Does Bird Predation Enhance the Impact of Green Muscle® (Metarhizium acridum) used for Grasshopper Control?*

Abstract

A study at Khelcom, Central Senegal, from September 2008 till June 2009 tested two different dose rates (25 and 50 g conidia/ha) of the mycoinsecticide Green Muscle® (Metarhizium acridum) and an untreated control, against grasshoppers in a field trial on nine 400-ha plots in three blocks. The study area was a deforested sylvo-pastoral reserve, 12.5% cultivated, whereas the remainder was fallow or grassland in succession. Grasshopper densities were calculated by executing transect and quadrat counts on plots. The grasshopper community structure was assessed by systematic sweep-net sampling on plots. From these data grasshopper biomass on plots was calculated for each sampling date. Bird numbers were counted on the same transects by the Distance Sampling technique and their densities calculated from Effective Strip Widths (ESW). Energetic requirements of individual bird species were calculated from digestibility-corrected Field Metabolic Rates (FMRs) and for acridivorous species their daily intake of grasshoppers was calculated. Grasshopper densities were very high, with up to 90 ind./m2 in September and 30–35 ind./m2 in October. Numbers and biomass decreased on treated plots as grasshoppers became infected, and remained significantly different from control plots for three months (until January). The relative importance of grasshopper consumption by birds increased between October (high grasshopper densities) and December (medium densities) from an initial 0.06 (±0.03) %/day to a ceiling of 1.6 (±0.9)% per day. Total grasshopper removal during the dry season was 70 %, whereas during the rainy season this was < 1%. Birds specifically captured large and medium-bodied grasshoppers, but rarely small-bodied species, whose numbers initially increased. These findings were corroborated by field observations and by analysis of regurgitated pellet contents of Montagu's Harriers, i.e., small-bodied grasshoppers were only 1.4 – 2.6% of all grasshoppers taken, whereas they constituted 61 – 68% of random samples from the field. Densities of acridivorous Palaearctic migratory birds, in particular White Stork, Ciconia ciconia, Montagu's Harrier, Circus pygargus and Lesser Kestrel, Falco naumanni, were very high and unprecedented elsewhere. Their numbers largely exceeded the 1% criterion for international importance.