Anthropogenic chemical inputs (e.g., sodium chloride) modify plant demography by directly influencing plant performance and reproductive capacity. Inputs of these chemicals, especially as environmental gradients, may also indirectly modify plant demography by altering interactions with other species. Plant performance is greatly limited by herbivory however, the extent to which environmental chemical gradients may modify plant-herbivore interactions remains unclear. We examined how sodium chloride (hereafter “salt”) modifies plant growth and plant-herbivore interactions for tomato (Solanum lycopersicum cv. “Moneymaker”). We also explored how plant induction, modified via methyl-jasmonate (MeJA) application, is altered by soil salinization. First, we tested how a two-week exposure to a gradient in soil salinization (0 mM, 50 mM, 100 mM, and 150 mM) altered tomato stomatal conductance and relative height growth rate. We then used a 24-hour no-choice feeding assay with beet armyworm (Spodoptera exigua) to evaluate how a gradient in soil salinity and induction via MeJA modified generalist herbivory on tomato leaf tissue (i.e., proportion leaf tissue consumed). We also quantified the effects diet has on S. exigua mortality rates; herbivores were kept ad libitum on their assigned diet following feeding trials.
Results/Conclusions
Over a two-week salt addition treatment period, S. lycopersicum growth rates at 0 mM and 50 mM soil salinization concentrations were approximately double S. lycopersicum growth rates at 100 mM and 150 mM soil salinization concentrations (greater soil salinities were also associated with lower S. lycopersicum stomatal conductance. Greater soil salinities (100 mM and 150 mM salt) also corresponded to a 50-65% reduction in proportion leaf tissue consumption compared to the controls (0 mM salt). Herbivores consuming only leaves from 100mM and 150 mM salt-treated tomatoes were three and four times more likely to die during the 196-hour observation period than larvae fed leaves from 0 mM salt concentration plants. Treatment with MeJA resulted in 60% less leaf consumption in tomatoes treated with 0 mM salt, but MeJA application had no significant effects on leaf consumption at greater soil salinities. Our results suggest variation in soil salinity may modify patterns of herbivory by generalist insect herbivores. Anthropogenic caused salt gradients (e.g., proximity to winter road maintenance, agricultural legacies) may generate unique ecological stress legacies in natural plant populations that exhibit slower relative growth, but also experience less herbivory.