Trophic impacts of a rolled-rye mulch on the lettuce aphid (Nasonovia ribisnigri) and the predatory midge (Aphidoletes aphidimyza) in lettuce grown in organic soils

Lying in Southern Québec (Canada) are small patches of highly fertile organic soil producing 90% of Canadian lettuce. However, this precious resource is steadily diminishing due to conventional farming practices. To protect this asset, practices, including rolled rye mulches, are being slowly implemented without full knowledge of their impacts on field trophic chain actors. Existing studies of such impacts are almost exclusively done on mineral soil and with soy or corn leaving a knowledge gap regarding other soil and crop types. During summer 2021, we attempted two in situ experiments to assess the impacts of a rolled-rye mulch on colonization rate and population growth of lettuce aphids (Nasonovia ribisnigri). Colonization rates were monitored by visual observation of aphids and their predators while population growth and metatibial lengths were measured in manually introduced aphid populations in exclusion cages. Rye mulch was found to negatively impact lettuce aphid colonization rate, population growth, metatibial length as well as lettuce weight and was positively correlated with predator colonization rate.