Effect of Disease Resistance on Secondary Herbivores: Aphid Abundance on Hybrid and Non-hybrid Coffee Varieties in the Chagga Homegardens

Abstract

Coffee accounts for over 20% of Tanzania's income from foreign exchange and has been identified as an essential crop for the future of food security in Africa. Coffee production in the Kilimanjaro region, one of Tanzania's most important coffee-producing areas, is facing threats due to declining coffee prices, climate change, and outbreaks of fungal disease. Fungal-resistant hybrid coffee varieties have the potential to increase productivity and yields, although the ecological impact of their widespread introduction has not yet been determined. This study estimates the differences in aphid abundance and probability of aphid presence on hybrid and non-hybrid coffee plants in the Chagga homegardens. For this study, 1,119 coffee plants were sampled in 45 farms across 3 villages in the Kilimanjaro region. Hybrid coffee plants were 74.7% less likely to have the fungal disease coffee leaf rust (CLR) than non-hybrid plants (p<0.001). However, hybrid status was not found to be a significant factor in predicting aphid presence (p=0.88) or abundance (p=0.71). Factors that were significant in predicting aphid abundance included farm size (p<0.001), insecticide use (p<0.001), village (p=0.001), and ant count (p<0.001). The results of this study suggest that hybrid coffee has no significant bottom-up effect on the dynamics of the Chagga homegarden ecosystem.