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ABSTRACT When phylogenetically close, two competing species may reproductively interfere, and thereby affect their population dynamics. We tested for reproductive interference (RI) between

two congeneric haplo-diploid spider mites, _Tetranychus evansi_ and _Tetranychus urticae_, by investigating their interspecific mating and their population dynamics when they competed on the

same plants. They are both pests of tomato, but differ in the host plant defences that they suppress or induce. To reduce the effect of plant-mediated interaction, we used a mutant tomato

plant lacking jasmonate-mediated anti-herbivore defences in the competition experiment. In addition, to manipulate the effect of RI, we introduced founder females already mated with

conspecific males in mild RI treatments or founder, virgin females in strong RI treatments (in either case together with heterospecific and conspecific males). As females show first-male

sperm precedence, RI should occur especially in the founder generation under strong RI treatments. We found that _T. urticae_ outcompeted _T. evansi_ in mild, but not in strong RI

treatments. Thus, _T. evansi_ interfered reproductively with _T. urticae_. This result was supported by crossing experiments showing frequent interspecific copulations, strong postmating

reproductive isolation and a preference of _T. evansi_ males to mate with _T. urticae_ (instead of conspecific) females, whereas _T. urticae_ males preferred conspecific females. We conclude

that interspecific mating comes at a cost due to asymmetric mate preferences of males. Because RI by _T. evansi_ can improve its competitiveness to _T. urticae_, we propose that RI partly

explains why _T. evansi_ became invasive in Europe where _T. urticae_ is endemic. SIMILAR CONTENT BEING VIEWED BY OTHERS ADAPTIVE DIVERGENCE AND POST-ZYGOTIC BARRIERS TO GENE FLOW BETWEEN

SYMPATRIC POPULATIONS OF A HERBIVOROUS MITE Article Open access 09 July 2021 MULTIPARASITISM ENABLES A SPECIALIST ENDOPARASITOID TO COMPLETE PARASITISM IN AN UNSUITABLE HOST CATERPILLAR

Article Open access 11 March 2025 MATING BEHAVIOUR, MATE CHOICE AND FEMALE RESISTANCE IN THE BEAN FLOWER THRIPS (_MEGALUROTHRIPS SJOSTEDTI_) Article Open access 15 July 2021 INTRODUCTION In

some animal species, males readily mate with heterospecific females, supposedly because they are unable to discriminate between the species (Gro¨ning and Hochkirch, 2008), and they may even

prefer to mate with heterospecific females instead of conspecific females (for example, Ludden et al., 2004; Hochkirch et al., 2008). This may come at a cost to the females because it can

hamper or even block their reproduction and it may even cause populations of their species to be outcompeted. Thus, reproductive interference may alter the species composition in a community

(Kuno, 1992), but such an effect can be brought also about by other competitive processes. For example, communities of plant-inhabiting arthropods are not only determined by direct

competition, but also by indirect modes of competition, such as plant-mediated competition (Denno et al., 1995; Ohgushi, 2005; Denno and Kaplan, 2007) and predator-mediated competition

(Holt, 1977; Holt and Lawton, 1994; van Veen et al., 2006). Hence, to test for the occurrence of reproductive interference (RI) between species and its effect on the outcome of the

interspecific competition, it is necessary not only to perform crossing experiments between (closely related) species and to compare the population dynamics in presence and absence of other

species in the field (Gro¨ning and Hochkirch, 2008), but also to carefully design population experiments in which other competitive processes, such as plant-mediated and predator-mediated

competition, are excluded and in which the effect of RI is manipulated (for example, Kishi et al., 2009). To assess the role of RI, we studied two congeneric haplo-diploid species of spider

mites, _Tetranychus evansi_ Baker & Pritchard (Acari: Tetranychidae) and _Tetranychus urticae_ (Koch) (Acari: Tetranychidae) (Figure 1). Both species are important pests of solanaceous

crops, especially of tomato plants (_Solanum lycopersicum_ L.), the first species being a host plant specialist and the second being a generalist (Helle and Sabelis, 1985; Navajas et al.,

2012). Each of the two species spins a protective web on the leaves of their host plant, and under such webs groups of spider mites feed, mate, reproduce and develop. Before mating males

guard the last moulting stage of the females and mate directly upon emergence of the female from the exuvium. Proximity to the emerging female is an important determinant of male mating

success (for example, Potter et al. 1976). _Tetranychus evansi_ from South America has become an invasive species first in Africa and then in Europe (Boubou et al., 2012; Navajas et al.,

2012). Nowadays, its distribution overlaps with that of the closely related species _T. urticae_ in these regions (Helle and Sabelis, 1985; Navajas et al., 2012). Hence, these two species

are expected to interact with each other when sharing host plants in the field. Both species belong to the same genus _Tetranychus,_ and in spider mites, RI has been observed between species

from the same or even of different genera (Fujimoto et al., 1996; Takafuji et al., 1997; Navajas et al., 2000; Ben-David et al., 2009). RI is, therefore, likely to affect the outcome of

competition between _T. evansi_ and _T. urticae_. Using these two species of spider mites provides us with advantages in designing a competition experiment that allows testing for the effect

of RI on spider mite dynamics. First, spider mites show strong first-male sperm precedence (Boudreaux, 1963; Helle, 1967), indicating that secondary matings are less effective and that

females do not exhibit cryptic male choice by selecting among sperm of various males for fertilisation of their eggs. This allowed us to manipulate the founder females that were introduced

to the competition experiment as follows. In one experiment, we created a mild RI treatment in the founder generation by releasing heterospecific and conspecific males with females that had

previously mated with conspecific males. These females can, therefore, reproduce irrespective of secondary matings with heterospecific males. In a parallel experiment, we created a strong RI

treatment in the founder generation by releasing heterospecific and conspecific males with virgin females, which can then mate with conspecific and heterospecific males, the latter of which

can potentially result in decreased reproduction. A second advantage of using the two species of spider mites is that there is much knowledge of the impact they have on host plant defences:

within leaves of tomato plants many strains of _T. urticae_ induce the jasmonate(JA)-mediated plant defences (Li et al., 2002; Ament et al., 2004; Kant et al., 2004, 2008), whereas _T.

evansi_ downregulates these defences, even below household levels (Sarmento et al., 2011a), thereby making the host plant more profitable for both _T. evansi_ and _T. urticae_ (Sarmento et

al., 2011b). By using mutant _def-1_ tomato plants that are deficient in mounting JA-mediated plant defences (Howe et al., 1996; Li et al., 2002; Ament et al., 2004; Kant et al., 2008), we

were able to reduce the effect of JA-mediated plant defences in our competition experiments, thereby gaining more analytic power to detect RI. Also, we were able to exclude predator-mediated

competition by introducing only the two spider mite species on the plants in an isolated environment (climate room). Hence, by taking advantage of these features from our experimental

system, we studied the effect of RI on the competition between _T. evansi_ and _T. urticae_. This was done by comparing the population growth of the two species between experiments initiated

by founder females differing in reproductive status (virgin or mated with conspecific males) on mutant _def_-1 tomato plants and in absence of natural enemies of the spider mites. We also

investigated their mating behaviour, their reproductive compatibility, and male mate choice when males were offered females of these two species simultaneously, in order to analyse the

results from our competition experiments. MATERIALS AND METHODS PLANTS Tomato seeds (_S. lycopersicum_ cv. Castlemart) were sown in 12-cm pots and watered two times per week. After

maintaining the plants in a greenhouse for 3 weeks, they were transferred to a climate chamber (25 °C; 60% relative humidity (RH); 16:8 h light:dark photoperiod). When the tomato plants were

4–5-week-old, leaves were detached and then used to maintain the colonies. The interspecific competition experiments were carried on _def-1_ mutant tomato plants that have a Castlemart

genetic background (Howe et al., 1996). These _def-1_ plants were also grown from seeds and maintained in the same way as the Castlemart plants. We used this plant genotype because the

_def-1_ mutation impairs the accumulation of JA and thereby also the JA-mediated plant defence mechanisms induced after wounding by herbivorous arthropods (Howe et al., 1996). In this way,

we reduced the effect of plant-mediated competition between the two phytophagous species. MITES _Tetranychus urticae_ and _T. evansi_ were collected from _S. nigrum_ L. in Málaga

(N36°34′29″, W5°57′35″), Spain, August 2010. Harvested plant material was placed in separate zip-bags and transported to Amsterdam. All adult females died during transport, however, there

were still mites in different developmental stages on the sampled leaves. Subsequently the plant material was transferred to separate trays with tomato leaves and incubated in a quarantine

climate box to allow mites to complete the development (25 °C; 60% RH; 16:8 h light:dark photoperiod). These quarantine cultures were checked every 2 days and adult females were transferred

to clean tomato leaves and moved to a climate room providing the same conditions. When the colonies were established, 10 males per colony were sampled and placed in 70% ethanol. The species

identity of both strains was confirmed on the basis of the aedeagus (the male reproductive organ) morphology (Gutierrez and Etienne, 1986). Whereas _T. evansi_ was reared on detached tomato

leaves (cv. Castlemart), _T. urticae_ was reared on detached common bean leaves (_Phaseolus vulgaris_ L.) on wet cotton wool in a plastic box under constant climatic conditions (25 °C; 60%

RH; 16:8 h light:dark photoperiod). At least 1 month before use in the experiments, _T. urticae_ was moved to detached tomato leaves under the same climatic conditions. Females of the two

species to be used for the experiments were collected in the teleiochrysalis stage (i.e. moulting stage preceding adult phase; thus definitely before mating occurs immediately after adult

emergence) so that they were virgins after their last moult. Hence, we could always exactly infer their mating status from subsequent exposure to males. Males of the two species were

obtained from eggs laid by virgin females. Because spider mites reproduce by arrhenotokous parthenogenesis, virgin females produce haploid eggs that develop into males (Helle and Sabelis,

1985). CROSSING EXPERIMENTS Leaf discs (1.5 cm in diameter), punched from tomato plants (4–5 week-old), were placed on wet cotton wool in a Petri dish, and used as an arena for either mating

or rearing of offspring. To observe premating behaviour of spider mites, a single teleiochrysalis female was placed onto a leaf disc where it moulted into the adult phase within 24 h.

Subsequently, three virgin males, 1–7-day-old since their last moult, were introduced for mating. The occurrence and duration of copulations was recorded for 0.5 h using a camera (Leica IC80

HD, Leica Microsystems GmbH, Germany) connected to a binocular microscope and a computer with adequate software (LAS EZ software version 2.0.0, Leica Microsystems GmbH). In an independent

experiment, we assessed reproductive compatibility between females and males in exactly the same set-up (one female, three males, leaf disc). They were allowed to copulate and oviposit for 5

days. Egg and juvenile survival was checked every 48 h, and gender was checked when they developed into adults. The leaf discs were replaced with fresh ones every 4 days. Using this set-up,

we carried out mating and crossing experiments between _T. urticae_ and _T. evansi_, and also within each of the two species as a control. In addition, we assessed reproduction by virgin

females of each of the two species. We evaluated the compatibility of _T. urticae_ and _T. evansi_ by comparing the number of eggs deposited, the offspring survival and the sex ratio (%

females) between intraspecific and interspecific crosses. Prezygotic reproductive barriers were inferred from differences in sex ratios, whereas postzygotic barriers were inferred from

juvenile survival. The crossing experiments were all performed under constant climatic conditions (25 °C; 60% RH; 16:8 h light:dark photoperiod). MALE MATE CHOICE In spider mites, females

have a preference when given a choice between males differing in relatedness and bacterial infection status (Vala et al., 2004; Tien et al., 2011). However, females usually cannot express

this preference because males guarding females in the teleiochrysalis stage immediately mate upon female emergence (Potter et al., 1976). We, therefore, assumed that male mate choice

prevails. A teleiochrysalis female of _T. evansi_ and one of _T. urticae_ were introduced on a leaf disc (1.5 cm). After the two females moulted to the adult phase, one _T. evansi_ male or

one _T. urticae_ male, moulted 2 days earlier, were released on the leaf arena. Male behaviour was observed until copulation occurred. The pairs in which copulation did not occur within 0.5 

h after male introduction were excluded from further analysis. This experiment was carried out under constant climatic conditions (25 °C; 60% RH; 16: 8 h light:dark photoperiod).

INTERSPECIFIC COMPETITION ON PLANTS To investigate the competitive relation between the two species, we observed population growth of the mites over a period of 4 weeks (2–3 generations) for

the case where two species were introduced together on a tomato plant and for the case where each of the two species mites were introduced separately on a tomato plant. To detect the effect

of RI, we modified the reproductive status of founder females in two treatments: they were either virgin or mated with conspecific males. The experimental design is summarized in Table 1.

As the spider mites increase exponentially and as they, by doing so, cause death of the tomato plant within a few weeks, we introduced a small number of mites (four females and four males)

on a tomato plant as founders. To reduce the effect of JA-mediated plant defence, we used an intact _def-1_ tomato plant as the host plant. Details of the process of the experiment were as

follows. Four females and four males were released together on a leaflet of the third or fourth leaf from an intact _def-1_ tomato plant (3-week-old). Their establishment on the leaflet was

checked for 3 days after mite introduction. When mites were missing during these first 3 days (for example, by accidental death or by falling down), they were replaced to ensure a fixed

number of founder mites (thereby reducing the probability that small initial differences in numbers were magnified through exponential growth). We carried out six treatments, differing in

species composition (‘_T. urticae_’, ‘_T. evansi’_ or ‘mix’) and the mating status of founder females (‘virgin’ or ‘mated’) (Table 1). In the first treatment (mix, virgin), two females and

two males of each species were released, of which the females were virgin. This served to assess competition under conditions where RI could operate strongly from the start of the experiment

(strong RI treatment). To obtain weak RI, we released two mated females and two males of each species (mix, mated; mild RI treatment). Here, RI could mainly arise among the offspring of the

mated females. A further four treatments served as controls and consisted of either four mated or four virgin females and four males of each species separately (_T. urticae_ virgin, _T.

urticae_ mated, _T. evansi_ virgin and _T. evansi_ mated). In all treatments, females and males were 2-day-old since they reached adulthood. To obtain mated females, 1-day-old virgin females

were introduced on leaf arenas with males of the same species, and they were kept together for 1 day to give them sufficient opportunity to copulate. The ratio of males and females on each

mating arena was 1:2. After releasing the mites on the plant, the adult females of each species were counted once per week over a period of four weeks. Females of the two species were

differentiated based on their body colour (resulting from pigments in the haemolymph and internal tissue, not in the cuticle or the gut) (Figure 1). The experiments were carried out under

constant climatic conditions (25 °C; 60% RH; 16:8 h light:dark photoperiod). STATISTICAL ANALYSIS The number of pairs where copulation occurred was compared between intra- and interspecific

crossings for each of the two species of males, using Fisher’s exact test for count data. To test for effects of the species of the male (_T. urticae_ or _T. evansi_) and crossing type

(intra- or interspecific) on the duration of copulation (s), we used a two-way analysis of variance (two-way ANOVA). To test for effects of female species (_T. urticae_ or _T. evansi_) and

crossing type (intra- or interspecific) on the number of eggs, we also used a two-way ANOVA. Juvenile survival rate and offspring sex ratio were analysed using generalised linear models with

a binomial error distribution using the species of the female (_T. urticae_ or _T. evansi_) and mating type (intra- or interspecific) as factors. The effect of each factor was assessed with

likelihood ratio tests, comparing the saturated model with the model without the factor. Male mate choice was analysed using exact binomial tests for each species, taking the probability to

choose a conspecific female equal to 0.5 under the null hypothesis. For the analysis of interspecific competition on plants, generalised linear mixed-effects models (with a Poisson error

distribution, ‘glmer’ in package ‘lme4’ from the statistical package R) were used because repeated measurements were taken over time. To compare the growth curve (change of the number of

females over time on a plant) among the control treatments (i.e. in the absence of interspecific competition), a model was constructed with species (‘_T. urticae_’ or ‘_T. evansi_’), the

reproductive status (‘virgin’ or ‘mated’), time (weeks after mite introduction) and their interactions as fixed factors and with replicate as random factor. To compare the growth curve

between strong RI treatment (mix, virgin) and mild RI treatment (mix, mated), a model was constructed with species (‘_T. urticae_’ or ‘_T. evansi_’), the reproductive status (‘virgin’ or

‘mated’), time (weeks after mite introduction) and their interactions as fixed factors and with replicate as random factor. As we want to know if RI affects the competition (the relation of

growth curves) between _T. urticae_ and _T. evansi_, we tested the effect of higher-order interaction (species × time × reproductive status) on the number of female mites in the latter

model. If the competition between _T. urticae_ and _T. evansi_ changes depending on the reproductive status (virgin or mated), the higher-order interaction (species × time × reproductive

status) should be significant. To confirm if simply the reproductive status of the founder females affects the relation of growth curve between these two species, we tested the effect of the

higher-order interaction in the former model. In the tests, we compared the saturated model with the model without higher-order interaction by a likelihood ratio test. We followed the

procedure described by Faraway (2006) in constructing these models. The analyses were performed with the statistical package R ver. 2.14.2 (R Development Core Team (2011)). RESULTS CROSSING

EXPERIMENT Interspecific copulation occurred between females of _T. urticae_ and males of _T. evansi_ as well as between males and females of the respective species, although the frequency

of copulation of _T. urticae_ males with _T. evansi_ females was significantly lower than that of intraspecific copulations in _T. urticae_ (Fisher’s exact test, _T. urticae_ (♀) × _T.

urticae_ (♂) vs _T. evansi_ (♀) × _T. urticae_ (♂): _P_=0.024, _T. evansi_ (♀) × _T. evansi_ (♂) vs _T. urticae_ (♀) × _T. evansi_ (♂): _P_=1.000; Figure 2). The duration of the copulation

in interspecific combinations was 2.5 times shorter than that in intraspecific pairs and this difference was significant (two-way ANOVA, crossing type: _F_1,122=340.44, _P_<0.001, male

species: _F_1,122=3.65, _P_=0.06; Figure 3). The number of eggs produced after interspecific crossings did not differ significantly from that after intraspecific crossings, although the

number of eggs laid by _T. urticae_ females was significantly larger than that by _T. evansi_ (two-way ANOVA, crossing type: _F_1,142=0.83, _P_=0.36, female species: _F_1,142=4.33,

_P_=0.039; Figure 4a). The juvenile survival rate of offspring from interspecific crossings was lower than that from intraspecific crosses, but this effect was bordering significance

(likelihood ratio test, _χ__2_=3.68, _P_=0.06; Figure 4b). The effect of female species on the juvenile survival rate of offspring was not significant (likelihood ratio test, _χ__2_=0.003,

_P_=0.96; Figure 3b). Unmated females and females of interspecific crossings produced predominantly male offspring, but females from intraspecific crosses produced female-biased offspring

(likelihood ratio test, cross type: _χ__2_=983.19, _P_<0.001, female species: _χ__2_=0.16, _P_=0.69; Figure 4c). Only a single replicate of the crossing between _T. evansi_ females and

_T. urticae_ males yielded three females, which represented genuine hybrids (Figure 4c). MALE MATE CHOICE Most _T. urticae_ males first copulated with conspecific females (exact binomial

test, _P_<0.001; Figure 5). However, most _T. evansi_ males first copulated with _T. urticae_ females rather than with conspecific females (exact binomial test, _P_<0.001; Figure 5).

COMPETITION ON PLANTS In treatments where each of the two species was introduced separately on a _def-1_ tomato plant (controls; Table 1), the number of females increased with time and the

growth rate of _T. evansi_ was higher than that of _T. urticae_. The difference between growth curves of _T. urticae_ and _T. evansi_ did not depend on the reproductive status, as the

interaction between species, time and reproductive status did not have a significant effect (likelihood ratio test, species × reproductive status × time: _χ_2=0.645, df=1, _P_=0.422; Figures

6c and d). When the two species were introduced together on the same plants (treatments of strong RI and mild RI; Table 1), there was a significant effect in the interaction between

species, reproductive status and time (likelihood ratio test, species × reproductive status × time: _χ_2=19.261, df=1, _P_<0.001; Figures 6a and b). This means that the competitive

relation between _T. urticae_ and _T. evansi_ (the difference between the growth curves of _T. urticae_ and _T. evansi_) significantly differs between strong and mild RI treatments (Figures

6a and b). DISCUSSION We found that under strong RI treatment, the colony growth of _T. evansi_ was similar to that of _T. urticae_ (Figure 6a), whereas under mild RI treatment, _T. evansi_

was inferior to _T. urticae_ (Figure 6b). These results suggest that _T. evansi_ counteracted _T. urticae_ by interfering with the reproduction of this competitor. This inference is

supported by the results of crossing experiments and male mate choice experiments: interspecific copulation readily occurred between these two species (Figures 2 and 3), they are

reproductively incompatible (postmating, prezygotic reproductive barriers; Figure 4) and _T. evansi_ males preferred _T. urticae_ females to conspecific females, whereas _T. urticae_ males

preferred conspecific females to _T. evansi_ females (Figure 5). Given that _T. evansi_ suppresses and _T. urticae_ induces host plant defences, the infestation of the plants by _T. evansi_

makes the host plant more profitable for _T. urticae_ (Sarmento et al., 2011b). Indeed, earlier experiments showed that the oviposition rate of _T. urticae_ increased on tomato leaves

previously infested by _T. evansi_, whereas _T. evansi_ showed a decrease in oviposition on leaves previously infested by _T. urticae_ (Sarmento et al., 2011a). Thus, their co-occurrence on

tomato plants may benefit _T. urticae_ but comes at a cost to _T. evansi_. Nevertheless, _T. evansi_ was found to outcompete _T. urticae_ (Sarmento et al., 2011b). Moreover, field

observations in Spain showed that _T. evansi_ invasion replaced native _Tetranychus_ species including _T. urticae_ (Ferragut et al., 2013). Hence, given that _T. evansi_ suppresses tomato

plant defence, the question arises how _T. evansi_ prevents _T. urticae_ to profit from this. One explanation for the successful invasion of _T. evansi_ into Europe is that it can prevent

_T. urticae_ from taking advantage of the suppressed plant defences by covering the exploited leaf area with a more dense web than that of _T. urticae_ and by producing even more web in

response to _T. urticae_, thereby reducing access of this competitor to the leaf covered by _T. evansi_ web (Sarmento et al., 2011b). However, the web produced by _T. evansi_ may be more

effective when populations are large (note that both species produce web and live under the web) and it may still be questioned to what extent it has a role at low mite densities. We suggest

that other processes, such as RI, shown in this article, may explain why _T. evansi_ prevents _T. urticae_ from profiting from the _T. evansi_-induced suppression of plant defences and also

why _T. evansi_ succeeded in invading and displacing other _Tetranychus_ species already established. RI likely works against invasive species, because colonisation of invasive species

typically starts from small number of founders (Sakai et al., 2001), especially when the introduction occurs accidentally by human activity as in the case of _T. evansi_ (Boubou et al.,

2012). RI is frequency dependent and the majority has an advantage when inter-sexual interactions between species are reciprocal (Kuno, 1992). However, most of the inter-sexual interactions

between species are asymmetric (Gro¨ning and Hochkirch, 2008), and this can allow invasive species to overcome the disadvantage of small colony size. Recently, biotype B of the whitefly,

_Bemisia tabaci,_ invaded and displaced indigenous biotypes of the species in Zhejiang (China; indigenous biotype is ZHJ1) and Queensland (Australia; indigenous biotype is AN; Liu et al.,

2007). Behavioural observations revealed asymmetric mating interactions between invasive and indigenous biotypes: when the proportion of males in the offspring increased by adding males of

the same or different biotype to a pair, the pair of biotype B increased the number of copulations and consequently increased the proportion of female progeny regardless of the biotype of

additional males, however, the pair of indigenous biotypes (ZHJ1 and AN) decreased the number of copulations and the proportion of female progeny when additional males were biotype B (Liu et

al., 2007). This asymmetric mating interaction and the capacity to adjust the sex ratio in favour of the population increase in biotype B are expected to be key features of biotype B

driving its widespread invasions and displacement, and this expectation is supported by long-term field surveys and population experiments in cages (Liu et al., 2007). In our study, _T.

evansi_ males showed mating preference to _T. urticae_ females, whereas _T. urticae_ males prefer conspecific females. It is unknown why _T. evansi_ males prefer females of _T. urticae_. We

hypothesize that _T. urticae_ females produce more of a similar sex pheromone to attract males near emergence from the last moulting stage. Whatever the explanation may be, the preference of

_T. evansi_ males to copulate with _T. urticae_ females is possibly one of the key features of _T. evansi_ driving its invasion and displacement in areas endemic to _T. urticae_. To exclude

or reduce indirect plant-mediated interactions between the two mite species via mite-induced plant defences, we used a _def-1_ tomato mutant that is deficient in JA accumulation after

wounding and hence lacks the expression of genes in response to increased JA. In doing so, we expected that _T. urticae_ could not benefit from the suppressed plant defences induced by _T.

evansi_ on the same leaves and that _T. evansi_ would not incur a cost by the plant defences induced by _T. urticae_. However, the results did not completely meet our expectations. Comparing

the population dynamics of _T. evansi_ under mild RI treatment (Figure 6b) and the control (_T. evansi_, mated; Figure 6d), it can be seen that the population growth of _T. evansi_ was

hampered by the presence of _T. urticae_. In contrast, the same comparison for _T. urticae_ shows that colony development of _T. urticae_ was promoted by the presence of _T. evansi_ (mild

RI: Figure 6b; _T. urticae_, mated: Figure 6d). This suggests that _T. urticae_ induces plant defences that hamper the reproductive performance of other spider mites, not only through JA

signalling. Previous studies showed higher reproductive performance of spider mites on a _def-1_ tomato mutant compared with a wild tomato (Li et al., 2002; Ament et al., 2004), but recent

studies show circumstantial evidence for anti-herbivore defences mediated by salicylic acid, another signal transducer mediating the expression of plant defence genes (Farouk and Osman,

2011, but see Kawazu et al., 2011). Although we recognize the possibility of a role for plant defences that are not mediated by JA, we emphasize that this does not change our conclusion

regarding the impact of RI. We expect RI to be much more common than shown so far, if future experiments are based on experimental designs that exclude other factors (such as plant defence

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692–700. Article  CAS  Google Scholar  Download references ACKNOWLEDGEMENTS We thank Dr Merijn R Kant, Dr Lívia Silva Ataíde and Mrs Dan Li from the University of Amsterdam for their useful

suggestions on the study of interspecific competition on plants and Dr Michiel van Wijk for his help in setting up the video recording equipment. Dr Arne Janssen and Dr Sara Magalhães

provided many useful comments on a first draft of the manuscript. Jan van Arkel made the excellent photos of the two mite species shown in Figure 1. JMA was funded as a postdoc via NWO Earth

and Life Sciences (ALW) TOP (854.11.005) and YS was funded as a postdoc via the budget of MWS for his Royal Academy of Sciences (KNAW) professorship (selected in 2006 for 5 years and

prolonged from 2012 to 2015). AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, Amsterdam, The Netherlands Y Sato, J M

Alba & M W Sabelis Authors * Y Sato View author publications You can also search for this author inPubMed Google Scholar * J M Alba View author publications You can also search for this

author inPubMed Google Scholar * M W Sabelis View author publications You can also search for this author inPubMed Google Scholar CORRESPONDING AUTHOR Correspondence to Y Sato. ETHICS

DECLARATIONS COMPETING INTERESTS The authors declare no conflict of interest. RIGHTS AND PERMISSIONS Reprints and permissions ABOUT THIS ARTICLE CITE THIS ARTICLE Sato, Y., Alba, J. &

Sabelis, M. Testing for reproductive interference in the population dynamics of two congeneric species of herbivorous mites. _Heredity_ 113, 495–502 (2014).

https://doi.org/10.1038/hdy.2014.53 Download citation * Received: 30 July 2013 * Revised: 08 April 2014 * Accepted: 10 April 2014 * Published: 28 May 2014 * Issue Date: December 2014 * DOI:

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