Crop heterogeneity may not enhance biological control of rice pests in landscapes rich in semi-natural habitats

Abstract

Agricultural landscapes are undergoing profound changes worldwide, including crop homogenization and loss of semi-natural habitats, which may be detrimental to arthropod natural enemies of crop pests, although interactive effects between these drivers remain understudied. We assessed the relative contribution of crop diversity, mean field size and the proportion of semi-natural habitats on the abundance and diversity of rice pests and their arthropod natural enemies (predators versus parasitoids) across four spatial scales (100, 250, 500 and 1000 m) in 17 rice fields in Hainan Island, China. The overall low abundance of pests and natural enemies measured in rice fields could indicate high biological control level. Semi-natural habitats (including some recently restored ones) were largely represented in the studied landscapes (average proportion of 65 %) and may be biodiversity reservoirs. Indeed, decreased mean field size and increased semi-natural habitat proportion promoted rice pests at the largest spatial scale only, possibly by enhancing their mobility and use of alternative resources. Predators’ response was weak and indicated a positive influence of semi-natural habitats (or crop diversity in absence of semi-natural habitats) at small spatial scales. Parasitoid abundance was low, but tended to support the resource concentration hypothesis (reduced by crop diversity and increased by mean field size). Overall, crop heterogeneity favoured pests, but not natural enemies, its two components reduced mean field size and increased crop diversity were somewhat complementary and modulated by the proportion of semi-natural habitats but depended on spatial scale.