Composition and Functional Groups of Insects in Grain Crops from South-Ern Guanajuato

INTRODUCTION

Mexico, being a mega-diverse country, is represented by a high diversity of animal and plant species, an example is insects that represent 75% of known species [1], of which an estimated 47,768 species are described, the best-known insects are grouped in the orders Coleoptera, Hemiptera, Homoptera, Diptera, Lepidoptera Orthoptera, and Odonata, having families of economic and ecological importance in agriculture [2]. In addition to representing various forms of life and habits, they also form diverse functional groups, including phytophages, predators, parasites, and decomposers, amongst others [3, 4].

The heterogeneity of the landscapes, the characteristics of the habitats that are part and in turn surround the agroecosystems, increase the diversity and have a significant influence on the organisms that are within the agroecosystem, particularly the insects, which can increase or decrease their ecological functions. Egerer et al. [5] found that the structure and diversity of the landscapes that surround an agroecosystem modify the abundance and diversity of predatory insects. In their study, they found a greater diversity of coccinellids in the agroecosystems highly anthropized by human activities that increased the variety of landscapes. In turn, Oliveira et al. [6], reported that a secondary forest surrounding citrus crops harbored a greater abundance of parasites of the Braconidae family, compared to the edges of secondary forest-citrus plantation landscapes. The foregoing highlights the role that vegetation has on the diversity and composition of insects in agroecosystems, in addition to the ecosystem services they offer concerning the crops, some of them being: pollination, predation, and degradation of organic matter, among others.

Despite recognizing the important role of biodiversity in agroecosystems, there is a lack of information from experiments and observational studies to generate a theoretical overview of the biodiversity-agroecosystem interrelation. Muriel and Vélez [7] have provided evidence that suggests non-linear relationships between levels of diversity and ecological factors of agroecosystems such as the abundance, richness, and diversity of beneficial and/or harmful insects, as well as other aspects that have to do with sustainability. Leyva and Lores [8] also indicate the need for new methods that allow measuring agrobiodiversity from a comprehensive perspective. This suggests that research on agroecological phenomena requires more and new approaches.

The Mexican state of Guanajuato is located in the middle of the national territory, bordered by two biogeographic regions, the Nearctic and the Neotropical, which implies a remarkable natural wealth through endemism and the exchange of flora and fauna. However, according to CONABIO [9] in its anthropogenic transformation index, the state of Guanajuato has its ecosystems at a degree of unsustainable. Given the vocation and agricultural boom of the state, it is necessary to promote viable and sustainable agroecology production strategies with the environment and the agroecosystems themselves. For the Bajio and southern Guanajuato region, there are a few studies with an agroecological approach on the biological control of pests, the analysis of ecological aspects, and the diversity of insects associated with crops, for example, the works of Salas-Araiza et al. [10], Guzmán-Mendoza et al. [3], Ramos-Patlán et al. [11] and León-Galván et al. [12] in which aspects of the habits, composition, and diversity of species are analyzed for some crops in the region, such as corn, sorghum, and carrot.

Thus, the absence of functional ecology studies expressed in the organization of guilds or ecological interactions of the agricultural entomofauna of a region of productive importance such as the state of Guanajuato is notable which implies the application of a little-explored approach [13] to address the issue of functional ecology applied to agroecosystems and understand the population dynamics of both pests and beneficial insects that interact in these environments. Therefore, this study aims to know the composition of insect families, as well as the functional groups that make up the entomofauna of three-grain crops (corn, wheat, and barley), in the community of El Capulin, Salvatierra, in the state of Guanajuato, Mexico. It is hypothesized that the composition of the entomofauna will be dominated by phytophages families, the presence of beneficial insects being low and that agronomic management plays an important role in this.

MATERIALS AND METHODS

Study site

The work was carried out in the town of El Capulín, municipality of Salvatierra, Guanajuato (Figure 1). The climate is semi-warm, subhumid with rains in summer, with lower humidity (67.3%), and temperate sub-humid with rains in summer with medium humidity (32.7%). The maximum temperature is 33.4 ° C, the minimum of 2 ° C and an annual average of 18.1 ° C. Annual rainfall ranges from 700 to 800 mm. The characteristic soil types in the study site correspond to Vertisol (87.2%), Phaeozem (7.5%), and Solonchak (1.4%) [14]. Representative native vegetation is subtropical scrub and deciduous forest, which surround various grain and vegetable crops.

Field design and specimen collection

The fieldwork was carried out in three irrigation production systems with two sampling periods. The first from August to November 2018 and the second from February to May 2019.  Before the collection of the insects, work transects were delimited in each of the crops: corn (2018 cycle), wheat, and barley (2019 cycle). For this case, five transects of 20 x 30 m2 were drawn in the three crops there was a total area of 9,000 m2, an area considered adequate to understand abundance patterns of insect populations [5]. The samplings were systematic every 10 days, with a duration of one day, and the sampling technique consisted of doing zigzag-shaped walks on the entire surface of the transect with the possibility of homogeneously collecting all insects, potentially found in the sampling site. In addition to the above, a distance of 10 meters was left between crops and transects to have data independence [15], also because transects are the most useful monitoring method [16].

Collection of specimens and treatment

The insects were collected with the help of aerial entomological and striking nets, as well as with stainless-steel tweezers, and in the case of small species (aphids), the collection was done with the help of brushes or with the use of vacuum cleaners. The collected insects were placed in 250 ml plastic bottles with 70% alcohol for their preservation. Each vial was labeled with the following information: place, date, crop, sampling type, transect, and time of day. Later, the specimens were transported to the Biology Laboratory of the Higher Technological Institute of Salvatierra, and the Entomology Laboratory of the University of Guanajuato, Irapuato-Salamanca campus. With the help of a stereoscopic microscope with a single objective lens and an integrated lamp, identification was made at the family level, working with higher-order taxa such as family, genus, or tribe. This facilitates the ordering of organisms into functional groups based on the feeding ecology, which is useful to detect changes in the function of the ecosystem as suggested by Grimbacher et al. [17] and Gómez et al. [18], and this can be extrapolated to agroecosystems [19]. The functional groups were characterized according to the proposal by others such as Oliveira [20] and Cumbria and Rodríguez [21] of the Functional Food Groups (GFF´s) and modified for this study. This consists of grouping insects based on their trophic relationships but does not take into account their ecological role. For this case, the insects were grouped into two groups: 1-EFG´s (ecological functional groups), based on their ecological relationships and their functional role, characterizing seven EFG´s: Herbivores (H), predators (D), parasitoids (PAR), pollinators (POL), vectors (V), decomposers (DES) and generalists (G), and 2- TFG´s (functional guilds trophic), considering their diet. There are nine guilds: phytophages (P), insectivores (I), polyphagous (POF), necrophagous (N), hematophagous (H), carnivores (C), nectarivores (NEC), and mycophagous (M).

Data analysis

The composition of the entomofauna in the crops was represented with range-abundance graphs or Whittaker curves, which order the species (in this case families), from the most to the least abundant, that is, they are based on the relative abundance of each family, Rank abundance curves are useful as biodiversity indices complement [22]. Finally, a principal component analysis was developed to see the relationship between the EFG and the TFG's and to see the behavior of the composition of the families of insects in the three crops. Statistical analysis was carried out with the Past version 4.0 statistical program.

RESULTS AND DISCUSSION

Abundance of insects

A total of 6,596 specimens were collected, and 11 orders and 59 families were identified. The largest number of families, 53, was presented in corn, followed by wheat with 25 and barley with 24. Table 1 shows the families found by order and their presence in the three crops. Diptera had the most families with 18, followed by Hemiptera with 13, and Coleoptera with 10.

Composition of families

The composition of families was represented by rank-abundance or Whittaker curves, and the families were ordered from the most to the least abundant. The corn crop presented the highest number of families (n = 53), with Labiidae being the most abundant with n = 647 individuals, followed by Cicadellidae (n = 591), Chloropidae (n = 459) and Culicidae (n = 332). In the case of the Aphididae family, there was a greater number of individuals in wheat (n = 1,343) and barley (n = 632 individuals) (Table 1 and Figure 2).

Functional groups

The insects were grouped into two categories, depending on their ecological relationships, having a total of seven EFG´s, and in nine TFG´s; trophic guilds, depending on their diet. For the GFF's, the largest number of families was presented in corn, the most representative groups being herbivores, predators, and generalists. The TFG´s had in corn, the highest abundance in phytophages, insectivores, entomophagous organisms, and polyphages (Figure 3).

The principal component analysis for EFG´s, showed a relationship between the groups, explaining Component 1, 72.7% of the correlation, while Component 2, explained, 22.5%, giving 95.2% of the total variation explained, with a total correlation of 96.6%, and a cophenetic correlation coefficient of 0.997. Ecologically, it is defined in the crops that the presence of herbivores and predators are related to corn, while for barley and wheat, there is the presence of generalist insects and decomposers, and other groups (pollinators, parasites, and vectors) (Figure 4). For the TFG´s, Component 1 explained a relationship of 97.6%, and Component 2, 1.7%, with the total explained correlation being 99.3%, and the cophenetic correlation of 1.00. It is shown that the trophic groups for corn are composed of phytophages, insectivores, entomophages, and polyphages, while wheat and barley present a more diverse composition that includes nectarivores, ghouls, decomposers, carnivores, hematophagous, mycophagy, and decomposers (Figure 4).

Table 1. Registered families for the three crops. The numbers in the boxes are equal to the proportional abundance of families in each crop.

Note: EFG´s: Herbivores = H, Predators = D, Parasitoids = PAR, Pollinators = POL, Vectors = V, Decomposers = DES and Generalist = G. TFG´s: Phytophages = P, insectivores = I, decomposers = DES, polyphages = POF, ghouls = N, hematophages = H, carnivores = C, nectaviborous = NEC, and mycophagous = M.