DOI: 10.5061/dryad.t24gg
title: Data from: Fitness consequences of indirect plant defence in the annual weed, Sinapis arvensis

description: 1. Plant traits that enhance the attraction of the natural enemies of
 their herbivores have been postulated to function as an ‘indirect
 defence’. An important underlying assumption is that this enhanced
 attraction results in increased plant fitness due to reduced herbivory.
 This assumption has been rarely tested. 2. We investigated whether there
 are fitness consequences for the charlock mustard Sinapis arvensis, a
 short-lived outcrossing annual weedy plant, when exposed to groups of
 large cabbage white (Pieris brassicae) caterpillars parasitized by either
 one of two wasp species, Hyposoter ebeninus and Cotesia glomerata, that
 allow the host to grow during parasitism. Hyposoter ebeninus is solitary
 and greatly reduces host growth compared with healthy caterpillars,
 whereas C. glomerata is gregarious and allows the host to grow
 approximately as large as unparasitized caterpillars. Both healthy and
 parasitized P. brassicae caterpillars initially feed on the foliage, but
 later stages preferentially consume the flowers. 3. In a garden
 experiment, plants damaged by parasitized caterpillars produced more seeds
 than conspecific plants damaged by unparasitized caterpillars.
 Reproductive potential (germination success multiplied by total seed
 number) was similar for plants that were not exposed to herbivory and
 those that were damaged by parasitized caterpillars and lower for plants
 that were damaged by healthy unparasitized caterpillars. However, these
 quantitative seed traits negatively correlated with the qualitative seed
 traits, individual seed size and germination success, suggesting a
 trade-off between these two types of traits. 4. We show that parasitism of
 insect herbivores that feed on reproductive plant tissues may have
 positive fitness consequences for S. arvensis. The extent to which plant
 fitness may benefit depends on parasitoid lifestyle (solitary or
 gregarious), which is correlated with the amount of damage inflicted on
 these tissues by the parasitized host.
description: seed and plant
 characteristics in response to herbivore treatmentIn the experiment
 Sinapis arvensis plants were exposed to one of four treatment: no
 herbivory (=ct), healthy Pieris brassicae caterpillars (=Pier),
 caterpillars parasitized by the gregarious Cotesia glomerata (=Cot), and
 caterpillars parasitized by the solitary Hyposoter eneninus (=Hyp). Plants
 were initially grown in the greenhouse but at the onset of flowering, they
 were transferred to tents placed in a garden to which pollinators were
 added. At this time point, the plants were exposed to the treatments. The
 experimental unit was a tray with 6 plants (=replicate), There were 4
 replicates per treatment. When insect development was completed, we
 counted 1) the number of undamaged seed pods and 2) the number of damaged
 seed pods. Plants were then transferred to a garden in which the control
 plants were already present. When the seedpods had matured and dried, they
 were collected. Seed pods that had grown additionally were collected one
 month later (=new seed pods). Seeds were separated from their pods and
 weighed (=total seed mass in g). Per plant, cohorts of 20 seeds were
 weighed and their mean was calculated (=mean individual seed mass in
 microgram). We calculated the number of seeds per plants by dividing the
 weight of all seeds produced by a plant by its individual seeds mass
 (=number of seeds). After 2 years of storage in the dark, germination
 success of the seeds was determined by randomly choosing 50 seeds from
 each plant and allow them to germinate on wet filter paper. Germination
 success was calculated as the fraction of seeds that germinated out of 50.
 Reproductive potential was then calculated as the product of seed number
 and germination success. The data on the seed characteristics are in the
 ‘seed data per tray’ sheet, individual seed mass are in the sheet ‘seed
 mass of individual seeds) and the data on the seed pods are in ‘seed pods’
 sheet.dryad.xlsx