Wild and cultivated flora are vital natural resources for humans because they provide multiple benefits. Seeds, flowers and fruit can be used directly by humans, while other products such as nectar can be processed by bees to produce honey¹. Use of honey began in prehistory when people harvested honeycombs from beehives in holes or cracks in trees and rocks². Before European contact in the 16^th Century, beekeeping in Mexico concentrated on native bees (meliponiculture). It was not until the early 20^th Century, about 1920, that modern apiculture practices employing Apis mellifera began to spread³.

Wulfrath and Speck⁴, and Ordetx et al⁵ published the first studies on flora in Mexico used by A. mellifera, including nationwide inventories of melliferous plants. Regions such as the Yucatan Peninsula have been intensively studied to identify nectariferous-polliniferous flora^6-10. Additional studies on apiculturally important flora have been done in states such as Michoacán¹¹, Colima¹², Guerrero¹³, Chiapas¹⁴ and Veracruz¹⁵.

Various local studies of melliferous flora have been done throughout Mexico, the south having received much more attention than the north. In the state of Tamaulipas, in Mexico’s northeast, a preliminary list was collated of the plants visited by Apis mellifera L. in the El Cielo Biosphere Reserve¹⁶. This was followed at the turn of the century by a catalog of the main nectariferous and polliniferous species in the state¹, and then a list of 147 wild and cultivated polliniferous and nectariferous plant species¹⁷.

Floral diversity in Tamaulipas has been estimated at 5,000 wild species¹⁸; to date 4,278 species have been recorded¹⁹. The state’s flora is distributed in twenty vegetation types defined by the Rangeland Coefficients Technical Advisory Commission (Comisión Técnico Consultiva de Coeficientes de Agostadero - COTECOCA), and there are extensive citrus orchards and other introduced melliferous agricultural crops¹. Although Tamaulipas contains extensive floral resources it has not met its full potential for honey production; in other words, current honey production is not proportional to the existing plant resources. One reason for this underutilization is limited knowledge of melliferous native and introduced plant species¹.

Honey production in Tamaulipas ranks eighth nationwide and apiculture and honey consumption has increased significantly in recent years. For example, 14,069 beehives were registered in 2000, which increased to 17,764 in 2008 and 22,000 in 2010^20,21. There are currently 350 registered producers in the state belonging to twelve beekeeping associations, which have an overall annual honey production of 716 t, valued at approximately 30 million pesos (~1.5 million dollars)²².

The present study objective was to expand current knowledge of melliferous flora diversity in Tamaulipas, concentrating on the nectariferous and polliniferous plant species visited by A. mellifera in the different seasons and at various study sites. This data will help beekeepers to take full advantage of floral resources and more efficiently manage them, potentially resulting in greater honey production.

Located in northeast Mexico (22°12’31”, 27°40’52” N; 97°08’38”, 100°08’51” W), Tamaulipas is the seventh largest state in the country (7,982,900 ha). To the north is the border with the United States of America, to the south the states of Veracruz and San Luis Potosí, to the west the state of Nuevo León and to the east the Gulf of Mexico²³.

Its varied topography includes dry, semi-dry, warm, semi-warm and temperate climates. Semi-dry warm and dry very warm climates predominate on the coastal plain. In mountainous areas sub-humid semi-warm to sub-humid temperate climates occur, depending on slope orientation and altitude. The most characteristic soils of Tamaulipas are phaeozems, vertisols, luvisols, xerosols, cambisols, regosols, rendzines and lithosols. Others such as gleysols are common throughout the coastal zone and fluvisols are found on the banks of rivers and streams²³.

Tamaulipas includes a large portion of the Northeast Coastal Plain, which extends south from the Rio Grande along the Gulf of Mexico coastline. This is bordered to the west by the Sierra Madre Oriental mountains, with altitudes as high as 3,450 m. The twenty vegetation types defined by the COTECOCA include jungle, forest, bush, palm groves, grasslands, halophyte and wetlands groups, as well as agricultural areas¹. Of the state’s total area, 557,566 ha are used for irrigated agriculture; 1,118,412 ha for seasonal agriculture; 852,454 ha for forestry; 4,683,528 ha for livestock production; and 770,940 ha for other uses¹.

Field work was done between 2012 and 2015, during all four annual seasons. A sample of 27 apiaries was selected in eleven municipalities with different vegetation types (natural and agricultural) and physiographic characteristics (Table 1, Figure 1). Each apiary location was GPS logged (Garmin GPS73 geopositioner), using the Universal Transverse Mercator (UTM) coordinate system (Zone 14, WGS 84 datum). Flowering specimens were collected from a 2 km area surrounding each apiary, a distance based on the estimated average flight distance of A. mellifera¹². Specimens were collected using scissors, plastic bags and a botanical press. Plant species frequently visited by A. mellifera were recorded by visually monitoring bee flower visits to a specific plant species for 5 to 10 min^24,25.

Figure 1 Apiary location in study area 

Collected botanical specimens were botanized and deposited in the Francisco González Medrano Herbarium of the Institute of Applied Ecology of the Autonomous University of Tamaulipas (Universidad Autónoma de Tamaulipas). First-hand data on local floral resources was collected from local beekeepers during field trips. They provided information on the plant species visited by bees and their flowering time and duration.

Using field data, databases and the literature, a species inventory was produced containing information on species grouping by life form (tree, bush, herbaceous); growth form (erect, ascending, decumbent, prostrate, creeping, climbing, rosette, epiphytic and floating); origin (native, introduced); and resource production (nectariferous, polliniferous or both). Species flowering period was recorded as well as the surrounding vegetation type. Family classification was done based on the international system established by the Angiosperm Phylogeny Group APG III²⁶.

A total of 215 species (including 1 subspecies and 1 variety) were found to be of interest for apiculture in Tamaulipas; these belong to 173 genera and 60 vascular plant families (Annex 1). The best represented family is Fabaceae (traditionally known as Leguminosae) with 35 species (16.28 %), followed by Asteraceae with 26 species (12.09 %). More than 50 % of these melliferous plant species belong to just eleven families (Table 2). The predominant genera were Acacia (6 species) and Croton and Mimosa (5 species each).

Most (87.91 %) of the recorded species are native and the rest (12.09 %) are introduced. Of the 215 species, 91 are herbaceous, 74 are shrubs and 50 are trees. Growth forms varied with 169 erect, 24 climbing, 6 ascending, 6 prostrate, 5 rosette, 2 floating, 1 creeping and 1 epiphytic species.

Melliferous plants were identified in 26 different vegetation types, including agricultural crops. Species diversity was highest (58 species) in secondary vegetation, although the natural vegetation types with the highest number of species were the dry tropical forest and Tamaulipan thorn scrub (Table 3). Cultivated species were not very diverse, with twelve agricultural crops and thirteen ornamentals. Of the 215 collected species, most (n= 97) are nectariferous, followed by the nectariferous-polliniferous (n= 86) and polliniferous species (n= 32).

During the annual seasonal cycle flowering tended to decrease during the colder seasons. For example, in Tamaulipas 355 species are reported to flower in the spring, 364 in the summer, 288 in the autumn and 233 in the winter. During the study period the month with the most floral resources was June, with 130 available species, and that with the least was December, with 64 species (Figure 2).

Figure 2 Monthly flowering distribution of important melliferous plant species in Tamaulipas 

The 215 species recorded here indicate that melliferous plant richness in Tamaulipas is notable, particularly since northern Mexico generally has a less diverse flora than southern Mexico. The present inventory constitutes the largest number of nectariferous-polliniferous plant species reported to date for Tamaulipas: far more than the 174 reported by Lara¹⁶, the 150 reported by Villegas et al¹ and the 146 reported by González-Rodríguez et al¹⁷. As observed in these previous studies, the most important melliferous plant families are the legumes (Fabaceae) and the compounds (Asteraceae), which has also been reported in other states^12,13,27.

As mentioned previously¹⁹, the largest proportion of melliferous species consists of natives since they are the most common floral resources in Tamaulipas. However, during winter citrus orchards (especially oranges) become a highly relevant nectar source for A. mellifera due to their vast extension in the state’s central area¹.

Herbaceous plants (91 species) represented a higher proportion than did shrubs and trees, a trend reported elsewhere^28,29. In terms of growth form, erect plants were the most common at the studied sites; in contrast, climbing plants have been reported to account for a large proportion of melliferous species in the states of Yucatan, Michoacán, Veracruz, Guerrero and Chiapas^7,10,13-15.

Secondary vegetation contains the greatest diversity of melliferous plant species in Tamaulipas (26.98%), perhaps due to the large number of herbaceous plants present in this and other vegetation types; a similar trend has been reported in Michoacán²⁸ and the Valley of Mexico²⁹. Of note is that weed species such as Argemone spp. and Helianthus annuus subsp. annuus are common year-round in Tamaulipas. Weeds have also been reported as important melliferous plant species in countries such as India³⁰. Of the natural vegetation communities, the dry tropical forest offers the most floral resources in the state; El Cielo Biosphere Reserve is an excellent example of this¹⁶. Both the dry tropical forest and the Tamaulipan thorn scrub are among the most widely distributed vegetation types in the state²³.

Nectariferous plants were the most diverse in the present study. This coincides with a report on melliferous flora in the state of Colima¹¹, although nectariferous-polliniferous species have been found to be more diverse in other studies^17-30.

In contrast to previous studies^11,27, melliferous floral resources were most abundant in Tamaulipas during summer. June was the most productive month since most species were flourishing, regardless of life form and vegetation type. However, all these species do not always provide abundant floral resources, as is the case with mesquite (Prosopis spp.) and citrus (Citrus spp.). Indeed, based on the current apicultural calendar, these species have the greatest influence on monofloral honey production in Tamaulipas during the February-April period.

Tamaulipas clearly offers extensive melliferous floral resources, as shown in the present study of species visited by Apis mellifera. The Fabaceae and Asteracea families provide the most floral resources for bees. The highest proportion of melliferous species are native (87.91 %) and herbaceous (42.32 %), and are mainly nectar producers. Secondary vegetation and dry tropical forest are the most important plant communities for honey production in the state, particularly during the summer. The data provided in the present study can be the foundation for more efficient apiculture practices in Tamaulipas by allowing beekeepers to manage apiaries and thus take greater advantage of melliferous floral resources year-round.