Contents - LAC Biosafety

296 Insect pests in plantations: case studies
affected districts. In March 1998, out of an average growing stock of 262 sal
trees/ha (above 20 cm diameter at breast height), 3–66% were infested. While the
infested trees constituted 2.6–9% of trees within the girth class 20–60 cm, it
constituted 34.4–55.9% within the girth class 61–120 cm and 69.4–78.3% within
the girth class 121–180 cm, showing a definite preference of the insect for trees
of higher girth classes. This trend has also been noted by some earlier workers.
It appears that any factor which imposes stress on trees, such as drought,
crowding, overmaturity etc., which compromises the tree’s ability to produce the
defensive resin flow, may trigger an outbreak. Singh and Thapa (1988), who
reported outbreak of a geometrid caterpillar Ascotis selenaria imparata that caused
total defoliation of patches of sal stands prior to the beginning of the monsoon,
remarked that such patches could prove susceptible to attack by the sal
borer beetles which start emerging with the beginning of the rains. Another
favourable factor is rain. As noted earlier, sal borer adults emerge with the onset
of rains and are active and fly during mild rains. The number of eggs laid and
their hatching and survival rate are higher at high humidity. It is obvious that
years of high rainfall are very favourable for the build-up of sal borer
populations. The circumstances that lead to the collapse of the outbreak are
also matters of speculation. Decline in the number of susceptible trees, drought
years and build-up of a predatory beetle (see below) are suspected to play a role.
Natural enemies Information is scanty on the natural enemies of the
sal borer; apparently, there are not many. An elaterid beetle Alaus sordidus
is recorded as a predator of H. spinicornis and of other cerambycid borers of some
trees. The adult A. sordius emerges with the onset of monsoon and lays eggs on
the bark of trees attacked by the sal borer. The predacious larvae attack the sal
borer larvae between the bark and sapwood; older larvae enter the larval tunnels
and pupal chambers of the sal borer. One A. sordidus larva can destroy up to 10 sal
borer larvae/pupae. The predator is not abundant initially but during sal borer
epidemics its population builds-up steadily. Up to 10–15% vacant sal borer pupal
chambers have been found occupied by A. sordidus (Beeson, 1941). The fungus
Beauveria bassiana was isolated from H. spinicornis and in laboratory tests it caused
75–78% mortality of young larvae within six days of exposure (Sharma and Joshi,
2004).
Control The sal borer is a chronic, endemic pest, i.e. the insect is always
present in small numbers in sal areas, usually infesting fallen, unhealthy or
dying trees. Living, healthy trees are infested only during population outbreaks.
Infestation of up to one per cent of the growing stock, i.e. an average of 2.5 trees
per hectare, is considered normal. For management purposes, a population