Amending soil with insect exuviae improves herbivore tolerance, pollinator attraction and seed yield of Brassica nigra plants

Abstract

To promote circularity in agriculture, the residual –streams from the production of insects as feed and food, such as insect exuviae (moulted skins), can be a sustainable novel organic soil amendment for crop production. Organic soil amendments can influence soil composition by providing nutrients and stimulating the growth of beneficial microorganisms. Soil composition significantly impacts plant growth and resistance against herbivores. However, little is known about the effect of soil composition on flower visitors and seed production. Here, black soldier fly (BSF; Hermetia illucens) exuviae was added to soil to investigate the effects on plant growth, plant resistance against two insect herbivores, and the consequences for attraction of pollinators and seed yield. Brassica nigra seeds were sown in field soil or in field soil mixed with powdered BSF exuviae. Three-week-old plants and the soil of each pot were planted in a common garden. The effects on vegetative and flower traits, interactions with flower visitors and seed yield were quantified. In addition, the performance of the leaf-chewing larvae of Pieris brassicae and the piercing-sucking aphid Brevicoryne brassicae were assessed. Brassica nigra plants grown on soil amended with BSF exuviae showed increased growth, had more flowers and flower visitors, and higher seed production. Herbivory neither affected number of flowers nor seed production, suggesting tolerance and compensatory growth responses to herbivory compared to plants growing in soil without the addition of BSF exuviae. After five weeks fewer aphids were found on plants growing in amended soil. Our findings show that BSF exuviae added to soil can positively affect tolerance to herbivory and seed production of B. nigra, an insect-pollinated plant species. Moreover, adding insect exuviae as soil amendment in the field benefits plant-pollinator mutualism and seed yield even during herbivore attack in the field. Use of by-products from insect production as soil amendment can contribute to sustainable agricultural practices along with the conservation of ecosystem services.