Proceedings of 6th International Fruit Fly Symposium
6–10 May 2002, Stellenbosch, South Africa
pp. 105–110
Response of female Medfly, Ceratitis capitata (Diptera: Tephritidae), to
olfactory stimuli from various host plants in a wind tunnel
Serge Quilici* & Thuy Nguyen Ngoc
CIRAD Réunion, Pôle de Protection des Plantes, 7 Chemin de l’IRAT, 97410 Saint-Pierre (France)
The Medfly, Ceratitis capitata (Wiedemann), is a highly polyphagous pest of economic importance
on Réunion Island as in many other parts of the world. Using a flight tunnel, a study was conducted
on the response of C. capitata females to volatile compounds emitted by different host plants.
Results confirm that the odours of host fruit play an important role in the female host-plant location behaviour. The response varied depending on fruit species: guava (Psidium guayava L.), citrus
(Citrus spp.), coffee (Coffea arabica L.), chilli (Capsicum frutescens L.), Indian almond (Terminalia
catappa L.) and Spanish cherry (Mimusops elengi L.). Host plant leaves also induced a weaker
response and a limited attractiveness of non-host fruit odour was also shown.
INTRODUCTION
Kairomones from plant origin have been shown
to play a significant role in attracting females of
various tephritid species during host-plant location process (Prokopy & Roitberg 1989; Fletcher
& Prokopy 1991). This perception of, or attraction
to, plant volatiles is well-known in oligophagous
tephritid species but also in polyphagous ones
such as Bactrocera tryoni (Froggatt) (Eisemann &
Rice 1992), the Caribbean fruit fly, Anastrepha
suspensa (Loew) (Nigg et al. 1994), the oriental
fruit fly, Bactrocera dorsalis Hendel (Cornelius et al.
2000) or the Mediterranean fruit fly (Medfly),
Ceratitis (Ceratitis) capitata (Wiedemann) (Light
et al. 1988; Light et al. 1992; Prokopy & Vargas
1996; Katsoyannos et al. 1997; Jang 1997; Warthen
et al. 1997). Light et al. (1992), using electroantennography, showed that the Medfly female
antennae could perceive the volatile constituents
of nectarine.
In field cages, Prokopy & Vargas (1996) showed
a strong attractiveness for females of ripe intact
or crushed coffee fruit, compared with five lowerranking host fruits (guava, banana, tangerine,
papaya and avocado) and three non-host fruits.
Previous studies have also shown that wind tunnels
constitute a suitable tool for analysing the response of fruit flies to these semiochemicals (Fein
et al. 1982; Jang & Light 1991; Jang et al. 1997).
This field of research proves useful for a better
understanding of the insect–plant relationship in
tephritid species of economic importance and
may lead to the identification of promising new
female attractants for monitoring or control.
On Réunion Island, C. capitata is a tephritid of
economic importance for fruit crops, particularly
in the lowlands on the leeward western side of
*To whom correspondence should be addressed.
E-mail: serge.quilici@cirad.fr
the island, which has a drier climate favourable
to this species (Etienne 1982).
This study aimed at looking for host fruits highly
attractive to Medfly females, using a wind tunnel.
The first objective was to specify the methodology for studying the response of Medfly female
to olfactory stimuli in a wind tunnel with respect
to the influence of wind speed on female response.
Once adequate parameters were defined, we
compared the oriented flight of Medfly females
in response to olfactory stimuli from various host
or non-host plants, in no-choice or choice situations. Our purpose was to assess the relative
attractiveness of previously studied host fruits,
such as coffee or guava, and of some common
tropical host fruits present on Réunion Island,
such as Indian almond (Terminalia catappa L.)
(Combretaceae) or Spanish cherry (Mimusops
elengi L.) (Sapotaceae). In addition, we also
examined whether ripe host fruits were preferred to unripe ones, and if host-plant leaves
presented any attractiveness for females.
MATERIALS AND METHODS
The strain of C.capitata used in these experiments
was started from samples of infested fruit collected
on Réunion Island and larvae were subsequently
reared on artificial diet in the laboratory for more
than 50 generations. Adults are reared at 25 ± 3°C
and 70 ± 15% RH, in large perspex cages (50 ×
50 × 60 cm), with two egg-laying devices placed
at the top of the cage. Each device consists of a
red plastic funnel punctured with numerous small
holes enabling the females to deposit their eggs
into it. Flies are provided with water and food
consisting of cane sugar and enzymatic yeast
hydrolysate (ICN Biochemicals, U.S.A.). Eggs are
collected daily and placed on an artificial diet for
larval development (Etienne 1982). At the end of
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Proceedings of the 6th International Fruit Fly Symposium
larval development, the plates containing larvae
are placed in large boxes containing a layer of sand,
where pupation occurs. After pupation, the sand
is sifted in order to collect the pupae.
According to their date of emergence, adult flies
of both sexes used were grouped daily in cubical
cages (30 × 30 × 30 cm) in order to constitute
cohorts of known age; only females were used in
this study. For all experiments, females were used
when 10 days old, an age when they had reached
their sexual maturity. They were naive, that is they
had neither previous contact with any host plant,
nor egg-laying experience before the experiment.
The experiments were conducted in a plastic
(Lexan) wind tunnel (l = 255 cm; b = 80 cm; h =
45 cm) placed in a room were temperature was
maintained at 25 ± 1°C. The airflow was brought
from the outside into the tunnel by a ventilator,
then extracted to the outside. The airspeed
was measured using a telescopic anemometer
equipped with a thermic sensor (Model 8330,
Velocicheck, TSI). A set of cold light neons, placed
50 cm above the tunnel, was equipped with a
regulator providing a light intensity which varied
from 0 to 5400 Lux. Light intensity was measured
with a luxmeter (Lux Meter LX-101, Bioblock
Scientific).
For all experiments, females were individually
tested . Each female was placed in a small glass
tube (5 × 1.5 cm) which was carefully placed
down the airflow at a distance of 1.2 m from the
odour source(s). In all experiments, in addition
to the odour, we used one or two red plastic
spheres (diameter 7 cm) placed at the upwind
extremity of the tunnel 2 cm above the tunnel
ceiling. For the study of female response to
olfactory stimuli in a choice situation, the spheres
were punctured with small holes to allow a
good flow of the volatiles, and the odour was
sent inside the sphere.
The observation began when the female reached
the opening of the tube and ended after a period of
two minutes, or when the female landed on the
sphere. In each experiment, 25 females were
tested in a series of five for each treatment. Each
female was used only once.
Preliminary methodological study
For methodological studies the source of odour
consisted of 150 g of ripe guava (Psidium guayava
L.) fruits cut in slices and placed in a metallic grid
box situated behind the filter, at the upwind side
of the tunnel. The response of females to the
same olfactory stimulus (guava odour) was com-
pared at the airspeed of 0, 20, 30 and 40 cm/sec.
Then the response of females to the same stimulus was compared at three different periods of the
day: morning (08:00–10:00), middle of the day
(12:00–14:00) and afternoon (16:00–18:00). Finally
the response of females to guava odour was
compared under four light intensities: 300, 600,
1200 and 2400 Lux, at an airspeed of 20 cm/sec,
the experiment being conducted in the middle
of the day (10:00–12:00).
Female response to olfactory stimuli
Based on the preliminary methodological study,
the conditions chosen for all subsequent experiments were: airspeed – 20 cm/sec; light intensity
– 600 Lux, period of the day–10:00–12:00. For all
experiments, three replicates of 25 flies, individually tested, were carried out for each treatment.
No-choice situation. To study the response to
olfactory stimuli in a no-choice situation, the flies
were exposed to two series of ripe host or non-host
fruits odours. The fruits were placed in a metallic
grid box situated behind the filter at the upwind
side of the tunnel. In a first series of experiments,
the odour source was composed of ripe fruits of
either guava (Psidium guayava L.), orange (Citrus
sinensis (L.), chilli (Capsicum frutescens L.), or pumpkin (Cucurbita pepo L.).For the control (clean air) no
fruit was placed in the odour compartment. In a
second series of experiments the females were
offered another series of ripe fruit odours: Spanish
cherry (Mimusops elengi L.), Indian almond
(Terminalia catappa L.), coffee (Coffea arabica L.),
pumpkin (Cucurbita pepo L.) or no odour (control).
The weight of the fruit sample used for the different treatments was 150 g for Indian almond, Spanish cherry, guava, orange, pumpkin or coffee, and
40 g for chilli. To allow for a better volatile emission, small fruits (Indian almond, Spanish cherry,
chilli) were punctured with a needle while bigger
fruits (guava, orange, pumpkin) were cut into
slices.mm
Choice situation. For this series of experiments
the odour sources were situated outside the tunnel, in two plastic bags with entry and exit holes
for airflow. The entries were connected by a silicone pipe to a vacuum pump of variable flow injecting clean air at a speed of l l/min, while the
exits were connected with the tunnel by silicone
pipes. Once they have passed above the odour
sources, the airflows charged with volatiles are
then sent inside the two red spheres placed within
the tunnel.mmm
In a first series of experiments, ripe fruits of
Quilici & Nguyen Ngoc: Response of female Medfly to olfactory stimuli in a wind tunnel
107
Fig. 1. Response of Ceratitis capitata females to volatiles of ripe guava fruit at different wind speeds (n = 25).
Figures followed by the same letter do not differ significantly (Fisher test, P = 0.05).
various host plants were compared in a two-choice
situation: orange vs clementine, guava vs chilli,
guava vs Spanish cherry, guava vs coffee, guava vs
Indian almond and guava vs orange. The response
to unripe vs ripe fruits was compared in a second
series of experiments, successively for clementine,
chilli, Spanish cherry, coffee and Indian almond.
Finally, on a third series of experiments, the
response of females to different plant organs
was evaluated for Indian almond then for coffee
(ripe fruit vs control, leaf vs control and ripe fruit vs
leaf ).
Data analysis
For the different experiments, analysis were
carried out to compare the percentages of females
reaching the sphere. A Chi-square test with Yates’
correction or an exact Fisher test (when females
showed a weak response) with P = 0.05 were
performed using the software S-plus 2000.
RESULTS
Preliminary methodological study
The percentage of females reaching the sphere
was higher at a windspeed of 20 cm/sec., though
the only significant difference was between 20 and
40 cm/sec (Fig. 1). As a result of this preliminary
test, an air speed of 20 cm/sec. was chosen for all
subsequent experiments. The latter were conducted in the midday period (10:00–12:00), using
a light intensity of 600 Lux.
Female response to olfactory stimuli
For all experiments, the response of females did
not differ between the three replicates (Chi-square
tests), so all three were analysed together.
No-choice situation
In the first series of experiments, the response
of females to fruit odours was significantly higher
Fig. 2. Response of Ceratitis capitata females to volatiles of ripe fruits from different host or non-host plant
(n = 3). Figures followed by the same letter do not differ significantly (Chi-square test, P = 0.05).
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Proceedings of the 6th International Fruit Fly Symposium
Fig. 3. Response of Ceratitis capitata females to volatiles of ripe fruits from different host or non-host plants
(n = 3). Figures followed by the same letter do not differ significantly (Chi-square test, P = 0.05).
than the response to the control. The percentage
of females flying upwind or reaching the target
sphere was significantly higher when the odour
source consisted of ripe chillis or guava fruits than
when ripe pumpkin fruits were used (Fig. 2). The
response to ripe orange fruit odour, significantly
lower than that to ripe chilli fruit, was not significantly different to ripe pumpkin or guava fruits.
In the second series of experiments, ripe
fruits of Indian almond (Terminalia catappa) and
Spanish cherry (Mimusops elengi) induced a significantly higher response than ripe pumpkin fruits
(Fig. 3). The response to Indian almond fruits
was significantly higher than that to coffee fruits,
but not significantly different than the response to
Spanish cherry fruits. Ripe coffee fruits induced
an intermediate response between the other
host fruits and the non-host fruit.
Choice situation
Females placed in a choice situation in the
presence of odours of unripe or ripe fruits of
Clementine or Spanish cherry showed a significantly stronger response to ripe fruits (Fig. 4). For
all other fruits tested, the responses of females
to unripe or ripe fruit were not significantly different.
Females showed no significant difference in their
response to ripe fruits of guava vs orange, chilli,
coffee and Spanish cherry respectively. Similarly,
Fig. 4. Choice of landing site by Ceratits capitata females between two red spheres emitting odour of unripe or
ripe fruits (n = 3). Figures followed by the same letter do not differ significantly (Chi-square test, P = 0.05).
Quilici & Nguyen Ngoc: Response of female Medfly to olfactory stimuli in a wind tunnel
109
Fig. 5. Choice of landing site by Ceratitis capitata
females between two red spheres emitting odours of
guava or Indian almond ripe fruits (n = 3). Figures
followed by the same letter do not differ significantly
(Chi-square test, P = 0.05).
for citrus fruits, the response of females to ripe
fruits of Clementine or orange was not significantly
different. By contrast, ripe Indian almond fruit was
more attractive than ripe guava fruit (Fig. 5).
In a choice situation, females showed a significantly higher response to ripe fruit or to Indian
almond or coffee leaves, compared with the
controls. When females were simultaneously
offered ripe fruit and leaves in a choice situation,
they showed a higher response to fruit in the case
of Indian almond, while the difference was not
significant in the case of coffee (Fig. 6).
DISCUSSION
Our results confirm that the odour of various
host fruits shows a strong attractiveness for
female Medfly. Interestingly, though coffee has
been shown to contain volatiles attractive to the
female Medfly and is presumed to be an ancestral host of Medfly in the Americas (Prokopy &
Vargas 1996), we could show that, in a choice
situation, the female Medfly is more attracted to
ripe fruits of Indian almond than to ripe coffee
berries. In future studies, it would be worth investigating the chemical compounds present in the
odour of Indian almond ripe fruits. Though the
response of female to ripe fruits of Spanish cherry
is not significantly greater than to ripe coffee
berries, this host fruit also shows a good attractiveness and it would also be worth analysing its
volatiles. In the conditions of Réunion, fruits of
both Indian almond and Spanish cherry are
heavily attacked by C. capitata. The former are
also heavily attacked by the Natal fruit fly, Ceratitis
rosa (Karsch), which is less common on Spanish
cherry fruits.
Ripe fruits of Clementine or Spanish cherry were
Fig. 6. Choice of landing site by Ceratitis capitata
females between two red spheres emitting odours of
coffee plant organs, or no odour (n = 3). Figures
followed by the same letter do not differ significantly
(Chi-square test, P = 0,.05).
more attractive than unripe fruits of the same
species. Though Prokopy & Vargas (1996) showed
that the odour of ripe or near-ripe coffee fruit was
significantly more attractive than the odour of
unripe fruit, we failed to find such a difference
between these two types of fruits in our experimental conditions, which may be due to the
phenological stage of the fruits used. In the case
of Indian almond, though ripe fruit attracted
more females, the difference was not significant.
Interestingly, the tendency was inverse in the case
of chilli fruit.
The tests in a choice situation showed that,
compared to a control, the leaves of both coffee
and Indian almond were attractive for female
Medfly. This attractiveness of leaves seems
higher in the case of coffee (for which the attractiveness of leaves is not different from that of
ripe fruit) than in the case of Indian almond.
CONCLUSION
This work enabled us to specify the methodology for wind tunnel assays for studying the
response of female Medfly to olfactory stimuli.
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Proceedings of the 6th International Fruit Fly Symposium
Our results confirm that odours from host fruits
probably play a major role in the host-plant location behaviour of the females in this highly polyphagous species. Among the different host or
non-host plants tested, a strong attractiveness of
the ripe fruits of Indian almond, and to a lesser
degree of Spanish cherry was proven.
This study provides elements for future research
aiming at identifying volatiles potentially attractive
for the female Medfly, and that could be used to
improve monitoring or bio-technical control
methods against this economically important
pest. Given the very large host range of Medfly, it
would be worth investigating in future studies the
response of female Medfly to other poorly known
tropical host fruits that are recorded as being
heavily attacked by the fly in some countries.
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