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Foliage feeders in temperate and boreal forests

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  1. Introduction.
  2. Diversity.
  3. Feeding ecology.
  4. Population dynamics.
    1. Life-history traits.
    2. Population regulation.
    3. Population cycles.
  5. Impacts of foliage-feeding insects on trees.
  6. Conclusion.

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 in?uence 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 in?uence 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 signi?cant economic harm by accelerating tree mortality, reducing productivity and increasing ?re 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, ?ightlessness, egg- clustering, low host plant speci?city, 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 intraspeci?c and interspeci?c 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. [...]

[...] In addition, the timing of the defoliation and the age or location of the affected foliage cans also in?uence the severity of impact. Hardwood and conifer trees differ greatly in their ability to tolerate severe defoliation. Healthy hardwood trees can generally recover from defoliation, even if 100% of the foliage is consumed. Most hardwood trees are able to produce a second set of leaves a few weeks after the initial foliage is lost a process referred to as ?re?ush.' As a rule, hardwood trees do not re?ush until roughly 60% or more of the canopy has been consumed or otherwise damaged. [...]

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