Influence of Plant Diversity on the Numerical Response of Eriopis connexa (Coleoptera: Coccinellidae) to Changes in Cereal Aphid Density in Wheat Crops
Cereal aphids cause economic injury to wheat crops. In Argentina, Eriopis connexa is an indigenous ladybird. In the present study, the numerical response of E. connexa to changes in aphid density on wheat crops with high and low plant diversity was investigated. The study was carried out in Balcarce, Buenos Aires, Argentina, from September to December 2007 and 2008, on two wheat crops with either a higher plant diversity (HPD) with refuge strips or a lower plant diversity (LPD) without refuge strips. Crops were sampled every week and the abundance of aphids and E. connexa was recorded. The dominant aphid species were Schizaphis graminum, Metopolophium dirhodum, and Sitobion avenae. Eriopis connexa showed a linear increase in the numerical response to an increase in aphid density, which varied in space and time. The abundance of E. connexa increased in relation to the crop development and aphid population and was higher in the HPD than in the LPD system. This predator increased its reproductive numerical response only in 2008, with a significant liner response in the HPD system. This suggests that the potential of E. connexa as a predator of cereal aphids also increases directly in proportion to landscape vegetal diversity. 1. Introduction Cereal aphids (Hemiptera: Aphididae) are among the most important pests on wheat crops in the world, since they are found in all temperate regions and cropping systems and have the potential to reduce yields [1, 2]. Depending on the species and the year, aphids can cause economic damage by direct feeding, by injection of toxins that cause yellowing, stunted growth, curled leaves, and malformations, or by transmission of viruses that cause indirect plant injury [1–4]. As a result, aphid damage in wheat crops causes losses in yields and grain quality [5–11]. Insecticide application is the strategy selected by farmers to control aphids in wheat crops around the world. However, the frequent use of a wide range of pesticides has caused many side-effects, including loss of biodiversity, secondary pest outbreaks, the development of insect resistance to insecticides, residual toxicity, and suppression of natural enemies [12–14]. As a consequence, promoting the activity of predators and parasitoids in growing systems to maintain aphid populations below the economic injury level remains a desirable goal [15]. Several groups of natural enemies can control aphid populations in wheat crops, such as parasitoid wasps (Hymenoptera: mainly Aphidiidae) and a very large number of predators such as hoverfly larvae (Diptera: Syrphidae),
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