Impact of caterpillars and plant-parasitic nematodes on Chinese tallow tree are more strongly affected by nematode density than by caterpillar identity

The relative importance of herbivore identity and density in determining the outcome of above-belowground herbivore interactions has not yet been fully explored. Here, we tested the effects of foliar insect identity (the specialist Gadirtha fusca vs. the generalist Spodoptera litura ) on the number of root knots and the effects of nematode density on the performance of the two leaf herbivores. We found that plant-mediated interactions between the two caterpillars and the nematode were highly density-dependent. Herbivory by G. fusca facilitates the root-knot nematode Meloidogyne incognita , but only under the low density of nematodes. Conversely, infestation of M. incognita inhibited the performance of both caterpillars, which both grew smaller on nematode-infested plants and caused lower damage on plants under the low density of nematodes compared to plants with no nematodes. Both G. fusca or S. litura and the high density of nematodes herbivory either alone or in combination all greatly reduced plant growth and biomass of tallow compared to control plants with no herbivory. However, the combinative herbivory between the caterpillars (especially G. fusca ) and the low density of nematodes exerted antagonistic effects on tallow tree that plant growth and biomass were similar or even higher than for control plants with no herbivory. Flavonoids and tannins varied among tissues and were also greatly influenced by nematode density. These results suggest that interactions among leaf and root organisms and their effects on the host plant appear to be more affected by root herbivore density than leaf herbivore identity.