Documentation and predatory potential of spiders in rice ecosystem and impact of insecticides on spiders

Abstract

The survey conducted to document the spiders in the paddy fields of Thiruvananthapuram district during the Virippu and Mundakan seasons of 2005-’06, brought out sixty five species of spiders, which fall into two major groups, hunters and web builders and belonged to eleven families and seven foraging guilds. The species composition of hunters was more than the web building spiders, although the latter was numerically dominant. The dominant guild was orb web weavers. Araneidae was the family with more species composition, followed by Tetragnathidae and Salticidae. The predominant genus being Tetragnatha, and species, Tetragnatha mandibulata Walckenaer. The other dominant species were Tetragnatha maxillosa Thorell, Argiope anasuja Thorell, Neoscona rumpfi Tikader & Biswas, Telamonia dimidiata Simon, Bianor carli Reimoser, Oxyopes javanus Thorell, Peucetia viridana Stoliczka, Pardosa pseudoannulata Böesenberg & Strand and Thomisus projectus Tikader. Seventeen species of spiders were reported for the first time from the rice fields of Kerala, and it included the three dominant spiders, viz., N. rumpfi, B. carli and T. projectus. Spiders were also observed in the field bunds and border weeds, and from the webs built among plants, in between the plants and on individual leaves. The seasonal occurrence of spiders during the Virippu and Mundakan seasons did not show any remarkable difference. Among the eleven families, six were more populated viz., Tetragnathidae, Araneidae, Salticidae, Lycosidae, Oxyopidae and Thomisidae. The predominant family was Tetragnathidae. The vegetative and reproductive stages of the crop harboured more spiders and the highest population was recorded during the reproductive stage. The crop growth stages had significant influence over the spider population, where as the weather parameters had no correlation. The ten spiders showed a definite preference for the different hoppers, bugs and lepidopterans when evaluated for their prey preference. The spiders A. anasuja, P. viridana, T. dimidiata and T. projectus broadened their preference to the orthopteran and coleopteran pests. The predatory potential of the spiders showed that the spiders which consumed more number of prey insects per day in a mixed diet took comparatively lower numbers when the prey insects were provided individually. A. anasuja was the most potential predator and P. viridana was the spider with highest rate of hyper predation and the most cross predated insect predator was C. lividipennis. The parasitoids were not consumed by the spiders. The topical application of chemicals recorded higher mortality of spiders than when released on the treated plants. Among the chemicals, imidacloprid 0.005 per cent proved to be safer and triazophos 0.05 per cent more toxic. Azadirachtin 0.004 per cent had more toxicity among the neem formulations. In the field, better recolonization of spiders was noticed in the neem products treated plots than the synthetic pesticides, imidacloprid proved to be the safest chemical for the spiders and all the insect natural enemies and was quite promising for the pests. From the results obtained, it can be concluded that pest management will be effectively executed in the field by these potential predators, which are abundant and had specific adaptations which overpowered the other natural enemies in the rice fields. In a naturally balanced rice ecosystem, in a pesticide free environment, these carnivores can survive and when assembled into groups, they could contribute to significant reductions in pest numbers, which have a positive effect on crop production and is beneficial to farmers. Imidacloprid 0.005 per cent could be utilized for pest control when infestation is severe without harming the spider fauna. Hence, conservation biological control could be practiced along with spider mediated IPM.