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Olfactory cues in host nest detection by the social parasite Polistes sulcifer (Hymenoptera, Vespidae)
BEHAVI~U~~AL ELSEVIER
Behavioural Processes 36 (1996) 2 13-2 I8
PROCESS
Olfactory cues in host nest detection by the social parasite Polistes sulcifer ( Hymenoptera, Vespidae) R. Cervo
*,
F. Bertocci, S. Turillazzi
Dipartimento di Biologia Animale e Genetica, Unioersitic di Firenze, Via Romana 17, 50125 Firenze, Italy Accepted 29 May 1995
Abstract
Sometimes the nests of the paper wasp Polistes dominulus are parasitized by the obligate social parasite sulctfer. It is not known how, in the spring, this parasite searches for established nests of its host
Polistes
species. This study investigates the capacity to detect the host nest by olfactory cues alone. experiments P. sulcifer females were allowed to choose different options hidden from view: dummy, various portions of the host nest (larvae, pupae and material), nests or immature brood different sympatric species (P. dominulus, P. nimpha and P. gallicus). The parasites proved to perceiving nest odour and of discriminating between different species of Polistes. The odour of host brood, rather than the nest material, elicits the greatest response in the parasites. Keywords:
Social parasitism;
Cuckoo wasp; Host nest detection;
In laboratory host nest and pertaining to
be capable of the immature
Olfactory cue; Polistes sulcijer; Vespidae
1. Introduction Extensive literature exists on the topic of nestmate recognition in Polistes paper wasps from the behavioural point of view (see Gamboa, 1996). In addition, much work has recently been carried out on odours that mediate recognition. Several papers mention the surface lipids in adults, immature brood and nests of some species of Polistes wasps (Espelie and Hermann, 1990; Espelie and Himmelsbach, 1990; Bonavita-Cougourdan et al., 1991) and the role these chemicals play in nest and kin recognition processes (Espelie et al., 1990; Singer and Espelie, 1992; Espelie et al., 1994; Lorenzi et al., 1996). On the contrary, no information is available on host recognition ability in Polistes social parasites. Obligate permanent social parasites are species which rely on the workers of other species
* Corresponding
author
0376.6357/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0376-6357(95)00030-5
214
R. Ceruo et al. /Behavioural
Processes 36 (1996) 213-218
to rear their own reproductives. Among Polistes, only three species of obligate social parasites are known. One of these, P. sulcifer, parasitizes the nest of the most common European species, P. dominulus. Evidence for host-specificity in this parasite was found in field data and laboratory tests (Cervo and Turillazzi, 1996). The method of host nest invasion by this parasite has already been analyzed (Turillazzi et al., 1990) but what drives its host-searching behaviour and nest localization remains to be investigated, even if Scheven (1958) hypothesized that Polistes sulcifer females choose the host nest solely on the basis of host female odour. Studies conducted on bumblebee obligate social parasites demonstrate that they orient themselves towards the odour of the host bees and are capable of discriminating between different species of bumblebees (Fisher, 1984). In addition, Fisher et al. (1993) attempted to determine the possible source of host odour which the searching parasites perceived. In this paper, we investigate the capacity of P. sulcifer females to detect a host nest by olfactory cues alone.
2. Materials
and methods
In the spring of 1989 and 1990, 32 females of P. sulcifer (20 and 12 respectively) were collected from two localities in Italy (Lessinia near Verona, northern Italy, and Versilia, at the feet of the Apuan Alps, central Italy). The wasps were caught during flight activity (presumably before nest usurpation) or immediately after they had invaded a host nest. The parasites were transferred to the laboratory and put in separate glass boxes (15 X 15 X 15 cm) with half of the top consisting of a metal grid. They were fed with water and honey ad libitum for the entire experimental period. The Polistes nests used for the experiments were collected in the field, in the surroundings of Florence (central Italy), then placed with their inhabitants in similar laboratory boxes. Water, honey and Tenebrio molitor larvae were supplied ad libitum together with paper for nest building. All the cages were set in a room with natural light and temperature with additional illumination from 6.00 a.m. to 6.00 p.m. supplied by 100 W light bulbs. The characteristics.of Polistes nests used in the two sets of experiments (1989- 1990) are given in Table 1. As P. dominulus cell size is larger than that of the
Table 1 Characteristics experiments
1st experiment P. dominulus 2nd experiment
(no. of cells, larvae and pupae) of the nests used in the two sets of trials (1989-1990) 1989
1990
Number of:
Number of:
for the first and second
Cells
Larvae
Pupae
Cells
50
8
8
46
5
7
4 9 6
1 1
33 45 40
11 15 6
4 2 6
P. dominulus
30
P. nimpha
44
P. gallicus
38
3
Larvae
Puoae
R. Cerw et al./Behaoioural
Processes 36 (1996) 213-218
215
other two species, the P. dominulus nest we used was smaller (in size and in immature brood content) than the P. nimpha and P. gallicus nests. The experimental procedure consisted in simultaneously offering the parasites a choice among different sources of odours. In 1989 each animal was tested in six experiments, while in 1990 each animal was also tested in two additional trials (experiment 7 and 8). Choice experiments were conducted in a container similar to the breeding boxes. A thin, porous paper screen (thickness = 0.16 mm) was applied externally over the metal grid so that the parasites could not see the sources of odour. The various choices were placed on the screen, about 10 cm apart in the double option experiment and about 2 cm from each other in the triple choice experiment. During the experiments, three of the cage sides were masked with opaque covers, whilst the fourth was left free to allow observations. A 100 W bulb lamp, approximately 25 cm over the experimental cage, illuminated the apparatus during the experiment. In each experiment the female parasites were tested individually for 10 min (except in the experiment with nest broods of different species, in which the parasites were tested for 5 min). A fresh porous paper screen was used for each parasite tested. As previous observations (Turillazzi et al., 1990) revealed that P. sulcifer females exhibit a pronounced antenna1 response on approaching the host nest, we measured the time spent in antennation under the options offered in the experiment, immediately after the parasites were inserted into the test cage.
3. Results Results of the choice experiments are given in Table 2. In the first experiment, we tested whether the parasites could detect the presence of a P. dominulus nest hidden from view by the paper screen. To avoid the possibility that the wasps were attracted by the nest’s shadow, we placed a cardboard silhouette of the same size (dummy) on the paper screen. The wasps antennated significantly more under the real nest than under the dummy (Wilcoxon test, Z = - 5.77, P < 0.001). When simultaneously offered nests of different species (P. dominulus, P. gallicus and P. nimpha), but all at the same stage of development, the parasites antennated significantly longer under the nest of P. dominulus (Kruskal-Wallis test, P < O.OOl>, the host species of this parasite. The next step was to first use the nest material and then immature brood pertaining to P. dominulus, covered with plastic caps, as baits on the paper screen which were tested against an empty cap (dummy). Only 17 out of the 32 wasps reacted, but those that did antennated significantly more under the cap with nest material (Wilcoxon test, Z = -4.35, P < 0.001). In the immature brood experiments, parasites antennated significantly more under caps with 4 larvae (Wilcoxon test, Z = -4.9, P < 0.001) and 4 pupae (Wilcoxon test, Z = -4.34. P < 0.001) than under empty caps. However, there was no significant difference when the parasites had to choose between 6 larvae and 6 pupae (Wilcoxon test, Z = -0.88, n.s.>. The same technique was used to test the parasites using immature broods (4 larvae + 2 pupae) from different species as the odour source. Average antennation time was longer under the immature brood taken from P. dominufus than from the other two Polistes species. The difference was not significant for P. galficus (Wilcoxon test, Z = -0.99, ns.), but was significant (Z = 1.69, P < 0.05) for P. nimpha.
216
R. Ceruo et al. / Behauioural Processes 36 (1996) 213-218
Table 2 Average time spent by the parasites in antennation Choices I st experiment Host nest Dummy 2nd experiment Host nest P. nimpha nest P. gallicus nest 3rd experiment Host nest material Dummy 4th experiment Host larvae Dummy 5th experiment Host pupae Dummy 6th experiment Host pupae Host larvae 7th experiment Host brood P. nimpha brood 8th experiment Host brood P. gallicus brood Standard deviation, significant for each experiment.
behaviour under the different
choices for each test
Mean
S.D.
P
Nt
Nr
55.8 5.8
44.9 11.3
< 0.001
32
30
16.9 3.3 3.6
13.8 5.8 5.4
< 0.001
32
29
11 1.5
14.2 3
< 0.001
32
17
34.1 4.6
32.4 5.3
< 0.001
31
26
40.9 5.4
64.3 14.7
< 0.001
31
23
25 13.3
56.9 15.4
NS
32
24
24.2 9.2
20.5 11.7
< 0.05
12
9
18.3 16.4
NS
12
11
25.3 17.2
,
level, number of parasites tested (Nt) and the number of responding
parasites (Nr) are given
4. Discussion The results demonstrate that P. sulcifer females can recognize the odour of a P. dominulus nest, and that they are capable of distinguishing between the odours of host and non-host species. Our experiments also suggest that the odour comes chiefly from the immature brood, rather than from the nest material. The role that sight plays in finding and choosing a nest has not yet been analyzed. It could be hypothesized that the parasites use sight during the initial phases of nest localization, perhaps by following their hosts back home. However, the present study reveals that once in the vicinity of the nest, the parasites can detect whether it belongs to the right species by chemical cues alone. Our results are similar to those Fisher and co-workers found in females of Psithyrus (Fisher, 1984; Fisher and Greenwood, 1990; Fisher et al., 1993). The limited space in which our experiments were conducted did not allow us to establish the distance from which the parasites can perceive nest odour. Preliminary evidence from trials recently conducted in a wind tunnel suggest that P. sulcifer females can rapidly detect a nest hidden from view, set at a distance of about 1 m in an air flow of approximately 35 cm per s.
R. Cervo et al. / Behauioural Processes 36 (1996) 213-218
217
It is well documented that Pokes cuticular hydrocarbons, whose primary function is to prevent desiccation, also play a determinant role in the social context. Since cuticular hydrocarbon are also known to serve as kairomonal cues for parasitoids (Blomquist and Dillwith, 1985>, it is possible that obligate social parasites also use their host’s cuticular lipids as cues in host nest detection. Pofistes wasps, like other insects, produce blends of hydrocarbons of rather high molecular weight, consequently such substance are non-volatile and only operative within a limited space (Blum, 1987; Howard, 1993). Between nestmates, cuticular hydrocarbons act as surface pheromones, which could be perceived by body contact or at the most within a confined space (Blum, 1987; Howard, 1993). If the same hydrocarbons are the specific substances the parasites use in host nest localization, their range of detection must be wider. Normal air circulation would only ensure perception by the parasites in the immediate vicinity of the nest - on the other hand a more volatile compound could work alone or in concert with cuticular lipids as a kairomone. As P. dominulus nests are usually built in sheltered positions, it is also possible that the wasps deposit chemicals around the area of the nest entrance on their arrival. This type of nest detection is reported by Cedeberg (1983) in the parasitic bee Psithyrus rupestris. The author showed how this bumblebee social parasite follows the trails of nest entrance extracts of the host species. Thus, Polistes parasites which approach the host nest by walking could perceive the host cuticular lipids within a restricted distance from the nest. Whatever the chemical source for host nest detection may be, the capacity of P. sulcifer females to recognize the species and the stage of development of an usurpable nest, without needing to land, is undoubtedly an advantage, since they avoid useless and risky usurpations of non-utilizable nests. We reported that this parasite chooses the larger and more developed nests from those available (Cervo and Turillazzi, 1996). Odour concentration may be higher in larger nests, making them easier to detect than smaller ones. Further experiments are needed to evaluate the minimum quantity of odour the parasites need to detect a nest, and to determine the source of the chemical cues which allow host recognition.
Acknowledgements This study was supported with funds from the MURST Coster-Longman for translation of the English text.
(60%).
We wish to thank
Dr. C.
References Blomquist, G.J. and Dillwith, J.W., 1985. Cuticular lipids. In: G.A. Kerkut and L.I. Gilbert (Editors), Comprehensive Insect Physiology, Biochemistry and Pharmacology, Pergamon Press, Oxford and New York, pp. 117- 154. Blum, MS., 1987. Specificity of pheromonal signs: a search for its recognitive bases in terms of a unified chemisociality. In: J. Eder and H. Rembold (Editors), Chemistry and Biology of Social Insects, J. Pepemy, Miinchen. Bonavita-Cougourdan, A., Theraulaz, G., Bag&es, A.-G., Roux, M., Pratte, M., Provost, E., ClCment, J.L., 1991. Cuticular hydrocarbons, social organization and ovarion development in a polistine wasp: Pofistes dominufus Christ. Comp. Biochem. Physiol., 1OOB: 667-680. Cedeberg, B., 1983. The role of trail pheromones in host selection by Psithyrus rupestris (Hymenoptera: Apidae). Ann. Ent. Fenn., 49: 1 I-16. Cervo, R. and Turillazzi, S., 1996. Host nest preference and nest choice in the cuckoo paper wasp Pofistes sufcifer (Hymenoptera: Vespidae). J. Insect Behav., 9(2): 305-314.
218
R. CURLY)et al./ Behaoioural Proce.w.~ 36 (19961 213-218
Espelie, K.E. and Hermann, H.R., 1990. Surface lipids of the social wasp Polistes annuluris and its nest and nest pedicel. J. Chem. Ecol., 16: 1841-1852. Espelie, K.E. and Himmelsbach, D.S., 1990. Characterization of pedicel, paper, and larval silk from nest of Polistes annuluris. J. Chem. Ecol., 16 (12): 3467-3477. Espelie, K.E., Wenzel, J.W. and Chang, G., 1990. Surface lipids of social wasp Polistes metricus say and its nest and nest pedicel and their relation to nestmate recognition. J. Chem. Ecol., 16(7): 2229-2241. Espelie, K.E., Gamboa, G.J., Grudzien, T.A. and Bura, E.A., 1994. Cuticular hydrocarbons of the paper wasp, Polistes fuscutus: a search for recognition pheromones. J. Chem. Ecol., 20 (7): 1677-1687. Fisher, R.M., 1984. Recognition of host nest odor by the bumble bee social parasite Psirhyrus ashtoni (Hymenoptera: Apidae). J. N.Y. Entomol. Sot., 91: 503-507. Fisher, R.M. and Greenwood, D.R., 1990. A chemical basis for host recognition in cuckoo bumble bees. In: G.K. Veeresh, B. Mallik and C.A. Viraktamath (Editors), Social insects and the environment, Oxford & IBH Publishing, New Delhi. Fisher, R.M., Greenwood, D.R. and Shaw, G.J., 1993. Host recognition and the study of a chemical basis for attraction by cuckoo bumble bees (Hymenoptera: Apidae). J. Chem. Ecol., 19(4): 771-786. Gamboa, G.J., 1996. Kin recognition in social wasps. In: S. Turillazzi and M.J. West-Eberhard (Editors), Natural History and Evolution of the paper wasps, Oxford University Press, Oxford, pp. 161- 177. Howard, 1993. Cuticular hydrocarbons and chemical communication. In: D.W. Stanley-Samuelson and D.R. Nelson (Editors), Insect Lipids: Chemistry, Biochemistry and Biology, University of Nebraska Press, pp. 179-226. Lorenzi, M.C., Bag&es, A.-G., and Clement, J.L., 1996. The role of cuticular hydrocarbons in social insects: is it the same in paper wasps? In: S. Turillazzi and M.J. West-Eberhard (Editors), Natural History and Evolution of the paper wasps. Oxford University Press, Oxford, pp. 178- 189. Scheven, J., 1958. Beitrag zur Biologie der Schmazotzefeldenwespen. Insectes Sot., 5(4): 409-438. Singer, T.L. and Espelie, K.E., 1992. Social wasps use nest paper hydrocarbons for nestmate recognition. Anim. Behav., 44: 63-68. Turillazzi, S., Cervo, R. and Cavallari, I., 1990. Invasion of the nest of Polistes dominufus by the social parasite Sulcopolistes sulcijer (Hymenoptera, Vespidae). Ethology, 84: 47-59.
Behavioural Processes 36 (1996) 2 13-2 I8
PROCESS
Olfactory cues in host nest detection by the social parasite Polistes sulcifer ( Hymenoptera, Vespidae) R. Cervo
*,
F. Bertocci, S. Turillazzi
Dipartimento di Biologia Animale e Genetica, Unioersitic di Firenze, Via Romana 17, 50125 Firenze, Italy Accepted 29 May 1995
Abstract
Sometimes the nests of the paper wasp Polistes dominulus are parasitized by the obligate social parasite sulctfer. It is not known how, in the spring, this parasite searches for established nests of its host
Polistes
species. This study investigates the capacity to detect the host nest by olfactory cues alone. experiments P. sulcifer females were allowed to choose different options hidden from view: dummy, various portions of the host nest (larvae, pupae and material), nests or immature brood different sympatric species (P. dominulus, P. nimpha and P. gallicus). The parasites proved to perceiving nest odour and of discriminating between different species of Polistes. The odour of host brood, rather than the nest material, elicits the greatest response in the parasites. Keywords:
Social parasitism;
Cuckoo wasp; Host nest detection;
In laboratory host nest and pertaining to
be capable of the immature
Olfactory cue; Polistes sulcijer; Vespidae
1. Introduction Extensive literature exists on the topic of nestmate recognition in Polistes paper wasps from the behavioural point of view (see Gamboa, 1996). In addition, much work has recently been carried out on odours that mediate recognition. Several papers mention the surface lipids in adults, immature brood and nests of some species of Polistes wasps (Espelie and Hermann, 1990; Espelie and Himmelsbach, 1990; Bonavita-Cougourdan et al., 1991) and the role these chemicals play in nest and kin recognition processes (Espelie et al., 1990; Singer and Espelie, 1992; Espelie et al., 1994; Lorenzi et al., 1996). On the contrary, no information is available on host recognition ability in Polistes social parasites. Obligate permanent social parasites are species which rely on the workers of other species
* Corresponding
author
0376.6357/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0376-6357(95)00030-5
214
R. Ceruo et al. /Behavioural
Processes 36 (1996) 213-218
to rear their own reproductives. Among Polistes, only three species of obligate social parasites are known. One of these, P. sulcifer, parasitizes the nest of the most common European species, P. dominulus. Evidence for host-specificity in this parasite was found in field data and laboratory tests (Cervo and Turillazzi, 1996). The method of host nest invasion by this parasite has already been analyzed (Turillazzi et al., 1990) but what drives its host-searching behaviour and nest localization remains to be investigated, even if Scheven (1958) hypothesized that Polistes sulcifer females choose the host nest solely on the basis of host female odour. Studies conducted on bumblebee obligate social parasites demonstrate that they orient themselves towards the odour of the host bees and are capable of discriminating between different species of bumblebees (Fisher, 1984). In addition, Fisher et al. (1993) attempted to determine the possible source of host odour which the searching parasites perceived. In this paper, we investigate the capacity of P. sulcifer females to detect a host nest by olfactory cues alone.
2. Materials
and methods
In the spring of 1989 and 1990, 32 females of P. sulcifer (20 and 12 respectively) were collected from two localities in Italy (Lessinia near Verona, northern Italy, and Versilia, at the feet of the Apuan Alps, central Italy). The wasps were caught during flight activity (presumably before nest usurpation) or immediately after they had invaded a host nest. The parasites were transferred to the laboratory and put in separate glass boxes (15 X 15 X 15 cm) with half of the top consisting of a metal grid. They were fed with water and honey ad libitum for the entire experimental period. The Polistes nests used for the experiments were collected in the field, in the surroundings of Florence (central Italy), then placed with their inhabitants in similar laboratory boxes. Water, honey and Tenebrio molitor larvae were supplied ad libitum together with paper for nest building. All the cages were set in a room with natural light and temperature with additional illumination from 6.00 a.m. to 6.00 p.m. supplied by 100 W light bulbs. The characteristics.of Polistes nests used in the two sets of experiments (1989- 1990) are given in Table 1. As P. dominulus cell size is larger than that of the
Table 1 Characteristics experiments
1st experiment P. dominulus 2nd experiment
(no. of cells, larvae and pupae) of the nests used in the two sets of trials (1989-1990) 1989
1990
Number of:
Number of:
for the first and second
Cells
Larvae
Pupae
Cells
50
8
8
46
5
7
4 9 6
1 1
33 45 40
11 15 6
4 2 6
P. dominulus
30
P. nimpha
44
P. gallicus
38
3
Larvae
Puoae
R. Cerw et al./Behaoioural
Processes 36 (1996) 213-218
215
other two species, the P. dominulus nest we used was smaller (in size and in immature brood content) than the P. nimpha and P. gallicus nests. The experimental procedure consisted in simultaneously offering the parasites a choice among different sources of odours. In 1989 each animal was tested in six experiments, while in 1990 each animal was also tested in two additional trials (experiment 7 and 8). Choice experiments were conducted in a container similar to the breeding boxes. A thin, porous paper screen (thickness = 0.16 mm) was applied externally over the metal grid so that the parasites could not see the sources of odour. The various choices were placed on the screen, about 10 cm apart in the double option experiment and about 2 cm from each other in the triple choice experiment. During the experiments, three of the cage sides were masked with opaque covers, whilst the fourth was left free to allow observations. A 100 W bulb lamp, approximately 25 cm over the experimental cage, illuminated the apparatus during the experiment. In each experiment the female parasites were tested individually for 10 min (except in the experiment with nest broods of different species, in which the parasites were tested for 5 min). A fresh porous paper screen was used for each parasite tested. As previous observations (Turillazzi et al., 1990) revealed that P. sulcifer females exhibit a pronounced antenna1 response on approaching the host nest, we measured the time spent in antennation under the options offered in the experiment, immediately after the parasites were inserted into the test cage.
3. Results Results of the choice experiments are given in Table 2. In the first experiment, we tested whether the parasites could detect the presence of a P. dominulus nest hidden from view by the paper screen. To avoid the possibility that the wasps were attracted by the nest’s shadow, we placed a cardboard silhouette of the same size (dummy) on the paper screen. The wasps antennated significantly more under the real nest than under the dummy (Wilcoxon test, Z = - 5.77, P < 0.001). When simultaneously offered nests of different species (P. dominulus, P. gallicus and P. nimpha), but all at the same stage of development, the parasites antennated significantly longer under the nest of P. dominulus (Kruskal-Wallis test, P < O.OOl>, the host species of this parasite. The next step was to first use the nest material and then immature brood pertaining to P. dominulus, covered with plastic caps, as baits on the paper screen which were tested against an empty cap (dummy). Only 17 out of the 32 wasps reacted, but those that did antennated significantly more under the cap with nest material (Wilcoxon test, Z = -4.35, P < 0.001). In the immature brood experiments, parasites antennated significantly more under caps with 4 larvae (Wilcoxon test, Z = -4.9, P < 0.001) and 4 pupae (Wilcoxon test, Z = -4.34. P < 0.001) than under empty caps. However, there was no significant difference when the parasites had to choose between 6 larvae and 6 pupae (Wilcoxon test, Z = -0.88, n.s.>. The same technique was used to test the parasites using immature broods (4 larvae + 2 pupae) from different species as the odour source. Average antennation time was longer under the immature brood taken from P. dominufus than from the other two Polistes species. The difference was not significant for P. galficus (Wilcoxon test, Z = -0.99, ns.), but was significant (Z = 1.69, P < 0.05) for P. nimpha.
216
R. Ceruo et al. / Behauioural Processes 36 (1996) 213-218
Table 2 Average time spent by the parasites in antennation Choices I st experiment Host nest Dummy 2nd experiment Host nest P. nimpha nest P. gallicus nest 3rd experiment Host nest material Dummy 4th experiment Host larvae Dummy 5th experiment Host pupae Dummy 6th experiment Host pupae Host larvae 7th experiment Host brood P. nimpha brood 8th experiment Host brood P. gallicus brood Standard deviation, significant for each experiment.
behaviour under the different
choices for each test
Mean
S.D.
P
Nt
Nr
55.8 5.8
44.9 11.3
< 0.001
32
30
16.9 3.3 3.6
13.8 5.8 5.4
< 0.001
32
29
11 1.5
14.2 3
< 0.001
32
17
34.1 4.6
32.4 5.3
< 0.001
31
26
40.9 5.4
64.3 14.7
< 0.001
31
23
25 13.3
56.9 15.4
NS
32
24
24.2 9.2
20.5 11.7
< 0.05
12
9
18.3 16.4
NS
12
11
25.3 17.2
,
level, number of parasites tested (Nt) and the number of responding
parasites (Nr) are given
4. Discussion The results demonstrate that P. sulcifer females can recognize the odour of a P. dominulus nest, and that they are capable of distinguishing between the odours of host and non-host species. Our experiments also suggest that the odour comes chiefly from the immature brood, rather than from the nest material. The role that sight plays in finding and choosing a nest has not yet been analyzed. It could be hypothesized that the parasites use sight during the initial phases of nest localization, perhaps by following their hosts back home. However, the present study reveals that once in the vicinity of the nest, the parasites can detect whether it belongs to the right species by chemical cues alone. Our results are similar to those Fisher and co-workers found in females of Psithyrus (Fisher, 1984; Fisher and Greenwood, 1990; Fisher et al., 1993). The limited space in which our experiments were conducted did not allow us to establish the distance from which the parasites can perceive nest odour. Preliminary evidence from trials recently conducted in a wind tunnel suggest that P. sulcifer females can rapidly detect a nest hidden from view, set at a distance of about 1 m in an air flow of approximately 35 cm per s.
R. Cervo et al. / Behauioural Processes 36 (1996) 213-218
217
It is well documented that Pokes cuticular hydrocarbons, whose primary function is to prevent desiccation, also play a determinant role in the social context. Since cuticular hydrocarbon are also known to serve as kairomonal cues for parasitoids (Blomquist and Dillwith, 1985>, it is possible that obligate social parasites also use their host’s cuticular lipids as cues in host nest detection. Pofistes wasps, like other insects, produce blends of hydrocarbons of rather high molecular weight, consequently such substance are non-volatile and only operative within a limited space (Blum, 1987; Howard, 1993). Between nestmates, cuticular hydrocarbons act as surface pheromones, which could be perceived by body contact or at the most within a confined space (Blum, 1987; Howard, 1993). If the same hydrocarbons are the specific substances the parasites use in host nest localization, their range of detection must be wider. Normal air circulation would only ensure perception by the parasites in the immediate vicinity of the nest - on the other hand a more volatile compound could work alone or in concert with cuticular lipids as a kairomone. As P. dominulus nests are usually built in sheltered positions, it is also possible that the wasps deposit chemicals around the area of the nest entrance on their arrival. This type of nest detection is reported by Cedeberg (1983) in the parasitic bee Psithyrus rupestris. The author showed how this bumblebee social parasite follows the trails of nest entrance extracts of the host species. Thus, Polistes parasites which approach the host nest by walking could perceive the host cuticular lipids within a restricted distance from the nest. Whatever the chemical source for host nest detection may be, the capacity of P. sulcifer females to recognize the species and the stage of development of an usurpable nest, without needing to land, is undoubtedly an advantage, since they avoid useless and risky usurpations of non-utilizable nests. We reported that this parasite chooses the larger and more developed nests from those available (Cervo and Turillazzi, 1996). Odour concentration may be higher in larger nests, making them easier to detect than smaller ones. Further experiments are needed to evaluate the minimum quantity of odour the parasites need to detect a nest, and to determine the source of the chemical cues which allow host recognition.
Acknowledgements This study was supported with funds from the MURST Coster-Longman for translation of the English text.
(60%).
We wish to thank
Dr. C.
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