Pollinator supplementation and flower visitation
To create a P+/P- treatment we supplemented P+ cages (N=14) with bumblebee hives (Bombus terrestris L. , Natupol Seed, Koppert, https://www.koppert.com/natupol-seeds/) on the 22nd of June at the onset of flowering (BBCH-61). The hives contained approximately 2-5 workers foraging for pollen and nectar and 5-8 males collecting nectar. Hives were placed at 1 m above the ground facing east for the duration of crop flowering, until the 27th of July. At the end of the experiment, we found high variation in the abundance of bumblebees inside each hive (mean±sd =13.6±5.1), but there were no differences in bumblebee abundances between H+/H- treatments (t-test: p=0.4) and bumblebee abundances in the hives was not correlated with pollinator visitation rates (Pearson rho: -0.14).
To investigate effects of herbivores on pollinators we carried out pollinator visitation observations in each P+ cage in both herbivory treatments (N=14). Between the 23rd of June and the 10th of July, pollinators in P+ cages were surveyed 15 times. Surveys were carried out under good weather conditions (>15°C and no rain) between 1-6 pm. After each survey, the number of open flowers were counted, initially on 10 plants and from the 7th of July, when flowering was decreasing (survey round 7), in a 1 m2 quadrat (¼ of the cage). The same plants and quadrat were observed in every survey round. Pollinator visitation rate per flower and foraging behaviour were recorded for a duration of 10 min, initially on the 10 plants, where the number of flowers had been counted and later in the 1 m2 quadrat. For each visit, we noted whether pollinators were legitimately visiting flowers by inserting their proboscis through the front of the flower opening, visiting extra floral nectaries (EFN) located underneath the stipules, or robbing nectar by inserting their proboscis through a hole at the base of the flower tube (Tasei, 1976). Because of low nectar reserves in all the hives due to spillage during transportation, pollinator behaviour might have been affected in the first half of the experiment, with more nectar robbing behaviour than would have been the case if hives had not been sugar-starved. From the 7th of July, each colony was supplemented with sugar-water. The timing of sugar-water supplementation was included as a factor in the analyses of pollinator visitation and foraging behaviour.