Adaptive self- and conspecific superparasitism in the solitary parasitoid Leptopilina heterotoma (Hymenoptera: Eucoilidae)

One of the foraging decisions facing parasitoids is whetherto accept (superparasitize) or reject hosts that have alreadybeen parasitized. An important distinction is whether the hosthas been parasitized by the female parasitoid herself or bya conspecific. In solitary parasitoids, the pay-off from anegg laid in the latter host type (conspecific superparasitism)is the probability that the second egg wins the competitionfor the host and results in an offspring. The pay-off from anegg laid in the former type (self-superparasitism) increaseswith an increasing probability that another female will superparasitizethe host in the near future. When this probability equals one,self-superparasitism and conspecific superparasitism have thesame payoff. However, conspecific superparasitism will generallyhave a higher pay-off than self-superparasitism. It will thereforebe beneficial for a female parasitoid to be able to distinguishbetween a host she parasitized and one parasitized by a conspecific.The degree of benefit depends on the probability of conspecificsuperparasitism in the near future. Using an optimal diet model,I show that when a parasitoid encounters a patch containinga mixture of unparasitized and already-parasitized hosts, afemale that can distinguish between the two types of parasitizedhosts gains more offspring than a female without this ability.However, when parasitoids search a patch together with conspecifics,it is adaptive to self-superparasitize, and the pay-off fromthis ability may be negligible. It is therefore predicted thatwhen a female parasitoid searches a partially depleted patchalone, it will reject the hosts parasitized by itself more frequentlythan hosts parasitized by conspecifics. In contrast, femaleparasitoids searching together are predicted to accept hoststhat they parasitized themselves much more often. The resultsshow that the solitary parasitoid Leptopilina heterotoma (Hymenoptera:Eucoilidae) is able to distinguish between hosts that it parasitizedand hosts parasitized by conspecifics. The predictions of themodel are met in a second experiment that shows that L. heterotomaself-superparasitizes when the probability of conspecific superparasitismis high.     

Host discrimination modulates brood guarding behaviour and the adaptive superparasitism in the parasitoid wasp Trissolcus semistriatus (Hymenoptera: Scelionidae)

Because hosts utilized by parasitoids are vulnerable to further oviposition by conspecifics, host guarding benefits female wasps. The present study aims to test whether female adults regulate brood guarding behaviour by host discrimination in a solitary parasitoid Trissolcus semistriatus by presenting an intact or parasitized host egg mass to a female adult. Virgin females without oviposition experience have host discrimination ability, which enables them to adjust the number of eggs laid in the hosts. Mating experience increases superparasitism by female adults, whereas mated females achieve a higher discrimination ability as a result of oviposition experience and show a lower superparasitism rate. As expected, females exhibit brood guard after parasitizing an intact host egg mass, whereas those females visiting a previously parasitized host egg mass, do not. Because the survival of eggs in superparasitized hosts is relatively low, regulating brood guarding behaviour by host discrimination is adaptive for female wasps.     

Host-searching behavior ofVenturia canescens (Grav.) (Hymenoptera: Ichneumonidae): Superparasitism

The foraging behavior ofVenturia canescens, a solitary endoparasitoid of lepidopteran larvae, was investigated in the laboratory. FemaleVenturia canescens with a larger number of mature eggs to lay were found to have higher levels of superparasitism (measured as numbers of eggs laid per parasitized host). Increased parasitoid density was found to result in reduced levels of superparasitism by host-deprived (i.e.,undepleted) wasps. Females which had been allowed access to hosts before the experiment (depleted wasps) laid fewer eggs per parasitized host than undepleted wasps, although there was no significant difference in the levels of superparasitism among the depletion periods of 1, 2, 5, and 7 h. It was also found that an egg which was encountered less than 15 min after oviposition was much less likely to be avoided than one which was encountered after more than 15 min had elapsed.     

The effect of superparasitism on development of the solitary parasitoid wasp, Venturia canescens (Hymenoptera: Ichneumonidae)

Abstract.

• 1 The effects of superparasitism on the rate of development, adult size and mortality of Venturia canescens (Hymenoptera: Ichneumonidae) were investigated. Parasitoids were reared from third (L3) and fifth (L5) instars of one of its hosts, Plodia interpunctella (Lepidoptera: Pyralidae) containing one, two or four parasitoid eggs.
• 2 Superparasitism increased the development time of Venturia reared from both instars, but the developmental delay was more evident in wasps from L5 hosts.
• 3 The size of parasitoids from L3 hosts was unaffected by egg number, but wasps from both superparasitized L5 treatments were significantly smaller than those from singly parasitized hosts.
• 4 Parasitoid mortality was significantly higher in L5 than L3, but within instars did not differ significantly with egg number.
• 5 The results confirm that superparasitism may affect the fitness of both the adult female wasp and her progeny, and should therefore be incorporated into models of superparasitism as an adaptive foraging strategy.

     

Self and conspecific superparasitism by the solitary parasitoid Antrocephalus pandens

Abstract.

• 1 The fitness consequences of superparasitism for a solitary parasitoid depend on whether the host was first parasitized by itself (‘self-superparasitism’) or a different individual (‘conspecific superparasitism’). Self superparasitism is usually expected to be avoided.
• 2 A.pandens females showed no difference in their probability of superparasitism between self-parasitized and conspecifically-parasitized hosts. The probability of superparasitism decreased as time from the laying of the first egg in a host increased, from about 0.29–0.46 at a time interval of 1 h to 0.10–0.14 at 72 h.
• 3 The egg distribution of wasps foraging alone on a patch showed significant avoidance of superparasitism, but that of wasps foraging in the presence of conspecifics was not significantly different from a random distribution. This suggests that wasps switch from avoidance of superparasitism when alone to acceptance of all hosts when in a group.
• 4 When wasps foraged in a group, the hosts had many more ovipositor puncture marks than when wasps foraged singly. This suggests that either hosts were attacked several times per encounter, or that the wasps' encounter rate with hosts was much higher when in a group. If the latter is true, it is possible that, although the egg distribution suggested a higher rate of superparasitism when wasps foraged in a group, the ratio of acceptances to contacts of parasitized hosts may in fact have been lower.

     

Inter-larval Competition and its Subsequent Effect on Pediobius furvus (Hym.: Eulophidae) Broods for the Management of Graminaceous Stem Borers

The effects of superparasitism on the size, rate of development, progeny production, sex ratio and percentage parasitism of the parasitoid Pediobius furvus (Gah.) and the number of parasitized hosts producing mature parasitoids were investigated. High parasitoid egg densities in hosts were obtained using two host densities (1 and 2) and three arbitrary parasitoid densities (1, 2 and 5) under two exposure times (12 and 24 h). Superparasitism in P. furvus resulted in a reduction in the number of adults that matured and a reduction in the size and preponderance of male production. Dissections of the parasitized host pupae of the stem borer Chilo partellus (Swinhoe) showed no combat among developing wasps. Immature supernumeraries were eliminated by starvation and/or suffocation. The results showed that P. furvus larvae are not harmed by surplus food material in their hosts and that the female parasitoid preferred to lay more eggs into unparasitized than into parasitized host pupae.     

Inter- and intra-specific host discrimination in gregarious and solitary endoparasitoid wasps

In nature, most species of Lepidoptera are attacked by parasitoids, and some species may be hosts for several parasitoid species. When hosts are parasitized by more than one female of the same species (=superparasitism) or females of different species (=multiparasitism), then intrinsic competition occurs for control of host resources. To reduce competition, some parasitoids are able to recognize the difference between parasitized and unparasitized hosts. Inter- and intra-specific host discrimination were investigated in the two sympatric species, the gregarious Cotesia kariyai (Watanabe) and solitary Meteorus pulchricornis (Wesmael), endoparasitoids of the Oriental armyworm Mythimna separata (Walker). To measure host discrimination, choice experiments were conducted in which females of both species foraged and chose between healthy host larvae and hosts initially parasitized by either C. kariyai or M. pulchricornis. An olfactory test was also performed to examine the discrimination behavior of the two parasitoids. Our results showed that, in oviposition choice tests, both braconid female wasps were able to discriminate between unparasitized hosts and from four to seven day-old hosts previously attacked by conspecific and heterospecific wasps. On the other hand, superparasitism and multiparasitism occurred even in host larvae that were parasitized two days earlier. However, once the immature parasitoids hosts are at larval stage (1st and 2nd instar), super- and multiparasitism were avoided in the two-choice test, but the latter often occurred in the multiple-choice experiment. Host discrimination abilities may have been based on plant volatile signals incurred from damaged plants and internal mechanisms from four to seven post-parasitized hosts.     

Impact of mating status on egg-laying and superparasitism behaviour in a parasitoid wasp

Most parasitoid female wasps can distinguish between unparasitized and parasitized hosts and use this information to optimize their progeny and sex allocation. In this study, we explored the impact of mating on oviposition behaviour (parasitism and self‐ and conspecific superparasitism) on both unparasitized and already parasitized hosts in the solitary parasitoid wasp Eupelmus vuilleti (Crw.) (Hymenoptera: Eupelmidae). Virgin and mated females had the same oviposition behaviour and laid eggs preferentially on unparasitized hosts. The sex ratio (as the proportion of females) of eggs laid by mated females in parasitism and conspecific superparasitism was 0.67 ± 0.04 and 0.57 ± 0.09, respectively. Likewise, females laid more eggs in conspecific superparasitism than self‐superparasitism under our experimental conditions. These experiments demonstrate that E. vuilleti females can (i) discriminate between unparasitized and parasitized hosts and adapt the number of eggs they lay accordingly, and (ii) probably discriminate self from conspecific superparasitized hosts. Finally, mating does not appear to influence the host discrimination capacity, the ovarian function, or the oviposition behaviour.     

螟蛉盘绒茧蜂的过寄生行为及其对子代生长发育的影响

【目的】过寄生现象普遍存在于寄生蜂寄生过程中。本研究旨在探究螟蛉盘绒茧蜂Cotesia ruficrus的过寄生对子代发育的影响及影响过寄生行为的主要因素。【方法】室内研究了螟蛉盘绒茧蜂在其寄主稻纵卷叶螟Cnaphalocrocis medinalis3龄幼虫上过寄生行为的发生,不同产卵次数对寄主存活及子代蜂生长发育的影响,研究了不同接蜂时间和不同接蜂密度对过寄生发生的影响。【结果】螟蛉盘绒茧蜂存在过寄生行为,无论雌蜂有无产卵寄生经历,均能在被自身寄生过和同种不同个体寄生过的寄主内产卵。寄生蜂茧量随着被产卵次数的增加而增加,被产卵3~5次的寄主体内死亡的寄生蜂幼虫数随着增大。寄主在育出蜂前的死亡率随着被产卵寄生次数的增加而增加,被产卵5次时,寄主育出蜂前死亡率达50%。过寄生使螟蛉盘绒茧蜂子代蜂卵-蛹的历期延长,羽化率和雌雄性比下降,雌蜂体型随寄生次数的增加显著变小。过寄生率随着接蜂密度及接蜂时间的增加而增加。【结论】在寄主上产卵2次对螟蛉盘绒茧蜂子代发育是最适合的,产卵3次及以上为过寄生。过寄生使蜂子代发育适合度降低,不利于子代生长发育。在室内大量繁殖螟蛉盘绒茧蜂时,应减少接蜂密度和接蜂时间,从而减少过寄生的发生。     

Discrimination of the age of conspecific eggs by an ovipositing ectoparasitic wasp

The recognition and avoidance of already parasitized hosts is a major issue in parasitoid behavioural ecology. A key factor affecting the fitness reward expected from superparasitism is the probability that the second or subsequent egg laid on a host will win the contest with the first-laid egg. The present study investigated the ability of females of the solitary ecto parasitoid Anisopteromalus calandrae Howard (Hymenoptera: Pteromalidae) to (i) discriminate between unparasitized Callosobruchus maculatus (Fabricius) (Coleoptera: Bruchidae) hosts and those parasitized by a conspecific, and (ii) discriminate between a host parasitized by an egg just laid (2 h) and one parasitized by an egg about to hatch (28 h). However, they did not adjust their offspring sex ratio on already parasitized hosts compared to unparasitized ones. Our results show that A. calandrae females can discriminate between parasitized and unparasitized hosts, as they lay more eggs on the latter. The probability of the second or subsequent egg laid on a host (superparasitism) winning the contest with a conspecific increases as the time between the two ovipositions decreases. Consequently, parasitoid females should lay more eggs on recently parasitized hosts than on those that have been parasitized for a long time (i.e., when the first eggs are about to hatch), and that is indeed what they were found to do. To increase their fitness in spite of the presence of already parasitized hosts, A. calandrae females have developed highly discriminative capacities regarding the parasitism status of hosts.     

Adaptive superparasitism in solitary parasitoids: marking of parasitized hosts in relation to the pay-off from superparasitism

Abstract.

• 1 The pay-off from an egg laid in a parasitized host is an important parameter in models on adaptive superparasitism in solitary insect parasitoids.
• 2 For Leptopilina heterotoma, a parasitoid of larval Drosophila, the pay-off from a second egg laid in a host is 0.43 offspring when the interval between the two ovipositions is less than 3h. For longer intervals, this pay-off decreases to almost zero for an interval of 24 h.
• 3 When a female encountering a parasitized host is able to estimate the interval since the first oviposition, it is expected that she will take this into account in her host selection decisions. This is, however, not in the direct interest of the female that lays the first egg, and marks the host.
• 4 We studied whether superparasitism in hosts containing a young egg is more common than in hosts containing an older egg, when searching in a patch containing once-parasitized and unparasitized hosts.
• 5 The acceptance/encounter ratio of parasitized hosts increased for intervals longer than 6h, as predicted when the interests of the marking female and the longevity of the mark are taken into account.
• 6 Superparasitism occurred more often when parasitoids had previously searched a host patch 7 days before the experiment compared to when parasitoids had searched a patch 1 day before, a phenomenon predicted by dynamic optimal diet models.

     

Superparasitism and mutual interference in the egg parasitoid Anagrus delicatus (Hymenoptera: Mymaridae)

Abstract.

• 1 In nature, interference among Anagrus delicatus (Hymenoptera: Mymaridae) parasitoids reduced the per-capita number of hosts parasitized. Interference increased with parasitoid density.
• 2 Anagrus delicatus did not avoid parasitizing hosts that had recently been parasitized by conspecific wasps. Evidence indicated that this superparasitism was largely a random process, increasing with the ratio of parasitized to unparasitized hosts.
• 3 Individual parasitoid efficiency, the number of hosts killed per wasp per unit time, decreased with increasing wasp density. This occurred whether wasps searched the patch together (simultaneously) or one by one (sequentially), and was the result of an increase in time spent superparasitizing hosts at higher wasp density. This is known as indirect mutual interference.
• 4 Increasing numbers of parasitoids together on the same patch caused a significant decline in the rate and per-capita number of hosts parasitized. However, there was not a correspondent decline in searching efficiency with increasing wasp density (i.e. no direct mutual interference).
• 5 These forms of parasitoid density dependence should contribute to the stability of the host—parasitoid interaction.

     

Evidence for a marking pheromone in host discrimination byCephalonomia Stephanoderis (Hym.: Bethylidae)

The bethylidCephalonomia stephanoderis Betrem is an ectoparasitoid that prefers to oviposit on the prepupae and pupae of the coffe berry borerHypothenemus hampei (Ferrari) (Coleoptera: Scolytidae). It has the ability to distinguish unparasitized from parasitized hosts and rarely lays more than one egg per host. The mechanism of this host discrimination byC. stephanoderis was investigated under laboratory conditions. For this, parasitoid eggs that had been deposited on host pupae were removed and pupae were then offered (individually and collectively) to individual female wasps. A total of 92% of individually offered hosts and 93% of collectively offered hosts were not parasitized. It is concluded thatC. stephanoderis recognizes a marking pheromone deposited into or onto the host, preceding, during, or after oviposition which enables female parasitoids to avoid self and conspecific superparasitism.     

Superparasitism as an ESS: to reject or not to reject, that is the question

A stochastic model is formulated to determine the optimal strategy for a solitary parasitoid which has discovered an already parasitized host. The model assumes that the parasitoid can count both the number of eggs already present in a host and the number of conspecifics searching in the same patch. The survival probability of an egg is assumed to depend on the total number of eggs in a host. The decision to (super)parasitize depends both on the degree to which the discovered host already is parasitized and on the number of conspecific females searching in the same patch. We consider both the case that egg laying does not involve any costs for the parasitoid and the case that it involves some marginal costs. Uniform behaviour of all the conspecific parasitoids in a patch, i.e. laying one additional egg in all encountered larvae containing a particular number of eggs, appears to be a pure evolutionary stable strategy (ESS). If either the probability that a parasitoid emerges from a host decreases with an increasing degree of parasitism, at least from a particular number of eggs onwards, or if parasitism involves marginal costs, the maximum number of eggs for which it is still profitable to superparasitize a host once more is limited. This number increases with the number of conspecifics searching in the patch. Large marginal costs (i.e. the expected gain of not parasitizing now) decrease the profit of superparasitism. For newly emerged parasitoids the rejection of an already parasitized host is not advantageous as long as the marginal costs of parasitism are small, because the host can never contain an egg of its own.     

Superparasitism in the solitary ectoparasitoid Aptesis nigrocincta: the influence of egg load and host encounter rate

Theory predicts that the acceptance of hosts already parasitized by a conspecific will depend both on egg load and the availability of hosts. In the present laboratory study, we tested the effect of egg load and host encounter rate on the propensity of superparasitism in the solitary parasitoid Aptesis nigrocincta Gravenhorst (Hymenoptera, Ichneumonidae), a synovigeneous ectoparasitoid of prepupae of the European Apple Sawfly. Parasitoid females carry few voluminous eggs at a time and the egg maturation rate is less than one egg per day. Egg load was manipulated by giving females access to hosts one week prior to the start of treatments and host availability by giving females access to either one host cocoon every day or every other day. In the first treatment where females had a high egg load of 5.3 egg in their ovaries and encountered host cocoons at low rates, we found that parasitized hosts were accepted to the same degree as healthy hosts. In females with significantly decreased egg load (3.8 eggs) encountering hosts at the same rate we found a slight but non-significant decrease in the acceptance of parasitized hosts compared with healthy hosts. In contrast, A. nigrocincta females accepted significantly fewer parasitized hosts at a high host encounter rate that would lead them to the point of egg limitation in the near future. Within the range of egg loads tested, the host encounter rate appears to be the most important determinant for a females decision to oviposit onto hosts already parasitized by a conspecific.     

Artifical transfer and morphological description of virus particles associated with superparasitism behaviour in a parasitoid wasp

In parasitoids, the adaptive significance of superparasitism (laying of egg(s) in already parasitized hosts) has been the subject of strong controversy. The current view is to interpret this behaviour as an adaptation to increased competition for hosts, because the supernumerary egg still has a chance to win possession for the host. However, we recently discovered that in the solitary parasitoid Leptopilina boulardi, superparasitism is rather caused by an unknown infectious element: stable non superparasitizing lineages (NS) are transformed into stable superparasitizing lineages (S) after eggs from both lineages have competed inside the same host (superparasitism). In this report, we investigate the nature and location of the causative agent. Involvement of bacteria is unlikely because antibiotic treatments do not affect wasp phenotype and because bacterial 16S ribosomal DNA was not detected using PCR. We report successful injection experiments showing that the causative agents are located in wasp poison gland and ovaries and are stably inherited. Electron microscopic studies demonstrate that long filamentous virus particles located in wasp oviducts are strongly associated with superparasitism behaviour, leading to reconsider the adaptive significance of this behaviour in parasitoids. Interestingly, parasitoids are often infected with similar viruses for which no phenotypic effect has been documented. This raises the possibility that they could induce the same behavioural manipulation.     

Superparasitism in the solitary ectoparasitoid Aptesis nigrocincta: the influence of egg load and host encounter rate

Theory predicts that the acceptance of hosts already parasitized by a conspecific will depend both on egg load and the availability of hosts. In the present laboratory study, we tested the effect of egg load and host encounter rate on the propensity of superparasitism in the solitary parasitoid Aptesis nigrocincta Gravenhorst (Hymenoptera, Ichneumonidae), a synovigeneous ectoparasitoid of prepupae of the European Apple Sawfly. Parasitoid females carry few voluminous eggs at a time and the egg maturation rate is less than one egg per day. Egg load was manipulated by giving females access to hosts one week prior to the start of treatments and host availability by giving females access to either one host cocoon every day or every other day. In the first treatment where females had a high egg load of 5.3 egg in their ovaries and encountered host cocoons at low rates, we found that parasitized hosts were accepted to the same degree as healthy hosts. In females with significantly decreased egg load (3.8 eggs) encountering hosts at the same rate we found a slight but non‐significant decrease in the acceptance of parasitized hosts compared with healthy hosts. In contrast, A. nigrocincta females accepted significantly fewer parasitized hosts at a high host encounter rate that would lead them to the point of egg limitation in the near future. Within the range of egg loads tested, the host encounter rate appears to be the most important determinant for a females decision to oviposit onto hosts already parasitized by a conspecific.     

寄主大小及寄生顺序对蝇蛹佣小蜂寄生策略的影响

寄主大小模型认为寄生蜂后代性比与寄主大小相关,寄生蜂倾向于在大寄主上产出更多雌性后代,在小寄主上产出更多雄性后代.探讨了以家蝇蛹为寄主时,蝇蛹佣小蜂后代产量和性比变化;单次寄生情况下,寄主大小及寄生顺序对寄生蜂后代性比等影响.结果表明,蝇蛹佣小蜂的产卵期为(8.93±3.34)d,单头雌蜂能产雌性后代(34.11±16.34)头和雄性后代(11.04±8.87)头,且雄性百分比为0.24±0.11.随成蜂日龄的增大,寄生蜂产生雄性后代的比率显著增加.蝇蛹佣小蜂在寄生家蝇蛹时,会优先选择寄生个体较大的蛹;在单次寄生的情况下,蝇蛹佣小蜂倾向于在较大的家蝇蛹内产出更多的雌性后代.     

EVOLUTION AND MANIPULATION OF PARASITOID EGG LOAD

In proovigenic parasitoids such as Leptopilina boulardi, the female emerges with a limited egg load and no further eggs are produced during its adult life. A female thus runs the risk of exhausting this limited supply of eggs before the end of her life. Given that the production of an egg is costly, what is the evolutionarily stable egg load at emergence? This question has attracted a lot of attention in the last decade. Here, we analyze a model that allows us to track both the evolution and the population dynamics of a solitary, proovigenic parasitoid. First, we show how host–parasitoid dynamics feedbacks on the evolution of parasitoid egg load. Second, we use this model to consider the situation in which the parasitoid can be infected by a virus that manipulates the oviposition behavior of the females. In particular, we model the effect of the LbFV virus in L. boulardi, a virus that is known to enhance its horizontal transmission by increasing superparasitism (i.e., the laying of eggs in a host already parasitized). Specifically, we model (1) the effect of the virus on parasitoid egg load strategies , and (2) the evolution of egg load manipulation by the virus. This analysis yields two alternative, yet not mutually exclusive, adaptive explanations for the observation that females infected by the virus harbor higher egg loads than uninfected females. Infected females could either respond plastically to the infection status, or be manipulated by the virus. Further experimental work is required to distinguish between these two hypotheses. In a broader context, we present a general theoretical framework that allows us to study the epidemiology, the evolution, the coevolution, and the evolution of manipulation of various reproductive strategies of parasitoids.     

Host genotype affects the relative success of competing lines of aphid parasitoids under superparasitism

1. In solitary parasitoids, only one individual can complete development in a given host. Therefore, solitary parasitoids tend to prefer unparasitised hosts for oviposition, yet under high parasitoid densities, superparasitism is frequent and results in fierce competition for the host's limited resources. This may lead to selection for the best intra‐host competitors. 2. Increased intra‐host competitive ability may evolve under a high risk of superparasitism if this trait exhibits genetic variation, and if competitive differences among parasitoid genotypes are consistent across environments, e.g. different host genotypes. 3. These assumptions were addressed in the aphid parasitoid Lysiphlebus fabarum (Hymenoptera: Braconidae: Aphidiinae) and its main host, the black bean aphid, Aphis fabae (Scopoli) (Hemiptera: Aphididae). Three parthenogenetic lines of L. fabarum were allowed to parasitise three aphid clones singly and in all pairwise combinations (superparasitism). The winning parasitoid in superparasitised aphids was determined by microsatellite analysis. 4. The proportions of singly parasitised aphids that were mummified were similar for the three parasitoid lines and did not differ significantly among host clones. 5. Under superparasitism, significant biases in favour of one parasitoid line were observed for some combinations, indicating that there is genetic variation for intra‐host competitive ability. However, the outcome of superparasitism was inconsistent across aphid clones and thus influenced significantly by the host clone in which parasitoids competed. 6. Overall, this study shows that the fitness of aphid parasitoids under superparasitism is determined by complex interactions with competitors as well as hosts, possibly hampering the evolution of improved intra‐host competitive ability.