Estudo experimental da associação de Runcinioides argenteus (Araneae, Thomisidae) em Trichogoniopsis adenantha (DC) (Asteraceae)

AUTOR(ES)
DATA DE PUBLICAÇÃO

2001

RESUMO

In this study, some aspects of natural history of the tritrophic interactions of the system: Trichogoniopsis adenantha (DC) (Asteraceae) - herbivores and pollinators - sit and-wait predator, Runcinioides argenteus (Araneae, Thomisidae) were investigated in the Serra do Japi, southeast Brazil. To better understand this trophic system, the study included the plant phenology; investment in achene production and seasons of higher number of viable achenes (intact and fertile); population phenology of the predator R argenteus, and types of prey captured; and foraging sites used by this spider in relation to prey availability. Since the spider can prey on pollinators as well as predators of achenes and herbivorous of other floral tissues, it can interfere in the herbivore assemblage and affect the plant reproductive success. The principal objective ofthis study was to verify experimentaly with paired plants (with and wthout spider) whether plants with spiders present have a lower ratio of intra and interspecific co-occurrence among endofagous insects in flowerheads, and a higher number ofintact and non-fecundated achenes. The plant T. adenantha produced more reproductive branches and higher number of achenes per flowerhead in the rainy season (September-March). During April it was observed the higher rate of fertilized and intact achenes while the higher number of achenes injured was observed in October and December. The R argenteus population started to grow in the beginning of summer (December), reaching a peak in March, and decreased in autumn, with lowest density in winter and spring, when the individuaIs became adults. The highest recruitment occurred in November and December. Time-Iag analysis (with until 3 months delay) showed synchrony between beginning of the rains, the flowering period of T. adenantha, increase of abundance of arthropods (potential prey of the spider) and increase of R. argenteus density, with each class of organism somewhat delayed one to three months in respost to abundance to the direct1y related conditions. Of the 595 individuaIs of R argenteus observed, 76 (12,8%) were preying on arthropods belonging to several guilds, as herbivores, pollinators and predators. Most of these prey were apterous species, or those that remain in the branch for longer periods. R argenteus occupied substrats with higher prey ftequency, indicating that, like other thomisid spiders already studied, it can respond to the quality of foraging sites, according to Oprimal Foraging Theory. In general, in the experimental study the plants without R argenteus had a higher number of Trupanea sp. (Diptera, Tephritidae) per flowerhead, while an other common herbivore, Melanagromyza sp. (Diptera, Agromyzidae) was not affected. Plants without predators had higher proportion of infested flowerheads, and a higher number of attacked achenes. Nevertheless, the spiders had an effect against on1y one species of achene predator, the endophagous Trupanea sp., not changing the rates of achene damage made by Melanagromyza sp. or by caterpillars of Geometridae sp. (Lepidoptera). The spiders had not effect in the rate of fertilized achenes, indicating that their presence only benefited the plant

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