambuseae) and woody temperate (Arundinarieae) bamboos (Table 1). Soderstrom and Calderón (1971) first documented insect visits to the flowers of five herbaceous bamboo species (Table 1). They stated that the insects visiting the flowers of these species transfer pollen among flowers rather than the wind doing so, because these herbaceous bamboos live in the forest understory where the wind is a limiting factor for pollination. Koshy et al. (2001) recorded the visit of bees on six Paleotropical woody bamboos species (Table 1). They mention that the bees never touch the stigmas in Ochlandra species, Bambusa bambos (L.) Voss, B. vulgaris Schrad. ex J.C. Wendl., or Bambusa sp., but if the flowers are widely open, pollen transfer could occur. Huang et al. (2002) observed bees visiting the flowers of Phyllostachys nidularia Munro, a temperate woody bamboo. They mention that the bees fed on pollen grains and that the anthers released huge amounts of pollen, so the bees play an indirect role in pollen transfer. On the other hand, three studies of floral biology tested insect pollination in D. strictus a Paleotropical woody bamboo, A. aristulata and M. riedeliana, both Neotropical woody bamboos and found no evidence of insect pollination for those species (Nadgauda et al. 1993; Guilherme and Ressel 2001; Grombone-Guartini et al. 2011). However, those studies agree that insects play a role, though an indirect one, in pollination given that the bees produced a vibration in the anthers of the flowers, triggering pollen release and acting as devices for the dispersal of pollen.

Two different hypotheses have been proposed regarding the role of bees in bamboo flowers. The first states that the bees only feed on the pollen grains (Nadgauda et al. 1993). The second states that the bees could have an indirect biological role in wind pollination, because they induce the anthers to release pollen rapidly. These pollen grains are dispersed into the air, which increases the probability of their colliding with the stigmas. Even if the insects do consume some of the pollen grains, a huge number of pollen grains remain to be dispersed by the wind (Huang et al. 2002). In this study, we did not test these competing hypotheses directly with our field observations. However, if the first hypothesis was true, we would expect to see insects (bees) feeding on bamboo flowers, which we did not see. Our observations of G. paniculata and G. inermis support the second hypothesis in which bamboo species visited by bees have an increased probability of sharing their pollen grains with other plants, thus facilitating outcrossing and promoting gene flow among plants of the same populations or from different populations. It is remarkable that these polylectic bees somehow contribute to the pollination of bamboo, but above all, it must be noted that the pollen of bamboos is an important resource for these bees for feeding their offspring.