Insect population dynamics, parasitism and behaviour in response to different spectra of artificial light at night

Field experiment data

We established the insect communities in the field in 47.5 x 47.5 x 47.5 cm mesocosms and observed their population dynamics for 17 weeks (03. 06. 2017 to 04. 10. 2017). The experimental plant-insect communities consisted of broad beans (Vicia faba, L., var. the Sutton) as a resource for three aphid species: (1) the black bean aphid Aphis fabae (Scopoli), (2) the pea aphid Acyrthosiphon pisum (Harris), and (3) the vetch aphid Megoura viciae (Buckton). The aphid M. viciae was attacked by the parasitoid Aphidius megourae (Stary) and the communities were exposed to the natural hyperparasitoid community. Mesocosms were located 1.5 meters apart on benches 1 m above the ground in a contained field site close to the University of Exeter campus in Cornwall (UK).

For the 7 different spectra treatments we used coloured near-monochromatic LEDs, with a single peak emittance across the visible and near-UV spectrum at wavelengths of 385, 447, 469, 475, 518, 607 and 630 nm plus a dark control. The dark control was exposed to the natural background light levels at night. We kept photon flux around 7.1 μmol m-2 s-1, similar to the output of a white light LED at 20 lux, for all light treatments apart from the UV treatment (2.4μmol m-2 s-1). From an initial 7 replicates per treatment, we only included those where all species had been established in the beginning, leading to 6 replicates for the red and green light treatment and 5 replicates for the UV treatment. Artificial lights were only turned on at night, by means of a dusk-dawn sensor, switching on at 70 lux and off at 110 lux. Barriers between the cages prevented spill over of light to other mesocosms.

Over the 17 weeks, all species abundances were monitored by counting aphids and parasitoid mummies (for parasitoid abundance) on half of all plants per cage on a weekly basis. In weeks 12 and 14 we sampled 10% of the aphid mummies per mesocosm, kept these in gelatine capsules in the laboratory for the parasitoids to emerge and identified any emerging hyperparasitoid. At week 10 we placed a 2-week old bean plant in each mesocosm for 3 weeks after which we measured dry plant biomass. Each plant was placed in a netted cage to exclude aphids whilst in the mesocosm.

Parasitoid attack rate and behaviour

To test for parasitoid behaviour and their impact on the aphids we run two experiments : (i) 24-hour attack rate and (ii) parasitoid flight to light behaviour. (i) and (ii) were done for all light treatments used in the field experiment but kept at 20 lux with the addition of a white light LED treatment. For (i) we used 150 third instar M. viciae aphid individuals on single 2 week old plants within 47.5 × 47.5 × 47.5 cm cages. One female A. megourae parasitoid was placed in each cage for a 24-hour period and removed afterwards. This experiment was done in a controlled temperature room at 20°C and 16: 8 hours light: dark period. All aphid mummies that had formed after 2 weeks were counted. For (ii) we used 2 week old bean plants infested with 150 third instar M. viciae aphid individuals that were placed in a cage in complete darkness. Different light treatments were then applied over the top of the cage. 20 mated female A. megourae parasitoids were then released into the cage. After one hour the locations of the parasitoids (either on the plant or away from plant) were noted. Each treatment was replicated 6 times.