A sown grass cover enriched with wild forb plants improves the biological control of aphids in citrus

Abstract

There is increasing interest in the use of sown ground covers in agriculture to provide alternative resources to predators and parasitoids as part of conservation biological control. Nevertheless, there is limited evidence that this approach is effective in commercial orchards, where a wild complex of plants co-occur with the sown plant species. In citrus orchards, ground covers with grasses (Poaceae) were originally promoted to prevent soil erosion. Herein, we analyzed the effect of this sown ground cover on the biological control of Aphis spiraecola Patch (Hemiptera: Aphididae), the main aphid pest on citrus. We therefore first described the ground cover plant composition and their inhabiting aphids in four commercial citrus orchards. Second, we compared the presence of A. spiraecola and its natural enemies between these and four other commercial orchards with bare soil. While grasses represented ∼66% of the ground cover, the rest of the cover comprised mainly Malva sp. (13%), Oxalis sp. (5%) and Sonchus sp. (2%). Poaceae plants and Oxalis sp. harbored stenophagous aphids and Macrosiphum euphorbiae Thomas (Hemiptera: Aphididae), respectively, which appeared sooner in the system than citrus aphids. These aphids may serve as alternative prey or hosts for natural enemies, and thus could enhance the biocontrol of A. spiraecola. By contrast, Malva sp. and Sonchus sp. harbored the potential citrus pest Aphis gossypii Glover and other aphids that appear simultaneously with A. spiraecola. Therefore, by attracting them to the cover, this latter group could relieve the attack of natural enemies on A. spiraecola in the canopy. Although these wild plants may act as reservoirs for A. spiraecola as well as other aphid species that can disrupt the biocontrol services of natural enemies, overall, the sown cover was effective in terms of biological control of A. spiraecola in the citrus canopy. It promoted the early presence of predators in citrus canopies but did not promote the early presence of parasitoids. Predators attacked A. spiraecola colonies before their exponential increase. These attacks resulted in satisfactory aphid control, as citrus orchards with ground cover never exceeded the aphid economic threshold.