Oviposition
Female European elm bark beetles bore a tunnel parallel with the tree surface
{[230],[ 311],[332]}. In contrast H. rufipes females cut a gallery entrance tunnel in the inner bark at an angle perpendicular
to the grain of the wood {[444]}. When galleries are made, part of the recent growth ring is often eaten into {[234]}. As a result, years after the bark
has fallen away, the history of infestation can still be seen {[230]}. Table 12 shows some characteristics of maternal galleries for S. scolytus, S. multistriatus and S. pygmaeus. Variation in length of the maternal gallery can be due to the thickness
of the bark. Manojlovic et al. {[62]} showed that S. scolytus maternal halls in thin bark (less than 4 mm) were considerable shorter
than those in thicker bark.
| Photo 43: Adult H. rufipes forming a brood gallery and laying eggs (Courtesy of I.
L. Pines, Manitoba Natural Resources, Winnipeg, Manitoba, Canada).
|
The female elm bark beetle lays a series of white oval eggs in niches along
the maternal tunnel (Photo 43, {[311],[322],[332]}). S. scolytus oviposits 60-110 eggs per gallery, S. multistriatus 100-150 {[381]}. Most bark beetles produce relatively large eggs (e.g.
the eggs of S. rugulosus are 1/8 of the insect's body size, {[364]}). Usually, the egg-laying period
of the female beetle is not interupted. However, when circumstances are too wet or too dry, the beetle will leave the gallery {[381]}. Care of the
eggs and the larvae is restricted to the general protection afforded by the female's presence. She keeps the egg tunnel clean and offers defence against certain enemies (Photo 44). Burying of dead adults may prevent further
spread of pathogens in the gallery system {[364]}. The construction of the maternal tunnel together with egg laying takes S. scolytus females about three weeks {[322]}. The last egg is laid three weeks after
the first one, thus after the first ones have hatched. In addition, changes in temperature and the fact that not all beetles simultaneously start
gallery construction results in marked differences in larval development in one host tree {[381]}
| Photo 44: Several red dust piles on American elm bark from overwintering H. rufipes
(Courtesy of I. L. Pines, Manitoba Natural Resources, Winnipeg, Manitoba, Canada).
|
Table 12: Characteristics of Scolytid maternal and larval {[62],[132],[322],[344]}
Species
| Maternal galleries
| Larval galleries
| Number of larval galleries2
| Length (mm)
| Width (mm)
| Length (mm)
| Width (mm)1
| Scolytus scolytus
| 15-65
| 1.5-2.3
| 60-150
| 2.4-2.8
| 25-55
| S. multistriatus
| 15-65
| 1-2
| 35-95
| 1.3-1.9
| 15-105
| S. pygmaeus
| 11-33
| 0.7-1.3
| 23-43
| 0.8-1.4
| 7-63
|
1 The larval galleries are widest shortly before pupation
2 Number of larval galleries per one maternal gallery.
| 
