Contents - LAC Biosafety

322 Insect pests in plantations: case studies
by erratic lighter defoliations later in the season. Outbreaks are spectacular
events, creating the impression of severe growth loss, although the trees put
forth a new flush of leaves within weeks. Several attempts have been made in the
past to estimate the loss due to defoliation, based on artificial defoliation
experiments, field observations on the frequency of defoliations etc., which put
the loss figures variously at 6.6–65% of the normal increment (Mackenzie, 1921;
Beeson, 1931a, 1941; Champion, 1934). Nair (1986a) made a critical review of
these early attempts and concluded that the available estimates were not
reliable as they rested on untenable assumptions.
In a detailed experimental study, Nair et al. (1996a) estimated the growth in
volume increment in replicated plots in a young teak plantation at Nilambur, in
Kerala, India. Over a five-year period sets of plots were either exposed to natural
insect defoliation or protected from the insect using insecticide. One set of plots
was fully protected by applying insecticide whenever there was threat of damage
by either H. puera or E. machaeralis, while another set of plots was protected only
against H. puera. Defoliation by the two insects was well separated in time which
facilitated such selective protection. A third set of plots with no protection
served as an untreated control. Differences in volume increment were estimated
by the following method. The experiment was started at the time of routine
4th year mechanical thinning, when as per standard silvicultural practice
alternate rows of trees in a plantation are thinned to facilitate growth.
Measurements made on the thinned trees were used to calculate the initial
volume of the standing experimental trees. There were a total of nine plots, with
100 trees per plot, of which half were felled in the 4th year. The girth under bark
of each felled tree was measured at every 50-cm interval, from which the wood
volume of the tree was calculated. Then the mathematical relationship between
the wood volume, on the one hand, and the girth at breast height and total tree
height, on the other, was determined by fitting the most suitable prediction
equation. This equation was used to arrive at the initial volume of the standing
experimental trees from measurements of their girth at breast height and total
tree height. Similar measurements were carried out at the time of the second
mechanical thinning, when 50% of the remaining trees were felled, to obtain
the volume of the trees at the end of the experimental period.
The study showed that during the experimental period the trees protected
against H. puera put forth a mean annual volume increment of 6.7 m 3 /ha
compared with 3.7 m 3 /ha of unprotected trees, a gain of 3 m 3 of wood/ha per
annum. Thus, in young plantations of teak, loss due to defoliation caused by
H. puera was estimated at 44% of the potential volume increment. They also
projected that protected trees would be ready for harvest at the age of 26 years
instead of the usual 60 years, provided other necessary inputs were given.