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.