The Influence of Host Plants on the Mating Behaviour of
the Cotton Bollworm, Helicoverpa armigera (Hübner)
(Lepidoptera: Noctuidae)
by
Olivia Louise Kvedaras
B. Sc, UNE
A thesis submitted for the degree of Doctor of Philosophy
of the University of New England
School of Rural Science and Agriculture
The University of New England
Armidale, NSW
August 2002
Abstract
The cotton bollworm, Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) is considered
Australia’s most destructive agricultural pest. An understanding of the mating behaviour of this
pest in relation to host plants and the volatile chemicals they emit may prove relevant for
determining suitable refuges to be planted in order to produce susceptible moths as part of the
resistance management plan for transgenic (INGARD®) cotton, which expresses a gene for
Bacillus thuringiensis toxin. This thesis briefly reviews the literature associated with management of
H. armigera in Australian cotton, especially in relation to resistance management for transgenic
cotton. The physiological and behavioural aspects of mating in insects are also reviewed, with
particular attention to the influences of host plants.
The influence of host plant volatiles and age on reproductive maturation, calling
behaviour and pheromone production of female insects was studied in the laboratory.
Reproductive maturation was determined by measuring the width of the near terminal oocyte
after the first night of calling. The relationship between maturation and calling (pheromone
releasing behaviour) was similar whether host plant volatiles were present or not. Flowering
pigeon pea volatiles had no significant effect on the cumulative number of females that had
initiated calling by 1, 3, 4 or 5 days of age. At 2 days of age significantly more females
commenced calling in the presence of flowering pigeon pea volatiles than in their absence.
Calling patterns were not largely influenced by the presence of host plant volatiles. A trend for
earlier first calling during the scotophase, and more bouts of calling, when host volatiles were
present was evident. Effects were not large in comparison to the variation between individual
moths. As females aged the time of first call was earlier in the scotophase both in the presence
and absence of host plant volatiles from flowering pigeon pea. There was also a significant
increase in the time spent calling. At all ages females in the presence of flowering pigeon pea
volatiles had more bouts on average than those females in the absence of plant volatiles,
however, the difference, across ages, was not statistically significant.
In wild (F1) females significantly more pheromone was produced in females aged 2 and 3
days in the presence of host volatiles than females of the same age in the absence of host
volatiles.
In the presence of host plant volatiles there was significantly more pheromone
produced in 2 and 3 day old females than 1 and 5 day old females. Four day old females
produced significantly less pheromone compared to 3 day old females. Laboratory reared females
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aged 2 – 3 days generally produced more pheromone when in the presence of host plants, and
females aged 4 – 5 days produced less, however these differences were not statistically significant.
There was a trend for less pheromone to be produced as females aged in the absence of host
plant volatiles although this was also not significant. In the presence of host plant volatiles
females aged 2 days produced significantly more pheromone than females aged 5 days suggesting
a decrease in pheromone titre as females age.
Male responses to females in the presence and absence of host plant volatiles were
studied in the laboratory and field. In laboratory wind tunnel studies males responded strongly to
females calling, as indicated by their capture in traps containing females placed in the upwind end
of the wind tunnel. There were no significant effects of host plant volatiles on the proportion of
males entering the traps for any of the six host plants tested at different growth stages.
Influences of host plants and synthetic plant volatiles on male responses to synthetic
pheromones were studied using a standard design of pheromone trap in the field. In one
experiment potted sunflower plants at various stages of maturity were placed adjacent to the
traps. Catches were not significantly different between traps placed with the three stages of
sunflower or the control on any trapping night, or when pooled over the eight nights of the
experiment. Another series of field experiments examined the effects of two synthetic plant
volatiles, phenylacetaldehyde and (Z)-3-hexenyl acetate, formulated in Sirene ® and placed alone
or together with pheromone lures in pheromone traps. For all trials there were significant
differences in the numbers of males captured between treatments. Generally, the same pattern
across all nights was observed with the pheromone only treatment catching the greatest numbers
of moths, followed by, in order of highest to lowest catch, pheromone + (Z)-3-hexenyl acetate,
pheromone + phenylacetaldehyde, (Z)-3-hexenyl acetate and phenylacetaldehyde. These results
indicated that both plant volatiles tended to inhibit rather than synergise male responses to
pheromones. This result is in contrast to previous studies with related species using (Z)-3hexenyl acetate, but consistent with more recent studies on Spodoptera frugiperda using
phenylacetaldehyde.
Methods were developed for studies aimed at determining mating success of H. armigera
females in different crops in the field. Various wing-clipping methods to immobilise female
moths on mating tables were compared. The removal of the right fore- and hind- wing was most
effective. Wing-clipped females were less likely to be mated than normal winged females in
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laboratory studies. However, enough mating occurred for the wing-clipped females to be used in
field experiments. There was no consistent effect of age of females (2-5 days) or density (1, 2 or
3 per mating table) on mating success.
Mating tables containing wing-clipped, laboratory-reared virgin females were used to
determine the mating success of H. armigera in various crops in the Darling Downs, Queensland.
Ten experiments, each lasting one night and using three or four adjacent fields containing
different crops, were conducted over two cotton seasons. Light and pheromone traps were
operated in each field concurrently with the mating tables. There were no significant differences
in the mating success of virgin females in mating tables with the exception of two experiments.
In one of these there were significantly more females mated in flowering soybeans than flowering
cotton and fallow land. Pre-flowering soybeans were intermediate. In the other, the highest
numbers of females mated were in flowering cotton compared to mature maize or fallow land,
although mating in general was very low for all crops. In an additional three experiments, there
were similar trends for higher levels of mating in crops more suitable for oviposition, though
these trends were not statistically significant. In the light traps, on all nights there were significant
differences in the number of males and/or females trapped between the crops. The differences
generally followed expected trends for H. armigera host plant preferences with flowering and preflowering crops (sunflower, soybean and sorghum) favoured over cotton, fallow land and mature
crops. For most experiments there was increased male activity in the period after midnight, as
evidenced by the light traps. In many experiments there were crop x time interactions that
indicated that male mate-searching activity was extended to unfavourable host crops later in the
night, perhaps accounting for mating in the captive females in these crops. Female activity
generally decreased after midnight or remained relatively similar to the PM period, and there were
fewer crop x time interactions. For the pheromone traps on nearly all nights there were
significant differences in the number of males trapped between the crops. Across all nights, both
mating success in captive virgin females, and catches in pheromone traps, were negatively
correlated with the percentage of females in light traps, and positively correlated with the levels
of mating in those females. These results are discussed in the context of the relationship of
mating behaviour to the ecological strategy of H. armigera, and its implications for the use of
refuges in management of resistance to transgenic cotton.
It is concluded that current
recommendations on the choice of crops for refuges, which are based on population genetics, are
also soundly based from the perspective of mating behaviour.
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