Host plant population size determines cascading effects in a plant–herbivore–parasitoid system

Multi-trophic consequences of mass flowering in two bamboos (Poales: Poaceae)

Mass flowering (masting) has been hypothesized to be an adaptive strategy to satiate florivores/granivores. However, few studies have corroborated this by examining seed predation in multiple flowering patches of varying sizes across a wide geographical range over multiple years. Moreover, the trophic consequences of masting for the parasitoids of florivores/granivores and their feedback effects are poorly understood. Here, we used the nationwide masting of two bamboo species, Sasamorpha borealis var. borealis and Phyllostachys nigra var. henonis, in Japan and compared florivory and seed sets in multiple flowering patches during the masting year and the following sporadic flowering years. We found lower florivory damage in both bamboo species and higher seed set for Sasamorpha borealis var. borealis in patches with massive and spatiotemporally isolated flowering. Additionally, the relative level of parasitism of florivores increased considerably in the sporadic flowering year, particularly in large flowering patches of Sasamorpha borealis var. borealis. Our results indicate the importance of spatiotemporal isolation during masting for satiating two dipteran florivores and suggest that parasitoids might rapidly suppress the extent of florivory in the sporadic flowering years after masting. Collectively, our study highlights the importance of considering multi-trophic consequences in understanding the adaptive significance of masting.

Pre-dispersal seed predators boost seed production in a short-lived plant

Pre-dispersal seed predation diminishes fitness and population growth rate of many plant species. Therefore, plants have developed multiple strategies to reduce the harmful effects of this type of herbivory. The present study aims to determine the effect of pre-dispersal seed predators (PSPs) on the fitness of a short-lived herb, and to discern the mechanisms allowing the plants to reduce the impact of pre-dispersal seed predation. Knowing that the interplay between pre-dispersal seed predators and plants is strongly shaped by the presence of other co-occurring organisms, we tested whether detritivores modulate plant responses towards pre-dispersal seed predators. To do so, we experimentally manipulated in the field pre-dispersal seed predators and detritivores interacting with the short-lived herb Moricandia moricandioides. We found that detritivores did not alter the response of plants to PSPs. Strikingly, the plant overcompensated for pre-dispersal seed predation, almost doubling the number of seeds produced. Plant response to PSPs led to substantial changes in shoot architecture, reproductive traits, chemical defences in leaves and seeds and in seed nutrient content. The overcompensating mechanism seems to be meristem activation, which allowed plants to produce more reproductive tissue, and increasing the proportion of ovules that became seeds, a response which specifically compensates for pre-dispersal seed predation. As far as we know, this is the first experimental evidence of a positive effect of PSPs on plant lifetime fitness as a consequence of plant overcompensation.

Flowering schedule in a perennial plant; life-history trade-offs, seed predation, and total offspring fitness

Optimal timing of reproduction within a season may be influenced by several abiotic and biotic factors. These factors sometimes affect different components of fitness, making assessments of net selection difficult. We used estimates of offspring fitness to examine how pre-dispersal seed predation influences selection on flowering schedule in an herb with a bimodal flowering pattern, Actaea spicata. Within individuals, seeds from flowers on early terminal inflorescences had a higher germination rate and produced larger seedlings than seeds from flowers on late basal inflorescences. Reproductive value, estimated using demographic integral projection models and accounting for size-dependent differences in future performance, was two times higher for intact seeds from early flowers than for seeds from late flowers. Fruits from late flowers were, however, much more likely to escape seed predation than fruits from early flowers. Reproductive values of early and late flowers balanced at a predation intensity of 63%. Across 15 natural populations, the strength of selection for allocation to late flowers was positively correlated with mean seed predation intensity. Our results suggest that the optimal shape of the flowering schedule, in terms of the allocation between early and late flowers, is determined by the trade-off between offspring number and quality, and that variation in antagonistic interactions among populations influences the balancing of this trade-off. At the same time they illustrate that phenotypic selection analyses that fail to account for differences in offspring fitness might be misleading.

Molecular analysis reveals high compartmentalization in aphid-primary parasitoid networks and low parasitoid sharing between crop and noncrop habitats

The ecosystem service of insect pest regulation by natural enemies, such as primary parasitoids, may be enhanced by the presence of uncultivated, semi-natural habitats within agro-ecosystems, although quantifying such host-parasitoid interactions is difficult. Here, we use rRNA 16S gene sequencing to assess both the level of parasitism by Aphidiinae primary parasitoids and parasitoid identity on a large sample of aphids collected in cultivated and uncultivated agricultural habitats in Western France. We used these data to construct ecological networks to assess the level of compartmentalization between aphid and parasitoid food webs of cultivated and uncultivated habitats. We evaluated the extent to which uncultivated margins provided a resource for parasitoids shared between pest and nonpest aphids. We compared the observed quantitative ecological network described by our molecular approach to an empirical qualitative network based on aphid-parasitoid interactions from traditional rearing data found in the literature. We found that the molecular network was highly compartmentalized and that parasitoid sharing is relatively rare between aphids, especially between crop and noncrop compartments. Moreover, the few cases of putative shared generalist parasitoids were questionable and could be due to the lack of discrimination of cryptic species or from intraspecific host specialization. Our results suggest that apparent competition mediated by Aphidiinae parasitoids is probably rare in agricultural areas and that the contribution of field margins as a source of these biocontrol agents is much more limited than expected. Further large-scale (spatial and temporal) studies on other crops and noncrop habitats are needed to confirm this.

Small-scale resource tracking in a population of a long-lived insect

How plant-feeding insects distribute themselves and utilize their host plant resources is still poorly understood. Several processes may be involved, and their relative roles may vary with the spatial scale considered. Herein, we investigate small-scale patterns, namely how population density of a gall midge is affected by individual growth form, phenology, and microsite characteristics of its herb host. The long-lived plant individuals vary much with regard to number of shoots, flower abundance, and flowering phenology. This variation is connected to site characteristics, primarily the degree of sun exposure. The monophagous insect galls the flowers of the host plant – an easily defined food resource. It is a poor disperser, but very long-lived; diapausing larvae can stay in the soil for many years. Galls were censused on individual plants during 5 years; from a peak to a low in gall population density. Only a very small fraction of the flowers produced (<0.5%) were galled even in the peak year. Nevertheless, most plant individuals had galls at least 1 year. In a stepwise multiple regression, plant size (number of shoots) was found to be the most important predictor of gall density (galls/flower). However, gall density decreased more than one order of magnitude over the plant size range observed. There was also a weak effect of plant phenology. Early flowering plants had lower gall densities than those starting later. Sun exposure had no direct effect on gall density, but a path analysis revealed indirect effects via the timing of flowering. Gall population change was highly synchronous in different parts of the study area with no significant decrease in synchrony with distance.