Allee effects and population regulation: a test for biotic resistance against an invasive leafroller by resident parasitoids

Resident natural enemies can impact invasive species by causing Allee effects, leading to a reduction in establishment success of small founder populations, or by regulating or merely suppressing the abundance of established populations. Epiphyas postvittana, the Light Brown Apple Moth, an invasive leafroller in California, has been found to be attacked by a large assemblage of resident parasitoids that cause relatively high rates of parasitism. Over a 4-year period, we measured the abundance and per capita growth rates of four E. postvittana populations in California and determined parasitism rates. We found that at two of the sites, parasitism caused a component Allee effect, a reduction in individual survivorship at lower E. postvittana population densities, although it did not translate into a demographic Allee effect, an impact on per capita population growth rates at low densities. Instead, E. postvittana populations at all four sites exhibited strong compensatory density feedback throughout the entire range of densities observed at each site. As we found no evidence for a negative relationship between per capita population growth rates and parasitism rates, we concluded that resident parasitoids were unable to regulate E. postvittana populations in California. Despite a lack of evidence for regulation or a demographic Allee effect, the impact of resident parasitoids on E. postvittana populations is substantial and demonstrates significant biotic resistance against this new invader.     

Facultative gregarious development in a solitary parasitoid wasp,Dendrocerus carpenteri: larvae may share nutritional resources

Solitary parasitoids generally produce only one offspring per host. Dendrocerus carpenteri (Curtis) (Hymenoptera: Megaspilidae) develops as an idiobiont ectoparasitoid on prepupae and pupae of primary aphid parasitoids inside the mummified aphid host. Females normally deposit a single egg but superparasitize when suitable hosts are scarce. We show that facultative gregarious development may occur but is constrained by resource competition between larvae. The probability of more than one offspring surviving increased with the intensity of parasitism; an age difference of ≤9 h between older and younger first instars did not promote gregarious development. Two female parasitoids and, rarely, up to three male parasitoids could develop together. Average body size in terms of dry mass did not differ between singly developing females and the combined mass of two females sharing host resources, but the combined mass of gregarious males was greater than that of their singly developing counterparts. Females were 3× more likely to eclose from single than gregarious mummies. The amount of host resources available per larva declines with increasing clutch size, in turn causing a corresponding reduction of adult size and size‐dependent fitness attributes. We suggest that competition for limiting host supplies may influence the transition from solitary to gregarious development and should be considered in models of clutch size evolution in parasitoid wasps.     

The effect of multi‐species host density on superparasitism and sex ratio in a gregarious parasitoid

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Resource rivalry between brood mates of a facultative gregarious parasitoid Dendrocerus carpenteri

Larvae of Dendrocerus carpenteri Curtis (Hymenoptera: Megaspilidae) develop as solitary ectoparasitoids on the prepupae and pupae of primary aphid parasitoids inside the aphid mummy. First instars are aggressive and kill potential competitors; however, facultative gregarious development of two, and occasionally three, larvae may occur under superparasitism. To test the hypothesis that gregarious larvae share host resources equally, adult dry mass is compared between three brood types: ‘double’ mummies containing (i) two males; (ii) two females; or (iii) one male plus one female, respectively. Resource rivalry varies with the sex of the competing larvae. Surviving adults differ significantly in size if both wasps are of the same sex, male or female. A minimum amount of resources is required for a larva to be viable; this threshold does not differ between sexes and is independent of the sex of a competing larva. The outcome of competition between a male larva and a female larva varies with the amount of the available resources, with neither sex being inherently dominant over the other. Females are eight times more likely to be larger than a male competitor if the amount of host resources is ≥0.116 mg, whereas males can win over a female competitor in terms of adult size if the available resources are <0.116 mg. It is suggested that rivalry between larvae for limiting host resources constrains the transition from solitary to gregarious development and should be considered in studies of parasitoid life‐history evolution.     

Latitudinal gradients and the shaping of life‐history traits in a gregarious caterpillar

The present study aimed to investigate how the impact of several factors linked to geography would shape life‐history traits in a gregarious species, using the pine processionary moth (PPM) Thaumetopoea pityocampa as a model system. PPM has a wide geographical distribution over the Mediterranean Basin, and it is a strictly gregarious species throughout larval development, where the total reproductive output of each female forms a colony. We reviewed both published and unpublished data on PPM from all over its distribution in the Mediterranean Basin and extracted data on fecundity, egg size, egg parasitoid mortality, flight period, and development time. These life‐history traits were then related to location, expressed as latitude and altitude, local average temperatures, and host tree species. We found that PPM fecundity increaseed with latitude, concomitant with an increase in the length of development and an earlier onset of adult flight. These results are the opposite of that found in other Lepidoptera species with a wide geographical distribution, as well as in insects in general. We propose that a large colony size in PPM is important at higher latitudes because this confers an advantage for thermoregulation and tent building in areas where larvae have to face harsher conditions during the winter, thus shifting the optimal trade‐off between the number and size of eggs with latitude. However, host tree species also affected the relationship between egg number and size and the optimal outcome of these traits is likely a compromise between different selection pressures. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100 , 224–236.     

How two different host species influence the performance of a gregarious parasitoid: host size is not equal to host quality

1. Hyssopus pallidus Askew (Hymenoptera, Eulophidae) is a gregarious ectoparasitoid of the two tortricid moths species Cydia molesta Busck and C. pomonella L. (Lepidoptera, Tortricidae). It paralyses and parasitizes different larval instars of both species inside the apple fruit, which leads to the death of the caterpillar. 2. We assessed the influence of host species characteristics and host food on the performance of the parasitoid female in terms of clutch size decisions and fitness of the F(1) generation. 3. A comparison of clutch size revealed that female parasitoids deposited similar numbers of eggs on the comparatively smaller C. molesta hosts as on the larger C. pomonella hosts. The number of parasitoid offspring produced per weight unit of host larva was significantly higher in C. molesta than in C. pomonella, which is contrary to the general prediction that smaller hosts yield less parasitoid offspring. However, the sex ratio was not influenced by host species that differed considerably in size. 4. Despite the fact that less host resources were available per parasitoid larva feeding on C. molesta caterpillars, the mean weight of emerging female wasps was higher in the parasitoids reared on C. molesta. Furthermore, longevity of these female wasps was neither influenced by host species nor by the food their host had consumed. In addition we did not find a positive relationship between adult female weight and longevity. 5. Parasitoid females proved to be able to assess accurately the nutritional quality of an encountered host and adjust clutch size accordingly. These findings indicate that host size is not equal to host quality. Thus host size is not the only parameter to explain the nutritional quality of a given host and to predict fitness gain in the subsequent generation.     

Consequences of abandonment for a regional fen flora and mechanisms of successional change

Abstract. Until the 1960s, species-rich vegetation on minero-trophic peaty soüs (fen sites) were characteristic of the alluvial plains in Schleswig-Holstein (Northwest Germany). Today, many of these habitats undergo successional changes due to abandonment. Vegetation development after abandonment can be characterized as a sequence of different successional stages and described in terms of a successional model. Successional stage I includes grazed, mown and recently abandoned sites without dominants. Stages II and III are characterized by the dominance of highly competitive herbaceous species whüe stage IV consists of woody vegetation. Ca. 3000 phytosociological relevés were assigned to the respective successional stages. Mean cover values were calculated for 250 species of the regional fen flora and assigned to successional categories according to their changes in cover in the successional series. According to our results 141 species decrease during succession, while 100 species were restricted to early successional stages and 85 species increased. Abandonment of all fen sites in Schleswig-Holstein will probably lead to the regional loss of 23 species of the fen flora. To identify mechanisms underlying successional change, the successional categories were correlated with life history traits and ecological requirements of the species. Results indicate that both light competition and limitation of sexual reproduction of small-seeded species might play a major role in the decrease and extinction of species during succession. Finally, conservation strategies for endangered species in a cultural landscape are discussed.     

The potential of mass rearing of Monoksa dorsiplana (Pteromalidae) a native gregarious ectoparasitoid of Pseudopachymeria spinipes (Bruchidae) in South America

In Chile and Uruguay, the gregarious Pteromalidae (Monoksa dorsiplana) has been discovered emerging from seeds of the persistent pods of Acacia caven attacked by the univoltin bruchid Pseudopachymeria spinipes. We investigated the potential for mass rearing of this gregarious ectoparasitoid on an alternative bruchid host, Callosobruchus maculatus, to use it against the bruchidae of native and cultured species of Leguminosea seeds in South America.The mass rearing of M. dorsiplana was carried out in a population cage where the density of egg-laying females per infested seed was increased from 1:1 on the first day to 5:1 on the last (fifth) day. Under these experimental conditions egg-clutch size per host increased, and at the same time the mortality of eggs laid also increased. The density of egg-laying females influenced the sex ratio which tended towards a balance of sons and daughters, in contrast to the sex ratio of a single egg-laying female per host (1 son to 7 daughters). The mean weight of adults emerging from a parasitized host was negatively correlated with the egg-clutch size, i.e. as egg-clutch size increased, adult weight decreased.All these results show that mass rearing of the gregarious ectoparasitoid M. dorsiplana was possible under laboratory conditions on an alternative bruchid host C. maculatus. As M. dorsiplana is a natural enemy of larval and pupal stages of bruchidae, the next step was to investigate whether the biological control of bruchid C. maculatus was possible in an experimental structure of stored beans.     

Consequences of constitutive and induced variation in plant nutritional quality for immune defence of a herbivore against parasitism

The mechanisms through which trophic interactions between species are indirectly mediated by distant members in a food web have received increasing attention in the field of ecology of multitrophic interactions. Scarcely studied aspects include the effects of varying plant chemistry on herbivore immune defences against parasitoids. We investigated the effects of constitutive and herbivore-induced variation in the nutritional quality of wild and cultivated populations of cabbage (Brassica oleracea) on the ability of small cabbage white Pieris rapae (Lepidoptera, Pieridae) larvae to encapsulate eggs of the parasitoid Cotesia glomerata (Hymenoptera, Braconidae). Average encapsulation rates in caterpillars parasitised as first instars were low and did not differ among plant populations, with caterpillar weight positively correlating with the rates of encapsulation. When caterpillars were parasitised as second instar larvae, encapsulation of eggs increased. Caterpillars were larger on the cultivated Brussels sprouts plants and exhibited higher levels of encapsulation compared with caterpillars on plants of either of the wild cabbage populations. Observed differences in encapsulation rates between plant populations could not be explained exclusively by differences in host growth on the different Brassica populations. Previous herbivore damage resulted in a reduction in the larval weight of subsequent herbivores with a concomitant reduction in encapsulation responses on both Brussels sprouts and wild cabbage plants. To our knowledge this is the first study demonstrating that constitutive and herbivore-induced changes in plant chemistry act in concert, affecting the immune response of herbivores to parasitism. We argue that plant-mediated immune responses of herbivores may be important in the evaluation of fitness costs and benefits of herbivore diet on the third trophic level.     

How feeding performance and energy intake change with a small increase in the body size of the three-spined stickleback

Changes in the foraging behaviour due to variation in the body size of the three-spined stickleback Gasterosteus aculeatus were investigated. All sizes of fish had a high probability of attacking prey whenever encountered. The probability of eating the prey increased with the size of the fish, as the larger fish had larger jaws and a greater stomach capacity. Therefore, as fish increased in size there was an increase in the probability of successful prey capture. The level of satiation did not have an effect on the prey handling time, which is contrary to other studies and is probably a result of the large prey sizes. The physical size of the prey meant that the handling times were long regardless of the motivational level of the fish. The larger fish took in more energy and at a faster rate, although the time to reach satiation was similar for all fish sizes. The advantage that large fish appear to have in successfully gaining large prey is negated by their greater metabolic requirement. The changes in feeding performance induced by small increases in body size could have important consequences for intraspecific competition, habitat Use and risk of predation.     

Evidence of a hemolymph-born factor that induces onset of maturation in Manduca sexta larvae

Insect metamorphosis is a complex developmental transition determined and coordinated by hormonal signaling that begins at a critical weight late in the larval phase of life. Even though this hormonal signaling is well understood in insects, the internal factors that are assessed at the critical weight and that drive commitment to metamorphosis have remained unresolved in most species. The critical weight may represent either an autonomous decision by the neuroendocrine system without input from other developing larval tissues, or an assessment of developmental thresholds occurring throughout the body that are then integrated by the neuroendocrine tissues. The latter hypothesis predicts that there could be one or more developmental threshold signals that originate from developing tissues and ultimately induce the onset of metamorphosis. However, there is no evidence for such a signal in the organisms for which the critical weight is well described. Here we test for the evidence of this factor in Manduca sexta (Lepidoptera: Sphingidae) by transferring hemolymph from individuals that are either post- or pre-critical weight into pre-critical weight 5^th instar larvae. We found that hemolymph from a post-critical weight donor induces a shortening of development time, though the mass at pupation is unaffected. This suggests that metamorphic commitment occurring at the critical weight is at least partially coordinated by signaling from developing tissues via a hemolymph-borne signaling factor.     

A unimodal curve for primate group size along a gradient of population density

When plotted along a gradient of population density, the mean group size in populations of several primate species has a unimodal distribution, i.e., mean group size is greater at intermediate population densities than at higher or lower population densities. In this study I present a mathematical model to clarify the cause of this relationship. Population density is assumed to affect group size by enhancing between- or within-group competition and by changing the number of neighboring groups around each group. The mean group size is predicted to decline as population density increases above a critical value, owing to the increasing number of neighboring groups.     

Coexistence of ecologically similar colonising species. II. Population differentiation in Drosophila aldrichi and D. Buzzatii for competitive effects and responses at different temperatures and allozyme variation in D. aldrichi

Drosophila aldrichi and D. buzzatii are cactophilic species that colonised Australia about 55–60 years ago. They are sympatric only in Australia. Thus they may be in the process of adapting to new environments and to each other, and diversifying among local, possibly isolated, populations. Larval competitive effects for three populations of each species (Roma, Planet Downs, and Binjour) were measured on semi-natural cactus rots at three temperatures, with preadult viability, developmental time and adult body weight scored for each sex and species. Populations of both species varied in their responses to the other species as competitor, and one D. buzzatii population (Roma) reduced larval performance of D. aldrichi significantly more than did other D. buzzatii populations. Geographic divergence for the three traits was similar in both species, with a relative performance index derived from these traits highest for Roma, second for Binjour, and least for the Planet Downs population of each species. The Roma D. aldrichi population was the most different from the other populations for the performance index and in terms of genetic distances derived from allozyme frequencies. Additionally, comparisons of climatic variables among the population localities showed that the Roma environment was most different from the others. Differential natural selection in different areas of the cactus distribution may be a major cause of population divergence in both species. Drosophila aldrichi is superior for some fitness components at the highest temperature. Thus temperature variation throughout the cactus distribution may contribute to the different ranges of these two species, with competitive exclusion of D. aldrichi in the southern, cooler region of the cactus distribution, but coexistence in the northern, warmer region.     

Consequences of removing a keystone herbivore for the abundance and diversity of arthropods associated with a cruciferous shrub

Abstract. 1. The effect of the removal of Timarcha lugens (Chrysomelidae), one of the main herbivores of Hormathophylla spinosa (Cruciferae), on the abundance of co-occurring phytophagous insects, the abundance of non-phytophagous arthropods (detritivores, predators, and parasitoids), and the structure and diversity of the entire arthropod community, was studied for 3 years (1999–2001).
2. There was competition between T. lugens and co-occurring herbivores; the removal of T. lugens was correlated with an increase in the abundance of sap-suckers, flower-feeders, and, above all, folivores.
3. Timarcha lugens also had an indirect effect on arthropods belonging to other trophic levels; the abundance of predators increased significantly after the removal of T. lugens .
4. Community composition was affected by the experimental removal. In addition, the diversity of the overall community increased after removal of T. lugens .
5. The study demonstrated experimentally that T. lugens has a significant effect not only on other species belonging to the same trophic level, but also on the abundance of species belonging to higher trophic levels, and, consequently, on the entire structure and diversity of the complex community in which it is immersed.     

The effects of host weight at parasitism on fitness correlates of the gregarious koinobiont parasitoid Microplitis tristis and consequences for food consumption by its host, Hadena bicruris

Gregarious koinobiont parasitoids attacking a range of host sizes have evolved several mechanisms to adapt to variable host resources, including the regulation of host growth, flexibility in larval development rate, and adjustment of clutch size. We investigated whether the first two mechanisms are involved in responses of the specialist gregarious parasitoid Microplitis tristis Nees (Hymenoptera: Braconidae) to differences in the larval weight and parasitoid load of its host Hadena bicruris Hufn. (Lepidoptera: Noctuidae). In addition, we examined the effects of parasitism on food consumption by the host. Parasitoids were offered caterpillars of different weight from all five instars, and parasitoid fitness correlates, including survival, development time, and cocoon weight, were recorded. Furthermore, several host growth parameters and food consumption of parasitized and unparasitized hosts were measured. Our results show that M. tristis responds to different host weights by regulating host growth and by adjusting larval development rate. In hosts with small weights, development time was increased, but the increase was insufficient to prevent a reduction in cocoon weight, and as a result parasitoids experienced a lower chance of successful eclosion. Cocoon weight was negatively affected by parasitoid load, even though host growth was positively affected by parasitoid load, especially in hosts with small weights. Later instars were more optimal for growth and development of M. tristis than early instars, which might reflect an adaptation to the life‐history of the host, whose early instars are usually concealed and inaccessible for parasitism on its food plant, Silene latifolia Krause (Caryophyllaceae). Parasitism by M. tristis greatly reduced total host food consumption for all instar stages. Whether plants can benefit directly from the attraction of gregarious koinobiont parasitoids of their herbivores is a subject of current debate. Our results indicate that, in this system, the attraction of a gregarious koinobiont parasitoid can directly benefit the plant by reducing the number of seeds destroyed by the herbivore.     

Benefits of self-superparasitism in a polyembryonic parasitoid

Macrocentrus grandii is a polyembryonic parasitoid, with embryos that divide asexually within the host (European corn borer, Ostrinia nubilalis) to produce broods of clonal offspring. From a biological control standpoint, polyembryony seems advantageous because each parasitized host yields multiple parasitoids with minimal time and egg investment. When we observed M. grandii in the field, however, we found that the parasitoid virtually always invested additional time and, if possible, stings into hosts that it had already stung, apparently reducing some of the advantages of polyembryony. We therefore investigated and found support for two potential benefits that can be gained by self-superparasitism in this system. First, multiple stings allowed production of mixed-sex broods: 27% of multiply-stung versus 0% of singly-stung hosts produced mixed-sex broods. Second, multiple stings increased mean parasitoid progeny produced per host, primarily by reducing the chance of complete brood failure. Our results indicate substantial benefit for a second sting, but little benefit for three or more stings, even though M. grandii was sometimes observed to invest more than two stings within a single host. However, we also found that within-host larval competition is prevalent, suggesting that supernumerary stings may pay off in competition against conspecific larvae. Such additional investment within a single host would be particularly beneficial when hosts, rather than eggs, are limiting, but would decrease the overall efficacy of M. grandii as a biological control agent.     

Size selective feeding and its limitations for the black rockfish, Sebastes inermis, in a demersal fish assemblage of Onagawa Bay, northeastern Japan

The size selective feeding of the black rockfish, Sebastes inermis, in a demersal fish assemblage of Onagawa Bay, located in northeastern Japan, was studied using laboratory experiments and stomach content analysis of the dominant demersal fish coexisting with the black rockfish in Onagawa Bay. The preferred food size of the black rockfish was determined in the laboratory experiments. The average food size taken by the black rockfish (150–227 mm long) under natural conditions was much smaller than that taken by fish in laboratory experiments (148–158 mm sized). The most preferred size of food taken by fish in laboratory experiments was rarely found under natural conditions. Analysis of the competition coefficients of fish species in the fish assemblage based on stomach content analysis suggested that the degrees to which the preferred food size was found depended upon the foods availability, by competition with other fish species and by the feeding ability of the fish. Intra- and inter-specific competition in a fish assemblage might be limiting factors on the food size selection of fish under natural conditions.     

Between-clutch interactions affect a benefit of group feeding for pipevine swallowtail larvae

Abstract.  1. In California, early instar larvae of the pipevine swallowtail ( Battus philenor ) develop at an accelerated rate when feeding in large groups compared with small groups due to a plant-mediated response to feeding group size. Larvae benefit from accelerated growth because the time larvae remain in early stages, where mortality is highest, is reduced. Occasionally, multiple clutches are laid on the same plant stem. Clutch size modification by females ovipositing on plant with previously laid clutches and the effect of kinship and group size on larval behaviour was examined. The direct and indirect interactions between clutches were investigated to determine if group size and time between clutch establishment affects the performance of early instar larvae.
2. Larger groups consume the young foliage more quickly and develop at an accelerated rate compared with smaller groups. Older foliage available to later clutches is an inferior food resource compared with younger foliage.
3. There was no evidence that females adjust clutch size in response to the presence of conspecific clutches.
4. Second groups of larvae readily joined previously established feeding groups. There were no observed behavioural differences between sibling and mixed-family groups.
5. The effect of a second group on the growth of the initial group was dependent on the size of both groups and the time interval between the arrival of the two groups.
6. Accelerated growth associated with larger feeding aggregations was absent when these groups were introduced to plants with previously established groups.
7. It is beneficial for ovipositing females to avoid plants with previously laid clutches because direct and indirect interactions with established clutches compromises larval performance.     

Predictable modification of body size and competitive ability following a host shift by a seed beetle

Interfertile populations of the seed beetle Callosobruchus maculatus differ genetically in several behavioral, morphological, and life-history traits, including traits that affect the intensity of larval competition within seeds. Previous studies have suggested that this variation depends on differences in host size. I performed a selection experiment in which replicate beetle lines were either maintained on a small, ancestral host (mung bean) or switched to a larger, novel host (cowpea). After 40 generations, I estimated survival, development time, and adult mass on each host, both in the presence and absence of larval competition. The shift to cowpea substantially reduced body size; irrespective of rearing host, adults from the cowpea lines were more than 10% lighter than those from the mung bean lines. Switching to cowpea also improved survival and reduced development time on this host, but without decreasing performance on the ancestral host. The most striking effect of the shift to a larger host was a reduction in larval competitiveness. When two even-aged larvae co-existed within a seed, the probability that both survived to adult emergence was > or = 65% if larvae were from the cowpea lines but < or = 12% if they were from the mung bean lines. The adverse effects of competition on development time and adult mass were also less severe in the cowpea lines than in the mung bean lines. By rapidly evolving smaller size and reduced competitiveness, the cowpea lines converged toward populations chronically associated with cowpea. These results suggest that evolutionary trajectories can be predictable, and that host-specific selection can play a major role in the diversification of insect life histories. Because host shifts by small, endophagous insects are comparable to the colonization of new habitats, adaptive responses may often include traits (such as larval competitiveness) that are not directly related to host use.