Spatial dynamics of teak defoliator (Hyblaea puera Cramer) - Cochin ...

2. At a still lower regional scale (for example the Indian Subcontinent) there is
a directional progression of outbreaks, which seems to be linked with the
movement ofmonsoon wind system.
3. At the local level (like the teak plantations of Nilambur), outbreaks occur
only during some part of the year. During the outbreak period, infestations
originate in a few small epicentres. Later, outbreaks spread td larger and
larger areas. While most of the outbreaks could be caused by previous
outbreaks, a few are not so. During the final phase of the outbreak period, a
few outbreaks occur that can only be explained either as caused by long­
range migration of moths or by aggregation of moths from the endemic
population. After this, the population density remains low until the next year
when the sequence of outbreaks is repeated. Thus, the teak defoliator
displays population cycles at a frequency ofone year.
Various explanatory hypotheses have been proposed for the dynamics
of forest lepidoptera that exhibit population cycles (Myers, 1988). These
include:
a) Variation in insect quality: Chitty (1967) proposed that density-
related selection on genetically controlled variation in behaviour and
physiology of animals could provide the basis for self regulation of
populations. Experimental proof is still non-existent for this hypothesis
(Myers, 1988).
b) Climatic release hypothesis: Uvarov (1931) and Andrewartha
and Birch (1954) proposed that weather and climate are major controlling
factors of insect abundance. Climate can cause direct and indirect impact on
population size, but the hypothesis remains untestable due to the difficulty in
. differentiating the impact due to climate from that caused by other factors
(Myers, 1998).
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