Behav Ecol Sociobiol (2002) 51:386–393 DOI 10.1007/s00265-001-0439-x
O R I G I N A L A RT I C L E
R. J. Morris · M. D. E. Fellowes
Learning and natal host influence host preference, handling time and sex allocation behaviour in a pupal parasitoid
Received: 16 July 2001 / Revised: 24 October 2001 / Accepted: 14 November 2001 / Published online: 23 January 2002 © Springer-Verlag 2002
Abstract The host choice and sex allocation decisions of a foraging female parasitoid will have an enormous influence on the life-history characteristics of her offspring. The pteromalid Pachycrepoideus vindemiae is a generalist idiobiont pupal parasitoid of many species of cyclorrhaphous Diptera. Wasps reared in Musca domestica were larger, had higher attack rates and greater male mating success than those reared in Drosophila melanogaster. In no-choice situations, naïve female P. vindemiae took significantly less time to accept hosts conspecific with their natal host. Parasitoids that emerged from M. domestica pupae spent similar amounts of time ovipositing in both D. melanogaster and M. domestica. Those parasitoids that had emerged from D. melanogaster spent significantly longer attacking M. domestica pupae. The host choice behaviour of female P. vindemiae was influenced by an interaction between natal host and experience. Female P. vindemiae reared in M. domestica only showed a preference among hosts when allowed to gain experience attacking M. domestica, preferentially attacking that species. Similarly, female parasitoids reared on D. melanogaster only showed a preference among hosts when allowed to gain experience attacking D. melanogaster, again preferentially attacking that species. Wasp natal host also influenced sex allocation behaviour. While wasps from both hosts oviposited more females in the larger host, M. domestica, wasps that emerged from M. domestica had significantly more male-biased offspring sex ratios. These results indicate the importance of learning and natal host size in determining P. vindeCommunicated by D. Gwynne M.D.E. Fellowes (✉) School of Animal and Microbial Sciences, University of Reading, PO Box 228, Whiteknights, Reading, Berkshire RG6 6AJ, UK e-mail:
[email protected] Tel.: +44-118-9875123, Fax: +44-118-9310180 R.J. Morris · M.D.E. Fellowes NERC Centre for Population Biology, Imperial College at Silwood Park, Ascot, Berkshire SL5 7PY, UK
miae attack rates, mating success, host preference and sex allocation behaviour, all critical components of parasitoid fitness. Keywords Conditional sex allocation · Drosophila melanogaster · Host choice · Musca domestica · Pachycrepoideus vindemiae
Introduction Parasitoids provide exceptional systems for examining questions about adaptive behaviour and its consequences (Godfray and Shimada 1999). The more successful areas of research have focused on the causes and consequences of parasitoid host choice behaviour, especially in terms of how natal host influences adult traits, how host quality affects sex allocation behaviour and the role of learning in determining host preference (reviewed in Godfray 1994; Fellowes et al., in press). These traits are part of a continuum of inter-related behaviours that have an enormous impact on the fitness of an individual wasp. Nevertheless, few studies have considered these factors together. Here, we ask how host species influences these traits in Pachycrepoideus vindemiae Rondani (Hymenoptera: Pteromalidae), a generalist solitary pupal parasitoid of cyclorrhaphous flies (Crandell 1939; Nöstvik 1954). P. vindemiae is an idiobiont parasitoid (i.e. the host stops developing on attack) and, therefore, host size and offspring size are strongly correlated (van Alphen and Thunnissen 1983; Fellowes et al. 1998). Size and fitness are generally correlated in the parasitic Hymenoptera, so as a result female host choice can affect the fitness of her offspring (e.g. Lampson et al. 1996; West et al. 1996; Bennett and Hoffmann 1998; Ueno 1998a; Boivin and Lagace 1999). Consequently, female P. vindemiae should be expected to preferentially attack larger hosts. As a result of haplodiploid sex determination (where unfertilised eggs become haploid males, fertilised eggs become diploid females), a female hymenopteran parasi-
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toid can decide which sex to place in the host. Conditional sex allocation theory predicts that the sex that benefits most from improved host quality should be placed in the superior host (Charnov 1979; Charnov et al. 1981). In the case of parasitoids, circumstantial evidence supports the hypothesis that female wasps generally benefit most from developing in larger hosts (e.g. West et al. 1996; Ueno 1998a, b, 1999), so the suggestion is that P. vindemiae females should preferentially place female offspring in larger hosts. In addition, we speculate that if female P. vindemiae judge host size (and hence quality) by self-reference (i.e. by comparing host size with their size), then large and small wasps may assess the quality of potential hosts differently. This may be seen in their sex allocation behaviour, with smaller wasps having a less male-biased offspring sex ratio, as smaller wasps will perceive proportionally more hosts as being of ‘good quality’. Finally, natal host may influence host preference, with wasps preferentially attacking hosts conspecific with their natal host. Here we consider two mechanisms that may promote natal host fidelity. First, the size of the female may influence host choice, either through influencing handling times or by introducing physical limitations on the size of host they can attack. For example, larger P. vindemiae hosts (such as Musca domestica L.) may be more difficult to attack than smaller hosts (such as Drosophila melanogaster Meigen), primarily as their thicker puparia may require a longer drilling time (e.g. Hagley et al. 1993 for Phygadeuon wiesmanni attacking pupae of Rhagoletis pomonella). Larger wasps (i.e. those that emerged from the larger host) should be less affected by this and, hence, will preferentially attack the better quality, larger host. In contrast, smaller wasps may be limited to attacking smaller hosts, as the increased fitness benefit gained by attacking a larger host is outweighed by the increased handling time, which reduces the number of hosts the wasp can attack. This is illustrated by Weis et al. (1989), who showed that larger Eurosta solidaginis (Diptera: Tephritidae) galls are more resistant to attack by the eurytomid parasitoid Eurytoma gigantea. However, while larger galls are less likely to be successfully attacked, parasitoids that emerge from larger galls are in turn larger, and these parasitoids with longer ovipositors are more likely to successfully attack larger galls. Smaller wasps are constrained to attack smaller hosts (reviewed in Abrahamson and Weis 1997). Second, the parasitoid may learn cues associated with a given host on emergence, and this can influence future host preference (Corbet 1985; Hérard et al. 1988; Vet and Groenewold 1990; Turlings et al. 1993; Hastings and Godfray 1999). For example, female Aphidius rhopalosiphi will preferentially attack hosts reared on the same plant cultivar as the hosts from which they emerged. These cues are either learned during emergence, when the parasitoid bites through the host exoskeleton, or after emergence, through antennation of the host mummy (van Emden et al. 1996). The ability to learn cues will increase the likelihood of encountering suitable hosts
(e.g. Vet and Groenewold 1990; Papaj and Vet 1990; Pérez-Maluf and Kaiser 1998; Steidle 1998; van Baaren and Boivin 1998; Vet 1999; Dutton et al. 2000; Oliai and King 2000). Using a three-species system [Pachycrepoideus vindemiae attacking two differently sized hosts, the small D. melanogaster (Diptera: Drosophilidae) and the larger M. domestica (Diptera: Muscidae)], we asked whether parasitoid natal host influenced offspring traits, which may influence interactions among species. Our first hypothesis was that natal host would influence P. vindemiae characteristics; wasps emerging from M. domestica should have increased fitness when compared with parasitoids emerging from D. melanogaster. We addressed this in a series of experiments. First, we studied the influence of natal host on P. vindemiae size. Then, taking wasps that had emerged from each host species, we (1) assayed attack rates on both host species in a no-choice situation; (2) measured handling times on both host species; and (3) investigated male mating success. Second, given that natal host does influence the above parasitoid traits, we indirectly examined whether female P. vindemiae reared on different hosts differed in their assessment of host quality. We did this by investigating the offspring secondary sex ratio that resulted when females were in a choice situation. Therefore, our second hypothesis was that females that emerged from D. melanogaster would have a relatively more femalebiased offspring sex ratio. Our third hypothesis was that natal host and experience (i.e. prior opportunity to attack) would alter host preference. This was tested by providing naïve parasitoids with either D. melanogaster pupae, M. domestica pupae, or no pupae, prior to providing them with a choice of attacking D. melanogaster or M. domestica pupae. If natal host determines future host preference, then naïve wasps should preferentially attack hosts conspecific with their natal host, and experience should not alter that preference. This could have adaptive significance when host distributions change slowly through time, so that when an ovipositing female attacks a host their offspring are also likely to encounter the same host species. However, if experience (i.e. learning) does determine host preference, then females that gain experience attacking hosts which are not conspecific with their natal host should preferentially attack that host species. Such behaviour may be more important when the relative abundance of potential hosts changes more rapidly.
Methods General An outbred population of P. vindemiae was maintained on D. melanogaster pupae for several years before the experiments commenced. D. melanogaster and M. domestica larvae were reared on a standard laboratory yeast-based medium and commercial canned dog-food respectively. In both cases, food was sup-
388 plied ad libitum. For convenience, parasitoids emerging from D. melanogaster hosts will be termed Dm-parasitoids, and parasitoids emerging from M. domestica will be termed Mdparasitoids. Parasitoids used in experiments were 5–10 days postemergence, had mated and were fed on honey and water. The D. melanogaster and M. domestica pupae used in experiments were 2 days old. All parasitoids and pupae were used in experiments only once. All experiments took place at 25±1 °C, under ambient humidity (ca. 65–70% relative humidity) and a 16:8 light:dark regime. Wasps take about 9 days on average from oviposition to emergence (Crandell 1939). After all experiments the pupae were placed into individual vials and kept until an adult fly or an adult wasp had emerged, or until it was clear that the host was dead, since nothing had emerged 3 months after the experiment ended. Unless stated otherwise, all females used in experiments had not previously attacked hosts.
stopwatch. All four combinations of Md- and Dm-parasitoids were tested and repeated with fresh parasitoids ten times. 2. One virgin female Md- or Dm-parasitoid was placed in a 7.5 cm tall×2.5 cm diameter vial with two virgin male parasitoids, one Md- and one Dm-parasitoid, which could be distinguished by eye (n=8 for female Md- and Dm-parasitoids). The natal host of the first male to mate was recorded.
Influence of natal host on secondary sex ratio Parasitoids that emerged from the host-preference experiments (described below) were sexed, and the numbers of male and female parasitoids emerging from M. domestica and D. melanogaster pupae were noted.
Influence of natal host on parasitoid traits
Influence of natal host and experience on host preference
Does host size determine parasitoid size?
Twenty-four hours before the start of the experiment, 5-day-old female Md- and Dm-parasitoids were divided into three groups, each consisting of 28–54 wasps. Prior to the experiment the females were reared in vials containing a honey and water solution and did not have access to potential hosts. Each wasp in the first group was individually kept in a glass vial with a water/honey solution; wasps in the second group were individually kept in similar vials, and also had access to two D. melanogaster pupae. The final group was kept in similar conditions to group 2 wasps, except they were provided with two M. domestica pupae. Wasps therefore varied in natal host, having emerged from either (1) D. melanogaster or (2) M. domestica, and experience, being either (1) naïve or experienced, having had the opportunity to attack, (2) D. melanogaster or (3) M. domestica pupae. For logistical reasons we did not record the fate of the pupae used in this part of the experiment, although previous work suggests that very few of the female wasps would not have oviposited in the offered hosts. We cannot rule out that some females only host-fed on the offered hosts (or indeed failed to attack), but since P. vindemiae is an ectoparasitoid we suggest that both host-feeding and oviposition are likely to lead to similar levels of experience (with endoparasitoids this is not necessarily so; see Nurindah et al. 1999). Ten M. domestica and ten D. melanogaster pupae were evenly spaced on the base of a 9-cm-diameter Petri dish. One female parasitoid was introduced into the Petri dish for 2 h. Thirty controls, each with ten M. domestica and ten D. melanogaster pupae were used to estimate the survival rate of flies in the absence of parasitoids. This was performed to ensure that deaths noted in the trials with P. vindemiae (recorded as number of emerged flies, number of emerged parasitoids and number of dead pupae) were indeed caused by an attacking parasitoid.
Puparium length and width were measured using a dissecting microscope for 100 M. domestica and 100 D. melanogaster pupae from cultures before the flies had emerged. Puparium thickness was estimated by dividing dry mass (measured for pupae from which flies had emerged) by surface area (estimated as an ellipsoid). Parasitoid size was measured in terms of adult head width. Thirty female and 22 male parasitoids emerging from each host species were measured from the attack rate experiments (described below). Influence of natal host on female parasitoid attack rates Twenty M. domestica pupae or 20 D. melanogaster pupae were evenly spaced on the base of a 9-cm-diameter Petri dish, in a checkerboard fashion. Previous observations had shown that this was many more hosts than a parasitoid could attack in the time allowed (Fellowes et al. 1998). One female Md- or Dm-parasitoid (n=39–42 for each combination of host and parasitoid type) was introduced into the Petri dish for 2 h. Thirty controls, each with ten M. domestica and ten D. melanogaster pupae, were used to estimate the survival rate of flies in the absence of parasitoids. This was performed to ensure that deaths noted in the trials with P. vindemiae (recorded as number of emerged flies, emerged parasitoids, and dead pupae) were caused by an attacking parasitoid. Influence of natal host on handling time The handling times of Md- and Dm-parasitoids on M. domestica and D. melanogaster pupae were measured (n=15–21 for each of Md- and Dm-parasitoids) using video-recording equipment attached to a microscope. To avoid any confounding effects of experience, one inexperienced female parasitoid was placed in a Petri dish with five M. domestica or five D. melanogaster pupae. Handling time was recorded for the first pupa oviposited in by the parasitoid using a digital stopwatch. Handling time was divided into acceptance time (time taken from the first touch of the pupa by the parasitoid until drilling commenced) and oviposition time (time from ovipositor insertion to ovipositor withdrawal; all recorded oviposition events resulted in the emergence of a parasitoid). Influence of natal host on mating success 1. One virgin male and one virgin female Md- or Dm-parasitoid were placed together in a 7.5 cm tall×2.5 cm diameter vial and the time until mating took place was recorded using a digital
Analysis Unless otherwise noted, all data were analysed using ANOVA. For handling time, the data were normally distributed (inspection of the data confirmed this) and no transformations were used. To avoid problems associated with the non-normal distribution of count data (i.e. number of hosts attacked), count data were analysed using Poisson errors, after checking for overdispersion (Crawley 1993). When using Poisson errors, the change in deviance resulting from the addition/deletion of values follows the asymptotic χ2 distribution, explaining the use of χ2 values rather than F-ratios when hypothesis testing (Crawley 1993). In analysing the data from the choice experiments, we arcsine square-root transformed all proportion data (expressed as the proportion of M. domestica pupae attacked out of all pupae available) prior to analysis and, where appropriate, planned comparisons were made within treatments (Sokal and Rohlf 1995). The results of the male–male competition experiment were analysed using sign tests (Sokal and Rohlf 1995).
389 Table 1 Mean (± SE) size of host puparia and parasitoids reared in Drosophila. melanogaster or Musca domestica. All data were compared using onetailed t-tests assuming unequal variances
Host traits
D. melanogaster M. domestica Test statistics
Puparium length (mm)
3.06±0.01 (n=100) 0.02±0.001 (n=100)
6.28±0.02 (n=100) 0.06±0.001 (n=100)
t198=113.49 P
