Behind the idle and quiet appearance of plants, warfare is an everyday issue. Herbivorous arthropods, below- and aboveground, continue to threaten a plant's existence, whereas their attack is countered by the plant in many ways. Plants defend themselves directly by modifying plant structure (e.g., cuticle thickness, leaf hairiness), lowering nutritional quality, decreasing digestibility and increasing toxicity, but also indirectly by promoting the effectiveness of enemies of the herbivores (Price et al. 1980). This indirect plant defense implies that plants provide chemical lures, shelter and/or food, whereas they gain protection in exchange (Sabelis et al. 1999a, b, c, d, 2002). Central American Acacia trees stand out as a landmark example (Janzen 1966). They have stipular thorns that are expanded and hollow and provide nesting sites for certain ants. In addition, they secrete nectar from large foliar nectaries and produce nutritive organs called Beltian bodies on the leaf pinnules. These food bodies are eagerly harvested by foraging ants and fed to their larvae. The ants in turn kill insect herbivores, repel mammalian herbivores, and destroy plants interfering with the Acacia tree. In this chapter, we focus on food provisioning as a strategy of the plant to boost the third trophic level and we discuss the conditions under which this particular mode of defense is favored by natural selection.

The argument that plants benefit from consumers of the foods they provide dates back to Thomas Belt in his book published in 1874.