Abstract
Plants are known to affect their soils in ways that can have positive or negative effects on themselves and other plants, often described as plant-soil feedbacks. However, less is known about the potential for herbivores to change the soil in ways that affect plant growth. Many sap-feeding insect herbivores produce a sugary secretion called honeydew. Here, we ask if seasonal pulses of accumulated aphid ( Aphis nerii ) honeydew reduce the availability of soil nitrogen below milkweed ( Asclepias fascicularis ) host plants, and how honeydew-conditioned soil affects the relative seedling growth of host plants and other, potentially competing plant species. We hypothesized that: (1) honeydew would reduce plant-available nitrogen in the soil, reflecting the expectation for increased microbial immobilization in response to high carbon inputs, and (2) reduced nitrogen availability would have a greater negative effect on the growth of cheatgrass ( Bromus tectorum) and yellow star-thistle ( Centaurea solstitialis ) than milkweed, reflecting the expectation of greater nitrogen limitation in disturbance-adapted, non-native, annual plant species compared with native, perennial plants. We found that the addition of honeydew at accumulated levels reduced total inorganic soil nitrogen by 48.8% relative to controls. Unexpectedly, milkweed seedlings were 90% taller and had 4.2 times greater dry biomass in honeydew-conditioned soil than in the control group, but we did not observe any effects on the growth of cheatgrass ( Bromus tectorum) and yellow star-thistle (Centaurea solstitialis ) seedlings. These results suggest the possibility of a positive plant-herbivore-soil feedback for milkweeds, mediated by aphid honeydew.