Mo, Jianhua
Description
The primary objective of this study was to provide some basic
information about the ecology and behaviour of Hypsipyla robusta Moore, a
serous shoot borer of Australian red cedar (Toona australis (F. Muell.)
Harmes). First, quantitative analyses were made on the temporal and spatial
patterns of infestation. This was followed by artificial rearing of the insect in
the laboratory, modelling of temperature-dependent development, studies
of the feeding behaviour of larvae, analyses of the...[Show more] die! patterns of
reproductive activities, and preliminary investigations of the host selection
behaviour of larvae and adults.
The infestation patterns of the insect were investigated with sample
data from a red cedar plantation. The temporal pattern of infestation levels
was closely correlated to rainfall: the larger the amount the rainfall the
higher the infestation levels. Temperature did not affect the general
infestation levels, but low daily minimum temperatures in the winter(< 6.5
0C) were always associated with low proportions of attacked trees. Attack by
the insect was concentrated on open-grown trees. However, attack on forestlocated
trees was also observed. Among trees planted in the open, attack was
largely a random process, i.e. all trees had similar likelihood of being
attacked. The only exception was trees 5 1.5 m tall, which were attacked
significantly less often than larger trees. For attacked trees, the percentages of
shoots attacked per tree decreased as tree size increased. Within an attacked
tree, shoots positioned among the upper tree crown were attacked
significantly more often than those positioned in the lower tree crown or
offshoots.
Damaged shoots are likely to retain the section starting from the
position of 1.0 em diameter to the shoot base. The most significant factors
affecting the height increments and changes in tree-form values of trees in
the heavily infested open plot were all associated with initial tree size or tree form: larger and less-well formed trees achieved larger height
increments and better-formed trees achieved less gain in tree-form values.
For trees with initial height in the same range, the average height
increment of trees in the forest during the same period was over twice as
much as that in the open plot.
The insect had been successfully reared on an artificial diet for 23
generations. The colony reared in this study compared favourably to other
artificially-reared colonies of H. robusta in terms of pupal weight and
fecundity. Duration of development was comparable to that observed for
individuals reared with host tissues. The number of larval instars varied
from five to seven, with most larvae completing six instars before pupating.
Mating in indoor cages was enhanced by exposing the moths to wind.
Mortality due to non-feeding was reduced by confining the neonates to
small rearing vials.
Mean rate of development of the combined larval and pupal period
in the temperature range of 16.4 oc - 28.7 oc was closely fitted by the linear
model. Based on the model parameters and data on duration of egg
development, the maximum number of annual generations of the insect
around the field study site was estimated at five to six. Variable
development rates among individual insects were evident at all the five
constant temperatures tested. Distribution of develop times at different
temperatures can be modelled by a common 3-parameter Weibull function,
which can be incorporated into field phenology models in estimating the
proportions of individuals completing development by a certain amount of
accumulated DD. A simpler but practical approach to modelling the
stochastic process of development was also demonstrated by directly fitting
the proportional development data to the logistic phenology model, which
can be used to estimate the proportions of individuals in each development
stages at a given time. Feeding bioassays using neutral substrates confirmed the existence of
feeding stimulants in the ethanol extracts of young shoots. Larvae fed more
intensively on agar-cellulose medium or filter paper treated with the
ethanol extracts than on the corresponding plain substrates in both nonchoice
and choice tests. The biting response could be elicited by the presence
of host odour alone. Feeding stimulants for the insect was not confined to
red cedar. The larvae readily bored into the shoots of three non-host
meliaceous species: Spanish Cedar (Cedrela odorata), Chinese toon (Toona
sinensis), and white cedar (Melia azadirachta var. australasica ), although
those bored into the shoots of the latter two species later died.
Feeding spots with respect to hust tissues changed as the larva aged.
Feeding by larvae of the first two instars were mostly found in terminal
foliage (buds and unexpanded foliage) or damaged tissues (leaf scars or other
damaged areas on the surface of shoots or stems). Pith-feeding (tunnelling)
started at later 2nd instar. Some larvae came out of the tunnels before
pupating. With potted plants, the preferred pupation sites was around the
base of the plant close to the soil level. On average, a larva initiated feeding
in 5.4 different locations during its life time, with a minimum of three and
a maximum of 11. Switching of feeding spots was most frequent during
early first instar and much of the 3rd and 4th instar.
Most moths emerged in the early hours of the scotophase (82%), the
rest before light-off. Female calling started at 3 hours after light-off and
peaked around 5.5-7.5 hours after light-off. There was an apparent trend for
earlier calling as the females aged. Mating started 1.5 hours later than calling
but reached its peak around the same time as calling. Mating was recorded
for females aged 1-6 days and for males aged 0-4 days. Females appeared to be
most receptive to males in the 2nd-3rd day following emergence. Egg-laying
was observed in all but the first 0.5 hour and the last 1.5 hours of the
scotophase. There appears to be no single, dominant peak in the die! pattern
of egg-laying. Despite being nocturnal, adults of the insect, irrespective of their sex and mating history, periodically underwent rest in the dark phase.
Virgin females were more active during the early half of the scotophase and
remained relatively stationary during the latter half of the scotophase.
Average duration of active intervals was significantly longer in mated
females than that in virgin females. Males were most active in the first
scotophase following emergence, and virgin females in the second
scotophase following emergence. The results suggest that mated females
may be responsible for host finding.
Volatiles from the young foliage of the host plant were attractive to
larvae when tested in the larval olfactometer. However, tests in the Y-tube
olfactometer and wind tunnel failed to detect apparent directional responses
toward host volatiles in virgin and mated females. Hypotheses were
proposed to explain the lack of olfactory responses of females to host
volatiles.
Implications of results of this study for the management of the insect
are discussed.
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