Populations of aphid natural enemies in agroecosystems, with special emphasis on the Multicoloured Asian Ladybird Harmonia axyridis Pallas (Coleoptera: Coccinellidae)

The international trade of goods and food can lead to the introduction of alien species in the importing countries. Even if in the most cases there is no negative impact induced by alien species, in some cases the new imported species can spread out of control. These species can cause economical and ecological damages. The Multicoloured Asian ladybird Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae) is one of these species which was intentionally introduced for biological control and has spread from its native range in Central and Eastern Asia to large parts of North and South America, Europe and Africa. The decline of native species is linked to the spread and the aggressive behaviour of H. axyridis. The objective of this thesis is the evaluation of aphid's predator population densities and predator diversities, with a special focus on H. axyridis in agroecosystems in Wallonia, in the South of Belgium. First, field crops preferences of H. axyridis were determined. Later aphid's predator population changes during time were evaluated. Finally H. axyridis population changes between two crop farming were evaluated. Our main findings were as follows. Results, based on insects collected since 2001, highlighted that the first collected H. axyridis was in 2002 and the population of H. axyridis is increasing until 2008. In the same time population of Adalia bipunctata, Propylea quatuordecimpunctata and Psyllobora vigintiduopunctata, three native species, are decreasing. Samplings in agroecosystems highlighted that aphid's predator community is composed of few dominant species: three coccinellids Coccinella septempunctata, P. quatuordecimpunctata and H. axyridis, one syrphid Episyrphus balteatus and one chrysopid Chrysoperla carnea. We have shown that H. axyridis do not invade all Wallonia crops at the same rate because corn and broad bean are more invaded that wheat and potato at both larval and adult stages. Moreover in corn, H. axyridis populations has strongly increased from 2009 to 2011. Finally, organic farming do not enhance abundance of H. axyridis but lead to increase the total abundance of aphid natural enemies.