Larval Competition, Adult Fitness, and Reproductive Strategies in the Acoustically Orienting Ormiine Homotrixa alleni (Diptera: Tachinidae)

Homotrixa alleni is a gregarious endoparasitoid fly that attacks adult male Sciarasaga quadrata (Orthoptera: Tettigoniidae) in southwestern Western Australia. Gravid female flies acoustically orient to their host's call and deposit live first-instar larvae upon or near their calling host. Up to 16 larvae may be found developing in the one host, and since only calling adult male S. quadrata are parasitized, host size and hence larval resources are essentially fixed at parasitism. This study examines parasitism by H. alleni in relation to intraspecific larval competition and adult fitness. The mean number of larvae emerging per host failed to increase significantly beyond a clutch size of four. Mean pupal weight and survival to the adult stage decreased linearly with increasing clutch size across the entire range of clutch sizes examined. Within a clutch, heavier pupae successfully completed pupal development significantly more often than lighter pupae. Pupal weight was directly related to adult size, with adult males being significantly larger than adult females at any given pupal weight. Female body size was positively correlated with fecundity. The size distribution of emerging females was normally distributed, while the distribution of searching gravid females collected at acoustic traps in the field was significantly skewed toward larger flies, suggesting yet another fitness benefit associated with large size. Using fecundity and survival to adulthood as our measure of fitness we calculated the optimal clutch size maximizing fitness per host to be seven, which exceeds the majority of observed clutch sizes in the field. Uncertainties associated with larvae successfully entering the host following larviposition are likely to reduce clutch sizes of H. alleni below this optimum in the field.     

Fecundity,reproductive effort,and pupal size in the profundal midgeChironomus cuccini (Diptera: Chironomidae)

Fecundity ofChironomus cucini increased linearly across a three-fold range of female pupal biomass. Females from a lake producing small pupae (Crystal Lake) had an average of 303 primary follicles, while larger pupae from Trout Lake had an average fecundity of 582. Pupae produced in Crystal Lake from larvae that received experimental food supplements were intermediate in size and fecundity. Pupal biomass explained 76% of the variation in fecundity. No difference in egg size was detected between pupae from the two lakes, and an average value of 2.1 g egg^–1 was used to calculate the total biomass allocated to eggs in each of 79 females dissected. This measure of reproductive effort scaled isometrically with pupal mass. On average, females allocated 48% of their biomass to eggs and 52% to somatic tissues. A comparison of average male biomass to female somatic biomass across 12 populations indicated that the biomass difference between the sexes is a biased predictor of fecundity.     

Evolutionary divergence of field and laboratory populations of Manduca sexta in response to host‐plant quality

1. The tobacco hornworm, Manduca sexta, has been an important model system in insect biology for more than 50 years. In nature, M. sexta successfully utilises a range of host plants that vary in quality. The consequences of laboratory domestication and rearing on artificial diet for fitness of phytophagous insects on natural host plants have not been explored. 2. We examine the evolutionary divergence of two domesticated laboratory populations and a field population (separated for more than 40 years, or > 250 laboratory generations) of M. sexta with respect to performance and fitness on two natural host plants: a typical host plant, tobacco (Nicotiana tabacum) and a novel host plant, devil's claw (Proboscidea louisianica). 3. For both field and laboratory populations, rearing on devil's claw resulted in animals with lower survival, smaller final size, longer development time, and reduced size‐corrected fecundity than animals reared on tobacco. Reductions in some fitness components (survival and fecundity) were greater for the laboratory population animals than the field population animals. 4. When reared on tobacco, the laboratory population animals had similar or larger pupal masses and slightly shorter development times than when reared on artificial diet, suggesting that laboratory domestication on artificial diet has not greatly affected the ability of M. sexta to perform well on a typical natural host plant. 5. Although field and laboratory populations exhibited qualitatively similar responses to host‐plant quality, i.e. reduced performance on devil‘s claw, the magnitude of this reduction differed across populations, with the domesticated laboratory populations having greater reductions in performance than the field population. The use of domesticated populations as models for responses of field populations may therefore be more appropriate for considering environmental conditions that are relatively benign or near‐optimal, than when exploring responses to extreme or stressful conditions.     

Grape variety affects larval performance and also female reproductive performance of the European grapevine moth Lobesia botrana (Lepidoptera: Tortricidae)

For insect herbivores, the quality of the larval host plant is a key determinant of fitness. Therefore, insect populations are supposed to be positively correlated with the nutritional quality of their host plant. This study aimed to determine if and how different varieties of grapes (including the wild grape Lambrusque) affect both larval and adult performance of the polyphagous European grapevine moth Lobesia botrana (Denis & Schiffermüller). Significant differences were found in larval development time, but not in pupal mass, adult emergence rate, or sex ratio. Although the fecundity of females is not different among varieties, females fed on some varieties produced eggs of different sizes which are correlated to their fertility. Thus, females adapt resource allocation to eggs depending on their diet as larvae. Using a fitness index, the average reproductive output was found to be highest for females reared on cv. Chardonnay. Females reared on wild grape produced a fitness index identical to the cultivated grapes. However, Lambrusque and Gewurztraminer separate themselves from the cultivated varieties according to our discriminant analyses. It is emphasized, through this study, that cultivars fed on by larvae should be considered in the population dynamics of L. botrana and that egg number is insufficient to determine host plant quality.     

Parasitism and constitutive defence costs to host life‐history traits in a parasitoid–host interaction

1. The level of an organism's investment in defences against natural enemies depends on the fitness costs of resisting parasitism and on the costs of maintaining defences in the absence of infection. Heritable variation in resistance suggests that costs exist, but very little is known about the nature or magnitude of these costs in natural populations of animals. 2. A powerful technique for identifying trade‐offs between fitness components is the study of correlated responses to artificial selection. We selected for increased resistance in the Indian meal moth, Plodia interpunctella, following parasitism by the koinobiont parasitoid, Venturia canescens, and measured the cost of resistance to parasitism and the cost of maintaining resistance in the absence of immune challenge during the next generation. 3. Parasitism decreased larval host size, growth, and developmental time and was significantly negatively correlated with the size of surviving host adults. Larvae of the next generation also had a reduced developmental period, whilst the duration of the invulnerable pupal instar was increased. There was no effect on host adult size and related fecundity in the F₁ generation.     

The effects of wild cruciferous host plants on morphology,reproductive performance and flight activity in the diamondback moth,Plutella xylostella (Lepidoptera: Yponomeutidae)

The wild crucifers,Rorippa indica andLepidium virginicum, are known to serve as host plants for the diamondback moth (DBM),Plutella xylostella, but they are less suitable than the preferred cultivated cruciferous plant, cabbage, in terms of adult body size and fecundity. The life history traits and flight activity of DBM adults grown on various host plants were investigated. The adults thus reared on each host plant were divided into three size groups (small, medium and large). In general, female adults grown on the wild crucifers were less fecund and lived longer than those reared on cabbage. Flight activity was higher in adults grown on wild crucifers than in those reared on cabbage. Male adults flew longer than females. Fecundity, longevity, flight activity and morphometrical characters of adults were positively correlated with pupal weight in individuals reared on the same host plant. A negative relationship was found between fecundity and flight activity in females of the same size group, but a positive one was observed in females reared on the same host plant.     

The effect of host plant on larval survivorship of the Alaskan swallowtail butterfly (Papilio machaon aliaska)

The Alaskan swallowtail butterfly, Papilio machaon aliaska (Lepidoptera: Papilionidae), uses three plant species as host plants. Cnidium cnidiifolium (Turcz.) Schischk. belongs to the family Apiaceae, the ancestral host‐plant family of the P. machaon group. Artemisia arctica Less. and Petasites frigidus (L.) Franch, in contrast, belong to the distantly related Asteraceae family and were colonized relatively recently by this group of butterflies. Previous work has shown that larval survival is highest on the novel host plants when natural enemies are present in the field. Here I examine whether P. m. aliaska fitness varied when individuals were reared on the three host plants in a common environment, free of environmental and ecological complications such as predation. I collected 12 P. m. aliaska females and measured their reproductive success when their progeny were reared on each of the three host plants. I assessed larval fitness on each of the hosts by recording the percentage pupation and percentage emergence as well as by measuring pupal mass. I found that larvae reared on the ancestral host plant, C. cnidiifolium, had higher fitness than did larvae reared on either of the novel host plants, A. arctica or P. frigidus. Larvae reared on C. cnidiifolium were more likely to pupate, achieved a greater pupal mass, and had higher emergence rates than larvae reared on the novel hosts. I interpret these results to mean that, in the absence of predation, the ancestral host plant is a better host for P. m. aliaska larvae than either novel host and I contend that this result does not appear to support the hypothesis that P. m. aliaska populations in central Alaska are divided into host races.     

PREDICTING HOST RANGE EVOLUTION: COLONIZATION OF CORONILLA VARIA BY COLIAS PHILODICE (LEPIDOPTERA: PIERIDAE)

Extensive sympatry is currently arising between the common sulfur butterfly, Colias philodice Latreille (Lepidoptera: Pieridae) and a potential leguminous host plant, Coronilla varia (L.). In laboratory trials, larval surviviorship and growth were higher on the primary host, Medicago sativa (L.), than on the nonhost C. varia. However, because females reared from C. varia were on average more fecund than females reared from M. sativa, fitness on C. varia (approximately as survivorship times fecundity) was commensurate with fitness on M. sativa. Thus, it is predicted that selection would favor oviposition on C. varia, if such behavior were to arise. In addition, significant among-family variation exists for several measures of larval performance on both C. varia and M. sativa, indicating that C. philodice can potentially respond to selection for increased performance on each species. Moreover, larval performance was significantly positively correlated across these species, suggesting that selection for increased performance on each species will facilitate, not constrain, evolution of increased performance on the other. It is concluded that behavioral rather than physiological barriers currently account for the absence of C. philodice from C. varia and that, if such barriers are overcome, C. philodice will expand its host plant range to include C. varia.     

Effects of host variety on blue willow beetle Phratora vulgatissima performance

Phratora vulgatissima adults exhibit feeding preferences among willow varieties, yet little is known of the effects of willow variety on larval and adult performance. The effects of host variety on adult fitness and fecundity as well as on larval mortality and development were studied under laboratory conditions for 35 willow varieties. The host variety significantly affected the number of days that adults survived and the rates of weight change. On average, males lost weight and females gained weight. The total number of eggs, eggs day¹, eggs clutch^?1 and the length of the oviposition period were also significantly affected by willow variety. Progeny from eggs laid by adults fed on the different willow varieties showed significant differences in days to pupation and pupal weight when subsequently reared on Salix × dasyclados or on the same variety as fed to the adults. However, there was no correlation between these parameters on S. × dasyclados and the variety fed to adults. The willow variety fed to larvae significantly affected larval mortality (four varieties caused 100% mortality), the shape of larval growth curves (as measured by predicted final weight and time to half the final weight), the number of days to pupation and pupal weight. There were significant positive correlations between previously determined adult P. vulgatissima feeding preferences of the 35 willow varieties and the following: number of eggs laid, length of the oviposition period, larval mortality and development and change in adult weight. There was a considerable degree of variation in these correlations and some varieties did not follow the general trend indicated by the size or sign of particular correlations, for example, having a high feeding preference ranking yet few eggs laid, low larval weight and longer time to pupation. The differences found between varieties for adult and larval performance in conjunction with previously established feeding preferences offer great potential for utilising plant resistance to P. vulgatissima as a means of strategic control. Despite the general correlation of feeding preference and performance parameters, the results found here suggest that it appears to be possible for plant breeders to circumvent this trend.     

Butterfly oviposition preference is not related to larval performance on a polyploid herb

The preference–performance hypothesis predicts that female insects maximize their fitness by utilizing host plants which are associated with high larval performance. Still, studies with several insect species have failed to find a positive correlation between oviposition preference and larval performance. In the present study, we experimentally investigated the relationship between oviposition preferences and larval performance in the butterfly Anthocharis cardamines. Preferences were assessed using both cage experiments and field data on the proportion of host plant individuals utilized in natural populations. Larval performance was experimentally investigated using larvae descending from 419 oviposition events by 21 females on plants from 51 populations of two ploidy types of the perennial herb Cardamine pratensis. Neither ploidy type nor population identity influenced egg survival or larval development, but increased plant inflorescence size resulted in a larger final larval size. There was no correlation between female oviposition preference and egg survival or larval development under controlled conditions. Moreover, variation in larval performance among populations under controlled conditions was not correlated with the proportion of host plants utilized in the field. Lastly, first instar larvae added to plants rejected for oviposition by butterfly females during the preference experiment performed equally well as larvae growing on plants chosen for oviposition. The lack of a correlation between larval performance and oviposition preference for A. cardamines under both experimental and natural settings suggests that female host choice does not maximize the fitness of the individual offspring.     

Dispersal and development of bagworm larvae (Lepidoptera: Psychidae) on three host plants

The present study investigated the development and dispersal of bagworm larvae, Thyridopteryx ephemeraeformis (Haworth) (Lepidoptera: Psychidae), on three hosts: arborvitae, Thuja occidentalis, blue spruce, Picea pungens, and juniper, Juniperus virginiana. The biomass of larvae was recorded at repeated intervals at 21 field sites. The relative quality of plants varied during larval development: early in the season, larvae were heavier on juniper and arborvitae than on blue spruce; late in the season, in contrast, larvae and pupae were lighter on juniper than on the two other plants. Larvae on blue spruce pupated later than those on arborvitae. In the laboratory, feeding rate and larval growth were not influenced by the density of larvae on the foliage of arborvitae up to a range which greatly exceeded that observed at any of our experimental sites. The incidence of dispersal among neonates emerging on arborvitae was high, as indicated by the high number of larvae captured on sticky traps, and the relatively low number of early instars on arborvitae foliage in relation to the reproductive output of females in the parental generation. The high rate of dispersal among early instars may be due to potential resource depletion over successive generations of bagworms.     

Selection on body size and sexual size dimorphism differs between host species in a seed-feeding beetle

Sexual size dimorphism varies substantially among populations and species but we have little understanding of the sources of selection generating this variation. We used path analysis to study how oviposition host affects selection on body size in a seed-feeding beetle (Stator limbatus) in which males contribute large ejaculates (nuptial gifts) to females. Females use nutrients in these ejaculates for egg production. Male body size, which affects ejaculate size, affects female fecundity and is thus under fecundity selection similar in magnitude to the fecundity selection on female body size. We show that when eggs are laid on a host on which larval mortality is low (seeds of Acacia greggii) fecundity predicts fitness very well and fecundity selection is the major source of selection on both male and female adult size. In contrast, when eggs are laid on a host on which larval mortality is high (seeds of Parkinsonia florida) fecundity poorly predicts fitness such that fecundity selection is relaxed on both male and female size. However, because egg size affects larval mortality on this poor host (P. florida) there is selection on female size via the female size --> egg size --> fitness path; this selection via egg size offsets the reduction in fecundity selection on female, but not male, body size. Thus, differences in host suitability (due to differences in larval mortality) affect the relative importance of two sources of selection on adult body size; fecundity selection on both male and female body size is lower on the poor quality host (P. florida) relative to the high quality host (A. greggii) whereas selection on female body size via effects of egg size on offspring survival (body size --> egg size --> fitness) is greater on the poor quality host relative to the high quality host. Because selection via the egg size path affects only females the difference in larval survival between hosts shifts the relative magnitude of selection on female vs. male size. Researchers working on other study systems should be alerted to the possible importance of subtle, but consequential, indirect selection on their study organisms.     

The direct effects of male killer infection on fitness of ladybird hosts (Coleoptera: Coccinellidae)

Male killing bacteria are common in insects and are thought to persist in host populations primarily by indirect fitness benefits to infected females, whereas direct fitness effects are generally assumed to be neutral or deleterious. Here, we estimated the effect of male killer infection on direct fitness (number of eggs laid, as a measure of fecundity, together with survival) and other life‐history traits (development time and body size) in seven ladybird host/male killer combinations. Effects of male killers on fecundity ranged, as expected, from costly to neutral; however, we found evidence of reduced development time and increased survival and body size in infected strains. Greater body size in Spiroplasma‐infected Harmonia axyridis corresponded to greater ovariole number and therefore higher potential fecundity. To our knowledge, this is the first report of direct benefits of male killer infection after explicitly controlling for indirect fitness effects. Neutral or deleterious fitness effects of male killer infection should not therefore be automatically assumed.     

Fitness effects of Wolbachia and Spiroplasma in Drosophila melanogaster

Maternally inherited endosymbionts that manipulate the reproduction of their insect host are very common. Aside from the reproductive manipulation they produce, the fitness of these symbionts depends in part on the direct impact they have on the female host. Although this parameter has commonly been investigated for single infections, it has much more rarely been established in dual infections. We here establish the direct effect of infection with two different symbionts exhibiting different reproductive manipulation phenotypes, both alone and in combination, in the fruit fly Drosophila melanogaster. This species carries a cytoplasmic incompatibility inducing Wolbachia and a male-killing Spiroplasma, occurring as single or double (co-) infections in natural populations. We assessed direct fitness effects of these bacteria on their host, by comparing larval competitiveness and adult fecundity of uninfected, Wolbachia, Spiroplasma and Wolbachia–Spiroplasma co-infected females. We found no effect of infection status on the fitness of females for both estimates, that is, no evidence of any benefits or costs to either single or co-infection. This leads to the conclusion that both bacteria probably have other sources of benefits to persist in D. melanogaster populations, either by means of their reproductive manipulations (fitness compensation from male death in Spiroplasma infection and cytoplasmic incompatibility in Wolbachia infection) or by positive fitness interactions on other fitness components.     

Effect of leaf nitrogen content of tomato plants on preference and performance of a leafmining fly

Summary In a study on intraspecific host plant acceptability, Liriomyza trifolii females that had previously been exposed to plants of high nitrogen content, showed a feeding and oviposition preference for plants of high nitrogen (Minkenberg and Fredrix 1989). Females showed a preference to feed and oviposit on the high middle leaves within plants. It was hypothesized that the preference between plants was related to a better performance of females and offspring on high nitrogen plants compared to low nitrogen plants. Different nitrogen dosages were applied to tomato plants, resulting in plants containing 3.4, 3.9, 4.6 or 4.9% leaf nitrogen. L. trifolii females responded to increased leaf nitrogen with significantly increased feeding and fecundity, longer oviposition periods, and higher feeding and oviposition rates. Their offspring on the same plants showed reduced developmental time, lower mortality and increased pupal size. Consequently, intrinsic rate of increase was positively linearly related to leaf nitrogen. Size of L. trifolii females appeared to be independent of fecundity, longevity and developmental time. Pupal length of males increased with increasing developmental time. These results indicate that L. trifolii is well adapted in its intraspecific host plant selection, because the ability to distinguish between plants with differences in leaf nitrogen content will directly lead to an increase in their fecundity, longevity and overall fitness. The implications of leaf nitrogen as a significant factor in the behavior and population dynamics of L. trifolii are discussed.     

Interpopulation differences in pupal size and fecundity are not associated with occurrence of outbreaks in Epirrita autumnata (Lepidoptera, Geometridae)

Abstract.

• 1 Among-population differences in pupal mass were studied in a geometrid, Epirrita autumnata. Some Epirrita autumnata populations regularly reach outbreak densities while others are never known to do so. Because adults do not feed, pupal mass of females correlates strongly with fecundity.
• 2 Larvae were collected from twelve field sites. Ten of our sample populations originated within the outbreak range of the species and represented different phases of outbreaks. Two populations originated outside the outbreak range.
• 3 Pupal mass of field-collected E. autumnata varied significantly among populations. The peak phase populations had the smallest pupae and the biggest were found in low density populations outside the outbreak range.
• 4 Offspring of moths from each population were reared under identical conditions in two larval densities. Significant differences were not found in pupal mass among populations. That is, the inherent size, correlated with fecundity of moths, was not different between populations originating within and outside the outbreak range, nor among collections from different densities or phases of the outbreaks.
• 5 Rearing density did not interact in a consistent way with population.
• 6 As far as size and fecundity are concerned, the results do not support Chitty's hypothesis that differences in genetic composition of the population at low and high density phases generate cyclic fluctuations of population density.
• 7 Because no hereditary or maternal differences were found in size and fecundity between E.autumnata originating within and outside the outbreak range, variation in reproductive capacity cannot explain why outbreaks occur only in some populations.

     

Condition and fecundity of the damselfly,Enallagma ebrium (Hagen): the importance of ectoparasites

Summary The extent, magnitude, and cause of natural covariation between degree of parasitism and other variables known or suspected of influencing host fitness (such as host age or body size) has been understudied. We demonstrate that degree of parasitism by larval water mites (Arrenurus spp.) was associated with reduced condition of males and with lowered fecundity of young females of the damselfly, Enallagma ebrium (Hagen) (Odonata: Coenagrionidae). We also demonstrate that degree of parasitism can covary with both age and size of host damselflies. We explain the putative causes of such natural covariation, and we suggest that degree of parasitism, host age, and host size can all interact to determine damselfly fitness. We expect that natural covariation between the host's phenotype and degree of parasitism will be frequently observed. Studies of such natural covariation will help researchers to assess better the importance of several variables on host reproductive success and to understand better the dynamics of host-parasite interactions.     

Effect of plant nutrition on aphid size,prey consumption,and life history characteristics of green lacewing

Plant quality can directly and indirectly affect the third trophic level. The predation by all the instars of green lacewing, Chrysoperla carnea （S.） （Neuroptera： Chrysopidae） on the cereal aphids, Rhopalosiphum padi （L.）, and Sitobion avenae （F.） at varying nitrogen fertilizer levels was calculated under laboratory conditions. Wheat plants were grown on four nitrogen fertilizer levels and aphids were fed on these plants and subsequently offered as food to the C. carnea. Aphid densities of 10, 30, and 90 were offered to first, second, and third instar larvae of green lacewing. Increased nitrogen application improved nitrogen contents of the plants and also the body weight of cereal aphids feeding on them. Aphid consumption by green lacewings was reduced with the increase in nitrogen content in the host plants of aphids. Predation of both aphid species by first, second, and third instars larvae of C. carnea was highest on aphids reared on plants with the lowest rate of fertilization, suggesting a compensatory consumption to overcome reduced biomass （lower aphid size）. Total biomass devoured by C. carnea on all nitrogen fertilizer treatments was not statistically different. Additionally, the heavier host prey influenced by the plant nutrition had an effect on the life history characteristics of green lacewings. The larval duration, pupal weight, pupal duration, fecundity, and male and female longevity were significantly affected by the level of nitrogen fertilization to the aphid＇s host plants, except for pupal duration when fed on S. avenae. This study showed that quantity of prey supplied to the larvae affects the prey consumption and thereafter the life history characteristics of green lacewings.     

No evidence for costs of being large in females of Orgyia spp. (Lepidoptera, Lymantriidae): larger is always better

Strong correlation between female body size and potential fecundity is often observed in insects. Directional selection favouring increased body sizes is thus predicted in the absence of opposing selection pressure. The evolutionary forces capable of counterbalancing such a 'fecundity advantage' are poorly documented. This study focuses on revealing the costs of large body size in the wingless females of Orgyia antiqua and O. leucostigma, two related species of lymantriid moths. Extreme behavioural simplicity of these animals allows systematic assessment of various fitness components in conditions that are close to natural. A linear relationship between pupal weight and potential fecundity was observed. This association was found to be independent of particular rearing conditions. There was no evidence that the relationship between fecundity and body mass becomes asymptotic when body sizes increases. No component of fitness showed a negative phenotypic correlation with body size; some displayed a weakly positive one. In particular, pupal mortality, adult longevity, mating and oviposition success, as well as egg hatching rate and egg size, were established as independent of body size in a series of field and laboratory experiments. There was a very high overall efficiency of converting resources accumulated during the larval stage to egg masses. With no costs of large adult size, selective forces balancing the fecundity advantage should operate in the course of immature development. The strong dependence of realized fecundity on body size is considered characteristic of the capital breeding strategy.     

Maintenance of body‐size variation and host range in the orange‐tip butterfly: evidence for a trade‐off between adult life‐history traits

1. The evolution of host range and preference in phytophagous insects is driven by a female's oviposition choice impacting her offspring's fitness. Analysis of the fitness of progeny on different host plants has commonly been restricted to the performance of immature stages. However, since host use can affect adult size, it is important to measure the ongoing effects of host choice on the resulting imagines. 2. The orange‐tip butterfly, Anthocharis cardamines, shows a strong preference for two host plants in Britain, Alliaria petiolata and Cardamine pratensis, which affect body size. Whilst females exhibit a strong positive size–fecundity relation, the impact of body‐size alteration is unknown in males. In this study, fitness effects of host plant choice for male A. cardamines were examined. 3. Males reared on C. pratensis were smaller and emerged earlier than those reared on A. petiolata, and early‐season males were smaller than late‐season ones in the field. Interestingly, regression analysis indicated that the earlier emergence of small males was a host‐mediated rather than a size‐mediated effect. Small size was associated with reduced male dispersal in a semi‐isolated wild population over a 3‐year period. 4. It is proposed that the earlier emergence associated with C. pratensis has evolved in response to depressed dispersal in isolated/semi‐isolated populations associated with this patchily distributed host. We suggest that adult life‐history traits are important for the maintenance of host range in this species, and offer a critique of Courtney's earlier hypothesis that host range is maintained by time‐limited oviposition behaviour.