DOI: 10.5061/dryad.35nn0
title: Data from: Population growth and sequestration of plant toxins along a gradient of specialization in four aphid species on the common milkweed Asclepias syriaca

description: Dietary specialization in insect herbivores has long been hypothesized to
 predict tolerance of plant defences, with more specialized herbivores
 being highly tolerant of and sometimes sequestering plant secondary
 compounds. Plant variation in secondary compounds should thus play an
 important and predictable role in shaping the performance and distribution
 of insect communities. We compared the performance of four naturally
 co-occurring aphid species on twenty genotypes of the common milkweed
 Asclepias syriaca. Genotypes of milkweed consistently differed in
 functional traits, including concentrations of toxic cardenolides, while
 the diet breadths of the four aphids ranged from broadly generalized to
 monophagous. The two more generalized species had the highest population
 growth rate overall, while growth rates decreased with increasing
 specialization. In contrast, honeydew exudation as a measure of phloem
 consumption increased with specialization; thus, resource-use efficiency
 was lower in specialist aphids. The two more generalized aphids grew best
 on genotypes with the highest plant growth rate (as an approximation for
 resource availability), while specialist aphids were not affected by plant
 growth. All four species contained apolar cardenolides in their bodies and
 excreted polar cardenolides, but only the most specialized aphid
 Myzocallis asclepiadis was negatively affected by increasing cardenolide
 concentrations of the host plant. Sequestration of cardenolides increased
 with diet specialization, with M. asclepiadis accumulating twice as much
 as any other species, perhaps explaining its susceptibility to plant
 cardenolides. Heritable plant traits differentially impacted co-occurring
 insect herbivores within the same guild. Generalist aphids were
 susceptible to variation in plant vigour but not defensive compounds.
 Increased host specialization resulted in lower resource-use efficiency,
 increased phloem throughput and ultimately higher cardenolide
 sequestration. Variation in these traits is thus likely to determine the
 relative distribution of generalist and specialist herbivores on plants in
 natural communities.
description: Sequential aphid
 population census dataTab-delimited file
 containing aphid census data on 20 plant genotypes. Each replicate plant
 is identified by a unique numeric code ('ID') to indicate
 repeated measures. 'Chamber' refers to one of two growth
 chambers. 'Census' refers to the five sequential censuses, while
 'Growth.days' is the number of days since aphid introduction.
 Aphid numbers are given as 'Juvenile', 'Apterous'
 (unwinged adult), 'Alate' (winged adult), and the summed
 total.Züst
 and Agrawal (2015)_Aphid
 data.txtSequential plant size
 dataTab-delimited file
 containing repeated plant size measurements of 20 plant genotypes. Each
 replicate plant is identified by a unique numeric code ('ID') to
 indicate repeated measures. 'Chamber' refers to one of two
 growth chambers. 'Census' refers to the five sequential
 censuses, while 'Age' is the number of days since planting.
 'Age.aph.int' is the plant age at time of aphid introduction
 (variation is due to some differences in day of planting). Plant size is
 given by the number of leaves, the number of stem nodes, and the plant
 height [cm].Züst and Agrawal (2015)_Plant
 data.txtChemistry
 dataTab-delimited file
 containing chemistry data of plant (Sample.type = Pl), aphid (Sample.type
 = Aph), and honeydew (Sample.type = Hd) samples. 'Chamber'
 refers to one of two growth chambers. For a subset of plant samples, the
 foliar C:N ratio is given in column 'C.N'. All columns headed
 with cX.X are individual cardenolide compounds [ng/mg sample], with X.X
 referring to their HPLC retention time.Züst and Agrawal
 (2015)_Chemistry data.txtHoneydew exudation
 dataTab-delimited file
 containing honeydew weights on all 20 plant genotypes and number of aphids
 present during time of collection. 'Aphid.start' and
 'Aphid.end' are actual numbers of aphids present at the
 beginning and end of honeydew collection, while 'Exposure.time'
 indicates the duration of collection in days. Columns headed
 'day1' through 'day5' are predicted aphid numbers
 based on linearly integrating start and end censuses, and
 'Cumulative.aphids' is the sum of predicted aphids present
 throughout honeydew collection.Züst and Agrawal
 (2015)_Honeydew data.txtGenotype trait
 meansTab-delimited file
 containing trait means for all 20 plant genotypes and aphid growth on
 these plants (RGR.an = A. nerii, RGR.mp = M.persicae, RGR.ma = M.
 asclepiadis). Plant traits include C:N ratio (C.N), absolute growth rate
 at time of aphid introduction (Plant.AGR) and total (Card.tot) cardenolide
 concentration [ng/mg]. In addition, the average sum of the four most polar
 (Card.pol) or six most apolar (Card.apo) cardenolides is
 given.Züst
 and Agrawal (2015)_Genotype
 means.txtComplete R code for Züst
 and Agrawal (2015)R code to recreate all
 analyses in Züst and Agrawal (2015), separated into sections 'aphid
 population growth', 'plant growth', 'honeydew
 exudation', 'chemistry means' and 'trait
 correlations', with each section requiring its own data file provided
 on Dryad.Züst and Agrawal
 (2015)_Rcode.r