Effects of host plant on life‐history traits in the polyphagous spider mite Tetranychus urticae

Studying antagonistic coevolution between host plants and herbivores is particularly relevant for polyphagous species that can experience a great diversity of host plants with a large range of defenses. Here, we performed experimental evolution with the polyphagous spider mite Tetranychus urticae to detect how mites can exploit host plants. We thus compared on a same host the performance of replicated populations from an ancestral one reared for hundreds of generations on cucumber plants that were shifted to either tomato or cucumber plants. We controlled for maternal effects by rearing females from all replicated populations on either tomato or cucumber leaves, crossing this factor with the host plant in a factorial design. About 24 generations after the host shift and for all individual mites, we measured the following fitness components on tomato leaf fragments: survival at all stages, acceptance of the host plant by juvenile and adult mites, longevity, and female fecundity. The host plant on which mite populations had evolved did not affect the performance of the mites, but only affected their sex ratio. Females that lived on tomato plants for circa 24 generations produced a higher proportion of daughters than did females that lived on cucumber plants. In contrast, maternal effects influenced juvenile survival, acceptance of the host plant by adult mites and female fecundity. Independently of the host plant species on which their population had evolved, females reared on the tomato maternal environment produced offspring that survived better on tomato as juveniles, but accepted less this host plant as adults and had a lower fecundity than did females reared on the cucumber maternal environment. We also found that temporal blocks affected mite dispersal and both female longevity and fecundity. Taken together, our results show that the host plant species can affect critical parameters of population dynamics, and most importantly that maternal and environmental conditions can facilitate colonization and exploitation of a novel host in the polyphagous T. urticae, by affecting dispersal behavior (host acceptance) and female fecundity.     

Factors determining the host plant range of the phytophagous mite, Tetranychus urticae (Acari: Tetranychidae): a method for quantifying host plant acceptance

The host plant acceptance of the phytophagous mite Tetranychus urticae was experimentally quantified. Host plant acceptance is described as the proportion of adult females settling on the test plant on which they have been placed. On the other hand, the host plant suitability of T. urticae on different plant species is expressed as the mean number of eggs produced by the females within 5 days (hereafter 'fecundity'). An inbred T. urticae line was tested with regard to host plant acceptance and fecundity on 11 potential host plants. These two variables were positively correlated across host plants; host plant species on which the fecundity was low were also those on which females settled less readily compared to host plants with high fecundity. The characteristics of host plant acceptance of the T. urticae are discussed in light of their potential food resource under natural conditions.     

Antipredator responses in Tetranychus urticae differ with predator specialization

The behavioural response of Tetranychus urticae to chemical cues from specialist predatory mites, Phytoseiulus persimilis, or generalist predatory bugs, Orius majusculus, on either bean or strawberry was studied in experimental arenas. Predators were placed on the leaf disc for 24 h and removed before T. urticae females were introduced. After 24 h, prey fecundity (number of eggs laid) and dispersal (number of prey drowned in the water barrier) were assessed. Chemical cues from the specialist predator resulted in reduced prey fecundity, significantly different from the generalist predator and control treatments. No interaction effect was found between plant species and prey fecundity, while significantly more eggs were laid on bean than on strawberry. Predator cues irrespective of predator specialization resulted in more prey dispersal than in the control. Findings emphasize the importance of specialization in the predator species complex for the degree and type of antipredator responses and resulting biological control.     

Arbuscular mycorrhizal symbiosis increases host plant acceptance and population growth rates of the two-spotted spider mite <Emphasis Type="Italic">Tetranychus urticae</Emphasis>

Most terrestrial plants live in symbiosis with arbuscular mycorrhizal (AM) fungi. Studies on the direct interaction between plants and mycorrhizal fungi are numerous whereas studies on the indirect interaction between such fungi and herbivores feeding on aboveground plant parts are scarce. We studied the impact of AM symbiosis on host plant choice and life history of an acarine surface piercing-sucking herbivore, the polyphagous two-spotted spider mite Tetranychus urticae. Experiments were performed on detached leaflets taken from common bean plants (Phaseolus vulgaris) colonized or not colonized by the AM fungus Glomus mosseae. T. urticae females were subjected to choice tests between leaves from mycorrhizal and non-mycorrhizal plants. Juvenile survival and development, adult female survival, oviposition rate and offspring sex ratio were measured in order to estimate the population growth parameters of T. urticae on either substrate. Moreover, we analyzed the macro- and micronutrient concentration of the aboveground plant parts. Adult T. urticae females preferentially resided and oviposited on mycorrhizal versus non-mycorrhizal leaflets. AM symbiosis significantly decreased embryonic development time and increased the overall oviposition rate as well as the proportion of female offspring produced during peak oviposition. Altogether, the improved life history parameters resulted in significant changes in net reproductive rate, intrinsic rate of increase, doubling time and finite rate of increase. Aboveground parts of colonized plants showed higher concentrations of P and K whereas Mn and Zn were both found at lower levels. This is the first study documenting the effect of AM symbiosis on the population growth rates of a herbivore, tracking the changes in life history characteristics throughout the life cycle. We discuss the AM-plant-herbivore interaction in relation to plant quality, herbivore feeding type and site and the evolutionary implications in a multi-trophic context.     

Evaluation of the predatory mite <Emphasis Type="Italic">Phytoseiulus macropilis</Emphasis> (Acari: Phytoseiidae) as a biological control agent of the two-spotted spider mite on strawberry plants under greenhouse conditions

The predatory mite Phytoseiulus macropilis is a potential biological control agent of the two-spotted spider mite (TSSM) Tetranychus urticae on strawberry plants. Its ability to control TSSM was recently assessed under laboratory conditions, but its ability to locate and control TSSM under greenhouse conditions has not been tested so far. We evaluated whether P. macropilis is able to control TSSM on strawberry plants and to locate strawberry plants infested with TSSM under greenhouse conditions. Additionally, we tested, in an olfactometer, whether odours play a role in prey-finding by P. macropilis. The predatory mite P. macropilis required about 20 days to achive reduction of the TSSM population on strawberry plants initially infested with 100 TSSM females per plant. TSSM-infested plants attract an average of 27.5 ± 1.0% of the predators recaptured per plant and uninfested plants attracted only 5.8 ± 1.0% per plant. The predatory mites were able to suppress TSSM populations on a single strawberry plant and were able to use odours from TSSM-infested strawberry plants to locate prey in both olfactometer and arena experiments. Hence, it is concluded that P. macropilis can locate and reduce TSSM population on strawberry plants under greenhouse conditions.     

Innate preference hierarchies coupled with adult experience,rather than larval imprinting or transgenerational acclimation,determine host plant use in Pieris rapae

The evolution of host range drives diversification in phytophagous insects, and understanding the female oviposition choices is pivotal for understanding host specialization. One controversial mechanism for female host choice is Hopkins’ host selection principle, where females are predicted to increase their preference for the host species they were feeding upon as larvae. A recent hypothesis posits that such larval imprinting is especially adaptive in combination with anticipatory transgenerational acclimation, so that females both allocate and adapt their offspring to their future host. We study the butterfly Pieris rapae, for which previous evidence suggests that females prefer to oviposit on host individuals of similar nitrogen content as the plant they were feeding upon as larvae, and where the offspring show higher performance on the mother's host type. We test the hypothesis that larval experience and anticipatory transgenerational effects influence female host plant acceptance (no‐choice) and preference (choice) of two host plant species (Barbarea vulgaris and Berteroa incana) of varying nitrogen content. We then test the offspring performance on these hosts. We found no evidence of larval imprinting affecting female decision‐making during oviposition, but that an adult female experience of egg laying in no‐choice trials on the less‐preferred host Be. incana slightly increased the P. rapae propensity to oviposit on Be. incana in subsequent choice trials. We found no transgenerational effects on female host acceptance or preference, but negative transgenerational effects on larval performance, because the offspring of P. rapae females that had developed on Be. incana as larvae grew slower on both hosts, and especially on Be. incana. Our results suggest that among host species, preferences are guided by hard‐wired preference hierarchies linked to species‐specific host traits and less affected by larval experience or transgenerational effects, which may be more important for females evaluating different host individuals of the same species.     

Development of a positive preference–performance relationship in an oligophagous beetle: adaptive learning?

The relationship between oviposition preference and larval performance is a central topic in insect–plant biology. In this study, we investigate whether the oligophagous flea beetle, Altica fragariae Nakane (Coleoptera: Chrysomelidae), exhibits a positive preference–performance relationship, and whether oviposition preference develops over time. We tested the beetles using four sympatric plant species: Duchesnea indica (Andrews) Focke (the normal host plant), Agrimonia pilosa Ledeb. (a secondary host plant), and Potentilla chinensis Ser. and Sanguisorba officinalis L. (host plants of two related Altica species) (all Rosaceae). In no‐choice experiments, both oviposition rate and offspring fitness parameters (eclosion rate, development time, and body mass) were highest on D. indica. Oviposition rate was much lower on P. chinensis than on A. pilosa, whereas offspring fitness parameters did not differ significantly between these two host plants. Offspring fitness were lowest for S. officinalis, and adult females refused to oviposit on this acceptable non‐host in a no‐choice situation. Repeated two‐choice experiments showed that the proportion of oviposition on one of the novel host plants decreased significantly over time when the alternative host plant was D. indica. In repeated two‐choice experiments using A. pilosa and P. chinensis, females mainly fed on A. pilosa but distributed their eggs equally over the two host plants, in accordance with the lack of difference in offspring fitness on those hosts. Together, these results showed that A. fragariae females develop a positive preference–performance relationship over time. We suggest that A. fragariae achieves this through adaptive learning of oviposition preference: not only does the female learn to discriminate among the host plants when there is a fitness difference for her offspring, but the female also fails to discriminate when there is no fitness difference.     

Host plant choice in the comma butterfly-larval choosiness may ameliorate effects of indiscriminate oviposition

In most phytophagous insects, the larval diet strongly affects future fitness and in species that do not feed on plant parts as adults, larval diet is the main source of nitrogen. In many of these insect-host plant systems, the immature larvae are considered to be fully dependent on the choice of the mothers, who, in turn, possess a highly developed host recognition system. This circumstance allows for a potential mother-offspring conflict, resulting in the female maximizing her fecundity at the expense of larval performance on suboptimal hosts. In two experiments, we aimed to investigate this relationship in the polyphagous comma butterfly, Polygonia c-album, by comparing the relative acceptance of low- and medium-ranked hosts between females and neonate larvae both within individuals between life stages, and between mothers and their offspring. The study shows a variation between females in oviposition acceptance of low-ranked hosts, and that the degree of acceptance in the mothers correlates with the probability of acceptance of the same host in the larvae. We also found a negative relationship between stages within individuals as there was a higher acceptance of lower ranked hosts in females who had abandoned said host as a larva. Notably, however, neonate larvae of the comma butterfly did not unconditionally accept to feed from the least favorable host species even when it was the only food source. Our results suggest the possibility that the disadvantages associated with a generalist oviposition strategy can be decreased by larval participation in host plant choice.     

Increasing host age does not have the expected negative effects on the fitness parameters of an egg parasitoid

Host age is an important determinant of host acceptance and suitability for egg parasitoids. As host embryonic development advances, the quality of resources available to the parasitoid offspring typically declines, usually resulting in reduced acceptance levels by foraging females and lower offspring fitness. We examined the ability of the parasitoid Telenomus podisi Ashmead (Hymenoptera: Scelionidae) to parasitize and develop in Podisus maculiventris (Say) (Hemiptera: Pentatomidae) eggs of different ages. In laboratory experiments, we measured the effect of host age (6, 24, 48, 72, 96, or 120 h old) on parasitism rate and offspring fitness parameters such as survival, development time, sex ratio, and size. Contrary to our expectations, parasitism rate did not differ between host age treatments, nor did sex ratio allocation, offspring size, or the fecundity of newly emerged female offspring. However, parasitoid offspring had a longer development time with increasing host age. This trend was stronger for males than for females, which we suggest could reduce the degree of protandry among offspring emerging from older host eggs, thus increasing the rate of virginity upon leaving the emergence patch and resulting in more frequent off‐patch mating by female offspring in nature. Overall, our results suggest that all stages of P. maculiventris embryonic development are suitable for acceptance and development of T. podisi. Unlike most species of egg parasitoids, T. podisi has evolved mechanisms to utilize host resources, regardless of host developmental stage, with relatively minor fitness consequences.     

Host plant utilization in the comma butterfly: sources of variation and evolutionary implications

A major challenge in the study of insect-host plant interactions is to understand how the different aspects of offspring performance interact to produce a preference hierarchy in the ovipositing females. In this paper we investigate host plant preference of the polyphagous butterfly Polygonia c-album (Lepidoptera: Nymphalidae) and compare it with several aspects of the life history of its offspring (growth rate, development time, adult size, survival and female fecundity). Females and offspring were tested on four naturally used host plants (Urtica dioica, Ulmus glabra, Salix caprea, and Betula pubescens). There was substantial individual variation in host plant preference, including reversals in rank order, but the differences were largely confined to differences in the ranking of Urtica dioica and S. caprea. Different aspects of performance on these two plants gave conflicting and complementary results, implying a trade-off between short development time on U. dioica, and larger size and higher fecundity on S. caprea. As all performance components showed low individual variation the large variation in host plant preference was interpreted as due to alternative oviposition strategies on the basis of similar performance hierarchies. This indicates that the larval performance component of host-plant utilization may be more conservative to evolutionary change than the preference of ovipositing females. Possible macro-evolutionary implications of this are discussed.     

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.     

Responses of ovipositing moths to host plant deprivation: life history aspects and implications for intercropping

1. When considering intercropping as a strategy to reduce pest oviposition, knowledge about the insect’s oviposition behaviour is very important. Physiological effects on the insect because of difficulties in finding a suitable oviposition site may also be important. 2. In the present study, the effects that delays in access to host plants have on lifetime fecundity on diamondback moth and leek moth were examined. The ability to postpone egg laying, fecundity and lifetime oviposition are discussed in relation to intercrop/cover crop as a strategy to reduce oviposition on crop plants. 3. When faced with host plant deprivation, the diamondback moth is relatively more dependent upon host plant stimuli for the onset of egg production. By contrast, leek moth is able to postpone egg production for a longer time. There even appeared to be a tendency for leek moth females to extend their lifetime when faced with host plant deprivation. 4. We conclude that leek moths have the ability to postpone production of eggs and lay them later in life when finally encountering host plants after a period of host plant deprivation. Therefore, the use of intercropping as a strategy to reduce oviposition is questionable. For such an insect, use of a trap crop might be a better option because the female will lay her eggs in the trap crop and not get the opportunity to lay them later in life when finally encountering crop plants.     

Consequences of mite feeding injury to beans on the fecundity and survivorship of the two-spotted spider mite (Acari: Tetranychidae)

Fecundity and survival of the two-spotted spider mite,Tetranychus urticae Koch, were examined on bean (Phaseolus vulgaris L.) plants that had been subjected to mite feeding injury in the laboratory. Different numbers ofT. urticae were restricted on the first two leaves of young bean plants, and spider-mite fecundity and survivorship was assayed on the third leaf. Each plant received four recently enclosed females, one female from each of four mite lineages. Using changes in the ratio of root mass to shoot mass of bean plants as a continuous measure of plant stress from spider-mite feeding, fecundity was positively related to stress for three out of four experiments. In two out of four experiments, survival of females was also positively related to stress, but reached an asymptote at slight or moderate stress levels. No evidence for induced resistance in beans was found. Mite lineage and the interaction between lineage and stress affected female survival but not fecundity. The implications of these results for understanding spider-mite outbreaks are discussed.     

Predator avoidance in phytophagous mites: response to present danger depends on alternative host quality

We studied whether volatiles released by putative host plants affect the antipredator response of an herbivorous mite, Tetranychus urticae, when the patch was invaded by Phytoseiulus persimilis. Tetranychus urticae laid a lower number of eggs on tomato leaves than on lima bean leaves, suggesting that lima bean is a preferred host food source for T. urticae. In addition, T. urticae preferred lima bean plant volatiles to tomato plant volatiles in a Y-tube olfactometer test. To investigate the antipredator response of T. urticae, we examined the migration of T. urticae from a lima bean leaf disc to a neighbouring leaf disc (either a tomato or lima bean leaf disc) when ten predators were introduced into the original lima bean disc. A Parafilm bridge allowed for migration between the leaf discs. No migrations occurred between leaf discs when there were no predators introduced to the original leaf disc. However, when predators were introduced migrations did occur. When the neighbouring leaf disc was upwind of the original disc, the migration rate of the mite from original lima bean leaf disc to a neighbouring tomato leaf disc was significantly lower than that to a neighbouring lima bean leaf disc. By contrast, when the neighbouring leaf disc was downwind of the original leaf disc, there was no difference in the migration rates between lima bean leaf discs and tomato leaf discs. The number of T. urticae killed by P. persimilis for each treatment was not different, and this clearly shows that the danger was the same in all treatments regardless of the decision made by T. urticae. From these results, we conclude that T. urticae change their antipredator response by evaluating the difference in host plant volatiles in the patch they inhabit.     

Host plant acceptance by the phytophagous mite Tetranychus kanzawai Kishida is affected by the availability of a refuge on the leaf surface

We used 11 wild plants to experimentally test the effects of leaf trichomes and leaf quality on host plant acceptance by a phytophagous spider mite, Tetranychus kanzawai, in the absence of predators. There was a positive correlation between leaf hair traits (height/density) and host plant acceptance. We used two model plants (Phaseolus vulgaris and Phaseolus lunatus; the former had higher and denser leaf hairs than the latter) to examine the effects of leaf hairs on the dispersal and fecundity of T. kanzawai in the presence of a predatory mite, Neoseiulus womersleyi. In the presence of N. womersleyi, significantly fewer T. kanzawai females dispersed from a P. vulgaris leaf than from a P. lunatus leaf. Moreover, in the presence of the predator the fecundity of T. kanzawai females on P. lunatus was significantly lower than on P. vulgaris, although in terms of host quality the two plants were equivalent. In the presence of N. womersleyi, T. kanzawai females on P. vulgaris spent more time on webs than those on P. lunatus. Moreover, webs seemed to be less accessible than leaf surfaces to the predator. These results suggest that leaf hairs provide a refuge for T. kanzawai adult females.     

Do assortative mating and immigrant inviability help maintain population genetic structuring of an herbivore on a crop and a wild relative?

Population genetic structuring is common among herbivorous insects and frequently is associated with divergent host plants, such as crops and their wild relatives. Previous studies showed population genetic structuring in corn leafhopper Dulbulus maidis in Mexico, such that the species consists of two sympatric, host plant-associated populations: an abundant and widespread "pestiferous” population on maize (Zea mays mays), and a small and localized "wild" population on perennial teosinte (Zea diploperennis). a maize wild relative with a limited distribution. This study addressed whether assortative mating and immigrant inviability mediate genetic structuring of corn leafliopper by comparing the mating and reproductive successes of pestiferous and wild females that colonize their nonassociated host plants against the successes of females colonizing their associated host plants. Assortative mating was assessed by comparing mating frequencies and premating and mating times among females of each population on each host plant: immigrant inviability was assessed by comparing, across two generations, the fecundity, survival, development time, sex ratio, and population growth rate among leafhopper populations and host plants. Our results showed that on maize, and compared to resident, pestiferous females, wild females were more likely to mate, and greater proportions of their offspring survived to adult stage and were daughters;consequently, the per-generation population growth rate on maize was greater for immigrant, wild leafhoppers compared to resident, pestiferous leafhoppers. Our results suggested that wild leafhoppers emigrating to maize have a fitness advantage over resident, pestiferous leafhoppers, while immigrant pestiferous and resident wild leafhoppers on teosinte have similar fitnesses.     

Aphid behavioral responses to virus‐infected plants are similar despite divergent fitness effects

We compared the settling preferences and reproductive potential of an oligophagous herbivore, the pea aphid, Acyrthosiphon pisum Harris (Hemiptera: Aphididae), in response to pea plants, Pisum sativum L. cv. ‘Aragorn’ (Fabaceae), infected with two persistently transmitted viruses, Pea enation mosaic virus (PEMV) and Bean leaf roll virus (BLRV), that differ in their distribution within an infected plant. Aphids preferentially oriented toward and settled on plants infected with PEMV or BLRV in comparison with sham‐inoculated plants (plants exposed to herbivory by uninfected aphids), but aphids did not discriminate between plants infected with the two viruses. Analysis of plant volatiles indicated that plants inoculated with either virus had significantly higher green leaf volatile‐to‐monoterpene ratios. Time until reproductive maturity was marginally influenced by plant infection status, with a trend toward earlier nymph production on infected plants. There were consistent age‐specific effects of plant infection status on aphid fecundity: reproduction was significantly enhanced for aphids on BLRV‐infected plants across most time intervals, though mean aphid fecundity did not differ between sham and PEMV‐infected plants. There was no clear pattern of age‐specific survivorship; however, mean aphid lifespan was reduced on plants infected with PEMV. Our results are consistent with predictions of the host manipulation hypothesis, extended to include plant viruses: non‐viruliferous A. pisum preferentially orient to virus‐infected host plants, potentially facilitating pathogen transmission. These studies extend the scope of the host manipulation hypothesis by demonstrating that divergent fitness effects on vectors arise relative to the mode of virus transmission.     

Larval food affects oviposition preference,female fecundity and offspring survival in Yponomeuta evonymellus

1. Yponomeuta evonymellus is a monophagous moth that feeds on Prunus padus which is native to Europe. In recent years, larval feeding and egg clusters have also been observed on non‐native Prunus serotina plants; however, survival of larvae on this new host is very low. 2. The objective of the present study was to determine how the feeding of larvae on each of the two host plants impacts oviposition, offspring survival and fecundity in Y. evonymellus. Our hypothesis was that, under controlled conditions, females will lay eggs on the host on which they fed as larvae. We also hypothesised that the lower survival of young larvae feeding on P. serotina was due to the smaller buds and leaves present in this species, relative to those of P. padus. 3. A dual‐choice experiment conducted under laboratory conditions demonstrated that females preferentially chose to oviposit on the plant species on which they fed as larvae. In the experiment, potential fecundity and offspring survival were significantly higher on P. padus than on P. serotina. The reduced performance of Y. evonymellus on P. serotina was correlated with a smaller bud mass and volume, lower leaf mass and surface area, and difficulty in constructing a protective tent against unfavourable weather conditions. 4. In summary, the identity of the host plant species during larval feeding determines adult oviposition preference for that host species. The survival of larvae on P. serotina growing in the nature is low, but for phenology‐related reasons.     

Effects of undersown clover on host-plant selection by Thrips tabaci adults in leek

The suppressive effects of undersown clover on Thrips tabaci Lindeman infestation in leek is known but not the stages in thrips population dynamics that are affected by intercropping and the mechanisms involved. Colonization or settling of adult onion thrips (T. tabaci) in monocropped leek (Allium porrum L.) and in leek intercropped with strawberry clover (Trifolium fragiferum L.) was studied in two potted plant experiments. Potted leek plants with and without undersown clover were placed for a short period (2 and 5 days) in monocropped or intercropped field plots when high thrips infestation was expected. Thrips adults were counted on all leaves and in the shaft during this observation period. Thrips populations were monitored weekly in the field plots throughout the entire growing season.In both potted plant experiments, there were consistently fewer thrips adults on intercropped leek plants than on monocropped plants. The plant growth parameters – number of leaves and stem diameter – were similar in all treatments and thus did not explain differences in thrips adults. Natural enemies were absent on both leek and clover, and thus cannot explain the differences in adult thrips numbers. Furthermore, no adults of T. tabaci were found on the clover in the potted plant experiment and only very few in the field experiment. Thus it was concluded that strawberry clover was not an effective trap crop for thrips.In the second potted plant experiment, clover was removed just before the leek plants were introduced to the field, thus eliminating direct physical, visual and olfactory interference by clover. After the undersown clover was removed, the leek plants harboured only one-third of the number of thrips adults, as compared to the monocropped plants. This study supports the notion that there are subtle links between intercropping and plant quality, and indicates that host-plant quality is an underlying cause of the reduction in adult thrips numbers in leek/clover intercropping.     

Interspecific competition counteracts negative effects of dispersal on adaptation of an arthropod herbivore to a new host

Dispersal and competition have both been suggested to drive variation in adaptability to a new environment, either positively or negatively. A simultaneous experimental test of both mechanisms is however lacking. Here, we experimentally investigate how population dynamics and local adaptation to a new host plant in a model species, the two‐spotted spider mite (Tetranychus urticae), are affected by dispersal from a stock population (no‐adapted) and competition with an already adapted spider mite species (Tetranychus evansi). For the population dynamics, we find that competition generally reduces population size and increases the risk of population extinction. However, these negative effects are counteracted by dispersal. For local adaptation, the roles of competition and dispersal are reversed. Without competition, dispersal exerts a negative effect on adaptation (measured as fecundity) to a novel host and females receiving the highest number of immigrants performed similarly to the stock population females. By contrast, with competition, adding more immigrants did not result in a lower fecundity. Females from populations with competition receiving the highest number of immigrants had a significantly higher fecundity than females from populations without competition (same dispersal treatment) and than the stock population females. We suggest that by exerting a stronger selection on the adapting populations, competition can counteract the migration load effect of dispersal. Interestingly, adaptation to the new host does not significantly reduce performance on the ancestral host, regardless of dispersal rate or competition. Our results highlight that assessments of how species can adapt to changing conditions need to jointly consider connectivity and the community context.