Cytokinin-containing seaweed extract does not reduce damage by an insect herbivore

Abstract

We examined two aspects of treating plants with a cytokinin-containing seaweed extract (SWE). In the first series of experiments, we tested the hypothesis that immature lima bean (Phaseolus lunatus L.) and tomato (Lycopersicon esculentum Mill.) plants provided with exogenous cytokinins could recover from defoliation by a generalist insect herbivore, Spodoptera exigua (Hübner), more rapidly than plants without cytokinin supplements. However, the SWE inhibited growth of lima beans at all levels of herbivore damage. The SWE neither inhibited nor stimulated growth of tomatoes following defoliation. Because SWE effects largely were neutral for tomato growth, we conducted a second series of experiments to test the hypothesis that SWE treatments alter the attractiveness of tomato foliage to S. exigua larvae. In these experiments, we determined consumption of, and preference for, SWE-treated tomato foliage by S. exigua larvae. Repeated root applications of SWE led to increased consumption and preference by S. exigua. Repeated foliar applications did not alter consumption or preference compared with controls. Spodoptera exigua larvae gained significantly more mass when feeding on SWE-treated foliage compared with controls. While these data indicate that plant responses to exogenous cytokinin-containing materials depend on taxa and application method, the practical uses of SWE appear limited given the negative effects on plant growth and increased attractiveness of treated foliage to herbivores.