Evidence at the landscape level links high predator/pest ratios to biocontrol services against aphids

Compared with external environment interferences, the interaction between arthropod natural enemies and pests is the key inherent driver determining the strength of biocontrol services. However, the extent to which this effect can suppress pests is still inconclusive. We combined two complementary experiments to determine how variation in an assemblage of generalist arthropod predators modified the level of biocontrol services for suppression of a key aphid pest in cotton fields. Using generalized linear mixed effect models (GLMM) and path analysis, we clarified the causal relationship between predator/aphid ratios (PAR) and (1) the resulting biocontrol services index (BSI) and (2) the aphid population growth index (APGI) as seen in field predator exclusion trials. We also measured the effect of PAR on APGI in more commercial cotton fields. Our results, at landscape level, indicate that when PAR values increased one unit, BSI values improved 34.1 %, and reduced aphid population growth 28.3 % (the standardized effect coefficient in path analysis) during two weeks in field cage-exclusion trials. The effect of high predator/aphid ratios on reducing the aphid population growth rates was also significant in more commercial cotton fields, which were sampled over a longer time interval (4 weeks). Our study confirmed that there was a causal relationship between the natural enemy/pest ratio and the level of biological pest control services for predators and aphid population growth in crop fields. PAR values (predator/aphid ratios) can, therefore, be used to predict the level of biocontrol services in this context instead of relying on more labor-intensive cage exclusion studies.