Naturwissenschaften (2011) 98:705–709 DOI 10.1007/s00114-011-0813-0
SHORT COMMUNICATION
Temperature and parasitism by Asobara tabida (Hymenoptera: Braconidae) influence larval pupation behaviour in two Drosophila species Céline Josso & Joffrey Moiroux & Philippe Vernon & Joan van Baaren & Jacques J. M. van Alphen
Received: 28 February 2011 / Revised: 26 May 2011 / Accepted: 27 May 2011 / Published online: 17 June 2011 # Springer-Verlag 2011
Abstract In holometabolous insects, pupation site selection behaviour has large consequences for survival. Here, we investigated the combined effects of temperature and parasitism by the parasitoid Asobara tabida on larval pupation behaviour in two of its main Drosophila sp. hosts differing in their climate origin. We found that larvae of Drosophila melanogaster—a species with a (sub)tropical origin—placed at 25°C pupated higher in rearing jars than those placed at 15°C. The opposite pattern was observed for Drosophila subobscura larvae—a species from temperate regions—which pupated lower, i.e. on or near the substrate at 25°C, than those placed at 15°C. When placed at 25°C, parasitized larvae of both species pupated closer to the substrate than unparasitized ones. Moreover, the Drosophila larvae that had been exposed and probably stung by A. tabida, but were not parasitized, pupated C. Josso : J. Moiroux : P. Vernon : J. van Baaren : J. J. M. van Alphen Université de Rennes 1, UMR CNRS 6553 ECOBIO, 263 Avenue du Général Leclerc, CS 74205, 35042 Rennes Cedex, France P. Vernon Station Biologique de Paimpont, Université de Rennes 1, UMR CNRS 6553, 35380 Paimpont, France J. J. M. van Alphen IBED, University of Amsterdam, P.O. Box 94248, 1090 GE Amsterdam, The Netherlands C. Josso (*) Université de Rennes 1, UMR INRA 1099 BiO3P, 263 Avenue du Général Leclerc, CS 74205, 35042 Rennes Cedex, France e-mail:
[email protected]
lower than the control unparasitized larvae. These results provide new insights of host behaviour manipulation by A. tabida larvae. Keywords Drosophila . Parasitoid . Temperature . Pupation behaviour
Introduction Temperature and parasitism are examples of abiotic and biotic conditions experienced by organisms which directly influence their geographical and habitat distribution (e.g. Gaston 2003, 2009; Schnebel and Grossfield 1992), as well as their life histories and as a consequence their fitness (Dillon et al. 2009; Denis et al. 2011). Parasitoids, common natural enemies of many insect species, display a large variety of mechanisms permitting them to manipulate their hosts’ behaviour and/or physiology to the benefit of their own development (for review, see Beckage and Gelman 2004). Their successful attacks systematically kill the host. When the latter is immobile as during the pupal stage, the pupation site selection during the late larval stage can be critical for survival (Schnebel and Grossfield 1992). In this way, this trait is supposed to be under strong selection in the host. Indeed, immobile pupae remain exposed to potentially harmful factors (desiccation, predation, hyperparasitism by pupal parasitoids, fungal infection, etc.) for varied periods of time. Pupation site selection can also be of critical importance for parasitoids, especially when they spend more time in the puparium than their host, being thus vulnerable to dangers for a longer time. The pupation behaviour of larvae has been well documented in Drosophila species, especially in the
706
Drosophila melanogaster group. The effects of many factors such as temperature, light, moisture, density, presence of predators or parasitism have been documented, but for the greater part of them independently (Sameoto and Miller 1968; Markow 1979; Vandal and Shivanna 2007; Sameoto and Miller 1968; Joshi and Mueller 1993; Seyahooei et al. 2009). Schnebel and Grossfield (1986, 1992) have, for example, underlined the complexity of the relation between temperature and pupation height. They showed that the response of a species in pupation height at one temperature extreme cannot be used to predict responses at other temperatures, and temperature effects on pupation height in one species do not consistently reflect the effects on other species within the Drosophila group. Concerning the parasitism effect, Seyahooei et al. (2009) hypothesized that females of the parasitoids Asobara sp. are able to manipulate the behaviour of their host larvae D. melanogaster to achieve their own optimal pupation strategy because it is the host larva that chooses the pupation site. In this paper, we will test the combined effects of temperature and parasitism on the pupation behaviour of two Drosophila species, Drosophila subobscura and D. melanogaster, which differ in climate origin, but which have a common main parasitoid, Asobara tabida (Carton et al. 1986). In Northwestern Europe (i.e. in temperate regions), D. subobscura Collin is the most abundant host species while in the Mediterranean regions, D. melanogaster Meigen is the main host (Mollema 1988). We expected that (1) temperature would affect the pupation site selection of the two Drosophila species differently because of their different climate origin. At a higher temperature, the risks associated with desiccation should result in a decrease of D. subobscura pupation height and a selection of pupation sites near the substrate in relatively moist conditions. D. melanogaster is expected to be more resistant to desiccation, and its larvae should be less constrained in choosing pupation sites. Hence, we expect them to be able to pupate further away from the substrate than D. subobscura larvae at the same temperature; (2) if the parasitoid larvae are able to manipulate host pupation behaviour to their own advantage (Seyahooei et al. 2009), parasitized host larvae might choose a different pupation site than unparasitized ones. At a higher temperature, as parasitized hosts of both species need to be protected from desiccation for a longer period of time, they are expected to pupate closer to the substrate than the unparasitized ones. We also expect A. tabida to affect pupation site behaviour of its two main hosts in the same way. (3) Finally, we compared pupation behaviour of unparasitized larvae with unparasitized ones that had been exposed to the parasitoid, to study if such exposure changes pupation site choice.
Naturwissenschaften (2011) 98:705–709
Material and methods Hosts and parasitoid strains Origin D. subobscura and D. melanogaster (Diptera: Drosophilidae) develop in fermenting substrates such as fruits. D. subobscura originates from temperate European regions and has colonized the west coasts of North and South America, whereas D. melanogaster has an African, subtropical origin and has become a cosmopolitan species (Kraaijeveld and Godfray 1999). Development time varies with temperature (e.g. 9 days on average at 25°C for D. melanogaster vs. more than 4 weeks at 15°C). A. tabida (Hymenoptera: Braconidae) is a holarctic larval solitary endoparasitoid that attacks Drosophila larvae (Kraaijeveld and Godfray 1997). Its main host species are successively D. subobscura and D. melanogaster on a scale from north to south of Europe (Kraaijeveld and Godfray 1997). Hosts and parasitoids were collected in the Netherlands using banana bait traps (i.e. a plastic container with a 3-cm diameter hole covered with a mesh with 2-mm openings). Samplings were made in Leiden (52°09′40 N; 4°29′29 E) in 1960 (D. melanogaster), 1980 (D. subobscura) and in Woerdense Verlaat (52°08′60″ N; 4° 52′00″ E) in 2007 (A. tabida). Rearings Hosts were reared in glass jars (h=13 cm, Ø=6 cm) on agar–yeast–sugar–Nipagine substrate (20 g–50 g–35 g–5 mL in 1 L) with a thick yeast layer (1 cm of baker’s yeast at 0.5 g/mL). Adult parasitoids were maintained in glass jars (h =8 cm, Ø =5 cm) on agar–Nipagine substrate (20 g–5 mL in 1 L) and fed ad libitum with 10% diluted acacia honey. Since their establishments, D. subobscura and D. melanogaster colonies have been cultured at 20°C and 25°C, respectively. In our experiments, both hosts and parasitoids were reared at 20 ±1°C, Relative Humidity (RH)=60± 10%, 16Light:8Dark. Pupation height experiments To test whether A. tabida affects the pupation behaviour of its two main hosts in the same way, we set up an experiment using the two host species (D. subobscura and D. melanogaster) tested independently with the parasitoid and a control group of non-parasitized hosts for each species. Each of the four combinations consisted of 20 replicates. In each experiment, 20 second instar Drosophila larvae reared at 20°C were offered on a thin yeast suspension and were placed in a jar (h=13 cm, Ø=6 cm) containing a layer of agar medium (3 cm). This is the
Naturwissenschaften (2011) 98:705–709
preferred host stage for oviposition by A. tabida females (van Alphen and Drijver 1982). The vials were stoppered with porous foam bungs similar in thickness in all replicates to avoid contrasted relative humidity between the vials and the outside. One mated parasitoid female was then placed in each jar and was allowed to parasitize hosts in 2 h before being removed, as it exceeds the time needed by an A. tabida female to parasitize 20 D. melanogaster larvae (van Alphen and Galis 1983). The control vials were left without parasitoids. The effect of temperature was studied by placing ten replicates at 15°C and at 25°C, for each combination (i.e. for each species, jars with unparasitized or parasitized larvae). All the jars were kept at 15°C or 25°C in Sanyo incubators MIR 253 (RH=60±10%, 16Light:8Dark) until hosts had pupated.
707
placed at 25°C pupated significantly higher than those placed at 15°C (z=20.11, p
