Diversity of arthropods associated to Brachiaria spp. and damage indexes of insect-pests

Diversidad de artrópodos asociados a Brachiaria spp. e índices de daños de insectos-plaga

Nurys Valenciaga,^I C. A. Mora,^I C. García,^I Dayleni Fortes,^I Aida C. Noda,^I

^IInstituto de Ciencia Animal, Apartado Postal 24, San José de Las Lajas, Mayabeque, Cuba.

ABSTRACT

A research was carried out in areas of Brachiaria spp., using a random block design, with five replications, in order to identify insect-pest associated to four varieties of Brachiaria: Brachiaria brizantha, Brachiaria brizantha cv. toledo, hybrid Brachiaria (Brachiaria ruziziensis x Brachiaria brizantha) cv. mulato 1 and Brachiaria decumbens cv. Basilisk, and evaluate damage indexes. Periodical random samplings were performed once a month for three years. A sample of 20 sweep/plot was taken and, according to the visual sampling, the damage caused by biting, chewing, sweep and sucking insects was determined. Results demonstrated superiority of Insecta class, with six orders, eight families and 21 identified morpho-species, in contrast to Arachnida class. The levels of provoked damage did not surpass degree two of damage scale, so it is considered as light. Basilisk variety showed less resistance to insect-pest attacks, although there was no economic damage. Species richness had a slight increase during rainy season (22), in contrast to dry season (20). Basilisk variety showed the highest number of individuals during dry season (130) and during rainy period (193). Shannon index ranged between 2.38 and 2.65. There was no associated dominant species, according to the values (0.09 to 0.12). It can be concluded the arthropods associated to Brachiaria genus, Insecta class shows higher expression, with an increase during rainy season. However, without dominance among present species, phytophagous and bioregulators, during the seasons, these species manage to live in harmony, without provoking economic damages among varieties and cultivars under study. It is recommended to continue studies on arthropod population in other varieties of Brachiaria, with other crop management.

Key words: Brachiaria spp., insect-pest, damages, diversity.

RESUMEN

Se condujo una investigación en áreas de Brachiaria spp., en la que se aplicó un diseño de bloques al azar, con cinco réplicas, para identificar los insectos-plaga asociados a cuatro variedades de Brachiaria: Brachiaria brizantha, Brachiaria brizantha vc. toledo, Brachiaria híbrido (Brachiaria ruziziensis x Brachiaria brizantha) vc. mulato 1 y Brachiaria decumbens vc. Basilisk y evaluar los índices de daños. Se llevaron a cabo muestreos aleatorios periódicos, con una frecuencia mensual, durante tres años. Se tomó una muestra de 20 redadas/parcela y se determinó, según muestreo visual,  el daño causado por insectos picadores-chupadores y masticadores. Los resultados demostraron la superioridad de la clase Insecta, con seis órdenes, ocho familias y 21 morfo-especies identificadas, en contraste con la clase Arachnida. Los niveles de daños provocados no superaron el grado dos de la escala de daños, lo que se considera ligero. La variedad basilisk fue la que mostró menos resistencia al ataque de insectos-plaga, aunque no provocó daños económicos.  La riqueza de especies tuvo ligero incremento en la época de lluvia (22), en contraste con la época seca (20). La variedad basilisk presentó el mayor número de individuos en la época seca (130) como en la de lluvia (193).  El índice de Shannon osciló entre 2.38 y 2.65. No hubo ninguna especie dominante asociada según lo confirman los valores (0.09 a 0.12). Se concluye que entre los artrópodos  asociados al género Brachiaria, la clase Insecta manifiesta mayor expresión, con incremento en la época de lluvia. Sin embargo, al no existir dominancia entre las especies presentes, fitófagos y biorreguladores, en las épocas del año, estas especies logran convivir armónicamente, sin provocar daños económicos entre las variedades y cultivares en estudio. Se recomienda continuar estudios poblacionales de artrópodos en otras variedades de Brachiaria, con otros manejos del cultivo.

Palabras clave: Brachiaria spp., insectos-plaga, daños, diversidad.

INTRODUCTION

In Cuba, since the 90´s, Brachiaria genus is considered as promising for Cuban animal husbandry (Gutiérrez et al. 1990). Several researches have demonstrated its excellent performance due to its high production of green mass and its acceptability for meat animals grazing (González 2001).  Others have focused on the study of some morphological and productive indicators related to seeds of different Brachiaria varieties (Suárez et al. 2013). However, it is necessary to know its degree of susceptibility or tolerance to the attack of harmful organisms. Therefore, this research was conducted to identify insect-pest associated with this genus and their damage levels on different species or cultivars.

MATERIALS AND METHODS

The research was conducted in areas of the Centro Experimental "Miguel Sistachs Naya", belonging to the Department of Grasses and Forages from the Instituto de Ciencia Animal, located at Zaldívar farm, San José de las Lajas, Mayabeque province, Cuba. This province is located at 22º 55’ N and 82º 0’ W, at 92 m o.s.l.

Experimental area.The research was performed in a typical red ferrallitic soil (Anon 1999) of  35 plots with 24.5 m² each. They were sown on May, 2009 with four varieties of Brachiaria: Brachiaria brizantha, Brachiaria brizantha cv. toledo, hybrid Brachiaria (Brachiaria ruziziensis x Brachiaria brizantha) cv. mulato 1 and Brachiaria decumbens cv. basilisk. 

Experimental design. Plots were located according to a random block design, with five replications.

Experimental procedure. Periodical random samplings were performed to associated arthropods, once a month for three years. A sample of 20 swarm/plot was taken, using an entomological net. Species with flying habits were captured and individualized in plastic bags with their identification and moved to the lab of pest management from the Department of Grasses and Forages from the Instituto de Ciencia Animal (ICA). In addition, according to the visual sampling, the damage caused by chewing and biting sucking insects was determined, with a scale of 0-3 degrees, in accordance with Calderón (1982).

In order to facilitate lab processing, samples were placed in a graduated oven at 40 °C up to provoke death of the collected organisms. Later, each specimen was separated (entomological collections and related keys) using a stereoscopic microscope (Zayas 1988 and Alayo and Garcés 1989), and taxonomically identified. Phytophagous insects and associated beneficial fauna (bioregulators) were identified in each studied area.

Statistical analysis. A data matrix in Excel was performed with populations of phytophagous and bioregulators associated with the system. The contingency table was used to test whether there was interaction between grasses and insects and, on the contrary case, the use of the proportions comparison method (chi square) was taken into account to analyze the factors separately. In order to establish differences among means, the test of Duncan (1955) was used for P <0.05.

RESULTS AND DISCUSSION

The main arthropods associated to Brachiaria genus were identified (table 1). Organisms were grouped, according to their taxonomic classification and their feeding habits, into phytophagous and bioregulators. The results demonstrate the superiority of Insecta class, with six orders, eight families and 21 identified morpho-species, in contrast to Arachnida class. The beneficial fauna was represented by four orders (three of Insecta and one of Arachnida), with 8 morpho-species in total.

Rates of damage caused by studied Brachiarias demonstrate that the damage did not exceed grade two of the scale used, so they are considered as light. The basilisk variety showed less resistance to insect-pest attack, manifesting the same way in all evaluated periods, although without causing economic damage.

Regarding the interaction between bioregulators and phytophagous, in the evaluated grasses, according to the sampled season (table 2), only during the rain of the third year, the proportions of collected arthropods varied significantly among varieties. Basilisk was the variety with the largest proportion (42.53%), which differed (P < 0.005) from the other varieties, which did not differ among themselves. As for the proportion of phytophagous and bioregulators, except for the dry season in the second year, there was a tendency of more amounts of bioregulators than phytophagous. This is not common, because, except syrphid flies, the rest of the associated beneficial fauna is predatory, and each individual requires a great number of phytophagous specimens to feed themselves properly. Therefore, the proportion of phytophagous should be superior. Nevertheless, in the case of bioregulators, they can interact with the associated phytophagous fauna with other crops. The reason for them to remain in these plants could be that the volume of foliage and physiology of plants provides them shelter.

Diversity of arthropods among Brachiaria varieties had equal performance in species richness, with a slight increase of two more species during rainy season (22), in contrast to dry season (20). Basilisk variety showed higher number of individuals compared to the rest of those evaluated, in dry (130) and rainy (193) season. Shannon index ranged between 2.38 and 2.65 among the varieties, conditioned by the species equality (uniformity), and not for having an associated dominant species. Therefore, the values ranged from 0.09 to 0.12.

Among the arthropods associated to Brachiaria, Insecta class showed higher expression, with increases during the rainy season. However, the absence of dominance among species in seasons, phytophagous and bioregulators manage to live in harmony, without causing economic damage among varieties and cultivars under study. It is recommended to continue studies on arthropod population in other varieties of Brachiaria and in other crop management.

REFERENCES

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Received: November 24, 2015
Accepted: January 5, 2016

Nurys Valenciaga, Instituto de Ciencia Animal, Apartado Postal 24, San José de Las Lajas, Mayabeque, Cuba. Email: nvalenciaga@ica.co.cu