Some Observations on the Predominance of Aphis spiraecola on Citrus in Northwestern Algeria

Aphis spiraecola is the main aphid species found on citrus in Algeria. This study was carried out on Citrus clementina in northwestern Algeria, during a two year period (2016-2017) in the first flushing period (spring). The aphid fluctuation of the populations and their natural enemies, especially the parasitoids, were evaluated based on a weekly sampling of 100 leaves taken on 10 trees (10 leaves/ tree). A. spiraecola colonized citrus trees since the beginning of flushing. The density per young leaf reached a maximum of 78.8 ± 23.4 aphids in 2016 and 44.4 ± 13.0 aphids in 2017 with an average density of 6.0 ± 1.5 aphids/cm² and 4.4 ± 0.6 aphids/cm², respectively, where a significant difference between years (P inferieur 0.05) was observed. The parasitism rate expressed in terms of number of A. spiraecola mummies remained very low, varying between 1.6% in 2016 and 3.0% in 2017 with no significant difference (P > 0.05) between years. Also, the emergence number of primary parasitoids was low for both years with 26.6% in 2016 and 10.8% in 2017. The primary parasitoids of A. spiraecola in 2016 were Lysiphlebus testaceipes and Binodoxys angelicae whereas only L. testaceipes was found in 2017. The total hyperparasitism rate varying between 16.7% in 2016 and 25.7% in 2017 did not differ significantly between years (P > 0.05). Mummies without adult emergence rate were found to be very high varying between 85 and 100%. This partial parasitic failure observed on A. spiraecola underlines many questions related with different factors (climate, ability of aphids to form winged populations to escape to their enemies, impact of hyperparasitoids). The new field of research is concerning the possible presence of endosymbiont organisms that could give to the aphid a defense reaction against its aggressors