Environmental factors associated with the distribution of Simulium perflavum (Diptera: Simuliidae) among streams in Brazilian Amazonia

Abstract

To investigate the aquatic distribution of Simulium perflavum Roubaud, 97 streams were sampled (1995-1996) over a wide (12°57' S-04°25' N; 67°42'-59°56' W) geographic area of Brazilian Amazonia. Simulium perflavum was collected in 40 (41.2%) of the streams sampled and reported for the first time from the states of Rondonia and Acre. Last instar larvae of S. perflavum were analyzed at the cytotaxonomic level, giving added certainty to the species-level identification. At each sampled site, 10 variables were measured (temperature, water velocity, depth, width, discharge, streambed-particle size, riparian vegetation, canopy cover, pH, presence of upstream impoundments). Principal component analysis (PCA) and logistic regression were used to determine significant associations between stream-site conditions and larval distribution. Based on stream-site characteristics the occurrence (presence/absence) of S. perflavum larvae could be correctly predicted 75.3% of the time. Larval distribution showed a significant (p < 0.001) positive association with small, slow, sandy bottomed, open, streams near impoundments. Results agree with similar work in the Nearctic region, suggesting that the general response of black fly distribution to environmental conditions might be universal in nature.