Thesis (BS Biology) -- University of the Philippines Mindanao, 2009

Earthworms are primary contributors to the stability of soil health and fertility. As a general representative of soil fauna, they are commonly selected as indicator for assessment of the environmental risk of agricultural practice. To compare the effects of manual and chemical weeding (using paraquat and glyphosate), and determine other factors that may affect earthworm populations in a banana plantation, a long term field experiment was conducted in two sites (15% and 25% slope). The field experiment was supplemented by an acute toxicity and pot experiments using glyphosate and Pontoscolex corethrurus as the test species. Based on two years of field observation, the earthworm species composition was found to differ between the two sites. The 15% site tends to have a higher soil organic matter content. Earthworm populations were on a declining trend in both sites. But there was no significant difference in earthworm count between manual and chemical plots (15% slope: F-ratio:0.01, P=0.95; 25% slope: F-ratio: 3:44, P=0.21). rainfall, organic mulch and weed cover were not significantly correlated with the earthworm counts. However, the declining pH in both sites could help explain the decline in earthworm populations. The contact filter paper test failed to show evidence of acute toxicity at glyphosate application rates up to 40x the normal concentration, but the maximum rate tested (50x) significantly caused a slow response to stimuli and sublethal effects. Glyphosate application rates up to 50x the normal application, the highest concentration tested, did not significantly affect the growth rate of the test species in the pot experiment (F=0.60, P=0.73) experiment suggest that both weed management treatments do not pose a significant threat to earthworms under the conditions studied. The negative effect of declining pH needs further study