Effects of root herbivory by an insect on a foliar-feeding species,mediated through changes in the host plant

Summary The effects of root herbivory by larvae of the scarabaeid, Phyllopertha horticola, on the growth of Capsella bursa-pastoris were examined. Individuals of Aphis fabae were reared on the leaves to determine what effect, if any, root feeding has on the performance of this insect. The experiment was conducted under two watering regimes (low and high). Low watering and root feeding caused water stress in the plants and this was reflected in a reduction in vegetative biomass and an increase in the proportion of material allocated to reproduction. Supplying plants with ample water in the high treatment enabled the water stress caused by root herbivory to be offset, but not completely overcome. Low watering and root feeding caused an increase in aphid weight and growth rate, while root feeding also increased fecundity and adult longevity. These effects are attributed to an improvement in food quality, measured by total soluble nitrogen, and caused by amino acid mobilization due to the water stress. The implications of these results in agricultural and ecological situations are discussed.     

Effects of clonal variation of the host plant,interspecific competition,and climate on the population size of a folivorous thrips

Summary The effects of clonal variation, interspecific competition, and climate upon the population size of Apterothrips secticornis was assessed by a series of observations and experimental manipulations. Three clones of the host plant, Erigeron glaucus, consistently supported different numbers of thrips during monthly censuses. When rosettes of the three clones were transplanted to a common garden, relative numbers of thrips on the clones remained the same as those observed where the clones grew in situ. The presence or absence of other hervivores had no effect on thrips numbers in the common garden. Plume moth caterpillars and thrips were observed to co-occur less often than expected in the field but this was caused by differences in habitat selection by these two species rather than being the result of interspecific competition. Populations of thrips were affected by climate, but analyses suggest that the host clone was a more important factor.     

Plant community response following the removal of the invasive Lupinus arboreus in a coastal dune system

The removal of invasive species is common in restoration projects, yet the long‐term effects of pest management programs are seldom assessed. We present results of a long‐term program to remove the invasive species Lupinus arboreus (lupin) from sand dunes in New Zealand. We evaluate the response of plant communities to lupin removal, by comparing total plant cover, the cover of non‐native and native plant species, and species richness between sand dune sites where lupin removal has occurred, not occurred, and where lupin has never been present. Neither lupin presence nor removal had a significant impact on the foredune environment. Following removal, total and other non‐native plant cover remained higher, and the cover of several native sand dune species remained lower compared with uninvaded sites in the deflation and backdune environments. These changes can be attributed to persistent effects associated with the invasion of lupin, but have also developed in response to lupin removal. The results of this study have implications for restoration projects in sand dunes. Pest management alone is unlikely to be sufficient to restore plant communities. Given the difficulties in restoring plant communities once an invasive species has established, managers should prioritize actions to prevent the spread of invasive species into uninvaded areas of sand dunes. Finally, the response to lupin invasion and removal differed between dune habitats. This highlights the importance of tailoring a pest management program to restoration goals by, for example, prioritizing areas in which the impacts of the invading species are greatest.     

An analysis of the coexistence of two host species with a shared pathogen

Population dynamics of two-host species under direct transmission of an infectious disease or a pathogen is studied based on the Holt–Pickering mathematical model, which accounts for the influence of the pathogen on the population of the two-host species. Through rigorous analysis and a numerical scheme of study, circumstances are specified under which the shared pathogen leads to the coexistence of the two-host species in either a persistent or periodic form. This study shows the importance of intrinsic growth rates or the differences between birth rates and death rates of the two host susceptibles in controlling these circumstances. It is also demonstrated that the periodicity may arise when the positive intrinsic growth rates are very small, but the periodicity is very weak which may not be observed in an empirical investigation.      

Density-dependent effects of larval dispersal mediated by host plant quality on populations of an invasive insect

The success of invasive species is often thought to be due to release from natural enemies. This hypothesis assumes that species are regulated by top-down forces in their native range and are likely to be regulated by bottom-up forces in the invasive range. Neither of these assumptions has been consistently supported with insects, a group which includes many destructive invasive species. Winter moth (Operophtera brumata) is an invasive defoliator in North America that appears to be regulated by larval mortality. To assess whether regulation was caused by top-down or bottom-up forces, we sought to identify the main causes of larval mortality. We used observational and manipulative field and laboratory studies to demonstrate that larval mortality due to predation, parasitism, and disease were minimal. We measured the response of larval dispersal in the field to multiple aspects of foliar quality, including total phenolics, pH 10 oxidized phenolics, trichome density, total nitrogen, total carbon, and carbon–nitrogen ratio. Tree-level declines in density were driven by density-dependent dispersal of early instars. Late instar larvae dispersed at increased rates from previously damaged as compared to undamaged foliage, and in 2015 field larval dispersal rates were related to proportion of oxidative phenolics. We conclude that larval dispersal is the dominant source of density-dependent larval mortality, may be mediated by induced changes in foliar quality, and likely regulates population densities in New England. These findings suggest that winter moth population densities in New England are regulated by bottom-up forces, aligning with the natural enemy release hypothesis.     

Structure and dynamics of the parasitoid community shared by two herbivore species on different host plants

The structure of the parasitoid community on phytophagous insects can be affected by host plant properties, such as chemical compounds, trichomes, and glandular hairs. To clarify effects of host plants on herbivores and the parasitoid community, I examined the structure and dynamics of the parasitoid community associated with two species of Caloptilia moths (Lepidoptera: Gracillariidae) that feed on different Rhododendron species (Ericaceae) for 3 years in a temperate secondary forest in central Japan. Caloptilia azaleella had overlapping generations in summer and overwintered as larvae on leaves of R. macrosepalum. Caloptilia leucothoes also had overlapping generations in summer, but it did not overwinter on the deciduous shrub R. reticulatum. The parasitoid community of C. azaleella larvae and pupae was composed of 18 species, whereas that of C. leucothoes was composed of seven species. Five species of parasitoids attacked both Caloptilia species. The most abundant parasitoid, Apanteles cf. xanthostigma (Hymenoptera: Braconidae), more frequently attacked C. azaleella than C. leucothoes larvae. In contrast, another abundant parasitoid, Acrysocharoides sp. (Hymenoptera: Eulophidae), more frequently attacked C. leucothoes than C. azaleella larvae. This differential parasitism by the most abundant parasitoid species may be responsible for the differential structure and dynamics of the parasitoid community between the Caloptilia species. The host plant of C. azaleella, R. macrosepalum, more frequently trapped and killed parasitoids (of similar size to Acrysocharoides sp.) on the glandular hairs of leaves than did R. reticulatum. The differential effect of host plants on abundant parasitoids may be related to the differential parasitism by the two abundant parasitoids shared by the herbivore hosts.     

Climatic change is advancing the phenology of moth species in Ireland

Recent increases in global temperatures have contributed to advancing phenology of plants and animals. These increases in temperature have been shown to affect the phenological phases (phenophases) of plants and birds in Ireland, but less is known about the effect on the phenophases of Irish insects. Records of the flight periods of 59 species of Irish moths over the past 35 years (1974–2009) were obtained from a public monitoring group. Observations were analysed across the country using generalized additive models (GAMs) weighted by total yearly population numbers for each species. The results of the statistical analyses showed that 45 of the 59 species studied have a significantly earlier first sighting date now than when observations began. With this earlier emergence, 44 of the 59 species also have a significantly longer flight season over the same 35‐year period. The extent of these changes varies across the country and by species life history. In particular, species emerging in spring are advancing at a much faster rate than species emerging during the summer. Many of these changes in first sighting are negatively correlated with rising temperatures in Ireland, particularly in late spring and early summer (May and June). The variation in phenological advancement in the moth species of Ireland is extremely complex and may be influenced more by species life history than by the phenology of interacting species, such as host plants.     

Shiny Cowbird parasitism of a low quality host: effect of host traits on a parasite's reproductive success

ABSTRACT.   The reproductive success of parasitic cowbirds ( Molothrus spp.) varies among host species and is influenced by the degree of synchronization in timing of egg laying, the duration of parasite and host incubation periods, and the ability of hosts to incubate and rear parasite young. We studied the reproductive success of Shiny Cowbirds ( Molothrus bonariensis ) that parasitized the nests of Creamy-bellied Thrushes ( Turdus amaurochalinus ) in the Monte desert region of Argentina. Shiny Cowbirds frequently parasitized Creamy-bellied Thrush nests (60%), and most cowbirds synchronized egg laying with that of thrushes (79%). Most parasitic eggs (80%) hatched within 1 d of the hatching of the first host egg, and more than 91% of the eggs survived until the end of the incubation. However, only 60% of the cowbird eggs hatched and 52% of young survived. The proportion of Shiny Cowbirds eggs laid in Creamy-bellied Thrush nests that resulted in fledged young was 0.03, including eggs and young lost due to predation or desertion. Despite this low reproductive success, Creamy-bellied Thrushes were heavily parasitized by Shiny Cowbirds in our study area. Shiny Cowbirds may continue to parasitize these thrushes because of diffuse selection or because Shiny Cowbird chicks are more likely to fledge from Creamy-bellied Thrush nests in years or areas with greater food availability when brood reduction does not occur.     

Exotic herbivores on a shared native host: tissue quality after individual, simultaneous, and sequential attack

Plants in nature are often attacked by multiple enemies whose effect on the plant cannot always be predicted based on the outcome of individual attacks. We investigated how two invasive herbivores, the hemlock woolly adelgid (Adelges tsugae) (HWA) and the elongate hemlock scale (Fiorinia externa) (EHS), alter host plant quality (measured as amino acid concentration and composition) when feeding individually or jointly on eastern hemlock (Tsuga canadensis), an important long-lived forest tree that is in severe decline. The joint herbivore treatments included both simultaneous and sequential infestations by the two herbivores. We expected resource depletion over time, particularly in response to feeding by HWA. In contrast, HWA dramatically increased the concentration and altered the composition of individual free amino acids. Compared to control trees, HWA increased total amino acid concentration by 330% after 1 year of infestation. Conversely, EHS had a negligible effect when feeding individually. Interestingly, there was a marginally significant HWA × EHS interaction that suggests the potential for EHS presence to reduce the impact of HWA on foliage quality when the two species co-occur. We suggest indirect effects of water stress as a possible physiological mechanism for our results. Understanding how species interactions change the physiology of a shared host is crucial to making more accurate predictions about host mortality and subsequent changes in affected communities and ecosystems, and to help design appropriate management plans.     

Performance of moth larvae on birch in relation to altitude, climate, host quality and parasitoids

We studied topographical and year-to-year variation in the performance (pupal weights, survival) and larval parasitism of Epirrita autumnata larvae feeding on mountain birch in northernmost Finland in 1993–1996. We found differences in both food plant quality and parasitism between sites ranging from 80 m to 320 m above sea level. Variation in food plant quality had particularly marked effects on larval survival. The advanced phenology of the birches in relation to the start of the larval period reduced pupal weights. Parasitism rates were different between years and between sites. The clearest site differences were in the proportions of different parasitoid species: Eulophus larvarum was most abundant at the lowest-altitude sites, and Cotesia jucunda at the highest. Differences in the performance of E. autumnata were related to temperature conditions: at higher temperatures, survival and the egg production index were lower, and larval parasitism was higher than at lower temperatures. The higher parasitism at higher temperatures was probably due to greater parasitoid activity during warmer days. In the comparison of different sources of spatial and annual variation in the performance of E. autumnata, the most important factor appeared to be egg mortality related to minimum winter temperature, followed by parasitism and, finally, the variation in food plant quality. If, as predicted, the climate gradually warms up, the effects of warmer summers on the outbreaks of E. autumnata suggest a decrease in outbreak intensity. Received: 4 January 1999 / Accepted: 22 March 1999     

Dynamic effects of parasitism by an endoparasitoid wasp on the development of two host species: implications for host quality and parasitoid fitness

1. The study reported here examined growth and developmental interactions between the gregarious larval koinobiont endoparasitoid Cotesia glomerata (Hymenoptera: Braconidae) and two of its hosts that vary considerably in growth potential: Pieris rapae and the larger P. brassicae (Lepidoptera: Pieridae). At pupation, healthy larvae of P. brassicae are over twice as large, in terms of fresh body mass, as those of P. rapae. 2. Clutch size of C. glomerata was manipulated artificially, and the relationship between parasitoid burden and the maximum weight of the parasitised host (= host–parasitoid complex) was measured. In both hosts, the maximum complex weight was correlated positively with parasitoid burden. Compared with unparasitised hosts, however, the growth of P. rapae was increased at significantly lower parasitoid burdens than in P. brassicae. Emerging wasp size was correlated negatively with parasitoid burden in both host species, whereas development time was less affected. 3. After larval parasitoid egress, the weight of the host carcass increased slightly, but not significantly, with parasitoid burden, although there was a strong correlation between the proportion of host mass consumed by C. glomerata larvae during development and parasitoid burden. 4. Clutch size was generally correlated positively with instar parasitised in both hosts, and greater in P. brassicae than in P. rapae. Sex ratios were much more female biased in L1 and L2 P. rapae than in all other host classes. Adult parasitoid size was correlated inversely with host instar at parasitism, and wasps emerging from P. brassicae were larger, and completed development faster, than conspecifics emerging from P. rapae. 5. The data reveal that parasitism by C. glomerata has profound species‐specific effects on the growth of both host species. Consequently, optimality models in which host quality is often based on host size at parasitism or unparasitised growth potential may have little utility in describing the development of gregarious koinobiont endoparasitoids. The results of this investigation are discussed in relation to the potential effectiveness of gregarious koinobionts in biological control programmes.     

Influence of an endophytic fungus on host plant selection by a polyphagous moth via volatile spectrum changes

Recently the role of micro-organisms as mediators of plant-herbivore interactions has been increasingly acknowledged in ecological research. We investigated the interaction between an unspecialized root fungal endophyte (Acremonium strictum) and the polyphagous moth Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae), in greenhouse and laboratory bioassays. Specifically we examined in tomato (Lycopersicon esculentum Mill.) the systemic effects of the endophytic fungus on the host selection behaviour of female moths for oviposition and the volatile profiles of host plants to understand the mechanisms acting in this multi-trophic model system. Both laboratory and field strains of H. armigera moths oviposited more on leaves of A. strictum inoculated plants as compared to endophyte-free plants, both in free flight cages and in tethered moth laboratory experiments; the moth’s preferences were significant between 10 and 18 days after inoculation. The analysis of volatile profiles showed strong quantitative differences between treatments. Endophyte inoculated plants emitted diverse terpenes and sesquiterpenes at significantly lower amounts as compared to endophyte free-plants, except for α-terpinene, which did not differ between the treatments, and trans-β-caryophyllene, which was emitted in significantly higher amounts on inoculated plants. β-Thujene and α-phellandrene accounted for 73.3 and 12.0% of total amounts of volatiles emitted from endophyte-free and inoculated plants, respectively. Our findings demonstrate that A. strictum is able to systemically influence the host selection of H. armigera moths for oviposition; conceivably mediated by the induced changes in volatile emissions (and probably additional biochemical parameters of the host plants, which have not been analysed so far). We argue for a more detailed assessment of micro-organisms invisibly colonizing plants, when studying plant-herbivore or multitrophic interactions.     

Effects of aphid-tending Argentine ants, nitrogen enrichment and early-season herbivory on insects hosted by a coastal shrub

The presence of the exotic Argentine ant, Linepithema humile Mayr (Hymenoptera: Dolichoderinae), nitrogen enrichment, and early-season herbivory by the specialist beetle Trirhabda bacharidis (Coleoptera: Chrysomelidae) have been shown, through separate experiments, to affect the densities of insect herbivores of the coastal shrub Baccharis halimifolia (Asteraceae), in Florida. Using a fully-factorial field experiment, we examined the relative importance of all three of these factors to the six most common insect herbivore species utilizing this host plant in a West Central Florida coastal habitat. The presence of ants affected more herbivore species than either early-season herbivory by larval T. bacharidis or nitrogen enrichment. Experimental reductions of L. humile resulted in reductions of an aphid, its coccinellid predators, and adult T. bacharidis, and increases of two species of leafminers and one species of stemborer. Due to the strong negative effects of stemborer herbivory on host plant survival, the increase in stemborer abundance led to increased host plant mortality. Early-season herbivory by larval T. bacharidis only affected the abundance of aphids and their predators, both of which were more abundant on trees with reduced early-season herbivory. Nitrogen fertilization had the most limited effects and only T. bacharidis larvae achieved higher densities on fertilized trees. Our results indicate that aphid tending by the exotic L. humile affects other insects on B. halimifolia more so than herbivory by the exploitative competitor T. bacharidis or nitrogen as a limiting nutrient.     

Effects of phosphorus supply on growth, phosphate concentration and cluster-root formation in three Lupinus species

Background and Aims

In some lupin species, phosphate deficiency induces cluster-root formation, which enhances P uptake by increasing root surface area and, more importantly, the release of root exudates which enhances P availability.

Methods

Three species of Lupinus, L. albus, L. atlanticus and L. micranthus, with inherently different relative growth rates were cultivated under hydroponics in a greenhouse at four phosphate concentrations (1, 10, 50 and 150 µm) to compare the role of internal P in regulating cluster-root formation.

Key Results

The highest growth rate was observed in L. atlanticus, followed by L. albus and L. micranthus. At 1 µm P, cluster-root formation was markedly induced in all three species. The highest P uptake and accumulation was observed in L. micranthus, followed by L. atlanticus and then L. albus. Inhibition of cluster-root formation was severe at 10 µm P in L. atlanticus, but occurred stepwise with increasing P concentration in the root medium in L. albus.

Conclusions

In L. atlanticus and L. albus cluster-root formation was suppressed by P treatments above 10 µm, indicating a P-inducible regulating system for cluster-root formation, as expected. By contrast, production of cluster roots in L. micranthus, in spite of a high internal P concentration, indicated a lower sensitivity to P status, which allowed P-toxicity symptoms to develop.     

Effects of spatial patterns on the pollination success of a less attractive species

Plant individuals rely on pollinator services for their reproduction and often have to share these services with co‐occurring neighbours, creating complex indirect plant–plant interactions. Many current theoretical models focus on the effect of floral resources’ density on the net outcome of these indirect plant–plant interactions, often neglecting the identity of plant species in the communities and especially the species’ spatial pattern. To fill this gap, we created a spatially explicit model whose goal was to study the interplay between relative densities and spatial distribution patterns of two plant species differing in their attractiveness for pollinators. Since theory predicts that pollinator behaviour strongly governs the outcome of pollination, we allowed the pollinators to systematically change their plant preferences based on their foraging experience. Thus the interplay between density and spatial patterns of plants was tested over a continuum of behaviours from specialists to generalists. Our most striking finding was that reproductive success of the less attractive species was affected in an opposite way by spatial patterns depending on whether the species had relatively low or high densities. Namely, when the less attractive species was highly abundant, its survival was higher when aggregated in large monospecific patches than when uniformly distributed. On the other hand, when the attractive species was more abundant, the less attractive species survived better when uniformly distributed. These results were consistent as long as the scale of the plant spatial aggregation was similar to or larger than the pollinators’ detection range. Our results suggest that aggregated plant spatial patterns manipulate pollinator behaviour by trapping them within monospecific patches. This effect was sufficiently strong to enhance the survival of a competitively inferior species and hence to act in a way similar to the more familiar niche or temporal separation among plant species.     

A plant pathogen reduces the enemy-free space of an insect herbivore on a shared host plant

An important mechanism in stabilizing tightly linked host-parasitoid and prey-predator interactions is the presence of refuges that protect organisms from their natural enemies. However, the presence and quality of refuges can be strongly affected by the environment. We show that infection of the host plant Silene latifolia by its specialist fungal plant pathogen Microbotryum violaceum dramatically alters the enemy-free space of a herbivore, the specialist noctuid seed predator Hadena bicruris, on their shared host plant. The pathogen arrests the development of seed capsules that serve as refuges for the herbivore's offspring against the specialist parasitoid Microplitis tristis, a major source of mortality of H. bicruris in the field. Pathogen infection resulted both in lower host-plant food quality, causing reduced adult emergence, and in twofold higher rates of parasitism of the herbivore. We interpret the strong oviposition preference of H. bicruris for uninfected plants in the field as an adaptive response, positioning offspring on refuge-rich, high-quality hosts. To our knowledge, this is the first demonstration that plant-inhabiting micro-organisms can affect higher trophic interactions through alteration of host refuge quality. We speculate that such interference can potentially destabilize tightly linked multitrophic interactions.     

Population quality,dispersal and numerical change in the gypsy moth,Lymantria dispar (L.)

Summary First instars from small and large gypsy moth eggs differ significantly in their head capsule width, weight, hatching time and the length of thoracic setae. Pupal weight and the developmental period of immature stages of the gypsy moth originating from small or large eggs do not differ significantly. The mean number of eggs per mass produced by females originating from small eggs is greater than that of females from large eggs although not statistically significant. Highly significant differences in mean egg size of egg masses of each type of female were also observed. The relationship between egg size and dispersal strategies are discussed.Paper No. 2229 Massachusetts Agricultural Experiment Station. University of Massachusetts at Amherst. This research supported (in part) from Experiment Station Project No. 355     

The effect of insect herbivory on seed production of Lupinus nootkatensis,an introduced species in Iceland

1. Lupinus nootkatensis is an exotic plant species that has been used for large‐scale sowing all around Iceland for land reclamation of eroded surfaces protected from livestock grazing.
2. Until the early 1990s, L. nootkatensis was free from any significant arthropod herbivory in Iceland, whereas, after 1991, many outbreaks of native insect species, primarily Ceramica pisi and Eupithecia satyrata, have been recorded. These outbreaks have caused repeated total defoliation of extensive areas of L. nootkatensis, although the effects on its development are mostly unknown.
3. We studied the effect of: (i) reduced herbivory; (ii) increased herbivory; and (iii) simulated increased herbivory, compared with (iv) unmanipulated herbivory, on defoliation and seed production of L. nootkatensis in a 3‐year field study within two sites at contrasting ages and successional stages.
4. The results obtained showed that: (i) seed production across all treatments was negatively related to defoliation; (ii) reduced herbivory had a positive effect on the number of flowering stems and seed yield; and (iii) these effects depended on age and/or the successional stage because they were only significant in the older L. nootkatensis site.
5. These findings indicate that arthropod herbivory may affect the invasiveness of L. nootkatensis in Iceland by reducing the seed production and the spatial distribution rate of late successional lupin communities.

     

Effects of linear furanocoumarins on the herbivore Spodoptera exigua and the parasitoid Archytas marmoratus: host quality and parasitoid success

We assessed the effects of three photoactivated linear furanocoumarins, secondary plant metabolites present in Apium spp. (Apiaceae) and other taxa, on the larval-pupal parasitoid, Archytas marmoratus (Townsend) (Diptera: Tachinidae) and the host Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae), in the presence and absence of ultraviolet radiation. These results then were compared with previously described responses of Archytas marmoratus and other hosts to different plant allelochemicals. Ultraviolet radiation had no consistent detrimental effects. Increasing concentrations of linear furanocoumarins (from 0 to 0.029% of fresh weight) increased mortality of both the host and parasitoid. Increasing linear furanocoumarin levels also prolonged larval development of surviving S. exigua, but had no effect on pupal developmental time or pupal mass. Consequently, there was no effect of increasing linear furanocoumarin levels on surviving parasitoid development time (from the time of host pupation) or size. These results indicate the effects of linear furanocoumarins are mediated through effects on the host. Because reported responses of Ar. marmoratus to other allelochemicals (e.g., flavones, methyl ketones) are mediated through host effects, these results support the hypothesis that Ar. marmoratus displays a generalized response to plant allelochemicals that are predictable based on host mediated effects.