Optimising Berlese funnel extraction for population estimates of adult Cyrtobagous salviniae from Salvinia molesta

Abstract

ABSTRACT The invasive floating aquatic macrophyte giant salvinia (Salvinia molesta) can rapidly alter ecosystem stability by forming a thick vegetative mat on the water surface. Management efforts are often required to clear waterways, and field releases of the salvinia weevil (Cyrtobagous salviniae) has been successful in restoring aquatic systems. Measuring weevil densities is critical to determine the success of this agent, thus routine Berlese funnel extractions from salvinia should be conducted. A laboratory study was designed to examine how to optimise adult weevil extraction under realistic salvinia biomasses and light intensities (heat source), and a second lab study was conducted to determine the effect of temperature and relative humidity on adult weevil behaviour. The objectives were to improve recommendations to ensure accurate population estimates in a reasonable period and understand what may drive adult weevil movement. We found that weevil movement from salvinia was initiated by increasing temperature above 40°C and decreasing relative humidity below 40%. For the purpose of expediting management decisions, we suggest completing extractions between 48 and 72 h, which requires biomass to be between 0.3 and 0.6 kg with halogen light bulbs of 60 or 75 watts. Additionally, we discovered that thermal stress to the insect was buffered by relative humidity, with individuals showing signs of stress at 3.5°C less when subjected to low initial humidity (∼55%), relative to high (100%). The change in thermal tolerance under different humidity scenarios may be explained by the weevil being a semi-aquatic insect that requires an aquatic plant host.