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93 Chapter 9 Insects and Conifers Insects as Feeders Coniferous trees are home to a variety of different insects. Their presence adds to the biodiversity of evergreen forest ecosystems, and for that we can be thankful. At the same time, a conifer forest in which nearly all the trees are home to millions of a particular species of bark beetle is a doomed forest that will soon disappear. Such a species is defined, by some people, as a “pest.” That is a judgment, not a definition, but for brevity the quotation marks around “pest” will be omitted in what follows. This chapter is primarily concerned with pest insects because they are common—if they weren’t, they wouldn’t be pests—and therefore much more likely to be encountered than rare species. Their lifestyles and behaviors are also much better known, because it has been financially worthwhile for forest researchers to concentrate on them. Most of these insects belong to one of four orders: Coleoptera (beetles), Lepidoptera (moths and butterflies), Hymenoptera (sawflies), and Homoptera (bugs). Note that “bugs” here does not mean all creepy crawlies and unidentified microbes; it means members of the insect order Homoptera , which is described later. The most important part of a pest insect from a forester’s point of view is, not surprisingly, its mouthparts, defined in terms of how they work. Thus beetles, moths, and sawflies all have chewing mouthparts, whereas bugs have piercing and sucking mouthparts. As for the insects that chew, 94 the world of northern evergreens some chew harder than others: it takes strength and effort to eat wood, and so this activity is confined to beetles and their larvae (grubs) for the most part. Some tiny beetles are (at present) the most destructive of all forest insects. The leaves of conifers are also fairly hard to bite into, with their thick, wax-coated epidermises: the insects that feed on them are primarily caterpillars and sawflies. The much weaker mouthparts of bugs can only penetrate a tree’s tissues where they are young and tender. Beetles Beetles form the largest of all insect orders: about 40 percent of all the world’s insects are beetles. At present, the most serious forest pests in northern North America are bark beetles, which have destroyed immense tracts of forest in the west, including Alaska, where various species attack all the pines, all the spruces, Douglas-fir, and western hemlock. In the east, other species attack tamarack and white spruce. Bark beetles are remarkably small, about the size of a grain of rice. Figure 9.1c shows a bark beetle beside two other common beetles that are described later.1 Where they are at work, signs of them are everywhere. The immature grubs tunnel into the soft, rich phloem of many conifers, leaving patterns of grooves, known as galleries, exposed when the bark of a dying tree flakes off. The galleries can be seen on the inner surface of fallen bark Figure 9.1. Common forest beetles. (a) White-spotted sawyer beetle (male). (b) Golden buprestid beetle. (c) Mountain pine bark beetle. (All on the same scale.) insects and conifers 95 and the outer surface of the uncovered wood. Some species (not all) make galleries with a very regular herringbone pattern (figure 9.2a). Other species appear to favor no particular pattern. Experts can often tell from the galleries which species of beetle made them. The galleries are started by the adult beetles, which have tunneled into the tree from the outside. The females lay their eggs at intervals, and when the eggs hatch, the newborn grubs start eating (and tunneling) on their own, at right angles to their mother’s gallery in the case of species that make herringbone patterns. The grubs grow as they progress, so their tunnels widen. At the tip, the grub pupates, and when the adult emerges, it tunnels off in a different direction not in the plane of the pattern (figure 9.2). Bark beetles are divided into so-called primary and secondary attackers . Primary attackers (mostly in the genus Dendroctonus) choose healthy, undamaged trees, whereas secondary attackers (in the genera Ips and Scolytus ) choose trees already weakened by primary attackers. Dendroctonus beetle populations are exploding throughout our area at present (2010). The outbreak is worst in the west. Mountain pine bark beetle (Dendroctonus ponderosae), one of the most destructive, attacks lodgepole, ponderosa, western white, and whitebark pines. Lodgepole Figure 9.2. (a) Bark beetle’s gallery. (b) Buprestid...

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Additional Information

ISBN
9780801463037
Related ISBN
9780801477409
MARC Record
OCLC
966802958
Launched on MUSE
2017-01-01
Language
English
Open Access
No
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