Interaction between cropping systems and white grub (Coleoptera: Scarabeoidea) status (pest or beneficial) on upland rice

摘要

In Madagascar, some soil-dwelling Scarab beetle larvae and adults cause significant damage to upland rice. Insecticidal seed-dressing affords an efficient protection but the economic cost and environmental impact of the technique is not documented, thus justifying research in view of breaking with it in some direct-seeding, mulch-based cropping (DMC) systems. Such a DMC system involving upland rice rotated with soybean since 1998-1999 was thus studied from 2002-2007 in two locations of the Highlands. After four years on this rotation, a positive effect of DMC compared to conventional tillage and of seed-dressing on pest attack and upland rice yield could be observed. No detrimental effect of seed-dressing on soil macrofauna diversity was observed. Over the next four years we observed an increase in macrofauna populations, herbivores, and detritivore/geophagous and predatory biota, particularly under DMC management, although differences in cropping system management did not translate into yield differences in 2005 and 2007. The food preference of larvae (“white grubs”) of three Scarab beetle species belonging to three different families were studied in laboratory as well as the variation of status of seven species among these families according to the soil content in organic matter. Bricoptis variolosa (Cetoniidae) and Hexodon unicolor (Dynastidae) were found to be obligatory detritivores at the larval stage, while Apicencya waterloti (Melolonthidae) was found to be obligatory root-feeder. Heteroconus paradoxus and three other Dynastid species ( Heteronychus plebeius, H.?bituberculatus & H.?arator rugifrons) were found to be facultative root-feeders. Beyond changes in the below-ground fauna composition induced by some DMC systems, the change of the white grub status according to the organic status of the soil, opens up new prospects for the management of upland rice agroecosystems.