Acacia mangium (Fabales: Fabaceae) is broadly used in restoration process of degraded lands in tropical and subtropical regions. Thus, our aim was to assess the spatial distribution of arthropods on tree crown (vertical- upper, median and lower canopy and horizontalnorth, south, east and west) and leaf surfaces (adaxial and abaxial) of A. mangium trees. Phytophagous arthropods and natural enemies were quantified biweekly in 20 trees during three years. The Shannon index (H') of phytophagous insects were higher on the abaxial surface of leaves on branches facing the west side and basal thirds, while the lowest index was found on the adaxial surface of leaves on branches facing north and on trunk of A. mangium. The natural enemies and pollinators presented the highest H' indexes on the abaxial surface of leaves on branches facing north on basal thirds of A. mangium, while the lowest index values were found on the adaxial surface of leaves on branches facing the other sides. Trigona spinipes Fabricius (Hymenoptera: Apidae, Meliponinae), Aethalion reticulatum (Hemiptera: Aetalionidae) and Pentatomidae sp.1 (Hemiptera) were the most abundant and with the lowest k-dominance on different parts of A. mangium trees. Camponotus sp.2 (Hymenoptera: Formicidae), Tetragonisca angustula (Latreille) (Hymenoptera: Apidae) and Polistes sp. (Hymenoptera: Vespidae) had higher abundance and lower k-dominance. These results may be a support for programs of pest control and maintenance of natural enemies and pollinators in future plantations of A. mangium. For instance, application of biopesticides may reach better results if aimed directly to the preferred sites of target organisms, beyond minimizing possible negative effects on non-target ones.
The arboreal species Acacia mangium Willd. (Fabales: Fabaceae) is used in the initial succession and to restore degraded areas in tropical and subtropical regions (Yu & Li 2007; Phan Minh et al. 2013). This plant presents high potential for restoring or enhancing soil fertility by N-fixing and making fixed nitrogen and other plant nutrients available to other plants (Galiana et al. 1998).
Acacia mangium can also be used in windbreaks to prevent wind and water erosion and to improve environmental conditions (Brandle et al. 2004). A plant species used in windbreaks should be perennial, grow rapidly, but not be invasive nor excessively competitive, while having a dense crown for maximum efficiency and protection against erosion (Brandle et al. 2004; Norisada et al. 2005). These trees can be a barrier to the movement of herbivorous insects and to impede their capacity to locate their host plants (Rao et al. 2000). The plant canopy in a wind break offers great diversity of resources to maintain biodiversity, and their branches and leaves affect the environment by affecting humidity and evapotranspiration and by being refugia for birds and insects (Brandle et al. 2004).
Interactions between Acacia spp. plants and insects have been studied (Kruger & McGavin 1998; Fleming et al. 2007; Palmer et al. 2007) and insects are the main organisms responsible for the decline in Arabic gum production from these plants in Sudan (Jamal 1994). On the other hand, conversion of natural forests to A. mangium plantations has negatively impacted communities of insect groups, such as termites, ants, flies and beetles (Tsukamoto & Sabang 2005).
The aim of this study was to assess the spatial distribution of arthropods along the vertical canopy (apical, middle and basal parts and trunks), horizontal orientation (branches facing north, south, east and west) and leaf surface (adaxial and abaxial) of A. mangium plants.
Shannon (H') biodiversity indexes of phytophagous and natural enemy arthropods on Acacia mangium as functions of positions (cardinal points ) of the tree's branches, heights within the canopy and upper or lower leaf surfaces during three years of sampling.
Material and methods
This study was carried out in a pasture area of the Institute of Agricultural Sciences at the Universidade Federal de Minas Gerais (ICA/UFMG), Brazil. Samplings occurred from Jan 2005 to Mar 2007 in an area with Aw climate, i.e., tropical savanna according to the classification of Köppen with a dry winter and a rainy summer and a dystrophic red-yellow latosol.
Windbreaks, 100 m long with 2 rows of A. mangium spaced 3 × 3 m were used. Seedlings of this species were prepared in a nursery and planted in Sep 2003 in 30 × 30 × 30 cm holes with 360 grams of natural reactive phosphate mixed into the subsoil of a Brachiaria decumbens Stapf. (Poales: Poaceae) pasture.
The phytophagous insects, natural enemies and pollinators insects in twenty 16-month old A. mangium trees were visually counted biweekly every yr. The arthropods were counted on the adaxial and abaxial leaf surfaces in the upper, median and lower apical canopy on branches facing north, south, east and west with a total of 12 leaves per canopy and nine per tree branch osition in each sampling. Arthropod collection also occurred on the trunks of 20 trees per sampling. All material collected was stored in flasks with 70% ethanol, separated by morphospecies and sent for identification.
The ecological indexes (number of individuals, richness, diversity and abundance of species) were calculated for the arthropod species identified. All ecological indexes were measured by calculating the dataset of taxa by samples in BioDiversity Pro Version 2 software. Diversity was calculated by the Shannon-Weaver formula: H = pi ln (pi). Abundance and species richness (S) were calculated by the Simpson formula: D = (.ni /N) * 100, where: pi = ni /N; ni = number of individuals per species; N= total number of individuals; S= richness (number of species present). k-Dominance were calculated by plotting the percentage cumulative abundance against log species rank (Lambshead et al. 1983). The k-dominance values indicate the dominance and evenness distribution of individuals among species (Gee et al. 1985).
Results and Discussion
Phytophagous arthropods presented their greatest biodiversity Shannon (H') index values on the abaxial (lower) leaf surfaces, on branches facing west, and in the lowest part of A. mangium canopies. In contrast, phytophagous arthropods presented the lowest H' values on the adaxial (upper) leaf surfaces, on branches facing north, and on the trunks of this plant (Table 1). The natural enemies and pollinators presented the greatest H' indexes on the abaxial surface of leaves on branches facing north on the lowest parts of A. mangium, while the lowest index values were found on the adaxial surface of leaves on branches facing the other directions (Table 1). It is likely that the preference of phytophagous arthropods for the above distributions in the canopy are related to a lower risk of parasitism (Ramanand & Olckers 2013) or predation by natural enemies, such as ants (Elbanna 2011). Furthermore, the presence of ants may attract pollinators (Gonzalvez et al. 2013), which might explain the greatest abundance of both at the same sites.
The greatest abundance of Trigona spinipes Fabricius (Hymenoptera: Apidae, Meliponinae), Aethalion reticulatum L. (Hemiptera: Aetalionidae) and Pentatomidae sp.1 (Hemiptera) on the abaxial (lower) surface of leaves (Table 2) is likely owing to the biomechanical properties on this area. In general, densities of phytophagous insects are negatively correlated with work to tear and shear leaves (Peeters et al. 2007). Thus, it would be expected to find the greatest abundance of such insects on the abaxial surface of leaves where the epidermis is thinner, favoring insect feeding and, consequently, a higher fitness (Fiene et al. 2013).
The largest population of herbivorous insects on the abaxial leaf surfaces may attract arthropod natural enemies and pollinators, such as Camponotus sp.2 (Hymenoptera: Formicidae), Tetragonisca angustula (Latreille) (Hymenoptera: Apidae) and Polistes sp. (Hymenoptera: Vespidae) (Table 2). The greatest biodiversity of phytophagous arthropos and the natural enemies amponotus sp.2, Polistes sp. and Podisus sp. (Hemiptera: Pentatomidae) on the west side of the canopy of A. mangium (Table 3) may be explained by the lowest impact of predominant wind currents from northeast/east in this region (Leite et al. 2006). Furthermore, parts of plants more exposed to winds may present thicker leaves owing to dehydration, which reduces feeding preference by phytophagous insects. Changes in the microclimate of arboreal systems increase air humidity and decrease the temperature and wind speed with positive effects on insect development (Rao et al. 2000). The quality of food resources may explain the greatest abundance of the bees, T. spinipes and T. angustula, on the east side of the canopy of A. mangium (Table 3) because leaves exposed to wind currents may present a higher evaporation, and thus increased nectar concentration in flowers (deBruijn & Sommeijer 1997).
K-dominance and abundance values (between brackets) of phytophagous and natural enemy arthropods on Acacia mangium plants as a function of the position of a tree branch relative to the cardinal points during three years of sampling.
K-dominance and abundance values (between brackets) of phytophagous and natural enemy arthropods on Acacia mangium trees as function of the height within the canopy during three years of sampling.
The phytophagous T. spinipes and Hemiptera were most abundant in the median and lower level canopy of A. mangium, respectively (Table 4), sites where leaves are likely older than on upper canopy. Older leaves are less chemically defended than young ones, which may favor the herbivory (Alba et al. 2013). Furthermore, the lower canopy may present higher humidities and lower temperatures (Rao et al. 2000), decreasing insect desiccation (Rao et al. 2000) and improving survival conditions. The same protecting factors against desiccation might have influenced the natural enemies Camponotus sp.2, T. angustula and Polistes sp., which were more abundant in the median than in the upper canopy and trunks of A. manigium trees, respectively (Table 4).
The lowest k-dominance and greatest abundance values of T. spinipes, A. reticulatum and Pentatomidae sp.1 on different A. mangium parts without damaging leaves and flowers of this tree agrees with that reported for Hemiptera with greater number of species on the canopy of Acacia spp. (Kruger & McGavin 1998). These insects are harmful to several plant cultures, being responsible for damaging sprouts for removal of fibers to construct their nests (Boica et al. 2004). Aethalion reticulatum sucks sap which affects fruit development and sprouting, beyond killing plants at high infestations (Brown 1976).
The natural enemies Camponotus sp.2 and Polistes sp. and the pollinator T. angustula were the most abundant and with the lowest k-dominance. Camponotus spp. are, in general, associated with sucking insects (Fernandes et al. 2005) such as A. reticulatum (Brown 1976), protecting them against predators and parasitoids (Renault et al. 2005), which explains their great abundance on A. mangium trees. Therefore, Camponotus spp. can indirectly affect host plants by hindering natural enemies impact. However, T. angustula is important for dispersing pollen and increasing plant genetic variability (Proni & Macieira 2004).
The potential of soil restoration or improvement by A. mangium trees in agroforestry systems can be explained by its benefit on soil structure and fertility due to N-fixing and increasing organic matter (Garay et al. 2003). Acacia mangium improves the conditions for the soil fauna, including earthworms (Pellens & Garay 1999; Tsukamoto & Sabang 2005) and for the commensalism between Pseudomyrmex spp. (Hymenoptera: Formicidae) and birds on this plant (de Ita and Rojas-Soto 2006). The ant presence can improve bird reproduction by reducing predation on their nests, whereas Acacia trees can supply branches, leaves and other materials for the nest building (Ndithia et al. 2007).
In conclusion, organisms, including phytophagous insects, natural enemies and pollinators presented the greatest diversities on the abaxial surface of leaves located on lower parts of the canopy of A. mangium. With regards to the canopy side, phytophagous insects presented a greater diversity on the west side, while natural enemies and pollinators presented a greater diversity on the north side. These results may support for programs of pest control and maintenance of natural enemies and pollinators in future plantations of Acacia mangium. For instance, application of biopesticides may reach better results if aimed directly to the preferred sites of target organisms, beyond minimizing possible negative effects on non-target ones.
To the taxonomistas “Drs. Antônio Domingos Brescovit (Instituto Butantã) (Arachnida), Ayr de Moura Bello (Coleoptera), Ivan Cardoso Nascimento (EMBRAPA- ILHÉUS-Centro de Pesquisas do Cacau, CEPLAC, Itabuna, BA) (Formicidae), Paulo Sérgio Fiuza Ferreira (Hemiptera) (UFV) e Luci Boa Nova Coelho (Universidade Federal do Rio de Janeiro (Cicadellidae)” for identifying the insects. To Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)” and “Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG).