The role of parasitoids in evolution of habitat and larval food plant preference by three Pieris butterflies

This article attempts to explain that parasitoids provide the evolutionary pressure responsible for relationships between habitat use and larval food plant use in herbivorous insects. Three species of butterflies of the genus Pieris, P. rapae, P. melete, and P. napi use different sets of cruciferous plants. They prefer different habitats composed of similar sets of cruciferous plants. In our study, P. rapae used temporary habitats with ephemeral plants, P. melete used permanent habitat with persistent plants, although they also used temporary habitats, and P. napi used only permanent habitat. The choice experiment in the field cages indicated that each of the three butterfly species avoided oviposition on plants usually unused in its own habitat, but accepted the unused plants which grew outside its own habitat. Their habitat use and plant use were not explained by intrinsic plant quality examined in terms of larval performance. Pieris larvae collected from persistent plants or more long lasting habitats were more heavily parasitized by two specialist parasitoids, the braconid wasp Cotesia glomerata and the tachinid fly Epicampocera succincta. The results suggest that Pieris habitat and larval food plant use patterns can be explained by two principles. The evolution of habitat preference may have been driven by various factors including escape from parasitism. Once habitat preference has evolved, selection favors the evolution of larval food plant preferences by discriminating against unsuitable plants, including those which are associated with high parasitism pressures. Received: December 3, 1998 / Accepted: January 20, 1999     

Comparative population studies of threePieris butterflies,P. rapae,P. melete andP. napi,living in the same area

The adult populations of three Pieris butterflies, P. rapae, P. melete and P. napi, were studied in an area of their coexistence throughout the flight seasons by using the mark-and-recapture method. The study area, about 3×1.5 km, was set up in a farm village surrounded by the mountainous area in Inabu, Aichi Prefecture. The habitats were qualified by the four factors, i. e., oviposition plants, adult nector plants, roosting-sites and light conditions. Between P. rapae and P. napi, there were sharp differences with regards to overall habitat preferences. P. melete had the widest preferences for all the habitat resources, which overlapped greately with requirements of P. rapae and P. napi. P. melete and P. rapae showed similar preferences for oviposition plants, but the former preferred shaded habitats while the latter preferred sunny places. P. melete and P. napi, having similar preferences for shaded situations, showed differences in the preferences for oviposition plants. Moreover, three species of Pieris were different in their preferences for adult nector plants. Thus, they were more likely to partition habitat resources rather than competing for them. The habitat structures of each species in respect of time, space and stability to weather changes were much different each other in the same area. The habitat of P. rapae was temporary, localized and unstable. While, that of P. melete was more permanent, widespread and stable than that of P. rapae. P. napi seemed to live in the intermediate habitat, i. e., permanent, localized and stable one.     

Spatio-temporal distribution patterns of two tachinid flies,Epicampocera succincta andCompsilura concinnata parasitizingPieris

Larvae of genus Pieris in the northern part of Kyoto City are parasitized by two tachinid flies:Epicampocera succincta, a specialist on genus Pieris, and Compsilura concinnata, a generalist with very wide host-range. We surveyed the parasitism rates of Pieris by both flies for two years at six study areas. In these study areas, there lived three host species in the genus Pieris: P. rapae, P. melete, and P. napi, but neither tachinid parasitized P. napi to any significant extent. In the mountainous district, P. rapae and P. melete coexisted and their populations were relatively continuous, while in the lowland, only P. rapae larvae were abundant in spring and autumn, but even they disappeared in summer. Parasitisms by E. succincta occurred mainly in mountainous district and never in the lowland. C. concinnata parasitized Pieris in all the areas, but its parasitisms occurred mainly in autumn. We analyzed the factors affecting the spatial and temporal patterns of parasitism rates and presumed that the temporal discontinuity of host population restricted the distribution of the specialist parasitoid.     

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.     

Cue hierarchy for host plant selection in Pieris rapae

During oviposition, female insects utilize a wide variety of cues to find an optimal host for their offspring. These cues may be encountered simultaneously or sequentially, and females must rely on a hierarchical categorization to rank cues for optimal ovipositional choice. During alighting, cues are often encountered at different distances, which may influence hierarchical order. Cues that are observed at longer distances and are experienced first may be more influential on oviposition choice than those experienced at shorter distances. We tested the effects of two long‐distance cues, plant size and habitat structure, and two short‐distance cues, plant nitrogen level and predator presence, on Pieris rapae L. (Lepidoptera: Pieridae) oviposition choice through a series of two‐ and four‐choice tests in both greenhouse and field settings. We found that the long‐distance cues of plant size and habitat structure had the largest impact on P. rapae oviposition in laboratory and field settings, with females preferring large plants without habitat structure. Plant nitrogen level had a smaller impact on oviposition, and the presence of predators did not affect oviposition choice. However, plant size and plant nitrogen level had a synergistic relationship, with more eggs laid on large high‐nitrogen plants compared to large low‐nitrogen or small high‐nitrogen plants, suggesting that optimal cues throughout the entire searching process may be important for P. rapae in choosing a host plant.     

The use of oviposition‐induced plant cues by Trichogramma egg parasitoids

1. Female parasitoids have evolved various foraging strategies in order to find suitable hosts. Egg parasitoids have been shown to exploit plant cues induced by the deposition of host eggs. 2. The tiny wasp Trichogramma brassicae uses oviposition‐induced cues from Brussels sprouts to locate eggs of the cabbage white butterflies Pieris brassicae and Pieris rapae that differ in their egg‐laying behaviour. These plant cues are elicited by male‐derived anti‐aphrodisiac pheromones in the accessory reproductive gland (ARG) secretions of mated female butterflies. However, the closely related generalist species Trichogramma evanescens does not respond to Brussels sprout cues induced by the deposition of P. brassicae egg clutches. 3. Here we showed in two‐choice bioassays that T. evanescens wasps respond to Brussels sprout cues induced by (i) the deposition of single eggs by P. rapae, and (ii) the application of ARG secretions from either mated P. rapae females, or from virgin female butterflies in combination with P. rapae's anti‐aphrodisiac compound indole. The wasps only associatively learned to respond to Brussels sprout cues after applying indole alone by linking those cues with the presence of P. rapae eggs. 4. Our results indicate that Trichogramma wasps more commonly exploit oviposition‐induced plant cues to locate their host eggs. Generalist wasps show less specificity in their response than specialists and employ associative learning.     

Comparative population studies of threePieris butterflies,P. rapae,P. melete andP. napi,living in the same area

Utilization of patchy habitats by adult populations of three Pieris butterflies, P. rapae, P. melete and P. napi was studied throughout the flight season in an area of their coexistence, about 3×1.5 km, in a farm village in the mountains in Inabu, Aichi Prefecture. Field study was by the mark-recapture method. Results were analyzed by dispersal distances and recapture duration decay curves for adults of different age-classes estimated on the basis of physical condition of their wings, together with supplementary information of daliy egg-laying rate of females, obtained in field cages. Sexually immature, mated femals of P. rapae after teneral stage showed a migratory flight. On the other hand, reproductive females and all males of P. rapae were strongly resident within suitable habitats, and reproductive females begun to lay eggs abundantly at sunny places of newly suitable areas within a short period. P. melete seemed to disperse gradually from emerged stites and females of this species continued to lay some constant numbers of eggs for more than ten days over a wider area. P. napi appeared more like P. melete than P. rapae. The habitats of the three species can be characterized as follows:P. rapae, temporary, continued for pre-reproductive females but localized for reproductive females and all males, and unstable;P. melete, permanent, widespread, and stable;P. napi, permanent, localized, and stable. The numbers of generations of P. rapae, P. melete and P. napi were estimated to be about six, three and three, respectively. Seasonal fluctuations in the number of adults were influenced by the stability of their habitats, i. e., the population size fluctuated sharply in P. rapae, but it was much more stable in P. melete and P. napi. In view of these results, it can be said that P. rapae fits the general characteristics of a r-strategist whereas P. melete and P. napi are more K-strategic than P. rapae.     

A comparative study of the egg-laying behaviour and larval development of Pieris rapae L. and P. brassicae L. on the same host plants

Summary Pieris rapae and P. brassicae feed on the same host plants and have synchronized seasons. P. brassicae, whose larvae are twice the size of P. rapae, lays eggs in clusters of 40–100 eggs whereas P. rapae lays single eggs. In this paper we examine how egg clustering may be advantageous for P. brassicae. The larval development of each species was studied, and found not to differ significantly. P. brassicae larvae were observed to migrate from their host plant after defoliating it. A comparison of the efficiency of host plant utilization by the two pierid species was undertaken by measuring the effect of larval feeding on the growth of their host plants (kale and brussel sprouts). The results show that egg clustering is advantageous for larval fitness in terms of host resource exploitation, and we suggest that P. brassicae is adapted for ovipositing on clumped vegetation, while P. rapae is selected for exploiting isolated plants.     

Differential selection of host plants by twoPieris species: the role of oviposition stimulants and deterrents

Oviposition responses ofPieris rapae L. andP. napi oleracea Harris to nine crucifers, one Capparidaceae and one Tropaeolaceae were directly compared under controlled conditions. Chemical fractions from these plants were also tested on both insects for the presence of oviposition stimulants or deterrents. The results showed that plant chemistry is a key factor in differential selection of potential hosts by thesePieris species. Some plant species were equally acceptable to bothPieris species. However,P. rapae preferred cabbage over most test plants whereasP. napi oleracea strongly preferred plant species that were avoided byP. rapae. The observed preferences were explained in most cases by the presence of stimulants and deterrents in extracts of the plants. The twoPieris species have apparently evolved differential sensitivities to the chemical stimuli that trigger or deter oviposition. The balance of positively and negatively interpreted sensory signals evoked by plant chemicals obviously plays an important role in acceptance or rejection of a plant by both species. The role of specific glucosinolates and differing structure-activity relationships is suggested.     

Does the Small White Butterfly (Pieris rapae L.) Aggregate Eggs on Plants with Greater Gas Exchange Activity?

Few studies have investigated insect egg-laying preferences in relation to photosynthesis or transpiration of their host plants. It has been suggested that intravarietal preferences of the small white butterfly (Pieris rapae L.: Pieridae) include larger plants with characteristically higher transpiration rates. Interestingly this species, like many other Lepidoptera, may detect biogenic CO₂ gradients associated with photosynthesis. We studied egg-laying preferences in working farm environments examining relationships among host choice, plant gas exchange activity, and plant size. Females discriminated between plants in monocultures on the basis of height. A balance of pre- and post–alighting preferences resulted in plants of medium size receiving eggs. Post–alighting preferences led to plants, but not alighted leaves, with higher rates of photosynthesis supporting eggs. These findings do not support a mechanistic basis for the use of gas exchange activity during host selection but, for the first time, indicate the greater physiological activity of crop plants that ultimately received the eggs of a pest insect.     

The role of host species, age and defensive behaviour on ovipositional decisions in a solitary specialist and gregarious generalist parasitoid (Cotesia species)

The main objective of this study was to determine the extent to which host acceptance behaviour as related to host species, age, and defensive behaviour might explain the differences in host use that exist between two congeneric and sympatric species of parasitic wasps. Cotesia glomerata (L.) (Hymenoptera: Braconidae) is gregarious and generalist on several species of Pieridae, whereas C. rubecula (Marshall) is solitary and specific to Pieris rapae (L.). Cotesia species differed in their responses to host species (P. brassicae (L.), P. napi (L.) and P. rapae) and developmental stage (early and late 1st, 2nd and 3rd instars). In no-choice tests, host acceptance by C. rubecula was higher for p. rapae and females did not distinguish among the 6 host ages. In contrast, when foraging for P. brassicae and P. napi, C. rubecula females more readily attacked early first instar. Cotesia glomerata showed a higher degree of behavioural plasticity towards acceptance of Pieris host species and host age than did C. rubecula. Cotesia glomerata females parasitized the three Pieris species and showed higher acceptance of first and second instars over third instar. Oviposition success was also influenced by host defensive behaviour. The frequency and the effectiveness of defensive behaviour rose with increasing age of the host, P. brassicae being the most aggressive Pieris species. Furthermore, the mean duration of C. glomerata oviposition was significantly reduced by the defensive reactions of P. brassicae, which would likely affect parasitoid fitness as oviposition time is positively correlated to clutch size in C. glomerata. Acceptance frequencies corresponded well to field reports of Pieris-Cotesia associations and to patterns of parasitoid larval performance, suggesting that the acceptance phase might be used as a reliable indicator of Cotesia host-specificity.     

Hypersensitivity reaction of Brassica nigra L. (Cruciferae) kills eggs of Pieris butterflies (Lepidoptera: Pieridae)

Summary Some individual plants of the mustard Brassica nigra in lowland California kill eggs of the Crucifer-specialist herbivores Pieris rapae and P. napi by producing a necrotic zone at the base of the egg, thereby apparently desicating it. This is a typical hypersensitivity reaction, but to an atypical stimulus. The eggs can be rescued by maintaining them in a saturated atmosphere. Attempts to demonstrate a bacterial or fungal agent associated with the reaction were unsuccessful.     

Support for mending the matrix: resource seeking by butterflies in apparent non-resource zones

In conserving organisms, a bipolar view has generally been adopted of landscapes, in which resources are allocated to patches of habitats and the matrix ignored. Allocating additional resources to the matrix would depend on two conditions: first, that organisms search for resources in landscapes regardless of differences in vegetation types and resource availability; second, that when resources occur in the matrix they are used by species. Behavioural data linked to biotope and substrate types, on three pierid species (Pieris brassicae, P. rapae, P. napi), have been examined to explore the relationship between flight modes and resource availability. Search flight and resource use, taken as measures of resource quests, occur extensively in all biotopes and over all substrates in addition to direct linear flight, more typical of butterflies when migrating between habitat units. Resource seeking and using even exceed direct flight in some biotopes that have been regarded generally as lacking in resources for these butterflies. For example, such is the case for P. brassicae in woodland and for P. rapae in scrub. This finding supports the view for repairing the matrix and enhancing the general countryside outside habitat patches with resources increasingly being made for organisms. The proviso is that species’ requirements are adequately researched and resourced so as not to become sinks.     

Coexistence and niche segregation by field populations of the parasitoids Cotesia glomerata and C. rubecula in the Netherlands: predicting field performance from laboratory data

Field experiments with foraging parasitoids are essential to validate the conclusions from laboratory studies and to interpret differences in searching and host selection behaviour of parasitoid species. Furthermore, field experiments can indicate whether the parameters measured in the laboratory are relevant to elucidation of the ecological processes under study, such as adaptation or species interactions. In previous extensive laboratory studies we studied plant- and host-searching behaviour, host acceptance, host suitability; host plant preference, and learning of two congeneric parasitoids of Pieris caterpillars: the generalist Cotesia glomerata, which has been reported to attack several Pieridae species, and C. rubecula, a specialist of the small cabbage white Pieris rapae. In the present field study our aim was to verify the importance of these previous laboratory findings for explaining the performance of these two species in the field. We investigated experimentally whether parasitism on three Pieris species varied with parasitoid species and with food plant of the caterpillars. We exposed different types of host plants, infested with different Pieris species, to parasitism by natural populations of Cotesia species, by setting the experimental plants out in Brussels sprouts cabbage fields. Furthermore we made direct observations of parasitoid foraging in the field. In general, the field results confirmed our predictions on the range of host plant and host species used in the field. The two Cotesia species appear to coexist through niche segregation, since C. glomerata was mainly recovered from P. brassicae and C. rubecula from P. rapae. Although C. glomerata is a generalist at the species level, it can be a specialist at the population level under certain ecological circumstances. Our study shows the importance of variation in host plant attraction and host species acceptance in restricting host plant and host diet in the field. Furthermore the results suggest that, at least in the Netherlands, specialisation of C. glomerata on P. brassicae may occur as a result of C. rubecula outcompeting C. glomerata in P. rapae larvae. Received: 8 July 1999 / Accepted: 31 January 2000     

Interspecific egg load assessment of host plants by Pieris rapae butterflies

Egglaying responses of Pieris rapae L. butterflies to the oviposition deterring pheromone (ODP) of Pieris brassicae L. were studied in the laboratory. Choice experiments with ODP treated leaves and control leaves revealed that females perform a strong preference to lay their eggs on the control leaves. This preference is maintained even when during the experiment the control leaf becomes covered with a large number of conspecific eggs. Choice experiments with cabbage leaves with and without P. rapae eggs seem to indicate the absence of intraspecific egg load assessment of host plants in P. rapae. The deterrent effect of the ODP of P. brassicae to P. rapae females persists for at least 8 days. Behavioural observations suggest olfactory hairs as well as gustatory hairs to be involved in the perception of the ODP but electrophysiological recordings of the various chemoreceptors are necessary to confirm this. Finally the prospects of application of this pheromone/kairomone in cabbage pest control are discussed.

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Appréciation de la charge interspécifique en oeufs sur la plante hôte par Pieris rapae

Résumé La réponse au laboratoire de P. rapae à la phéromone dissuadant la ponte (ODP) de P. brassicae a été étudiée par l'oviposition. Des expériences de choix entre des feuilles traitées à l'ODP et des témoins ont montré que les femelles préfèrent nettement les feuilles témoins. Cette préférence s'est maintenue même quand les feuilles témoins ont été recouvertes d'un grand nombre d'oeufs de P. rapae. Ceci peut indiquer l'absence chez P. rapae d'une évaluation de la charge de ses propres oeufs. L'effet dissuadant du ODP de P. brassicae sur les femelles de P. rapae persiste au moins 8 jours. Les observations comportementales suggèrent que des poils olfactifs aussi bien que des poils gustatifs sont impliqués dans la perception d'ODP mais une confirmation de cette hypothèse par enregistrements électrophysiologiques est nécessaire. Les perspectives d'utilisation de cette phéromone/kairomone dans la lutte contre les insectes du chou sont examinées.

     

The effect of host acceptability on oviposition and egg accumulation by the small white butterfly, Pieris rapae

The influence of host‐plant acceptability on oviposition rate, egg load, internal fat storage and longevity was studied in the small white butterfly, Pieris rapae L. (Lepidoptera: Pieridae). Newly emerged females and males were presented with either cabbage (Brassica oleracea L. var. gemmifera), which is a highly acceptable host for the small white, or wallflower (Cheiranthus cheiri L.), which is much less acceptable for oviposition. Individuals were dissected when 3, 6 and 9‐days‐old. Females kept on cabbage contained fewer mature eggs than those kept on wallflower and there was no change in the load of mature eggs over the time course studied. The number of immature eggs carried did not vary with host plant species, but did fall significantly between days 6 and 9. The body fat content of individuals declined with increasing age, but the decline was slower for individuals kept on the host of low acceptability. Individuals that were allowed to spend their natural lifespan on cabbage showed similar oviposition patterns over time, where the oviposition rate started high and gradually reduced until death. However, considerable differences in oviposition pattern were found in individuals kept on wallflower, varying from that found on cabbage to no oviposition at all. The implications of these findings are discussed in the light of existing oviposition theories. This leads to the conclusion that in one species the premises of existing theories on optimization of oviposition are not mutually exclusive but rather play in concert.     

Plastic changes in head size during juvenile development of the butterfly <Emphasis Type="Italic">Pieris napi</Emphasis>

Among herbivorous insects, the ability to change adaptive traits plastically in response to novel host plants is advantageous for coping with sudden environmental shifts. The host plants in our study were two closely related species, viz., Arabis flagellosa (tougher leaves, i.e., they are physically defended) and A. gemmifera (softer leaves that are physiologically defended). We demonstrated that young larvae of the butterfly Pieris napi are able to plastically change head size during development in response to changes in food plant species. When larvae were fed the physically defended A. flagellosa, the head sizes of third instar larvae emerging from eggs originally collected from leaves of both A. flagellosa and A. gemmifera became identically larger. When larvae fed on the physiologically defended A. gemmifera, the head sizes of third instar larvae emerging from eggs originally collected from leaves of both A. flagellosa and A. gemmifera became identically smaller. When leaves of A. flagellosa were presented to fourth instars reared on A. flagellosa, larvae with larger heads consumed more food than those with smaller heads. In contrast, when leaves of the physiologically defended A. gemmifera were presented to fourth instar larvae reared on A. gemmifera, larvae with smaller heads processed more food than those with larger heads. Hence, larvae of P. napi retain the capacity for adaptive plastic responses to novel host plant species.     

Butterflies and plants: preference/performance studies in relation to plant size and the use of intact plants vs. cuttings

Plants have evolved a number of defences to ameliorate herbivore attacks including chemicals induced by mechanical wounding. Such changes in plant chemical composition are potential confounding factors in experiments on plant – insect interactions, which often present cuttings of potential host plants to phytophagous insects. In particular, this could affect studies of female egg‐laying preference and larval performance, because the same plant chemicals that deter certain generalist insects can elevate attacks from more specialized insects. Furthermore, plant cuttings are by definition smaller than intact plants, and any female host size preference could thus affect experiments using plant cuttings. We first assessed female preference and larval performance of a specialist herbivore, Pieris napi (L.) (Lepidoptera: Pieridae, Pierini), confronted with either intact plants or leaf‐cuttings of four Brassicaceae host plants, Alliaria petiolata (Bieb.) Cavara & Grande, Barbarea vulgaris (L.) WT Aiton, Berteroa incana (L.) DC., and Brassica napus (L.). Egg and larval survival did not differ between intact plants and leaf‐cuttings, whereas larval growth was slightly, but significantly, faster on leaf‐cuttings. Females, however, significantly preferred to lay eggs on intact plants of all four hosts, although the preference hierarchy for the intact plants was largely mirrored by that for leaf‐cuttings. We then tested the female preference for different size‐classes of intact B. napus plants. Small individuals received more eggs than larger individuals, and follow‐up experiments showed that this difference was largely generated by a strong female preference for cotyledon leaves; there was no significant difference in female preference for large and small individuals when both carried cotyledons, and females landing on cotyledons were more likely to oviposit compared to when landing on a true leaf. Our study concludes that plant cuttings can serve as adequate proxies for live plants for preference/performance studies, but that experimentalists should be aware of the variation imposed both by plant handling and plant phenology for female oviposition preference.     

Development and evaluation of an enzyme-linked immunosorbent assay to detect Pieris rapae remains in guts of arthropod predators

An enzyme-linked immunosorbent assay (ELISA) was developed to detect remains of Pieris rapae L. (Lepidoptera: Pieridae) immature stages in the guts of field collected arthropod predators. The assay can be used to help ascertain the relative importance of arthropod predator species in suppressing P. rapae in cabbage, Brassica oleracea var. capitata L. The ELISA is sensitive to all immature stages of P. rapae, although first and fifth instars can be detected more readily than eggs or pupae and third instars showed intermediate detectability. Assays on whole body homogenates of predators readily detected predation on P. rapae first instars by all seven of the predator species tested, although response generally declined with increasing predator size. Together the results show that the P. rapae ELISA possesses a sufficiently high level of sensitivity and specificity to be a useful tool in helping to elucidate the roles of arthropod predator species in reducing populations of P. rapae in cabbage.