Host preference and suitability in the ... - Wiley Online Library

Ecological Entomology (2013), 38, 173–182

DOI: 10.1111/een.12006

Host preference and suitability in the endoparasitoid Campoletis chlorideae is associated with its ability to suppress host immune responses L I - B I N H A N, L I N G - Q I A O H U A N G and C H E N - Z H U W A N G State Key Laboratory of Integrated Management of Pest Insects and Rodents, Institute of Zoology, Chinese Academy of Sciences, Beijing, P. R. China Abstract. 1. The endoparasitoid wasp Campoletis chlorideae Uchida attacks many noctuid species which vary in their suitability but the host association of this wasp species is poorly understood. In this study, three sympatric noctuid species were chosen, Helicoverpa armigera (Hübner), Mythimna separata (Walker), and Spodoptera exigua (Hübner), to study host preference and suitability of C. chlorideae, examining the immunological compatibility between the parasitoid and the three host species. 2. Campoletis chlorideae parasitised all three species, but showed a much higher preference for H. armigera and M. separata than for S. exigua. In H. armigera, the young wasps developed perfectly and none was encapsulated. In M. separata, about 50% and 10% of them were encapsulated in single and double parasitisation experiments, respectively, and the cocoons were significantly lighter than those from parasitised H. armigera and S. exigua. In S. exigua, 84.8% of the young wasps were encapsulated. 3. In double parasitisation experiments, the phenoloxidase activity, the number of total haemocytes, and the number of plasmatocytes and granulocytes significantly decreased in the haemolymph of H. armigera and M. separata, but not in S. exigua. 4. These results indicate that H. armigera is the most suitable host, M. separata is moderately suitable, whereas S. exigua is an unsuitable host for C. chlorideae. The suitability of the parasitoid to three host species was closely related with the capacity of the wasp to inhibit the host immune system. 5. This study sustains the optimality theory and also evidences the correlation of host-selection behaviour of the parasitoid wasp and its immunosuppressive ability. Key words. Campoletis chlorideae, host preference, host suitability, immune

response.

Introduction Parasitoids of insects live inside or outside the body of their host and eventually kill them through physiological manipulation or physical consumption of host tissues (Beckage & Gelman, 2004; Pennacchio & Strand, 2006). Several steps are necessary for successful parasitism, including host habitat location, host location, host acceptance, host suitability, and host regulation (Vinson, 1976; Vinson & Iwantsch, 1980a,b). Correspondence: Chen-Zhu Wang, State Key Laboratory of Integrated Management of Pest Insects and Rodents, Institute of Zoology, Chinese Academy of Sciences, 1 Beichen West Road, Chaoyang District, Beijing,100101, China. E-mail: [email protected] © 2013 The Royal Entomological Society

The first three steps comprise the host-selection process and are expected to be selected in accordance with host suitability. Once a female parasitoid encounters a potential host, the optimality theory predicts that she will accept suitable hosts and reject unsuitable ones (Charnov & Stephens, 1988). This prediction is based on the principle that host suitability greatly influences the parasitoid reproductive fitness, and that selection favours host selection strategies that maximise reproductive fitness (Charnov & Stephens, 1988; Scheirs et al ., 2000, 2004; Lauro et al ., 2005). The correlation between host-selection behaviour and host suitability has been well observed in some parasitoid wasps (Kraaijeveld et al ., 1995; Rolff & Kraaijeveld, 2001; Dubuffet et al ., 2006). When females of Asobara tabida 173

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Li-Bin Han, Ling-Qiao Huang and Chen-Zhu Wang

Nees are offered a choice between Drosophila melanogaster Meigen and Drosophila subobscura Collin, the degree of preference for D. subobscura is linked to the difference in survival probability in the two host species (Kraaijeveld et al ., 1995). Further, the females from five lines selected for higher survival in the encapsulating host species D. melanogaster were compared with females from control lines. Results suggested that host selection behaviour evolved parallel to the ability to evade host encapsulation (Rolff & Kraaijeveld, 2001). In another parasitoid wasp Leptopilina boulardi Barb., a highly virulent line against D. melanogaster showed a strong preference for this host species, whereas another line less virulent against D. melanogaster but highly virulent against Drosophila yakuba Burla preferentially oviposited in D. yakuba, suggesting that the preference between host species match differences in virulence levels of the parasitoid species (Dubuffet et al ., 2006, 2008; Poirie et al ., 2009). However, host-selection behaviour is not always correlated with host suitability. For example in the parasitoid wasps Cotesia rubecula (Marshall), the percentage of encapsulation of this parasitoid is not consistent with host preference and host suitability in three Pieridae species: Pieris brassicae L., Pieris napi L., and Pieris rapae L. (Brodeur & Vet, 1995). A number of factors including host defence, host toxins, host nutrition, and environmental factors determine the suitability of a host (Vinson & Iwantsch, 1980). Because endoparasitoids directly interact with the host immune system, the suitability of a host depends on the ability of the parasitoid to suppress the host immune response (Strand & Pech, 1995). Unsuitable hosts often dispose of endoparasitoids by encapsulation, a process in which haemocytes form a multilayered cellular envelope around the parasitoid thus cutting sources of oxygen and nourishment and finally killing it. This process involves cooperation between one or more classes of haemocytes which are often granulocytes and plasmatocytes, and is probably mediated by cytokines and adhesion molecules (Carton et al ., 2008; Strand, 2008; Marmaras & Lampropoulou, 2009). Reciprocally, the endoparasitoids have evolved passive and active strategies for circumventing the host immune responses (Vinson, 1990; Strand & Pech, 1995). The passive strategies include developing in host tissues that protect the parasitoid from encapsulation. Many platygastrids, for example, oviposit into the gut of host, whereas some braconids (Asobara spp.) lay eggs in the host fat body (Carton & Nappi, 1997). Some other endoparasitoids also passively evade encapsulation because their progenies possess surface features that fail to elicit an immune response from the host. Some ovarian proteins, for instance, coat the eggs of several braconids of the genera Cotesia and Toxoneuron (Huw Davies & Vinson, 1986; Hayakawa & Yazak, 1997; Asgari et al ., 1998). The active strategies include alteration of haemocytes behaviour, elimination of haemocytes or inhibition of host humoural components (Vinson, 1990; Strand & Pech, 1995). PDV, venom, and teratocytes that may accompany the wasp eggs play a crucial role in compromising the host immune response by killing the haemocytes or disrupting their ability to react (Davies et al ., 1987; Lavine & Beckage, 1995, 1996), as well as inhibiting humoral immune

responses such as melanization and the production of antimicrobial peptides (Stoltz & Cook, 1983; Lavine et al ., 2005). Campoletis chlorideae Uchida is an endoparasitoid of the early instars of many noctuid species (Liu et al ., 2004; Yan & Wang, 2006). This parasitoid has been recognised as a key biological control agent of the cotton bollworm Helicoverpa armigera (Hübner) in the Yellow river valley and the Yangtze river valley in China. The oriental armyworm Mythimna separata (Walker) and the beet armyworm Spodoptera exigua (Hübner) are sympatric noctuid species with H. armigera in these areas. In the field, C. chlorideae occasionally parasitises M . separata, but rarely attacks S . exigua (He et al ., 1996). Interestingly, in the laboratory, we found that the production of C. chorideae cocoons varied greatly, being highest in H. armigera, moderate in M. separata, and low in S. exigua (Zhang, 2003). These observations raised the question of whether the host-selection behaviour of C. chlorideae matched the optimality theory and whether it is associated with immunological adaptation. In the present study, we evaluated the host preference and suitability of H. armigera, M. separata, and S. exigua for C. chlorideae in the laboratory, and investigated the encapsulation of C. chlorideae as well as the alteration of immune responses in the three species. We found a correlation between the host-selection behaviour of C. chlorideae and its ability to suppress host immune responses, suggesting a coevolution of behavioural and physiological adaptation under natural selection.

Materials and methods Insect culture Three noctuid species, H. armigera, M. separata, and S. exigua, were maintained at 26 ± 1 ◦ C 75% RH and a LD 16:8 h photoperiod. The larvae of H. armigera and S. exigua were reared on the artificial diet as described by Wang and Dong (2001) and M. separata on the artificial diet with corn leaf powder (Zhang et al ., 2010). The adults of the three species were fed with 10% honey solution. The colony of C. chlorideae was maintained on M. separata because the larvae of this species are easy to be mass reared in the laboratory (Tian et al ., 2008; Zhang et al ., 2010). Briefly, the late second and early third instar larvae were stung twice by mated female wasps, then reared in glass jars (20 cm diameter, 10 cm height) with the artificial diet and a piece of toilet paper on the bottoms at 24 ± 1 ◦ C 75 ± 5% RH and a LD 16:8 h photoperiod. After cocoon formation, the larvae were put into the wells of the 24-well cell culture plates individually until emergence of the adult wasps. About 20 pairs of adult wasps were reared in each of the transparent plastic round containers with a black cloth cover (10 cm diameter, 20 cm length), on a diet of 30% honey solution that was renewed every day. The same protocol was used to parasitise the early third instar larvae of H. armigera, M. separata, and S. exigua. Single or double parasitisations were performed in different experiments.

© 2013 The Royal Entomological Society, Ecological Entomology, 38, 173–182

Campoletis chlorideae host interactions Oviposition preference of C. chlorideae to the three noctuid species The two-choice test was adopted to measure the parasitism preference of C. chlorideae to the three noctuid species. Third instar larvae of the three species were presented in three combinations: (i) H. armigera and M. separata (Ha-Ms), (ii) H. armigera and S. exigua (Ha-Se), and (iii) M. separata and S. exigua (Ms-Se). Single larvae of each of the two species were placed simultaneously in a plastic Petri dish without a cover (6 cm in diameter), that was put in the central part of a container with 20 mated female wasps, which were all 4 days old with no previous oviposition experience. When a larva was attacked by a parasitoid, it was immediately replaced by a new individual of the same species. The test lasted 2 h, and the number of parasitised larvae of each species was recorded. New female parasitoids were used in each test. The experiments were conducted at 26 ± 1 ◦ C under a lamp (45 W) placed approximately 40 cm above the experimental arena. Three replications were run for each combination. Host suitability of three noctuid species for C. chlorideae Host suitability of H. armigera, M. separata, and S. exigua for C. chlorideae was evaluated with single and double parasitisation treatments. In the single parasitisation treatment, the larvae were attacked once by female wasps, and then immediately moved away. In the double parasitisation treatment, the larvae were attacked twice before being removed. An attack was defined as the insertion of the ovipositor of a wasp into the larval body. The parasitised larvae were all individually reared in the wells of plastic culture plates with artificial diets for about 6 days, and then moved to the new plates without food for pupation. After the parasitoid larva exited the host, cocoon formation rate, cocoon weight, adult emergence rate, and the sex ratio were recorded. Forty-eight individuals of each species were used in each treatment. All experiments were run in three replicates. Encapsulation of C. chlorideae in the three noctuid species To determine the encapsulation situation and developmental status of C. chlorideae in the haemocoel of the three species, the larvae with single and double parasitisations were dissected at 3, 24, 48, and 72 h post-parasitisation and observed under an Olympus BX51 inverted microscope and pictures were taken with a Nikon Coolpix 4500 camera. About 28 eggs or larvae of C. chlorideae were examined for each treatment. To assess whether encapsulation potentials are different among H. armigera, M. separata, and S. exigua, we also compared the encapsulation ability of the three species to injected Sephadex G25 beads. Each larva in the fourth instar was anaesthetised on ice and injected with 10–15 Sephadex G25 beads from the base of a proleg using a glass capillary. The caterpillar was dissected at 3 or 24 h post-injection to assess encapsulation. Five larvae were used at each time point for each species. The number of the encapsulated beads relative to the total number of beads recovered was recorded for five

175

larvae. The encapsulated beads were covered with a complete layer of haemocytes in spite of fact that the thickness of the layer was different, and the unencapsulated beads had no complete layer of haemocytes on the surface of the beads (Fig. S2). The experiments were replicated three times. To investigate the effect of parasitism on encapsulation ability, larvae of three species firstly were single and double parasitised by C. chlorideae, and then injected with Sephadex G25 beads at 24 h after parasitisation as described above. Larvae were then dissected 24 h later after injection to assess encapsulation of Sephadex G25 beads and the presence of parasitoids. Effect of parasitism on phenoloxidase activity The larvae of the three species were double parasitised and the haemolymph was collected individually at days 0, 1, 2, 3, 4, and 6 post-parasitisation. The phenoloxidase activity in the haemolymph was quantitatively determined by measuring the amount of dopachrome formed from L3, 4-dihydroxyphenylalanine (DOPA), as described by Stoltz and Cook (1983). Briefly, the host larvae were anaesthetised on ice and the haemolymph was gently squeezed on a sheet of Parafilm after puncturing the leg. One microlitre of haemolymph was diluted in distilled water 1 : 10 in BD Falcon 96-well cell culture plates (BD, New York). After incubation for 2 h at 4 ◦ C, 190 μl IPS (50 mm sodium phosphate buffer, 137 mm NaCl, and 3 mm KCl, pH 7.0) containing 1 μg μl−1 DOPA, were added and the mixtures were incubated at room temperature for 10 min. Phenoloxidase activity was then evaluated by measuring the optical density at 490 nm using the Microplate reader (Bio-Rad, Richmond, California) and expressed in photometric units (1 unit = 0.001 OD490 min μl−1 ). Ten to 13 individuals of each species were used for each time point. Effect of parasitism on total and differential haemocyte numbers Because of the limited amount of haemolymph available from third instar larvae, the total haemocyte count and differential haemocyte count were carried out in a single individual. The larvae of the three species were also treated with double parasitisation, and then the haemolymph was collected individually at days 0, 1, 2, 3 post-parasitisation. For total haemocyte counts, the haemolymph from each individual was initially squeezed on a sheet of Parafilm. For H. armigera and M. separata, 1 μl hemolymph was immediately transferred into 200 μl Grace’s medium (Sigma, New York) containing 1-phenylthiourea to inhibit melanisation. For S. exigua, because of the lower hemocyte numbers in haemolymph, 1 μl haemolymph was transferred into 50 μl Grace’s medium. The diluted haemolymph was loaded on a Neubauer haemocytometer under an inverted microscope (Olympus BX51, Japan) for total haemocyte counts. For differential haemocyte counts, 1 μl haemolymph from one individual was diluted into 200 μl medium on an alcohol-cleaned microscope slide and gently stirred to distribute the haemocytes. The slides were maintained in



humid boxes for 1 h. The differential haemocyte counts were then performed using an inverted phase-contrast microscopy (Zeiss, Oberkochen, Germany). More than 130–150 cells were identified from several randomly selected fields of view. Identification of granulocytes, plasmatocytes, and oenocytoids was accomplished based on their morphological characteristics as described by Davies et al . (1987). The percentages of each haemocyte type were transformed into cells ml−1 using the mean total haemocyte numbers. The data of total and differential haemocytes counts were both collected from 10 larvae of each species at each time point.

The number of parasitised larvae

176

60

a

a

50 a

40 b 30 20 10

All statistical analyses were conducted using SPSS software (SPSS.18). The data of oviposition preference were analysed using Student’s t-test. The data of cocoon formation rate, adult emergence rate, percentage of males, and cocoon weight were analysed using the one-way anova test. The data of cocoon formation rate, adult emergence rate, and percentage of males were transformed into a square root of arcsine before analysis. The encapsulation of Sephadex G25 beads was analysed using the General Linear Model. The phenoloxidase activity and the number of total haemocytes were also analysed against the species, host status, time point, and the interaction between them using the General Linear Model. The level for significant differences between the means of two data sets was P < 0.05. Results Oviposition preference of C. chlorideae for the three species Campoletis chlorideae parasitised all three species but showed different oviposition preferences (Fig. 1). In twochoice tests, we observed a significantly lower number of parasitised larvae of S. exigua with respect to those of H. armigera (Ha-Se, t = 13.408; d.f. = 4; P < 0.001) and M. separata (Ms-Se, t = 4.732; d.f. = 4; P = 0.009). Between H. armigera and M. separata, the first was preferentially attacked (t = 3.258; d.f. = 4; P = 0.036). Host suitability of the three noctuid species for C. chlorideae Cocoon production, cocoon weight, adult emergence, and the sex ratio of C. chlorideae from the three species at single and double parasitisations were evaluated for host suitability (Table 1). Except for cocoon production, all the other parameters did not vary significantly between single and double parasitisations. Cocoon formation rates differed markedly between the three species in the single parasitisation experiments, being 85.4%, 51.4%, and13.9% from H. armigera, M. separate, and S. exigua respectively (P < 0.01) (Fig. 1). In double parasitisation experiments, the cocoon formation rate greatly increased up to 93.7% for M. separata (F = 536.1; d.f. = 5, 12; P < 0.01), but did not change significantly for H. armigera and S. exigua (F = 536.1; d.f. = 5, 12; P = 0.057; F = 536.1; d.f. = 5, 12; P = 0.253). Moreover, we could not observe significant differences in

b

Ha Se

Ms Se

0 Ha Ms

Data analysis

b

Fig. 1. The number of Helicoverpa armigera (Ha), Mythimna separata (Ms), and Spodoptera exigua (Se) larvae parasitised by Campoletis chlorideae in two-choice tests. Means associated with a common letter within each pair of the diagram have no significant difference from each other.

cocoon formation rates between H. armigera and M. separata under the double parasitisation condition (F = 536.1; d.f. = 5, 12; P = 0.154). The cocoon weights were not significantly different between wasps from H. armigera and S. exigua, but were much heavier than those from M. separata (F = 58.277; d.f. = 2, 214; P < 0.01) (Table 1). The percentage of adult emergence from S. exigua was significantly lower than those from H. armigera and M. separata (F = 73.723; d.f. = 2, 10; P < 0.01), and the number of male wasps emerging from M. separata was much higher than those from the two other species (Table 1).

Encapsulation of C. chlorideae in three noctuid species In single parasitisation conditions, all wasps in H. armigera were not encapsulated (Figs 2a, 3a). Instead, 55.2% of the wasps in M. separata were encapsulated after 72 h, but they developed normally 3 h after parasitisation (Fig. 3a). In S. exigua, some wasp eggs showed signs of encapsulation after 3 h of parasitisation and some haemocytes attached as well as the surface of eggs were partially melanised (Fig. 2c). In S. exigua, 84.8% of the wasps encapsulated 72 h after parasitisation. Compared with the results in the single parasitisation condition, the only major difference in the double parasitisation experiments was the encapsulation rate of wasps in M. separata. All wasps developed well, but only 12.5% were encapsulated 72 h after parasitisation (Fig. 3b, Figure S1). As a control, we measured the encapsulation of Sephadex G25 beads in the three moth species (Figure S2), which was found to be about 30% after 3 h and 100% after 24 h without significant differences between the three hosts (Figure S3). Moreover, the encapsulation rate of Sephadex G25 beads significantly decreased in parasitised H. armigera and M. separata, but did not in parasitised S. exigua (Fig. 4 and Table 3).


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177

Table 1. Growth and development of Campoletis chlorideae in Helicoverpa armigera, Mythimna separata, and Spodoptera exigua.

H. armigera

S D S D S D

M. separata S. exigua

Cocoon formation rate (%)

Cocoon weight (mg)

Adult emergence rate (%)

Percentage of males

85.4 ± 1.7a 96.3 ± 2.3a 51.4 ± 1.8b 93.7 ± 2.5a 13.9 ± 2.7c 17.4 ± 2.4c

11.3 ± 3.4a 10.8 ± 2.9a 8.1 ± 2.0b 7.5 ± 2.6b 11.6 ± 2.8a 11.1 ± 2.8a

88.8 ± 3.4a 90.4 ± 2.9a 90.2 ± 4.2a 87.9 ± 3.2a 46.3 ± 2.1b 48.1 ± 3.3b

66.0 ± 1.5b 65.6 ± 2.0b 73.2 ± 1.3a 72.9 ± 1.3a 64.5 ± 1.9b 65.5 ± 1.7b

All numbers are mean ± SD; anova Fisher’s Least Significant Difference test was used to analyse the difference among the single parasitisation (S) and double parasitisation (D) treatments for three species in the same column. The data of cocoon formation rate, adult emergence rate, and percentage of males were square-root arcsine transformed before statistic analyses. Means associated with a common letter within the same column are not significantly different from each other (P > 0.05).

(a)

E HC

(b) E

E

HC

HC HC LH

(c)

LA LH

PME

LA TME

Fig. 2. Fate of Campoletis chlorideae eggs or larvae in Helicoverpa armigera, Mythimna separata, and Spodoptera exigua. The early third instar larvae were parasitised and then dissected at 3, 24, 48, and 72 h post-parasitisation. (a) All eggs and larvae successfully developed at different time points in H. armigera; (b) eggs developed normally at 3 h and encapsulated at 24, 48, 72 h post-parasitisation in M. separata; (c) eggs partially and totally melanised at 3 and 24 h, larvae totally and partially encapsulated at 48 and 72 h post-parasitisation in S. exigua. E, Egg; HC, haemocyte capsule; LH, larva head; LA, larva abdomen; PME, partial melanised egg; TME, total melanised egg.

Effect of parasitism on phenoloxidase activity There was an interaction among species, host status, and time point (Table 3). The activity decreased with the time elapsed after parasitisation, but the degree of decrease was different among the three species. The activity significantly declined in the haemolymph of H. armigera and M. separata, but did not in S. exigua (Fig. 5). Effect of parasitism on total and differential haemocyte numbers The number of total haemocytes differed significantly among the three species (Tables 2 and 3). Helicoverpa armigera presented the highest number of haemocytes whereas S. exigua gave the lowest numbers. For three species, the numbers of

total haemocytes increased with the time elapsed since the parasitoid attack. However, this increase was limited in H. armigera and M. separata attacked by the wasp. In S. exigua, parasitism had no effect on the numbers Table 4. Discussion The reproductive success of a parasitoid depends on its ability to select hosts in which its progeny has the highest probability of developing. Upon encountering different host species, parasitoids generally reject those of low quality because of the lower expected fitness returns (Pyke et al ., 1977; Charnov & Stephens, 1988). Extensive research has investigated the effect of size, stage, and nutritional status of the host on a parasitoid’s decision to oviposit in or on a host, but the influence of the host immune responses is not well understand. Here we evaluated



Percentage of unencapsulated eggs or larvae (%)

(a)

Ha

Ms

(a)

Se

Phenoloxidase activity (±SD)

178

120 100 80 60 40 20 0

24h

3h

72h

48h

parasitised group 10 8 6 4 2 0

d0

d2

d3

d4

d6

(b) Phenoloxidase activity (±SD)

(b) Percentage unencapsulated eggs or larvae (%)

d1

Times post-parasitisation


120 100 80 60 40 20 0

24h

3h

12 10 8 6 4 2 0

72h

48h

d0

100

non-parasitisation double parasitisation

a

single parasitisation

a

d2

d3

d4

d6

a a a

80 60

(c) Phenoloxidase activity (±SD)

Fig. 3. Percentage of unencapsulated Campoletis chlorideae in three noctuid species. (a) single parasitisation and (b) double parasitisation.

120

d1



Percentage

unparasitised group

12

12 10 8 6 4 2 0 d0

d1

d2

d3

d4

d6


40

b

Fig. 5. Effects of parasitisation on phenoloxidase activity in the haemolymph of parasitised and unparasitised Helicoverpa armigera (a), Mythimna separata (b), and Spodoptera exigua (c). A minimum of 10 larvae were examined per time point, and the activity is expressed in photometric units (1 unit = 0.001OD490 min μl−1 of haemolymph).

c

20 c

c

0 H. armigera

M. separata

S. exigua

Fig. 4. Percentage of encapsulation of Sephadex G25 injected in Helicoverpa armigera, Mythimna separata, and Spodoptera exigua unparasitised, single parasitised, and double parasitised by Campoletis chlorideae. Larvae were dissected at 24 h after injection. The data present the average percentage of beads (± SD) encapsulated (n = 3).

the host preference and suitability of three noctuid hosts, H. armigera, M. separata, and S. exigua, for C. chlorideae, and assessed the alterations of their immune responses. Results demonstrate that the host selection behaviour of C. chlorideae matches its varying degree of immunosuppressive ability in three host species, implicating a co-evolution of the parasitoid and its hosts in behavioural and physiological aspects.

The link between host preference and host suitability in host association of insects is one of the central issues in the ecological and evolutionary field. There are many studies on preference and performance, the correlation between the two and the evolution of host associations in plant-insect interactions (Futuyma & Keese, 1992). According to the oviposition preference-offspring performance hypothesis, a female will choose a host for oviposition on which larvae perform best (Jaenike, 1978). In many phytophagous insect species the ovipositing female selects the plant on which their offspring will feed. Although the genes determining host preference and performance are different, natural selection


Campoletis chlorideae host interactions

179

Table 2. Effect of parasitism by Campoletis chlorideae on the total haemocyte numbers (×104 hemocytes ml−1 , mean ± SD, n = 10) in the haemolymph of Helicoverpa armigera, Mythimna separata, and Spodoptera exigua. H. armigera

M. separata

S. exigua

Time

Unparasitised

Parasitised

Unparaitised

Parasitised

Unparasitised

Parasitised

0 day 1 day 2 day 3 day

1096.7 ± 494.0 1645.1 ± 484.5 3005.3 ± 590.9 2545.3 ± 751.1

1135.2 ± 402.8 805.7 ± 329.5* 915.6 ± 340.0* 940 ± 207.9*

785.1 ± 280.9 830.7 ± 364.5 2505.1 ± 771.5 1820.6 ± 583.7

765.4 ± 241.5 477.0 ± 310.2* 560.2 ± 266.5* 1305.1 ± 305.0*

112.5 ± 26.4 237.5 ± 71.2 400.0 ± 76.6 491.3 ± 90.0

122.5 ± 68.9 240.0 ± 40.3 450.0 ± 119.7 483.8 ± 101.7

Asterisk (*) indicates significantly lower total haemocyte numbers in the parasitised larvae (P < 0.05, t-test). Table 3. Effect of parasitism by Campoletis chlorideae on encapsulation response, phenoloxidase activity, and the number of total haemocytes of Helicoverpa armigera, Mythimna separata, and Spodoptera exigua. Encapsulation response

Species (1) Time points (2) Host status (3) Interaction (1 × 2) Interaction (1 × 3) Interaction (2 × 3)

Phenoloxidase activity

d.f.

F

P

2 – 2 – 4 –

447.13 – 626.65 – 113.962 –

< 0.001 – < 0.001 – < 0.001 –

d.f. 2 1 5 2 10 5

will favour the development of an accordance between the preference of a ovipositing female and performance of their offspring. However, there is a discrepancy between them in some cases (Wiklund, 1975; Chew, 1977; Jaenike, 1990), which is perhaps as a result of a lack of adequate genetic variation in the alleles that govern oviposition preference (Schoonhoven et al ., 2005). The theory behind host-parasitoid associations has many similarities to that of plant–insect associations. For a given parasitoid species, an array of hosts may be ranked with respect to preference and with respect to suitability. Variations in suitability among hosts are a potential source of natural selection on host preference. Here, in agreement with the optimality theory, C. chlorideae preferred to attack the most suitable host H. armigera, and, as a second choice, the moderately suitable M. separata, but seldom attacked the unsuitable host S. exigua. Our results further indicate that encapsulation of C. chlorideae in the three species is closely related to parasitism success, suggesting a correlation between host suitability and encapsulation. In A. tabida, when offered together with a non-encapsulating host D. subobscura, females from the selected lines showing a significantly higher level of counter defence more readily accepted the encapsulating host D. melannogaster for oviposition than females from the respective control lines (Rolff & Kraaijeveld, 2001). We suggest that host preference and host suitability of endoparasitoids may evolve parallel to its immunosuppression ability to the hosts. To protect the normal development of their progeny, the parasitoids usually disrupt host encapsulation by inhibition of phenoloxidase activity (Stoltz & Cook, 1983; Shelby et al ., 2000) and alteration of haemocyte numbers or behaviours (Davies et al ., 1987; Strand & Noda, 1991). These are usually carried out by the wasp-deriving virulent factors, especially by

F 2.775 164.068 7.491 22.735 3.618 6.815

< < < <