Among all plants, trees present the most diverse habitats for insects to occupy. Insects feed on all parts of the tree, i.e., vegetative structures such as leaves, stems, and roots and reproductive structures such as flowers, fruits, and seeds. Some insects are specialized to feed on phloem and/or xylem tissues, dead sapwood, and heartwood. Insects that feed on these structures and tissues vary in size from 1—2 mm (scale insects) to 6 cm (longhorned beetles). Life cycles (from egg to adult) can be completed in a few days or weeks (aphids) or be prolonged for 50 years (metallic wood borers).
Although all orders of insects are found in forest habitats, only a small number feed on the trees or are the predators and parasitoids of these taxa. Species from the following orders are generally referred to as "forest insects": Hemiptera, Isoptera, Orthoptera, and Thysanoptera (in the Exopterygota, which undergo incomplete metamorphosis) and Coleoptera, Diptera, Hymenoptera, Lepidoptera, and Raphidioptera (in the Endopterygota, which undergo complete metamorphosis).
Habitat diversity is greatly favored by the large size of trees, both in mass and in height. Thus, both the abiotic and the biotic environment can vary considerably from the roots to the upper canopy. For example, a bark beetle, Hylastes nigrinus, spends its life cycle feeding beneath the bark on the roots of Douglas-fir, Pseudotsuga menziesii, growing in California, whereas a cone midge, Contarina oregonensis, feeds in a gall in the seed coat within a cone. Old-growth Douglas-firs with a height over 100 m and a diameter of 3 m are not uncommon. Over 240 insect species from the above-mentioned orders are listed as feeding on Douglas-firs. However, only a few of these would be found at the same time on a large, living tree. Similar diversity of insects feeding on trees can be found in broadleaved deciduous forests and broadleaved evergreen forests. Deciduous oaks in England are fed upon by over 280 insect species, and some evergreen oaks in coastal California provide food for about 300 insect species.
This complexity is magnified further when the guilds of insects in the canopy are considered. The phytophagous insects are represented by chewing, mining, gall-making, and sap-sucking species. Chewing insects are found in the Coleoptera, Lepidoptera, Hymenoptera, and Orthoptera; mining species in the Hymenoptera, Lepidoptera, and Diptera; gall-making species in the Diptera and Hymenoptera; and sap-sucking species in the Hemiptera (Homoptera and Heteroptera) and Thysanoptera. Predators are found in the Coleoptera, Diptera, Hymenoptera, and Raphidioptera and parasites in the Diptera and Hymenoptera. Insects that feed on epiphytes such as lichens, algae, and mosses are found in the Psocoptera
(bark lice), Collembola (springtails), Dermaptera (earwigs), and Plecoptera (stoneflies). Mosquitos (Diptera) breed in water contained in tree holes, which are decayed cavities in the wood.
Many detritivores are found in the canopy as well as on other parts of the tree. They feed on many food sources such as protozoa, bacteria, fungi, nematodes, and small particles of plant and animal tissues. These species are found for example in the Psocoptera, Collembola, and Blattoidea.
Finally, many insects that do not feed on the trunk or foliage use them for a resting or hiding place, mating, pupation and/or hibernation, or estivation. These temporary residents have been classified as "tourists." Many of these insects feed on the surrounding vegetation. Their predators and para-sitoids would be included as tourists also. For example, ants and spiders, which are very important predators of tree-inhabiting insects, use tree trunks as roadways from the forest floor to the canopy in search of prey. Another example, a sawfly, Strongylogaster distans, that feeds on bracken fern in the Sierra Nevada of California, pupates in the bark crevices of nearby ponderosa pines.
In summary, about half of the insect orders are directly or indirectly associated with trees. As with humans, insects use trees for food, shelter, support, and travel.
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