Phenology of the invasive coccinellid Harmonia axyridis Pallas and other aphidophages in crops

The multicoloured Asian ladybird Harmonia axyridis Pallas (Coleoptera: Coccinellidae), previously introduced as a biological control agent to control aphids populations, is now frequently considered as an intraguild predator, consuming other aphids natural enemies. The interactions between this exotic ladybird and other aphidophagous species present in Belgian agro-ecosystems are mainly asymmetric in support of H. axyridis. An aphidophages sampling has been performed between 2009 and 2011 in four agrosystems such as broad bean, wheat, corn and potato. The sampling method consisted in the counting of aphids and all developmental stages of aphidophages present in quadrats of 1m² from April to September. Harmonia axyridis, the invasive coccinellid, was mainly observed in broad bean (21.84 ± 6.30 individuals/100m²) and corn (70.83±6.60 individuals/100m²) with other aphidophages such as Episyrphus balteatus (De Geer), Chrysoperla carnea sensus lato (Stephens), Coccinella septempunctata (Linné) and Propylea quatuordecimpunctata (Linné). In corn, our field data showed that immature stages and adults of H. axyridis were found throughout the summer. Adults were observed from mid-June to the beginning of September with two peaks, in the late June and in the beginning of August. The life history of larvae starts in late-June to the beginning of Augustus with only one peak (347.91 ± 59.99 individuals /100m²) in the beginning of July. The peak of H. axyridis arrives when no extraguild preys are present. At this moment, pupae of E. balteatus and C. septempunctata are the main food available to ensure the development of H. axyridis so that native aphidophages could be intraguild prey for H. axyridis. In the four crops, reproduction of H. axyridis starts after those of other aphidophages and so it does not benefit of available prey such as aphid: in corn and broad bean, C. septempunctata reproduce the first, in wheat it is E. balteatus and in potato it is C. carnea. That could be explained by several reasons: (1) H. axyridis reproduces firstly in arboreal habitats and after that comes in agrosystems, (2) the delay between aphidophage reproductions allows a reduction of interactions with other larvae during spring and therefore decrease mortality levels and (3) H. axyridis is able to use pollen as food substitute when no aphids are available.