Social change and indigenous pest outbreak in the Andes

Andean potato weevils (Premnotrypes spp.) are challenging agricultural sustainability and on-farm agrobiodiversity conservation throughout the Andean centre of crop domestication. Despite being native potato feeders in an 8,000 year old agricultural system, they were not reported as key pests until 1929. The causes leading to this putatively recent population buildup are poorly understood. An ethnoecological approach was used to address this imperative in a confederation of 16 pre-industrial farming communities in Bolivia. Fields were sampled for arthropod pests, pathogens and nematodes during planting season (February to March). Participatory observations, interviews and farmer workshops were conducted post harvest (June to September) with approximately 10 farmers per community (n=153). Potato weevil (Premnotrypes latithorax) was the dominant herbivore observed. The proportion of harvested potatoes infested with P. latithorax averaged 34.6 ± 8.8 %, with infestations averaging 49.2 ± 5.7% in the most heavily infested community. No weevil-resistant potato varieties were identified by farmers. In addition, multiple farmers reported losing their most susceptible varieties due to heavy infestations. Surveys revealed four non-exclusive potential causes of weevil outbreaks: (1) agricultural range expansion, (2) homogenization of potato genetic composition (3) land-use intensification and (4) climate change. The reported age of weevil infestations averaged 5.4 ± 2.5 years. This timing coincides with the reported dismantlement of common-field agriculture in the region. These findings suggest that land-use intensification following common-field dismantlement is the key mechanism leading to Andean potato weevil outbreaks in the region, and that weevil outbreaks impose strong directional selection for resistance in Andean potatoes, compromising on-farm conservation of potato genetic diversity.