Is the establishment threshold for the entomopathogenic fungus, Pandora neoaphidis, below the economic injury level of soybean aphid?

Soybean aphid, Aphis glycines Matsumura, is an invasive pest of cultivated soybean. Aphid outbreaks often require the use of broad-spectrum insecticides to avoid catastrophic economic loss. Pandora neoaphidis is a fungal pathogen infecting soybean aphid which has the potential to contribute to natural control of the aphid. We developed a model to estimate the mean per plant establishment density (ED₅₀) of soybean aphid required for the establishment of P. neoaphidis in field populations. Model simulations predicted the ED₅₀ would be 152 (95% confidence interval [CI]: 144-160) for V7 stage soybeans. During the summer of 2009, the threshold was validated in the field using single-soybean plant cages in which manipulated aphid densities ranged from 14 to 889 aphids per plant when P. neoaphidis inoculum was released. Aphids were monitored via bioassays to determine if fungus established. Distribution and per plant density of aphids were explored as predictors of fungal establishment; however, only the log₂-transformed aphid density was a significant predictor. Soybean aphid ED₅₀ for P. neoaphidis establishment was 48 (95% CI: 30-79) aphids per plant. For model and field data, estimated ED₅₀ for P. neoaphidis establishment is below the economic threshold (250 aphids per plant) and economic injury level (674 aphids per plant) indicating that P. neoaphidis can establish in sub-economic populations of soybean aphid. Thus, P. neoaphidis has the potential to infect and maintain soybean aphid density below the economic injury level.