Resource limitation of tephritid flies on lesser burdock,Arctium minus (Hill) Bernh. (Compositae)

Summary The intensity of resource exploitation by phytophagous insects is usually considered to reflect population size. For populations of two flowerhead-attacking tephritid flies, however, the resources utilised were not related to the numbers of searching adults. Tephritis bardanae Schrank attacked 11–13% of the total flowerheads each year, and Cerajocera tussilaginis (Fab) 17–65%, despite much wider and uncorrelated variation in adult numbers. Analysis of field data showed that the proportion of flowerheads used was not limited by poor flowerhead quality, but was restricted by (1) the synchronisation of adult activity with the appearance of flowerheads at the correct age for oviposition, and (2) by factors which influenced the ability of female flies to locate available heads. These restrictions were more severe in T. bardanae and explained its relatively low rate of infestation. Both tephritids tended to avoid flowerheads in open areas. The processes governing resource exploitation in each tephritid operated independently of the other, and a partial separation of the two species between flowerhead types and habitats arose simply because of their different timing of attack.     

Effects of herbivory on the growth and reproduction of Picris hieracioides in the Mediterranean region

Abstract. The impact of herbivorous insects on the growth and reproduction of Picris hieracioides (Compositae) was followed in three consecutive years by comparing plants treated with an insecticide and untreated plants in an old-field population abandoned for 15 yr (P15). In the third year the interactions between herbivory and competition were analysed as well. Pre-dispersal seed predation of seed heads was investigated in three adjacent old-fields abandoned for one (P1), 15 (P15) and 40 yr (P40) in the region of Montpellier, France. Fields differed in vegetation cover and floristic composition and also in density and above-ground biomass of Picris hieracioides plants (from 246 g per plant in P1, 13.3 g in P15, down to 3.3 g in P40, for the first year of study). The cumulative rainfall between January and July, the period of Picris growth, decreased from 1992 to 1994. Moreover, in 1992, three times as much rainfall occurred in May and June – the period of stem elongation – as in 1993 and 1994. As a consequence, environmental stress increased during the study period. Herbivorous invertebrates had a negative effect on the growth and reproduction of individual plants of Picris hieracioides in P15. The effect was not constant with time, being apparent in only two years out of three. These two years (1992 and 1994) had contrasting climatic characteristics, whereas the year with no apparent effect (1993) was climatically intermediate. When herbivory had effects on growth, the insecticide-treated plants had a significantly higher above-ground biomass and produced twice as many seed heads as the control plants. There was no interaction between herbivory and competition. The percentage of damaged seed heads was significantly lower in the P1-field than in the P40-field. The P15-field had intermediate levels of predation. Predation on seed heads increased whereas biomass and reproductive output decreased between 1992 and 1994 in the three old-fields. Therefore, the results on seed predation (but not for above-ground biomass and reproduction) tend to confirm‘the environmental stress hypothesis’ of Mattson & Haack (1987) that states that plants under stress are more liable to be attacked by grazers.     

Consequences for host-parasitoid interactions of grazing-dependent habitat heterogeneity

1. Environmental heterogeneity can produce effects that cascade up to higher trophic levels and affect species interactions. We hypothesized that grazing-dependent habitat heterogeneity and grazing-independent host plant heterogeneity would influence directly and indirectly a host-parasitoid interaction in a woodland habitat. 2. Thistles were planted randomly in 20 birch woodlands, half of which are grazed by cattle. The abundances of two species of seed herbivore and their shared parasitoid were measured, and related to habitat and host-plant heterogeneity. 3. The presence of cattle grazing created a structurally and compositionally distinct plant assemblage from the ungrazed seminatural situation. Grazing did not affect the number or dispersion of the host plant underpinning the host-parasitoid interaction. 4. The density of one insect herbivore, Tephritis conura, and its parasitoid Pteromalus elevatus was significantly increased by the presence of cattle; but another herbivore, Xyphosia miliaria, was unaffected. The percentage of parasitism of T. conura was increased in grazed habitat occurring at twice the rate found in ungrazed habitat. 5. The increase in T. conura abundance was correlated with increased species richness and cover of forbs in grazed sites. This effect of grazing-dependent habitat variation on host insect density cascaded up to parasitoid density and percentage of parasitism. Habitat heterogeneity had a further direct, positive effect on parasitoid density and percentage of parasitism after controlling for host-insect density. 6. Independent of grazing, heterogeneity in host-plant flowering, architecture and stature further affected T. conura and its parasitoid's densities. Parasitoid density was also affected by the dispersion of the host plant. 7. A combination of habitat and host-plant scale environmental heterogeneity influenced a host-parasitoid interaction indirectly and directly, providing a rare example of an anthropogenic disturbance positively affecting a tertiary trophic level. This finding highlights the need to consider not only the importance of bottom-up effects for top-down processes, but also the role of environmental heterogeneity arising from anthropogenic disturbance for trophic interactions such as parasitism.     

Consequences for a host–parasitoid interaction of host-plant aggregation, isolation, and phenology

Abstract.  1. Spatial habitat structure can influence the likelihood of patch colonisation by dispersing individuals, and this likelihood may differ according to trophic position, potentially leading to a refuge from parasitism for hosts.
2. Whether habitat patch size, isolation, and host-plant heterogeneity differentially affected host and parasitoid abundance, and parasitism rates was tested using a tri-trophic thistle–herbivore–parasitoid system.
3.  Cirsium palustre thistles ( n = 240) were transplanted in 24 blocks replicated in two sites, creating a range of habitat patch sizes at increasing distance from a pre-existing source population. Plant architecture and phenological stage were measured for each plant and the numbers of the herbivore Tephritis conura and parasitoid Pteromalus elevatus recorded.
4. Mean herbivore numbers per plant increased with host-plant density per patch, but parasitoid numbers and parasitism rates were unaffected. Patch distance from the source population did not influence insect abundance or parasitism rates. Parasitoid abundance was positively correlated with host insect number, and parasitism rates were negatively density dependent. Host-plant phenological stage was positively correlated with herbivore and parasitoid abundance, and parasitism rates at both patch and host-plant scales.
5. The differential response between herbivore and parasitoid to host-plant density did not lead to a spatial refuge but may have contributed to the observed parasitism rates being negatively density dependent. Heterogeneity in patch quality, mediated by variation in host-plant phenology, was more important than spatial habitat structure for both the herbivore and parasitoid populations, and for parasitism rates.     

Host-specificity of Candidate Agents for the Biological Control of Onopordum spp. Thistles in Australia: an Assessment of Testing Procedures

This paper describes a series of host-specificity tests carried out to evaluate the safety of eight insects for release in Australia as biological control agents of Onopordum spp. thistles; Larinus latus L., Lixus cardui Ol. and Trichosirocalus briesei Alonso-Zarazaga & Sanchez-Ruiz (Coleoptera: Curculionidae), Tephritis postica Loew and Urophora terebrans Loew (Diptera: Tephritidae), Botanophila spinosa Rondani (Diptera: Anthomyiidae), Tettigometra sulphurea Mulsant & Rey. (Hemiptera: Fulgoroidae) and Eublemma amoena (Hbn.) (Lepidoptera: Noctuidae). Preliminary tests were first carried out against key test plants in the country of origin to determine whether it was worth proceeding with formal testing under quarantine conditions in Australia. In the formal testing procedure, the test plant list was based solely on phylogenetic relationships. The life-stage of each insect tested was selected with respect to its biology and behaviour, e.g. only oviposition was tested for insect species with immobile endophagous larvae, while larval host utilization was tested for those with mobile larval stages. The initial laboratory experiments for each species used were caged no-choice tests, as confinement without choice can elicit extreme behavioural responses, making negative results extremely robust. Where positive results occurred, insects where re-exposed to the plant, but this time given a choice between the target and non-target plants. The results of these tests, plus data from open-field preliminary tests in the country of origin, helped interpret results and determine the risk posed by each candidate biological control agent to non-target plant species. Following submission, all eight candidates were approved for release by the Australian plant biosecurity and conservation authorities. Based on a comparison of the eight species tested, it is recommended that host-specificity testing be kept flexible and pragmatic, rather than moving towards a formulaic risk assessment procedure. Moreover, given the improved state of knowledge of plant phylogenies and the evolution of host utilization, it is time to base testing procedures purely on phylogenetic grounds, without the need to include less related test species solely because of economic or conservation reasons.     

Consequences of variation in flowering phenology for seed head herbivory and reproductive success in Erigeron glaucus (Compositae)

Summary We examined the relationship between flowering phenology, reproductive success (seed production only), and seed head herbivory for 20 similarly sized clones of Erigeron glaucus growing at Bodega Bay Reserve, northern California, USA. Although clones tended to reach peak flowering on the same date, they differed in the proportion of their total flowers produced around that date (flowering synchrony). Clones also differed in the number and density of flower heads presented at any one time to pollinators and herbivores (floral display). Both of these characteristics had consequences for herbivory and plant reproductive success. The proportion of flower heads damaged by insect herbivores was greater for clones that concentrated flowering activity during the main flowering period for the population as a whole (high synchrony) compared to clones that spread flowering out temporally. The primary reason for this result was that clones with low flowering synchrony produced a significant proportion of their flower heads during the fall and therefore, escaped attack by the tephritid fly, Tephritis ovatipennis. Clones with intermediate synchrony had lower seed success (total number of viable seeds produced over the year) than clones with either low or high synchrony. The proportion of flower heads damaged by insect herbivores and number of tephritid flies reared from flower heads were both negatively correlated to floral display while seed head mass and germination rates were positively related to display. Thus, clones which produced dense floral displays were favored both in terms of reduced herbivory and increased successful seed production.     

Host refuges and spatial patterns of parasitism in an endophytic host–parasitoid system

Abstract. 1. Larvae of Tephritis conura Loew (Diptera: Tephritidae) live gregariously in flower heads of Cirsium heterophyllum (L.) Hill (Cardueae). They are attacked by the endoparasitic wasps Eurytoma sp. near tibialis Boheman (Hymenoptera: Eurytomidae) and Pteromalus caudiger (Graham) (Hymenoptera: Pteromalidae).
2. The responses of the parasitoids to different host patch sizes were investigated from the analysis of field samples. At the host population level, overall probabilities of parasitism were independent of host numbers per flower head or showed a tendency to inverse density-dependence for both parasitoid species.
3. Measurements of ovipositor length in Eurytoma and P.caudiger indicated that parts of the flower head constitute a structural refuge from parasitism.
4. The accessibility of hosts in a flower head was found to differ markedly, depending on larval locations and flower head characters. In spite of this high variability, similar average percentages of larvae were accessible to the parasitoids in each patch size class.
5. High variability of oviposition success in laboratory experiments can be explained by random locations of hosts in the flower heads.