Interactions Between Chemical and Mechanical Defenses in the Plant Genus Bursera and Their Implications for Herbivores1

Traditionally, plant defenses have been divided into two main categories: chemical and mechanical defenses. The first category includes a variety of substances that are toxic, repellent, or that render plant tissues indigestible to animals. In the second category are a series of physical barriers to avoid being eaten. These include structures such as spines, trichomes, and hard, very sticky, or smooth surfaces. Studies dealing with either one of these two kinds of defenses typically assume they are independent of each other and that their actions on herbivores are additive (Dussourd and Denno, 1991; Farrell et al., 1991; Becerra, 1994b). Yet, in some plants, structures such as glandular trichomes and secretory canals are combinations of mechanical and chemical deterrence that may interact to entrap insects in sticky and toxic secretions (Southwood, 1986; Howe and Westley, 1988; Becerra, 1994a). In this article we report an investigation of plant secretory canals and their relationship with plant chemistry in the genus Bursera. We present data suggesting an interaction between the mechanical and the chemical components of this defense. Also we present data which suggest that the interaction between these components has profound repercussions on how Bursera’s herbivores defend themselves against their predators.