Bumblebees land remarkably well in red-blue greenhouse LED light conditions

Abstract

Red-blue emitting LEDs have recently been introduced in greenhouses to optimise plant growth. However, this spectrum may negatively affect the performance of bumblebees used for pollination, because the visual system of bumblebees is more sensitive to green light than to red-blue light. We used high-speed stereoscopic videography to three-dimensionally track and compare landing manoeuvres of Bombus terrestris bumblebees in red-blue light and in regular, broad-spectrum white light. In both conditions, the landing approaches were interspersed by one or several hover phases, followed by leg extension and touchdown. The time between leg extension and touchdown was 25% (0.05 s) longer in red-blue light than in white light, caused by a more tortuous flight path in red-blue light. However, the total landing duration, specified as the time between the first hover phase and touchdown, did not differ between the light conditions. This suggests that the negative effects of red-blue light on the landing manoeuvre are confined to the final phase of the landing.This article has an associated First Person interview with the first author of the paper.

Keywords: Bombus terrestris; Insect flight; Insect vision; Landing behaviour; Light spectrum; Pollination.

Fig. 1.

Experimental set-up and trajectory parameters of a flying bumblebee. (A) Overview of the experimental set-up: (1) landing tube, at 60 cm height; (2) feeding platform (5 cm diameter), positioned on a tripod at 60 cm height; (3) filming area (50×50×50 cm), filmed from the side and from above (using a silver front-surface mirror), including the x-, y- and z-axes of the coordinate system (green dotted lines). (B) Position of light sources, viewed upwards from within the set-up; the landing tube is on the right. The red and blue LED lights are laying on the top panel, while the white lights are positioned 25 cm above, at an angle of 30°. (C) A flying bumblebee including body pitch angle β, flight trajectory d and Euclidian distance d_E.

Fig. 2.

Example landing trajectory of a bumblebee. Top view (A–C) and side view (D–F) of a landing trajectory of a bumblebee flying in white light. Start of total landing (TL, yellow dot), start of leg extension (LE, orange dot) and touchdown (red dot). Video frames are of TL (A,D), start of LE (B,E), and touchdown (C,F). Flight speed throughout time for the complete flight (G) and from the start of TL (H).

Fig. 3.

Duration and tortuosity of landing phases. (A) Duration of TL; (B) duration of LE; (C) tortuosity of TL; (D) tortuosity of LE. Diamonds represent analysed landing manoeuvres in white light (green) and red–blue light (purple). Circles with error bars represent back transformed means with 95% confidence intervals. Linear mixed model test results: (A) F_1,87=1.67, n_w=43, n_rb=53, P=0.20; (B) F_1,95=7.45, n_w=44, n_rb=60, P=0.0076; (C) F_1,87=5.07, n_w=43, n_rb=53, P=0.027; (D) F_1,95=6.66, n_w=44, n_rb=60, P=0.011.

Fig. 4.

Euclidian distance to platform, flight speed and flight height of bumblebees. The average landing dynamics of bumblebees flying in red–blue light (purple) and in white light (green). (A–C) Temporal dynamics of the Euclidian distance to platform d_E (A), flight speed |U| (B), and flight height h (C), with t=0 s at touchdown. Results are shown per light condition as mean (black line) and standard errors of the means (colour band). Each panel consists of two views: temporal dynamics for −1.8<t<0 s including the mean and standard error of the start of the total landing for both groups (vertical lines with shading); temporal dynamics for −0.4<t<0 s including the mean and standard error of the start of leg extension for both groups (vertical lines with shading). (D–I) Back transformed means and 95% confidence intervals for both groups of the landing dynamics metrics: Euclidean distance at the start of TL (D) and LE (E); mean flight speed during TL (F) and LE (G); mean height during TL (H) and LE (I). Linear mixed model test results: (D) F_1,87=0.0013, n_w=43, n_rb=53, P=0.97; (E) F_1,95=0.26, n_w=44, n_rb=60, P=0.61; (F) F_1,87=4.50, n_w=43, n_rb=53, P=0.037; (G) F_1,95=0.59, n_w=44, n_rb=60, P=0.45; (H) F_1,87=0.060, n_w=43, n_rb=53, P=0.81; (I) F_1,95=4.14, n_w=44, n_rb=60, P=0.045.

Fig. 5.

Light spectra and bumblebee spectral sensitivity. (A) Relative photon radiance per wavelength in the white light condition (green line, left axis) and in the red–blue light condition (purple line, left axis), as measured with a spectrometer, together with the relative sensitivity of the achromatic system of bumblebees per wavelength (dashed line, right axis; Skorupski et al., 2007). (B) Relative achromatic input in the bumblebee brain in the white light condition (green) and in the red–blue light condition (purple).