Dutch Elm Disease | Cycle | Beetle | Life Cycle Oviposition Index document Life Cycle

    Larval stage

    After hatching each larva feeds on the phloem by making side-tunnels at right angles to the maternal gallery (Photo 45A, {[311],[332]}). In Table 12 some characteristics of the larval tunnels are given for three Scolytid beetles. The tunnels are first small, becoming wider as the larvae develop and move away from their birthplace {[322]}. The gallery system of S. multistriatus is elliptic, while the system of H. rufipes has a butterfly-like shape (Photo 45, {[230]}). However, the arrangement of the gallery systems can become irregular when the density of breeding beetles is high {[332]}.


    (A)


    (B)


    (C)


    (D)

    Photo 45:      A) Young H. rufipes larvae forming feeding tunnels (Courtesy of I. L. Pines, Manitoba Natural Resources, Winnipeg, Manitoba, Canada). B, C and D) Different stages of elm bark beetle gallery formation (Photo 45 B and C: Courtesy of H. Kaljee, OMEGAM Groenadvies, Amsterdam, The Netherlands).

    Webber et al. {[483],[548]} suggested that contact between O. ulmi s.l. and beetle larvae is harmful to larval development. Feeding on fungus-free tissue appears to be essential for the first phases of larval development (instar 1-3). During these phases gallery construction is aborted if the larvae have to feed on O. ulmi s.l. colonized bark. After reaching the later phases of development (instar 4-5), normal growth continues even if the larvae feed on infested phloem. Larval galleries appear to turn away from bark infected by  O. ulmi  s.l. (or other fungi) {[436]}. The larval mines are tightly packed with frass which tends to prevent physical contact between larva and parents {[364]}. Larvae of S. scolytus, S. multistriatus and H. rufipes are legless white grubs with a shiny amber-colored head capsule. The capsule of S. multistriatus larvae is no more than 1/2 the width of the body. The head capsule of the H. rufipes larva is 3/4 the width of the body {[230],[332]}. S. scolytus larvae are 4 mm or less in length, while S. multistriatus larvae are no longer than 2 mm {[332]}.

    Table 13:       Characteristics of Scolytid pupal chambers and the emergence in elm bark {[132]}

    Species

    Pupal chambers1

    Emergence holes in bark1

    Depth2

    Lenght

    Width

    Height

    Lenght

    Width

    Scolytus scolytus

    1.3-3.9

    5.9-7.5

    2.7-3.5

    2.3-2.9

    1.7-2.7

    1.9-2.5

    S. multistriatus

    0.9-3.5

    4.1-4.7

    1.2-1.8

    1.0-1.6

    1.0-1.8

    0.9-1.5

    S. pygmaeus

    n.d.

    2.2-2.8

    0.9-1.3

    0.7-1.1

    0.8-1.2

    0.7-1.1

    1           All measurements are given in mm.
    2              Measurements represent depth into the xylem
    n.d:      no data available

    It takes about 30 days before S. scolytus larvae are fully grown {[381]}. Shortly before pupation (from february-march to early summer) the larvae excavate a pupal chamber (Table 13) in the sapwood, the inner bark or dead outer bark {[354],[ 371]}. Subsequently the pupal chamber is physically cut off from the rest of the gallery system by a dense plug of frass {[371]}. The position chosen for pupation varies among Scolytus  species and probably depends on bark thickness and climate {[371]}. While many larvae of S. scolytus eventually pupate in the moist inner bark, S. multistriatus larvae pupate almost exclusively in the outer bark {[354]}. Occassionally the large elm bark beetle may cut a pupal chamber in the sapwood. Pupal chambers of the more northerly distributed S. laevis are primarily found in the sapwood, those of S. pygmaeus only in the bark tissue {[132],[370]}.

    A S. scolytus larva casts its skin four times during development. The new adult beetle needs to dry its chitine armour for 48 hours {[381]}. After the last sloughing the young adult excavates from the pupal chamber and subsequently emerges from the host tree leaving a hole in the bark (Table 13).

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