5.1.1 Colonization and life cycle of bark beetles

Bark beetles are one of the few insect groups that as adults bore into the host plant for the purpose of laying eggs (S.L. Wood, 1982). Bark beetle adults and larvae in northern temperate climates generally feed on phloem/cambium (phloeophagy) of conifers (Gymnospermae). In the more tropical zones, the majority of species feed on wood (xylem) and on phloem of broadleaved trees and shrubs (Dicotyledoneae). Monocotyledoneae are feed on by only a few tropical species of bark beetle. Species that feed on phloem are usually restricted to one or a few host species, whereas xylomycetophagous beetles (ambrosia beetles) that carry their own symbiotic fungi (which breaks down the xylem) may colonize a larger range of hosts (S.L. Wood, 1982).

Life cycle of bark beetles
Fig. 1. Generalized life cycles of phloem-feeding bark beetles. Dendroctonus (a), many Ips species, and Pityogenes, among other species (b) overwinter as larvae, pupae or callow (yellow) adults in the bark of the colonized tree. Tomicus piniperda (c) overwinter as adults in the outer bark of pines while some Ips species, e.g., I. typographus overwinter in forest litter as adults (d). In the spring, beetles of all species disperse and search for suitable host trees in which to reproduce. Beetles successful in colonizing a tree lay eggs, these develop to larvae, pupate and feed as callow adults until emergence. Depending on the season, beetles either overwinter or in warmer climates complete several generations during the summer. T. piniperda survive the summer by flying a short distance to the crowns of nearby trees and feeding in the stems of pine shoots (c); later in the autumn they crawl down the trunk and overwinter at the base (c).

Semiochemicals from both the tree and the beetle have many functions during the life cycle of a bark beetle (Fig. 1; for reviews see D.L. Wood, 1982; Borden, 1982; Lanier, 1983; Birch, 1984; Borden et al., 1986; Byers, 1989a, b; Raffa et al., 1993). Most of the following presentation involves species in the genera Dendroctonus, Tomicus, Ips, and Pityogenes. In general, adults of these species overwinter in either forest litter (Ips, Pityogenes) or the brood tree (Dendroctonus, Ips, Pityogenes, Fig. 1). In species that have several generations during the summer, emergence is from the brood tree. Tomicus piniperda, has a more complex life cycle in which adults overwinter in living, nonbrood trees (Salonen, 1973, Långström, 1983). After emergence the adults of all species attempt to locate a host tree (termed the dispersal flight), often by olfactory means, and determine if it is suitable for colonization and reproduction. This recognition of suitability may be in flight and also after landing on the tree, as will be discussed later in part 5.2. In the monogamous Tomicus and Dendroctonus (subfamily: Hylesininae), the females select the host and a site to begin oviposition galleries that are excavated in the phloem. In contrast, males of polygynous Ips and Pityogenes (subfamily: Scolytinae) begin the entrance hole (attack) and later accept several females. In most cases individuals of only one sex begin the attack; and release a species-specific blend of chemicals comprising an aggregation pheromone (Byers, 1989a). However, in D. brevicomis both the female and the joining male each produce a unique and synergistic pheromone component that when combined elicit maximal attraction response (Silverstein et al., 1968, Kinzer et al., 1969). In T. piniperda there is no evidence of an aggregation pheromone (Byers et al., 1985; Löyttyniemi et al., 1988); instead, host tree chemicals induce aggregation (discussed in part 5.2.3).

Once an individual or pair begin to release an aggregation pheromone, the likelihood that the tree is colonized depends on (1) the population level of beetles available for recruitment to the attack and (2) the resistance (health) of the tree and its ability to produce defensive resin (discussed in part 5.4). Beetles of many species have specialized areas of the integument or pouches where symbiotic fungi are carried and sometimes nourished until introduced inside the entrance tunnel where they grow into the tree (Happ et al., 1976; Whitney, 1982; Bridges et al., 1985; Paine and Stephen, 1987; Levieux et al., 1991). Some of the fungal species (genera Ceratocystis=Ophiostoma, Trichosporium) may attack the living tissues of the tree and paralyze the tree's ability to produce and exude resin for defense against the beetle (Mathre, 1964; Horntvedt et al., 1983; Paine, 1984; Raffa and Berryman, 1987; Paine and Stephen, 1987; Paine et al., 1988). Other fungal species of the beetle's mycangium grow in the galleries after the tree has been killed and appear important to the growth of the larvae (Bridges and Perry, 1985; Paine et al., 1988; Goldhammer et al., 1991). In ambrosia beetles, which generally attack unhealthy or dead trees, the adults and larvae feed on fungi lining the galleries instead of on the tree's tissues (Funk, 1970; Furniss et al., 1987; Kajimura and Hijii, 1992).

Successful colonization and reproduction by a bark beetle in a living tree requires release of enough aggregation pheromone to ensure the attraction of sufficient conspecifics to overwhelm the host tree defenses, but after killing the tree and securing mates, pheromone should not be released any longer in order to avoid further competition for bark areas (Berryman et al., 1985). Semiochemicals play a role in "cooperation" among beetles when killing the tree and in their avoidance of competition (discussed in part 5.5.4). "Pioneer" beetles that attack the tree first may suffer most from the tree's defensive resin, but these beetles may have no choice but to attack due to low fat reserves (discussed in 5.2). The later that a beetle arrives in the colonization sequence of the host, the poorer is the quality of the bark substrate due to (1) space utilization by established conspecifics (intraspecific competition) and (2) degradation by microorganisms (discussed in 5.2.4 and 5.4).

Under the bark, females lay eggs which hatch to larvae that feed on the phloem for several weeks. Chemicals from the plant and from microorganisms could affect survival at this time, but little is known about these interactions. However, once the tree is dead, there can be no natural selection by the insects to evolve different tree genotypes that produce chemicals harmful to beetles. The larvae pupate in the bark and become yellow, callow adults where they feed and mature until emerging. The beetles may begin a dispersal flight during the same season, or after overwintering in either the tree (Dendroctonus, Pityogenes and many Ips) or in the forest litter (I. typographus, Fig. 1). Tomicus minor and T. piniperda emerge from the bark and fly relatively short distances to the tops of pine trees where they bore into a shoot during the summer (Salonen 1973; Långström and Hellqvist, 1991; Fig. 1). In the autumn, beetles of T. piniperda crawl down the trunk and bore into its base to overwinter, whereas T. minor overwinters in the litter (Salonen 1973; Långström, 1983).
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Byers, J.A. 1995. Host tree chemistry affecting colonization in bark beetles, in R.T. Cardé and W.J. Bell (eds.). Chemical Ecology of Insects 2. Chapman and Hall, New York, pp. 154-213.