Bombus terrestris as pollinator-and-vector to suppress Botrytis cinerea in greenhouse strawberry

Bombus terrestris L. bumblebees are widely used as commercial pollinators, but they might also be of help in the battle against economically important crop diseases. This alternative control strategy is referred to as pollinator-and-vector technology. The present study was designed to investigate the capacity of B. terrestris to fulfil this role in greenhouse strawberry flowers, which were manually inoculated with a major plant pathogen, the grey mould Botrytis cinerea Pers.: Fr. A model microbiological control agent (MCA) product Prestop-Mix was loaded in a newly developed two-way bumblebee dispenser, and, in addition, the use of the diluent Maizena-Plus (corn starch) was tested.

Importantly, loading of the MCA caused no adverse effects on bumblebee workers, with no loss of survival or impairment of flight activity of the workers during the 4 week flowering period. Secondly, vectoring of Prestop-Mix by bumblebees resulted in a higher crop production, as 71% of the flowers developed into healthy red strawberries at picking (preharvest yield) as compared with 54% in the controls. In addition, these strawberries were better protected, as 79% of the picked berries remained free of B. cinerea after a 2 day incubation (post-harvest yield), while this percentage was only 43% in the control. Overall, the total yield (preharvest × post-harvest) was 2-2.5 times higher than the total yield in the controls (24%) in plants exposed to bumblebees vectoring Prestop-Mix. Thirdly, the addition of the diluent Maizena-Plus to Prestop-Mix at 1:1 (w/w) resulted in a similar yield to that of Prestop-Mix used alone, and in no negative effects on the bumblebees, flowers and berries.

This greenhouse study provides strong evidence that B. terrestris bumblebees can vector a MCA to reduce B. cinerea incidence in greenhouse strawberries, resulting in higher yields. Similar yields obtained in the treatments with Prestop-Mix and Prestop-Mix + Maizena-Plus suggest an equally efficient dissemination of the biocontrol agent into the flowers with only half the initial concentration of Prestop-Mix, which illustrates the importance of the diluent.