11.2.1. Leaf chewing
The damage caused by leaf-chewing insects is readily visible compared, for example, with that of many sap-sucking insects. Furthermore, the insects responsible for leaf tissue loss are usually easier to identify than the small larvae of species that mine or gall plant parts. By far the most diverse groups of leaf-chewing insects are the Lepidoptera and Coleoptera. Most moth and butterfly caterpillars and many beetle larvae and adults feed on leaves, although plant roots, shoots, stems, flowers, or fruits often are eaten as well. Certain Australian adult scarabs, especially species of Anoplognathus (Coleoptera: Scarabaeidae; commonly called Christmas beetles) (Fig. 11.1), can cause severe defoliation of eucalypt trees. The most important foliage-eating pests in north temperate forests are lepidopteran larvae, such as those of the gypsy moth, Lymantria dispar (Lymantriidae). Other important groups of leaf-chewing insects worldwide are the Orthoptera (most species) and Hymenoptera (most Symphyta). The stick-insects (Phasmatodea) generally have only minor impact as leaf chewers, although outbreaks of the spur-legged stick-insect, Didymuria violescens (Box 11.6), can defoliate eucalypts in Australia.
High levels of herbivory result in economic losses to forest trees and other plants, so reliable and repeatable methods of estimating damage are desirable. Most methods rely on estimating leaf area lost due to leaf-chewing insects. This can be measured directly from foliar damage, either by once-off sampling, or monitoring marked branches, or by destructively collecting separate samples over time (“spot sampling”), or indirectly by measuring the production of insect frass (feces). These sorts of measurements have been undertaken in several forest types, from rainforests to xeric (dry) forests, in many countries worldwide. Herbivory levels tend to be surprisingly uniform. For temperate forests, most values of proportional leaf area missing range from 3 to 17%, with a mean value of 8.8 ± 5.0% (n = 38) (values from Landsberg & Ohmart 1989). Data collected from rainforests and mangrove forests reveal similar levels of leaf area loss (range 3–15%, with mean 8.8 ± 3.5%). However, during outbreaks, especially of introduced pest species, defoliation levels may be very high and even lead to plant death. For some plant taxa, herbivory levels may be high (20–45%) even under natural, non-outbreak conditions.
Levels of herbivory, measured as leaf area loss, differ among plant populations or communities for a number of reasons. The leaves of different plant species vary in their suitability as insect food because of variations in nutrient content, water content, type and concentrations of secondary plant compounds, and degree of sclerophylly (toughness). Such differences may occur because of inherent differences among plant taxa and/or may relate to the maturity and growing conditions of the individual leaves and/or the plants sampled.
Assemblages in which the majority of the constituent tree species belong to different families (such as in many north temperate forests) may suffer less damage from phytophages than those that are dominated by one or a few genera (such as Australian eucalypt/acacia forests). In the latter systems, specialist insect species may be able to transfer relatively easily to new, closely related plant hosts. Favorable conditions thus may result in considerable insect damage to all or most tree species in a given area. In diverse (multigenera) forests, oligophagous insects are unlikely to switch to species unrelated to their normal hosts. Furthermore, there may be differences in herbivory levels within any given plant population over time as a result of seasonal and stochastic factors, including variability in weather conditions (which affects both insect and plant growth) or plant defenses induced by previous insect damage (Box 11.1). Such temporal variation in plant growth and response to insects can bias herbivory estimates made over a restricted time period.
