How plant neighborhood composition influences herbivory: Testing four mechanisms of associational resistance and susceptibility

The associational effects of host plant and mistletoe functional traits on leaf herbivory in mistletoe

Associational effects are a phenomenon in which herbivore damage on co-occurring plant species is influenced by neighboring plants. Mistletoes are a group of shrubs that obtain nutrients from host plants through haustoria. Despite the potential for mistletoe herbivory to be affected by associational effects with their hosts, the effects of host and mistletoe functional traits on mistletoe herbivory have been largely overlooked. This study aimed to evaluate the associational effects of host plants and the direct effects of mistletoe functional traits on mistletoe herbivory. To achieve this, we measured leaf herbivory and leaf traits of three mistletoe species (Dendrophthoe pentandra, Scurrula chingii var. yunnanensis, and Helixanthera parasitica) and their associated 11 host species during both dry and wet seasons. Our results showed that leaf herbivory of D. pentandra and S. chingii var. yunnanensis differed significantly on their respective host species, but H. parasitica did not. The relationships between mistletoe and the paired host herbivory differed between seasons, with a stronger positive relationship observed during the dry season. Furthermore, significant relationships were observed between paired leaf carbon, leaf nitrogen, and condensed tannin in mistletoes and their host plants, indicating that host plants can affect mistletoes' leaf functional traits. A group of mistletoe leaf traits provided significant predictions for leaf herbivory: leaves with higher leaf thickness and leaf total nitrogen showed higher herbivory. Overall, our study reveals that mistletoe leaf herbivory is directly affected by its leaf traits and indirectly affected by host associational effects, primarily through changes in mistletoes' leaf traits.

Synergistic effects of grass competition and insect herbivory on the weed Rumex obtusifolius in an inundative biocontrol approach

Outcomes of weed biological control projects are highly variable, but a mechanistic understanding of how top-down and bottom-up factors influence the success of weed biological control is often lacking. We grew Rumex obtusifolius, the most prominent native weed in European grasslands, in the presence and absence of competition from the grass Lolium perenne and subjected it to herbivory through targeted inoculation with root-boring Pyropteron spp. To explore whether the interactive effects of competition and inundative biological control were size-dependent, R. obtusifolius was planted covering a large range of plant sizes found in managed grasslands. Overall, competition from the grass sward reduced aboveground biomass and final root mass of R. obtusifolius about 62- and 7.5-fold, respectively, and increased root decay of R. obtusifolius from 14 to 58%. Herbivory alone increased only root decay. However, grass competition significantly enhanced infestation by Pyropteron spp. and, as a consequence, enhanced the impact of herbivory on aboveground biomass and final root mass. The synergistic effect was so strong that R. obtusifolius plants grown from initially smaller roots did no longer develop. Inoculating R. obtusifolius with Pyropteron species in grasslands should be further pursued as a promising inundative biological control strategy in the weed's native range.

Associational Effects and Indirect Interactions-The Dynamical Effects of Consumer and Resource Traits on Generalist-Resource Interactions

Trophic interaction modifications occur in food webs when the direct or indirect interaction between two species is affected by a third species. These behavioral modification effects are often referred to as associational effects. Changes in focal resource availability and consumption by a generalist herbivore can affect a range of outcomes from resource exclusion to multiple resources coexisting with the focal plant species. Here, we investigate the indirect interaction between a focal and alternative resource mediated by a generalist consumer. Using theoretical approaches we analyse the conceptual link between associational effects (both resistance and susceptibility) and the theory of apparent competition and resource switching. We find that changes in focal resource traits have the potential to affect the long-term outcome of indirect interactions. Inclusion of density-dependence expands generalist life-histories and broadens the range where, through associational effects, the availability of alternative resources positively influence a focal resource. We conclude that different forms of associational effects could, in the long-term, lead to a range of indirect interaction dynamics, including apparent competition and apparent mutualism. Our work aims to connects the theoretical body of work on indirect interactions to the concepts of associational effects. The indirect interactions between multiple resources need more thorough investigation to appreciate the range of associational effects that could result from the dynamical interaction between a generalist consumers and its focal and alternative resources.

Sequestration of Exogenous Volatiles by Plant Cuticular Waxes as a Mechanism of Passive Associational Resistance: A Proof of Concept

Numerous plant-derived volatile organic compounds (VOCs) induce the expression of resistance-related genes and thereby cause an "associational resistance" in neighbouring plants. However, VOCs can also be sequestered by plant cuticular waxes. In case that they maintain their biological activity, such sequestered VOCs could generate a "passive" associational resistance that is independent of any gene expression in the receiver. As a proof of concept, we used major components of the cuticular wax layers of the tree, Parkinsonia praecox, and conidia of Colletotrichum lindemuthianum, a fungal pathogen that has not been reported to infect P. praecox. Wax layers were re-constituted on glass slides and exposed to each of 20 pure VOCs for 1 d and then to ambient air for 1 d or 15 d. Gas chromatography-mass spectrometry (GC-MS) analyses showed that all 20 VOCs were sequestered by the re-constituted wax layers. Exposure to 18 of the VOCs significantly inhibited the germination of C. lindemuthianum conidia on these wax layers after 1 day of exposure to ambient air. Four of the VOCs: 4Z-heptenol, farnesene, limonene, and 2E-decenal, inhibited germination rates to less than 25% of the controls. After 15 d, all VOCs were still detectable, although at strongly reduced concentrations, and no significant inhibition of conidial germination could be detected anymore. Exogenous VOCs can be sequestered by the components of plant cuticular waxes and maintain their biological activity, at least over a certain time span: an effect that could generate a transient "passive associational resistance" to pathogens.

Low and high input agricultural fields have different effects on pest aphid abundance via different invasive alien weed species

We conducted field surveys to detect the population density of the most important invasive weed species and their associated virus vectoring aphids in crops grown under high input field (HIF) vs low-input field (LIF) conditions, with and without fertilizers and pesticides. The most frequent invasive weed species were Stenactisannua, Erigeroncanadensis and Solidagocanadensis. These species were hosts predominantly for the aphids Brachycaudushelichrysi and Aulacorthumsolani in both management systems. The 13% higher coverage of S.annua under LIF conditions resulted in a 30% higher B.helichrysi abundance and ~85% higher A.solani abundance compared with HIF conditions. Host plant quality was assessed by measuring peroxidase enzyme activity. There was a significantly increased POD activity at 10 μmol min−1 mg protein−1 unit in S.annua under LIF conditions, suggesting a higher stress by aphids under this management regime. The high colonization intensity of B.helichrysi on maize, potato and alfalfa crops were detected from both S.annua and E.canadensis. We conclude that new and faster methods need to be used to prevent colonization of such virus vectoring aphids and their host plants, even under low input regimes.