Insect consumers of tree foliage comprise one of the most abundant and diverse feeding guilds in forest ecosystems. Known as folivores, this guild is integral to the structure and functioning of forests. Folivores influence vital ecosystem processes in forests, including nutrient turnover, competition among plants, and stand structure. In addition, these insects are critical sources of food for many invertebrate and vertebrate predators. In this article, we will address foliage-feeding insects that affect trees in temperate and boreal forests. In these ecosystems, an estimated 10-30%of the total leaf area is annually removed by leaf- chewing forest insects. In some forest types, defoliating insects strongly influence productivity and the long-term dynamics of the ecosystem. Foliage-feeding insect species have little effect on tree health in most years. During outbreaks of some insect defoliators, however, the entire canopy can be consumed, some- times for several years in succession. While outbreaks may cause significant economic harm by accelerating tree mortality, reducing productivity and increasing fire risk, they may also play an important long-term role in maintaining healthy forests.
[...] While outbreaks occur at irregular intervals in some species, there are a fascinating subset of species whose populations rise and fall at regular intervals, known as cycles. Life-History Traits Several studies have attempted to assess whether or not outbreaks are a property of particular life-history attributes found in some forest insects. Among Lepidoptera, for example, gregariousness, flightlessness, egg- clustering, low host plant specificity, and nonfeeding adults are all found in greater frequency in species known to have outbreaks. However, there does not appear to be either a single trait or a suite of overarching traits that are uniformly associated with species that outbreak. [...]
[...] There are both intraspecific and interspecific differ- ences in the type and strength of volatiles released by trees, contributing to variability in the susceptibility of folivores to predators and parasitoids. Tree chemistry can also alter the susceptibility of folivores to pathogens. Gypsy moth larvae are less likely to succumb to NPV when feeding on oaks which are rich in hydrolyzable tannins than when feeding on other species with lower concentrations such as aspen. In some, but not all studies, increases in tannins following defoliation of oaks reduce susceptibility of gypsy moth to NPV. [...]
[...] While not important or diverse in temperate regions, leaf-cutter ants are the dominant herbivore in many tropical forests. Among the beetles (order Coleoptera), the diversity of leaf-feeders is richest in the large families Chrysomelidae and Curculionidae. Both adults and larvae in these families feed on foliage. Several other insect orders also contain species that can function as forest defoliators. These include grasshoppers, crickets, and walking-sticks from the order Orthoptera, and several families of flies (order Diptera). Other guilds of tree-feeding insects, such as sap-feeders and shoot borers, can also cause defoliation but will be described in other articles (see Entomology: Defoliators; Sapsuckers). [...]
[...] Some conifer feeders, such as jack pine budworm and yellow-headed spruce sawfly (Pikonema alaskensis), feed more heavily on needles at the top of the tree than in the middle or lower portion of the canopy. This can result in top-kill a condition in which the tree survives and continues to grow radially, but the leader and upper whorls of branches die. Because foliage-feeding insects reduce leaf area, photosynthesis is reduced during defoliation. This, in turn, leads to a decrease in the rate of radial growth. [...]
[...] Concentrations of primary compounds important to insects such as water and nitrogen, secondary compounds such as tannins and terpinoids, and physical properties such as toughness vary among leaves on an individual tree, among trees, and across entire forested landscapes. Foliage quality for herbivores also changes seasonally and is generally highest in the spring. As current-year needles or new leaves fully expand, the concentration of indigestible fiber and lignin increases. New growth on conifers is of much higher quality for many foliage-feeding insects than needles retained on the tree from previous years. Thus, folivorous insects encounter great temporal and spatial variation in the quality of leaves on which they feed. [...]
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