Plants link interactions between aboveground and belowground organisms. Herbivore-induced changes in plant chemistry are hypothesized to impact entire food webs by changing the strength of trophic cascades. Yet, few studies have explored how belowground herbivores affect the behaviors of generalist predators, nor how such changes may act through diverse changes to the plant metabolome. Using a factorial experiment, we tested whether herbivory by root-knot nematodes (Meloidogyne incognita) affected the aboveground interaction among milkweed plants (Asclepias fascicularis or Asclepias speciosa), oleander aphids (Aphis nerii), and aphid-tending ants (Linepithema humile). We quantified the behaviors of aphid-tending ants, and we measured the effects of herbivore treatments on aphid densities and on phytochemistry. Unexpectedly, ants tended aphids primarily on the leaves of uninfected plants, whereas ants tended aphids primarily at the base of the stem of nematode-infected plants. In nematode-infected plants, aphids excreted more sugar per capita in their ant-attracting honeydew. Additionally, although plant chemistry was species-specific, nematode infection generally decreased the richness of plant secondary metabolites while acting as a protein sink in the roots. Path analysis indicated that the ants’ behavioral change was driven in part by indirect effects of nematodes acting through changes in plant chemistry. We conclude that belowground herbivores can affect the behaviors of aboveground generalist ant predators by multiple paths, including changes in phytochemistry, which may affect the attractiveness of aphid honeydew rewards.

植物将地上生物与地下生物之间的相互作用联系在一起。据推测，草食动物引起的植物化学变化会通过改变营养级联的强度来影响整个食物网。但是，很少有研究探索地下草食动物如何影响通才捕食者的行为，或者这种变化如何通过植物代谢组的各种变化而起作用。通过阶乘实验，我们测试了根结线虫（Meloidogyne incognita）的食草是否影响了乳草植物（Asclepias fascicularis或Asclepias speciosa），夹竹桃蚜虫（Aphis nerii）和蚜虫趋向蚂蚁（Linepithema humile）之间的地上相互作用）。我们量化了蚜虫趋向的行为，并测量了草食动物处理对蚜虫密度和植物化学的影响。出乎意料的是，蚂蚁主要在未感染植物的叶子上生蚜虫，而蚂蚁主要在被线虫感染的植物茎的基部生蚜虫。在被线虫感染的植物中，蚜虫在吸引蚂蚁的蜜露中人均排泄了更多的糖。此外，尽管植物化学是特定于物种的，但线虫感染通常会降低植物次生代谢产物的丰富度，同时充当根中的蛋白质汇。路径分析表明，蚂蚁的行为变化部分是由线虫通过植物化学变化的间接作用驱动的。