Appendix 2: Detailed description of the data set
The real data came from a survey performed in 2008 and 2009 in the Eure department of France (49°25’N,
1°12’E). It comprised data on landscape composition, oilseed rape abundance and parasitism rate obtained from
35 non-overlapping landscapes centred on an oilseed rape field. The sites were selected according to the
variability of their proportions in grasslands, woodlands, and cultivated fields. Seven landscapes were studied
during two years (2008 and 2009), the central field being always different for each sampling date.
In each central field, a 1 ha zone was managed without pesticides and all observations were done within
this zone. Insects were sampled using D-vac suction tools. Abundance of insects per square meter was calculated
on the basis of plant densities, the latter being assessed in six 1 m2 quadrats along a transect.
Two sampling were assessed at two different developmental stages of the oilseed rape (we considered
that plant densities remained the same between the two sampling dates):

During the bud development stage of the oilseed rape (Growth stage 50), 50 plants were randomly
selected evenly spaced along a 100 m transect located within the unsprayed zone. Pollen beetles
were sampled on the selected plants. Pollen beetle sampling took place between 10:00 a.m. and
05:00 p.m. on a day with suitable weather (sunny, non-windy and dry). Insects were identified and
counted in the lab. Averaged pollen beetle abundance per square meter was 250 (min= 11, max=
650).

At the end of the flowering period of the oilseed rape (Growth stage: 65, 67), 30 plants were
randomly sampled along a transect to calculate the parasitism rates. On each plant, we extracted
and counted all pollen beetle larvae from flowers and buds to assess host abundance for parasitoids.
From these larvae we sub-sampled 150 second instar larvae and dissected them under the
microscope to check for parasitism. We identified the species of parasitoids eggs and larvae found
in pollen beetle larvae. Using the crop density and the total number of pollen beetle larvae on 30
plants enabled to calculate the abundance of hosts per square meter (the abundance of second instar
larvae of pollen beetle) for the parasitoid population. Parasitism rate ranged from 0 to 1.
Woodlands, grasslands, oilseed rape fields and previous oilseed rape fields were digitalised based on
aerial photographs and extensive field surveys. This allowed us to compute proportions of each habitat in
circular buffers with radii ranging from 250 m to 1750 m. Appendix 3 gives a few examples of digitalised maps
as well as the composition of each landscape.