Contrasting effects of host crops and crop diversity on the abundance and parasitism of a specialist herbivore in agricultural landscapes

Abstract

Landscape complexity affects herbivores in agroecosystems, but consequences on pest control services are variable. Carryover effects of landscape composition in previous years on herbivore control may be important, but have been seldom assessed. Understanding landscape complexity effects at different temporal and spatial scales is important to improve sustainable pest control services. We examined the effect of agricultural landscape complexity (e.g., the percentage of semi-natural habitats) on cereal leaf beetle (CLB), Oulema melanopus L., and its parasitism by Tetrastichus julis (Walker). From 2014 to 2015, we assessed CLB abundance and parasitism in 54 fields along a gradient of landscape complexity (2–70% of semi-natural habitats) in southern Alberta. We used generalized linear models to test the effects of percentage of crops and semi-natural habitats, and landscape and crop diversity on CLB abundance and parasitism at 0.5, 1, 1.5 and 2 km spatial scales. CLB abundance decreased with higher crop diversity at the 0.5 km scale and increased with CLB host crops in the current and previous years at multiple scales, supporting the resource concentration hypothesis. CLB parasitism increased with CLB abundance and in landscapes with increased canola and alfalfa during a year of low CLB abundance. CLB abundance had contrasting associations with semi-natural habitats: positive with woodlands and negative with pastures. Our study suggests that crop diversity reduces the abundance of this specialist pest in agricultural landscapes likely by a dual effect of reducing host crop area and increasing habitats with resources for parasitoids.