Early Leaf Abscission: A Neglected Source of Mortality for Folivores

We present evidence for a simple, yet heretofore overlooked, cause of mortality for folivorous insects, early leaf abscission. We discuss characteristics of both herbivores and host plants that are likely to influence folivore mortality via leaf abscission. Many plants have been shown to abscind leaves that are diseased or damaged (Jacobs 1962). The complete mechanism by which plants do so, however, is less clear. Apparently, leaf abscission involves interactions of many plant compounds including ethylene, auxins, and abscisic acid (Milborrow 1974). Whatever the physiological mechanism of leaf abscission, the consequence for a leaf-feeding insect is disjunction from the host plant. Separation from the plant can increase herbivore mortality in a number of ways. The excised leaf itself is probably a poorer nutritional source than an intact leaf (Haukioja and Niemeld 1977) especially in plants that can melanize, such as oaks. Since many herbivorous insects require more than one leaf for development, starvation may ensue unless the insect can relocate on the same or another suitable host plant. Herbivores detached from the host plant could be more susceptible to predation and fungal attack while on the ground. The size and density of the host plant could influence the ability of a detached folivore to relocate the host. Obviously, an insect displaced by leaf abscission is less likely to return to the foliage of a large, solitary oak than to any individual plant in a dense patch of herbs. From the plant's perspective, leaf abscission can be hypothesized as a trade-off resulting in the conservation of resources. If a leaf is damaged sufficiently, water loss might become prohibitive or photosynthate loss might be greater than the photosynthetic capability of the undamaged portion; consequently, abscission should occur when losses exceed gains. Long-lived plants that produce many leaves over their lifespan could abscise damaged leaves with relative impunity, but the cost would be high for annual herbs with only a few leaves. For example, Orians and Solbrig (1977) have demonstrated that plants with ephemeral leaves must compensate for constructing short-lived leaves with higher ates of photosynthesis. Furthermore, if there were some damage threshold for abscising leaves, large leaves should be less likely to abscind than small leaves with the same absolute area damaged by herbivores. The effect of leaf abscission on mortality of folivores hould depend not only on plant characteristics, but also on the relative mobility of the folivore. Species whose feeding habits restrict hem to a single leaf or a few leaves, such as leaf miners or gall formers, should experience greater mortality as a result of leaf abscission than should species that can readily move to other leaves. Moreover, mortality as a result of early leaf abscission should be greater in the more sedentary developmental stages (egg, larva, pupa). Folivorous insects that can