Causal relationship between biodiversity of insect population and agro-management in organic and conventional apple orchard

Abstract

Organic management of fruit orchards may increase biodiversity and therefore contributes to achieving an ecologically balanced and productive agroecosystem. In 2013–2015, using a standard methodology for field monitoring, our study investigated and described dynamics of selected insect indicator taxa in the soil, on orchard surface and apple trees in an organic apple orchard and a reference conventional orchard in the region of Plovdiv, Bulgaria. Aiming to determine the impact of agro-management on biodiversity, our study revealed statistically significant correlations between biodiversity (i.e., as indices of the diversity of Shannon (entropy) and Simpson (1-D)) and agro-management practices (i.e., as an agricultural intensification index (AI index)). We found that density and diversity of insect indicator taxa were high in organic soil and in the conventional soil, which was attributed to above-the-norms rainfall in 2014 and 2015 and agro-management practices such as mulching and organic fertilization. The cubic regression models showed positive correlations between the AI index and biodiversity indices of indicator taxa in organic soil (R 2 = 0.489 to 0.497) and on orchard surface (grassed inter-rows) (R 2 = 0.399 to 0.419). On organic trees, changes in population dynamics of beneficial insect taxa followed the changes of pest insect taxa and were related to food availability and climate conditions. Here, the best-fit linear regression models signified that ecological intensification through organic practices here expressed as high-AI index leads to a high diversity (i.e., high indices of Shannon and Simpson) of key beneficial insect taxa such as Coccinellidae, Chrysopidae, and Cantharidae which keeps the pest population below economic threshold levels. Farmers, therefore, should target practices leading to higher density and diversity of beneficial added by measures such as pheromone mating-disruption dispensers and selective bio-pesticides. Our study presents an example of how can biodiversity be assessed in such complex agro-ecological system as orchards are. However, we suggest re-designing the AI index to reflect important factors such as agroecological conditions (e.g., variable climate, soil fertility) and agro-management practices (e.g., time of mowing, irrigation regime, and type of pesticides and their application).