Neighboring plants can sometimes influence the amount of insect damage experienced by plants of interest (crops or weeds). This phenomenon ("associational effects") is the basis for some pest management strategies in agriculture and forestry and might occur through many mechanisms. For instance, neighboring plants may emit smells that repel insects, or attract predators that feed on insects. Although the possibilities are numerous, we understand little about how these specific mechanisms operate and the long-term consequences for plant communities. In a previous study, associational effects were observed between two common weedy plants (Carolina horsenettle and Tall goldenrod). This study will examine how goldenrod neighbors influence insect damage to horsenettle, focusing on four possible mechanisms. Results from this study will contribute to a larger study examining how insect pests influence the persistence of plants (such as the weedy horsenettle) in a community of other plants.
Understanding the mechanisms by which neighboring plants influence insect damage may lead to more effective management of weeds (where we may want to increase insect damage) and crops (where we may want to reduce insect damage). Results will be presented to agricultural researchers and farmers through seminars, newsletters, and workshops at the local agricultural experiment station and surrounding universities.
Neighboring plants can affect plant damage (and thus performance) through attracting or repelling insect herbivores. This phenomenon is known as associational susceptibility (AS) or resistance (AR), respectively and has been the basis for trap cropping and diversified planting in agriculture. Although a large literature tests for the existence of AR/AS and discusses AR/AS applications in agro-forestry, data on AR/AS mechanisms are largely anecdotal with the long-term consequences of AR/AS for plant performance and plant community structure (particularly in non-agricultural settings) poorly understood. The goal of this project was to examine how the local neighborhood can influence damage, and four mechanisms that might affect this influence on damage. We looked at how the total density of plants within the neighborhood, density of host-plants, and relative density (i.e. frequency) of host-plants influenced damage and AR/AS mechanisms. These features of the neighborhood are often confounded which can obscure the actual mechanisms leading to damage patterns so we used experimental designs that avoid this confounding. We focused our study on Solanum carolinense which is a weedy perennial plant, native to the south-east US, but considered invasive in other parts of the US, Europe and Asia. Solanum receives damage from both generalist and specialist leaf-feeding insects. Knowledge of how neighborhood composition influences susceptibility to herbivore damage may provide insight into effective management of Solanum with biocontrol agents. We used a series of field and greenhouse experiments to manipulate the density and frequency of two plants (focal plant: Solanum; non-focal Solidago altissima) and measure Solanum damage and four mechanisms suspected to affect damage through effects on host-plants or herbivores. These mechanisms include predator abundance and richness, microclimate (e.g. soil moisture, light intensity, and temperature), plant quality (measured as the relative growth rate of a specialist leaf feeding beetle larva (Leptinotarsa juncta) and the searching behavior of L. juncta adult females. We found that Solanum damage and all four AR/AS mechanisms were influenced by neighborhood composition but damage and the mechanisms responded differently to density and frequency of plants (Table 1). For example, Solanum damage and predator communities were positively influenced by the frequency of Solidago but some microclimate variables (e.g. light intensity and ambient temperature) were negatively affected by the density of Solidago. Plant density had a negative effect on plant quality (measured as the relative growth rate of a specialist Solanum beetle Leptinotarsa juncta), irrespective of the identity of the neighboring plant. These results suggest that different features of the neighborhood may affect herbivores and plants in complex ways, and that multiple mechanisms may operate simultaneously to influence plant damage patterns. Understanding how neighboring plants influence damage and plant performance has implications for invasive species management and insect pest management in agricultural settings. For example, the use of biocontrol agents (specifically insect herbivores) to reduce Solanum densities may only be effective in certain types of plant neighborhoods. Furthermore, results of the proposed research may help in the development of more effective insect pest management strategies. AR is used in agriculture and forestry, therefore understanding the particular mechanisms by which neighbors influence damage may offer insight into more effective land-management practices (e.g. crop density, fertilizer use, shade cropping, trap cropping) to maximize yield with minimal pesticide use. This research project has also contributed to training new scientists (one PhD student and two undergraduates). Over the course of this grant, we presented our research results in six different venues (e.g. classrooms, professional meetings, departmental seminars, meetings) in the US and Europe. We presented our results to both academics at various stages of their careers (e.g. undergraduates to scientists at professional meetings) and non-academics (e.g. meetings with agricultural workers and citizens at our field site).