Low-Cost-High-Efficacy Control of Faba Bean Aphids by Fungal and Chemical Insecticides Co-Applied at Low and Sublethal Rates.

The high costs of mycoinsecticides restrain their extensive application in green agriculture. Two six-week field trials were performed in spring to test synergistic effects of fungal-chemical interactions against faba bean aphid (Aphis fabae) populations in Zhejiang, China. The treatments (three 50-m² plots each) in each trial included Beauveria bassiana ZJU435 (Bb) and imidacloprid (ImD) applied biweekly at recommended rates (1.5 × 10¹³ conidia and 45 a.i. g ha^-1) and co-applied at reduced rates of 2/3Bb, 1/2Bb, and 1/3Bb plus 1/5ImD, which reduced the cost of the mycoinsecticide by 32-65%. During the first three weeks at 1.3-12.5 °C (daily means), the aphid population steadily increased to a peak in the blank control, and it was much more effectively suppressed by ImD than by Bb and Bb + ImD treatments. As the weather subsequently warmed to 8.7-21.0 °C, the aphid population was increasingly suppressed by the Bb and Bb + ImD treatments, which showed suppressive effects superior or close to those of ImD on days 35 and 42. Percent efficacy values (n = 36) of ImD, 2/3Bb + 1/5ImD, 1/2Bb + 1/5ImD, Bb, and 1/3Bb + 1/5ImD in two six-week trials averaged respectively 86%, 66%, 62.2%, 59.4%, and 58.4%, which significantly differed from one another (p < 0.01). Conclusively, low-rate co-applications of fungal/chemical insecticides offer a 'low-cost-high-efficacy' strategy to promote extensive mycoinsecticide application for sustainable aphid control.