Relationships between parasitoid host range and host defence: a comparative study of egg encapsulation in two related parasitoid species

Abstract. Parasitoid host range may proceed from traits affecting host suitability, traits affecting parasitoid foraging behaviour, or both. We tested the hypothesis that encapsulation can be used as a reliable indicator of parasitoid host range in two closely related larval endoparasitoids of Lepidoptera. Cotesia glomerata (L.) (Hymenoptera: Braconidae) is gregarious and a generalist on several species of Pieridae, whereas C. rubecula (Marshall) is solitary and specific to Pieris rapae (L.). We determined the effects of host species ( Pieris brassicae (L.), P. napi (L.) and P. rapae ) (Lepidoptera: Pieridae) and host developmental stage (early first, second and third instar) on encapsulation of parasitoid eggs. Host species and parasitoid species, as well as the resulting interaction between these two factors had significant effects on encapsulation of Cotesia eggs. Encapsulation in Pieris hosts was much lower for C. glomerata (<34%, except for second and third instar of P. rapae ) than for C. rubecula (>32%), even when the latter was parasitizing P. rapae. Encapsulation increased with the age of the larvae, although the only significant difference was for C. glomerata. Overall, P. rapae showed a stronger encapsulation reaction than P. brassicae and P. napi. Encapsulation levels of C. glomerata corresponded well to patterns of female host species and host age preference for oviposition and parasitoid larval performance. In contrast, percentages of encapsulation of C. rubecula were not consistent with host preference and host suitability. We argue that encapsulation alone is unlikely to provide a sufficient explanation for C. glomerata and C. rubecula host range.     

Effects of Pieris host species on life history parameters in a solitary specialist and gregarious generalist parasitoid (Cotesia species)

Host specificity and host selection by insect parasitoids are hypothesized to be correlated with suitability of the hosts for parasitoid development. The present study investigates the correlation between host suitability and earlier studied host-finding behaviour of two closely related braconid larval parasitoid species, the generalist Cotesia glomerata (L.) and the specialist C. rubecula (Marshall) (Hymenoptera: Braconidae). We compared the capability of both parasitoid species to parasitize and develop in three Pieris host species, i.e. P. brassicae (L.), P. rapae (L.) and P. napi (L.) (Lepidoptera: Pieridae). In laboratory experiments, we measured the effect of host species on fitness parameters such as survival, development, sex ratio and size of parasitoid progeny. The results show that C. glomerata is capable of developing in the three host species, with significant differences in parasitoid survival, clutch size and adult weight among Pieris species. The host range for development was more restricted for C. rubecula. Although C. rubecula is physiologically able to develop in P. brassicae larvae, parasitoid fitness is negatively affected by this host species, compared to its most regular host, P. rapae. A comparison of the present data on host suitability with earlier studies on host-searching behaviour suggests that the host-foraging behaviour of both parasitoid species not only leads to selection of the most suitable host species for parasitoid development, but also plays a significant role in shaping parasitoid host range.     

Development of the parasitoid,Cotesia rubecula (Hymenoptera: Braconidae) in Pieris rapae and Pieris brassicae (Lepidoptera: Pieridae): evidence for host regulation

Several recent models examining the developmental strategies of parasitoids attacking hosts which continue feeding and growing after parasitism (=koinobiont parasitoids) assume that host quality is a non-linear function of host size at oviposition. We tested this assumption by comparing the growth and development of males of the solitary koinobiont endoparasitoid, Cotesia rubecula, in first (L1) to third (L3) larval instars of its preferred host, Pieris rapae and in a less preferred host, Pieris brassicae. Beginning 3 days after parasitism, hosts were dissected daily, and both host and parasitoid dry mass was determined. Using data on parasitoid dry mass, we measured the mean relative growth rate of C. rubecula, and compared the trajectories of larval growth of the parasitoid during the larval and pupal stages using non-linear equations. Parasitoids generally survived better, completed development faster, and grew larger in earlier than in later instars of both host species, and adult wasps emerging from P. rapae were significantly larger than wasps emerging from all corresponding instars of P. brassicae. During their early larval stages, parasitoids grew most slowly in L1 P. rapae, whereas in all other host classes of both host species growth to pupation proceeded fairly uniformly. The growth of both host species was markedly reduced after parasitism compared with controls, with the development of P. brassicae arrested at an earlier stage, and at a smaller body mass, than P. rapae. Our results suggest that C. rubecula regulates certain biochemical processes more effectively in P. rapae than in P. brassicae, in accordance with its own nutritional and physiological requirements. Furthermore, we propose that, for parasitoids such as C. rubecula, which do not consume all host tissues prior to pupation, that parasitoid size and host quality may vary independently of host size at oviposition and at larval parasitoid egression.     

Sequential rapid adaptation of indigenous parasitoid wasps to the invasive butterfly Pieris brassicae

The introduction of a new species can change the characteristics of other species within a community. These changes may affect discontiguous trophic levels via adjacent trophic levels. The invasion of an exotic host species may provide the opportunity to observe the dynamics of changing interspecific interactions among parasitoids belonging to different trophic levels. The exotic large white butterfly Pieris brassicae invaded Hokkaido Island, Japan, and quickly spread throughout the island. Prior to the invasion, the small white butterfly P. rapae was the host of the primary parasitoid Cotesia glomerata, on which both the larval hyperparasitoid Baryscapus galactopus and the pupal hyperparasitoid Trichomalopsis apanteroctena depended. At the time of the invasion, C. glomerata generally laid eggs exclusively in P. rapae. During the five years following the invasion, however, the clutch size of C. glomerata in P. rapae gradually decreased, whereas the clutch size in P. brassicae increased. The field results corresponded well with laboratory experiments showing an increase in the rate of parasitism in P. brassicae. The host expansion of C. glomerata provided the two hyperparasitoids with an opportunity to choose between alternative hosts, that is, C. glomerata within P. brassicae and C. glomerata within P. rapae. Indeed, the pupal hyperparasitoid T. apanteroctena shifted its preference gradually to C. glomerata in P. brassicae, whereas the larval hyperparasitoid B. galactopus maintained a preference for C. glomerata in P. rapae. These changes in host preference may result from differential suitability of the two host types. The larval hyperparasitoid preferred C. glomerata within P. rapae to C. glomerata within P. brassicae, presumably because P. brassicae larvae attacked aggressively, thereby hindering the parasitization, whereas the pupal hyperparasitoid could take advantage of the competition-free resource by shifting its host preference. Consequently, the invasion of P. brassicae has changed the host use of the primary parasitoid C. glomerata and the pupal hyperparasitoid T. apanteroctena within a very short time.     

Carotenoids in thirty-eight species of Lepidoptera

Carotenoids in 38 species of Lepidoptera have been examined qualitatively and quantitatively. Comparisons are made between cryptic and aposematic species, pupae and adult, dried and fresh material, males and females, and certain polymorphic species. Although some selective storage is demonstrated, Lepidoptera chiefly store the carotenoids present in their food plants unchanged. It was found that the Large White ( Pieris brassicae ), a toxic species, stored more carotenoids than the less toxic Small White ( P. rapae ) while ab. coerulea ( P. brassicae ) contained only 25 % of the carotenoids found in typical specimens.     

Bird predation as a selective pressure on the immature stages of the cabbage butterflies, Pieris rapae and P. brassicae

The importance of bird predation as a selective pressure on the eggs, larvae and pupae of the cabbage butterflies, Pieris rapae L. and P. brassicae L. has been investigated in a rural garden and an allotment. Birds are the main predators of all stages in a well-tended garden whereas in a field the eggs and young larvae are mainly preyed upon by arthropods. The species of avian predators vary according to the stage of development and species of prey. Consequently the relative survival rate of P. rupae and P. brussicue varies with stage of development. This changing pattern of survival has in turn acted as a selective pressure on their two main braconid parasites. Many aspects of the size, shape, colour and behaviour of the larvae and pupae of both species of Pieris appear to be adaptations to the selective pressures exerted by bird predation.     

Host quality of different aphid species for rearing Diaeretiella rapae (McIntosh) (Hymenoptera: Braconidae)

This study aimed to evaluate the quality of the aphid Myzus persicae (Sulzer), Lipaphis erysimi (Kaltenbach) and Brevicoryne brassicae (L.) as hosts for the parasitoid Diaeretiella rapae (McIntosh). Parasitization by D. rapae was higher on M. persicae than on L. erysimi and B. brassicae. The time of development of D. rapae from egg to mummy or egg to adult male or female were shorter on M. persicae than on L. erysimi and B. brassicae. Moreover, D. rapae showed no significant differences in the emergence rate, sex ratio and longevity when reared on the three aphid species. Myzus persicae was the largest aphid host, with B. brassicae and L. erysimi being of intermediate and of small size, respectively. Diaeretiella rapae reared on M. persicae was larger than when reared on L. erysimi and B. brassicae, and females of D. rapae were significantly larger than males on M. persicae, but males of D. rapae were larger than females when reared on L. erysimi. No difference in size was detected between males and females in parasitoids reared on B. brassicae. Among the aphid species studied, M. persicae was found to be the most suitable to D. rapae.     

Innate responses of the parasitoidsCotesia glomerata andC. rubecula (Hymenoptera: Braconidae) to volatiles from different plant-herbivore complexes

To determine and compare innate preferences of the parasitoid speciesCotesia glomerata andC. rubecula for different plant-herbivore complexes, long-range (1-m) foraging behavior was studied in dual-choice experiments in a wind tunnel. In this study we tested the hypothesis that naive females of the specialistC. rubecula should show more pronounced preferences for different plant-herbivore complexes than females of the generalistC. glomerata. The herbivore species used were the pieridsPieris brassicae, P. rapae, P. Napi, andAporia crataegi and the nonhostsPlutella xylostella andMamestra brassicae. All herbivore species feed mainly on cabbage and wild crucifers, exceptAporia crataegi, which feeds on species of Rosaceae. Both parasitoid species preferred herbivore-damaged plants over nondamaged plants. NeitherC. rubecula norC. glomerata discriminated between plants infested by different caterpillar species, not even between plants infested by host-and nonhost species. Both parasitoid species showed preferences for certain cabbage cultivars and plant species. No differences were found in innate host-searching behavior betweenC. glomerata andC. rubecula. The tritrophic system cabbage-caterpillars-Cotesia sp. seems to lack specificity on the herbivore level, whereas on the plant level differences in attractiveness to parasitoids were found.     

A comparison of attack rates in a native and an introduced population of the parasitoid Cotesia glomerata

The attack rate of a population of the braconid parasitoid Cotesia glomerata, introduced into the USA over 100 years ago as a parasitoid of Pieris rapae, was compared with that of a native British population, which normally attacks P. brassicae, and with that of a P. rapae specialist, Cotesia rubecula. British C. glomerata attacked P. brassicae at a much higher rate than it attacked P. rapae. In comparison with British C. glomerata, C. rubecula showed a higher attack rate with P. rapae. American C. glomerata attacked P. rapae at a slightly higher rate than did British C. glomerata, but not at as high a rate as that achieved by C. rubecula. The differences in each comparison were statistically significant. The possible causes of the differences between British and American C. glomerata attacking P. rapae are discussed. They may be due to genetic or environmental effects. Egg load did not appear to be a factor limiting the number of hosts parasitized under the conditions of the experiments.     

Laboratory studies on the effects of pollen from Bt-maize on larvae of some butterfly species

Abstract:   Three lepidopteran species were tested to determine their susceptibility against the ingestion of pollen from genetically modified maize plants. To prove the existence of dose–response relations between the applied amount of pollen (Bt-maize) and the damages on the tested larvae, a method was developed which makes it possible to feed caterpillars with defined amounts of pollen. If their food plants were contaminated with pollen of a cultivar of the Bt-176 maize-line Pieris brassicae , Pieris rapae and Plutella xylostella -larvae fed less, grew more slowly and showed a higher mortality than caterpillars of an untreated control group. The 50% lethality (LD₅₀)-values were calculated for P.xylostella (L₄) with 19.2, for P. rapae (L₂) with 39.0 and also for P. brassicae (L₂) with 139.2 pollen of the transgenic maize Pactol CB. Studies with P. brassicae -caterpillars of different larval stages indicated, that older individuals showed a higher tolerance against pollen from Bt-maize than younger ones. It must be stated on the basis of the present studies, that ingestion of non-transgenic maize pollen has neither a positive nor a negative effect on caterpillars. It becomes clear from the information presented here that it is still difficult to make general statements about the endangering of butterflies, arising from cultivation of genetically modified maize lines. Further investigations on this issue are needed. Initially, the LD₅₀-values concerning the larvae of certain butterfly species have to be determined to anticipate the risks, and in addition the distances between habitats with caterpillar host plants and maize fields, and the abundance of these plants have to be considered.     

Behavioural and sensory responses to drimane antifeedants in Pieris brassicae larvae

Fifteen drimane compounds were tested for their feeding inhibiting activity in larvae of Pieris brassicae L. (Lepidoptera: Pieridae) when applied to leaf material of the host plant Brassica oleracea L. The antifeedant efficacy of the drimanes was related to their molecular structure in order to identify important functional groups. Of the drimanes tested, those with a lactone group on the B-ring were the most effective feeding inhibitors. Additionally, the sensory responses to 13 of the drimanes were measured. Neural activity was evoked in the deterrent cell in the medial sensillum styloconicum. Also, inhibition of sensory responses to feeding stimulants was found. Results of behavioural and electrophysiological tests were correlated in an attempt to elucidate the sensory code underlying feeding inhibition by drimanes in Pieris brassicae. It was concluded that the response of the deterrent cell in the medial sensillum styloconicum contributes significantly to inhibition of feeding behaviour in larvae of Pieris brassicae.     

Interactions of insecticidal toxin gene products from Xenorhabdus nematophilus PMFI296

Four genes on a genomic fragment from Xenorhabdus nematophilus PMFI296 were shown to be involved in insecticidal activity towards three commercially important insect species. Each gene was expressed individually and in combinations in Escherichia coli, and the insecticidal activity of the lysates was determined. The combined four genes (xptA1, xptA2, xptB1, and xptC1), in E. coli, showed activity towards Pieris brassicae, Pieris rapae, and Heliothis virescens. The genes xptA1, xptB1, and xptC1 were involved in expressing activity towards P. rapae and P. brassicae, while the genes xptA2, xptB1, and xptC1 were needed for activity towards H. virescens. When each of these three genes was expressed individually in E. coli and the cell lysates were used in insect assays or mixed and then used, insecticidal activity was detected at a very low level. If the genes xptB1 and xptC1 were expressed in the same E. coli cell and this cell lysate was mixed with cells expressing xptA1, activity was restored to P. rapae and P. brassicae. Similarly mixing XptB1/C1 lysate with XptA2 lysate restored activity towards H. virescens. Individual gene disruptions in X. nematophilus PMFI296 reduced activity to insects; this activity was restored by complementation with cells expressing either xptA1 or xptA2 for their respective disruptions or E. coli expressing both xptB1 and xptC1 for individual disruptions of either of these genes. The genes xptA2, xptC1, and xptB1 were expressed as an operon in PMFI296 and inactivation of xptA2 or xptC1 resulted in silencing of downstream gene(s), while xptA1 was expressed as a single gene. Therefore, the two three gene product combinations interact with each other to produce good insecticidal activity.     

Winter ecology of the cabbage aphid Brevicoryne brassicae (L.) (Homo., Aphididae) and its parasitoid Diaeretiella rapae (McIntosh) (Hym., Braconidae: Aphidiidae)

Abstract:  Flower strips near crops may stimulate natural enemies by the provision of nectar and hibernation sites. However, these habitats may also be beneficial for potential pest species. We investigated the dynamics of the cabbage aphid Brevicoryne brassicae (L.) (Homo., Aphididae) and its primary parasitoid Diaeretiella rapae (McIntosh) (Hym., Braconidae) in brussels sprout fields and adjoining flower plots in winter. A wide variety of 14 plant species were included in the study, each established as monoculture plots. Brussels sprout fields and flower plots were established at two sites. One site was located in an open agricultural landscape, the other in a landscape dominated by mixed forest. Brevicoryne brassicae and D. rapae were found on brussels sprout plants but not in the flower plots. Brevicoryne brassicae was initially more abundant in the open landscape, but as their densities declined rapidly in time, no living aphids were recovered at both sites by February. The density of aphids parasitized by D. rapae showed a similar trend, but densities of eight mummies per brussels sprout plant were still present by the end of February. These findings suggest that (i) flower species under investigation do not function as sources of B. brassicae and (ii) brussels sprout plants that are not harvested may not only harbour D. rapae populations that may sustain biological control, but are also likely to act as sources of B. brassicae infestation.     

Electrophysiological studies of gustation in lepidopterous larvae

Summary The electrophysiological responses of the maxillary gustatory sensilla of ten species of lepidopterous larvae to sugars, polyhydric alcohols, amino acids, organic acids, glycosides, and sterols were tested. Of the caterpillars examined three were monophagous (Calpodes ethlius, Ceratomia catalpae, Danaus plexippus), three were oligophagous (Pieris rapae, Papilio polyxenes, Malacosoma americana), and four were polyphagous (Helioihis zea, Estigmene acrea, Isia isabella, Porthetria dispar). The sensilla of all species are structurally similar to the extent of having four chemosensory bipolar neurons in each peg.It is not possible to account for the wide range of responsiveness to diverse compounds in terms of narrowly specific receptors characterized as water, sugar, salt, inositol, or amino acid receptors. Bach receptive cell was found to have only a limited degree of specificity. Not all amino acids stimulated the same cell. Some amino acids stimulated only one; some stimulated two; some inhibited all cells; some inhibited one while stimulating another. Differential sensitivity to amino acids varied from species to species and even between the two sensilla in an individual.The diversity of relationships found with amino acids existed also with respect to acids and glycosides. Ascorbic and hydrochloric acids at equivalent pH values each stimulated a different cell. Glycosides stimulated one or two cells depending on the species and sensillum.Synergism occurred nearly as often as inhibition. Among the instances were synergistic reactions between sucrose and inositol, proline and inositol, sucrose and sinigrin, glycine and sucrose. The occurrence of the phenomenon was specific to certain species and sensilla.Results point to the conclusion that the basis of difference in sensitivity to different compounds from species to species and from receptor to receptor reflects variations in the molecular structure of the receptor substrate. Whatever the mechanisms involved it is clear that caterpillars are able to detect not only compounds that may be of nutritive value but also non-nutritive compounds some of which are secondary plant substances. When the total taste sensitivities of the various species of caterpillars are compared, there is no obvious difference between polyphagous, oligophagous, and monophagous species.This work was supported by National Science Foundation Grant GB 1472.     

Behavioural and sensory responses to some neem compounds by Pieris brassicae larvae

Abstract. The efficacy of a commercial product (Margosan-O) to inhibit feeding in Pieris brassicae L. (Lepidoptera: Pieridae) larvae was compared to that of three pure triterpenoids of botanical origin: azadirachtin and salannin (occurring in Melia azedurach ), and toosendanin (occurring in M. toosendan ). In dual-choice behaviour tests the order of antifeedant efficacy was: Margosan-O > toosendanin > salannin > azadirachtin. The ranking order of their capacity to stimulate the deterrent receptor located in the medial sensillum styloconicum appears to be: Margosan-O = toosendanin > salannin > azadirachtin. In addition to stimulating the medial deterrent cell, toosendanin inhibits the sugar receptor and the glucosinolate receptor, both located in the lateral sensillum styloconicum. The amino acid receptor in this sensillum, however, is not affected. In contrast, neither azadirachtin nor salannin influences the sensitivity of any of the receptor cells in this sensillum.
The adaptation rate of the deterrent receptor is low in comparison to that of the two sugar receptors.
A significant correlation is found between the antifeedant indices of the four neem compounds tested at three different concentrations, and the responses of the deterrent receptor to these compounds at similar concentrations. It is concluded that when this insect species feeds on its natural foodplant (i.e. cabbage), the deterrent effect of neem compounds is mediated solely via the medial deterrent receptor, whereas inhibitory effects on the sugar and glucosinolate receptors do not play a significant role.     

Mono(ADP-ribosyl)ation of the N2 amino groups of guanine residues in DNA by pierisin-2, from the cabbage butterfly, Pieris brassicae

Pierisin-2 is a cytotoxic and apoptosis-inducing protein present in Pieris brassicae with a 91% homology in the deduced amino acid sequences to pierisin-1 from Pieris rapae. We earlier showed pierisin-1 to catalyze mono(ADP-ribosyl)ation of 2'-deoxyguanosine (dG) in DNA to form N2-(ADP-ribos-1-yl)-2'-deoxyguanosine, this DNA modification appearing linked to its cytotoxicity and ability to induce apoptosis in mammalian cell lines. In this paper, we documented evidence that pierisin-2 also catalyzed ADP-ribosylation of dG in DNA to give the same reaction product as demonstrated for pierisin-1, with similar efficiency. With oligonucleotides as substrates, ADP-ribosylation by pierisin-2 was suggested to occur by one-side attack of the carbon atom at 1 position of the ribose moiety in NAD toward N2 of dG. The presence of a unique ADP-ribosylation toxin targeting dG in DNA in two distinct species in a Pieris genus could be a quite important finding to better understand biological functions of pierisin-1 and -2 in Pieris butterflies and the generic evolution of these cabbage butterflies.     

Tritrophic interactions between cabbage cultivars with different resistance and fertilizer levels,cruciferous aphids and parasitoids under field conditions

Tritrophic interactions involving cabbage Brassica oleracea var. capitata cultivars +/- fertilizer, Brevicoryne brassicae (Linnaeus) and Myzus persicae (Sulzer), and the parasitoids Diaeretiella rapae (M'cIntosh) and Aphidius sp. were conducted in 1998 and 1999. Brevicorne brassicae was the dominant aphid species on all cultivars +/- fertilizer, except for some treatments in late season 1998. Ruby Ball (red-leaved with antixenosis factors for B. brassicae alates) +/- fertilizer was consistently less colonized by aphids in early stages of plant growth, although only significantly so compared with Derby Day (green-leaved, susceptible to aphids) without fertilizer for B. brassicae and Minicole (green-leaved with antibiosis factors for B. brassicae) with fertilizer for M. persicae. In early 1999, only B. brassicae was present and no significant differences between cultivars were seen. In the mid to late season 1998, the highest aphid infestations were usually found on Derby Day, although only significantly so for B. brassicae, in some treatments. In 1999, higher aphid infestations were observed on Derby Day in mid to late season and some significant differences were found for M. persicae as well as for B. brassicae. In both years, Ruby Ball had the greatest mummy:aphid ratios early season, with no consistent difference between the other cultivars. Later in the season, mummy:aphid ratios were generally highest on Minicole. Parasitism differed in seasonal occurrence and relative abundance. Diaeretiella rapae mummies were found earlier than Aphidius sp. There was evidence of a beneficial interaction between the degree of plant resistance and biological control in early to mid season.     

Nutritional suitability of individual carbohydrates and amino acids for adult Pieris brassicae

Abstract Many adult Lepidoptera are dependent on carbohydrate-rich solutions such as nectar and honeydew. These food sources can contain a range of carbohydrates as well as low levels of other compounds, including free amino acids, proteins and lipids. Little is known about the nutritional value of individual nectar sugars and amino acids for adult Lepidoptera. Ten sugars occurring in nectar and honeydew were tested with respect to their effects on a number of Pieris brassicae life-table parameters. In similar experiments it was investigated whether the butterfly derives nutritional benefits from mixtures of common non-essential or essential amino acids when provided either in water or in a sucrose solution. Food sources were found to have a significant effect on butterfly fertility, longevity, lifetime fecundity and daily oviposition rate. Whereas a number of sugars significantly increase longevity, melibiose and melezitose reduce P. brassicae longevity. Butterfly fecundity is compromized significantly by sucrose and melibiose. Glucose is the only sugar that has a positive effect on both longevity and fecundity as well as a number of oviposition parameters. Amino acids seem to have no or only a minor effect on the life-table parameters tested. However, when provided in sucrose, they appear to compensate for the negative effect of pure sucrose on fecundity. A comparison with an earlier study on the gustatory response of P. brassicae shows that there is only a weak correlation between the nutritional value of sugars and amino acid mixtures and their perceptibility.