This dissertation examined insect-plant, predator-prey, and predator-predator interactions as they relate to the invasive plant species Lythrum salicaria L. The first study determined the suitability of two L. salicaria herbivores, Myzus lythri and Galerucella calmariensis for the preimaginal development and survival and influence on adult weight of three predatory species commonly found in L. salicaria infested wetlands, Chrysoperla carnea, Coleomegilla maculata, and Harmonia axyridis. Myzus lythri is suitable prey for preimaginal survival of the three predatory species. Galerucella calmariensis larvae are highly suitable for development and survival of second instar to adult C. carnea and less suitable for H. axyridis;A greenhouse study determined that multiple herbivores (G. calmariensis and M. lythri) did not have an additive effect on L. salicaria as assessed by the number of leaves, dry aboveground biomass or internode length. However, the two predatory species (C. carnea and H. axyridis) decreased M. lythri survival. Chrysoperla carnea had no effect on G. calmariensis survival, but H. axyridis preyed on G. calmariensis when M. lythri was present. Finally, predator presence did not increase leaf area, number of leaves, above ground biomass, or internode length of L. salicaria . A field study using partial life table techniques determined that G. calmariensis preimaginal life stages experience high mortality from the egg to the second-third instar and mortality due to egg predation is below 3.1%. This is the first study to determine the pupation site of G. calmariensis pupae in L. salicaria surrounded by water. The pupation site was within the L. salicaria stem;To determine behaviors of predatory larvae when exposed to two L. salicaria herbivores, three laboratory experiments were performed. In a 24 hr time period, the majority of the behaviors observed were inactivity, feeding on prey, or feeding on another predator. Trophic interactions among introduced herbivores of L. salicaria and existing predators occurred on L. salicaria, which may affect the biological control of this invasive plant species.