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Multitrophic species interactions are shaped by both top-down and
bottom-up factors. Belowground symbionts of plants, such as arbuscular
mycorrhizal fungi (AMF), can alter the strength of these forces by
altering plant phenotype. For example, AMF-mediated changes in foliar
toxin and nutrient concentrations may influence herbivore growth and
fecundity. In addition, many specialist herbivores sequester toxins from
their host plants to resist natural enemies, and the extent of
sequestration varies with host plant secondary chemistry. Therefore, by
altering plant phenotype, AMF may affect both herbivore performance and
their resistance to natural enemies. We examined how inoculation of plants
with AMF influences toxin sequestration and performance of two specialist
herbivores feeding upon four milkweed species (Asclepias incarnata, A.
curassavica, A. latifolia, A. syriaca). We raised aphids (Aphis nerii) and
caterpillars (Danaus plexippus) on plants for six days in a fully
factorial manipulation of milkweed species and level of AMF inoculation
(zero, medium, and high). We then assessed aphid and caterpillar
sequestration of toxins (cardenolides) and performance, and measured
defensive and nutritive traits of control plants. Aphids and caterpillars
sequestered higher concentrations of cardenolides from plants inoculated
with AMF across all milkweed species. Aphid per capita growth rates and
aphid body mass varied nonlinearly with increasing AMF inoculum
availability; across all milkweed species, aphids had the lowest
performance under medium levels of AMF availability and highest
performance under high AMF availability. In contrast, caterpillar survival
varied strongly with AMF availability in a plant species-specific manner,
and caterpillar growth was unaffected by AMF. Inoculation with AMF
increased foliar cardenolide concentrations consistently among milkweed
species, but altered aboveground biomasses and foliar phosphorous
concentrations in a plant species-specific fashion. Increased herbivore
sequestration of cardenolides followed AMF-mediated increases in foliar
cardenolide concentrations. Aphid performance declined with increasing
foliar cardenolide concentrations, while caterpillar survival increased
with aboveground biomass. Our findings suggest that by altering plant
phenotype, the availability of AMF in soil has the potential to influence
both top-down (via sequestration) and bottom up (via plant defense and
nutr...
199 views reported since publication in 2019.