Interactive effects of mycorrhizal fungi, salt stress and exploitative competition on the herbivores of Baccharis halimifolia

Background/Question/Methods Plant stress and association with mycorrhizal fungi have been shown to influence plant quality significantly, yet their roles in influencing plant-insect interactions remain unclear.  Even less is known about how these factors might interact with or be modified by within-trophic level interactions.    Here we report the results of a factorial field experiment in which the effects of within-trophic-level interactions, plant stress, and mycorrhizae on three herbivores of the coastal plant Baccharis halimifolia were examined.  Plant stress was increased by adding salt to the soil, and availability of mycorrhizal fungi was increased by inoculating plant roots with a combination of arbuscular and ectomycorrhizae.  These treatments were applied to plants with either low or high densities of a chrysomelid beetle competitor (Trirhabda baccharidis).

Results/Conclusions For the two leaf miners, Amauromyza maculosa and Liriomyza trifolii (Diptera: Agromyzidae), increased soil salinity and high densities of the competitor Trirhabda baccharidis resulted in significant decreases in density.  Neither of these treatments affected the gall maker Neolasioptera lathami (Diptera: Cecidomyiidae).   Mycorrhizal fungi increased the densities of all three herbivores, possibly by increasing foliar nitrogen levels.  For the two leaf miners, there was evidence that mycorrhizae ameliorated the negative effects of salt stress.  There was also evidence that high levels of competition dampened the positive effects of mycorrhizae on the two leaf miners.