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

Abstract

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.

Keywords: Aphis fabae; Beauveria bassiana; aphid biocontrol; fungal–chemical interaction.

Figure 1

The faba bean crop located for initiation of field trials at Laishuitang Village, Qiaotou Town, Cixi City, Zhejiang. Left and right images show the budding stage of overwintered crop used for field trials in early spring and A. fabae (arrowed) damage to the tender shoots and buds of a plant, respectively.

Figure 2

Weather records over the six-week period of field trials with B. bassiana and imidacloprid co-applied for aphid control on faba bean crops in Qiaotou, Cixi, Zhejiang.

Figure 3

Suppression of A. fabae population by B. bassiana ZJU435 ES and 10% imidacloprid WP, which were applied alone at recommended rates (Bb and ImD) and co-applied at reduced rates in two field trials. The co-application rates were 2/3Bb, 1/2Bb, and 1/3Bb Bb plus 1/5ImD (2/3Bb + 1/5ImD, 1/2Bb + 1/5ImD, and 1/3Bb + 1/5ImD). Each field trial was sprayed biweekly over the six weeks. Different lowercase letters denote significant differences at p < 0.05 (Tukey’s test) among the treatments in each trial on each sampling day. Error bar: SD of the mean from three 50 m² plots per treatment.

Figure 4

Relative efficacies of B. bassiana ZJU435 ES applied alone at recommended rate (Bb) and at reduced rates (2/3Bb, 1/2Bb, and 1/3Bb) plus 1/5ImD for control of A. fabae populations on faba bean crops. (A) Relative efficacies of different treatments pooled by sampling occasions from two field trials (n = 6 per treatment for one-way ANOVA). (B,C) Treatment effect (n = 36 per treatment) and time effect (n = 30 per sampling occasion) revealed by two-way ANOVA of all relative efficacies pooled over the six-week period for the two field trials. Different uppercase letters indicate significant differences at p < 0.01 (Tukey’s test). Error bar: SD of the mean from 6 (A), 36 (B), and 30 (C) observations.