La comunidad de arañas del cultivo de soja en la provincia de Buenos Aires, Argentina

Abstract

The spiders are one of the most important predators that produce the mortality of the plague insects. The spider community was sampled with nets, plant pocketed and pit fall traps on soybean crop and the wild vegetation on the borders (2h manual capture). The samples are done during the crop development (December to mid-May) and, in the fallow period (mid-May to November), in order to know spiders specific composition, to compare specific diversity through Shannon index, and, to analyze the colonization process. Total specific richness was 37 species grouped in 13 families, and the dominant species was Misumenops pallidus (Keyserling) (Thomisidae). The higher diversity was recorded in wild vegetation (H = 2.829, and 25 species), then the herbal stratum of crop showed the highest affinity (H = 2.140, and 28 species), lowest diversity was recorded during the fallow period (H = 1.050, and 12 species). The early species colonizing the crop (December) belong to the families Thomisidae, Anyphaenidae and Oxyopidae, after them Araneidae and Corinnidae were recorded, the late colonizing were the Salticidae species. Thomisidae species were always the most abundant, predominating from February to April. The lowest density of species of spiders was found on December in the border crop, but the distribution, from the border to the center crop, from January to March, did not show any gradient. The occurrence of several species belonging to different guild in all the plots suggests that the spiders are an important limiting effect on insect populations, as soon as the importance of the preservation of natural areas in techniques of biological control.

Spiders of soybean crops in Santa Fe province, Argentina: influence of surrounding spontaneous vegetation on lot colonization

“…These results are not compatible with those reported by Minervino (1996) and Liljesthrom et al (2002), who found Thomisidae to represent about 50%. The discrepancy might be due to: 1) a different sampling method; these authors placed plants in individual plastic bags and probably the results could have been masked because of the reduced size of the Oxyopidae individuals; 2) different seeding method; they used conventional seeding, which has destructive effects on the soil, whereas in the present work, soil conditions were not altered because of the direct seeding method; 3) different climatic conditions (mid-south Buenos Aires province vs. mid-west Santa Fé province).…”

“…These traps consisted of 8 cm x 10 cm plastic containers, one fourth filled with ethylene glycol 30%, as a preserver. Forty traps were buried, at a rate of 5 equidistant traps in every margin; the remaining 20 traps were distributed in the same way within the crop (Liljesthrom et al, 2002). All captured material was preserved in 70 % ethyl alcohol to be identified at the laboratory.…”

“…Samples were taxonomically identified to the level of a family using a 40x binocular magnifier. The ShannonWeaver index (Minervino, 1996;Liljesthrom et al, 2002) was used to compare spider diversity in the crop and margins:…”

“…Spiders are more abundant in non cultured areas (Nyffeler & Benz, 1987;Desender et al, 1989;Heidger & Nentwing, 1989), and even more so in complex vegetation structures (Samu et al, 1999;Riechert, 1999), insecticide free (Withford et al, 1987;Mansour, 1987;Minervino, 1996;Liljesthrom et al, 2002). Moreover, they are highly susceptible to climatic conditions (Le Sar & Unzicker, 1978).…”

“…Minervino (1996) and Liljesthrom et al (2002) studied the spider community in soybean crops in the Buenos Aires Province. The aims of the present work are to determine the main spider families in a soybean crop in central Santa Fe, determine the influence of spontaneous vegetation on the crop's margins, compare diversity in both habitats at different layers and analyze the colonization of spiders towards the soybean crop.…”