Effects of Organic and Synthetic Fertiliser on Insect Herbivore Populations in Quinoa

Abstract

Fertilisers are used in agriculture to promote crop productivity. There are many nutrient sources in fertilisers, and different sources cause variation in plant defence and pest tolerance. Assessing how pests respond to different types of fertilisers is key to develop integrated pest management programs. Here, we assessed how two fertilisers applied to quinoa (Chenopodium quinoa Willd.) affected cowpea aphid (Aphis craccivora Koch.) and Lygus spp. survival and reproduction. Quinoa is a newly introduced crop in the USA, and fertiliser regimes using organic or synthetic nitrogen sources are being evaluated. We quantified growth and survival of each pest on organic- and synthetic-fertilised quinoa for two varieties in cages and the open field. We also quantified gene expression associated with three phytohormones involved in chemical defence to identify potential mechanisms of treatment effects. In cages, organic-fertilised quinoa had the fewest aphids and lowest Lygus survival. However, the effects were weaker in the open field, where aphids were marginally less abundant on organic-fertilised plants and no effects of fertiliser on Lygus were observed. There was also evidence for fertiliser altering defence gene expression, suggesting a possible mechanism impacting pest behaviour and fitness. Overall, our study shows that fertilisers from different sources may provide varying levels of pest control in quinoa crops.