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).
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).
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.