Impact of simulated herbivory on Alliaria petiolata survival, growth, and reproduction

In weed biological control, insect damage to target weeds can be simulated in invaded habitats to study potential responses of the plant to introduced natural enemies. In the present study, we investigated the impact of two levels of manual flower-shoot damage (shoots cut at tip or base) on Alliaria petiolata (garlic mustard) survival, size, and reproduction. Experiments were conducted in 2002 and 2003 using invasive field populations of A. petiolata under naturally varying plant densities. Plant survival was recorded, and size and reproduction parameters were measured. Manual flower-shoot damage had a significant effect on plant survival. In both years, fewer plants survived in the basal-cut treatment than in either the control (un-cut) or tip-cut treatment. Plant size and reproductive output were likewise reduced in the basal-cut treatment. In both years, total seed production was significantly lower in the basal-cut treatment than either the control or tip-cut treatment. When combined, increased mortality and reduced seed production of basal-cut plants greatly reduced the contribution these plants made to the seed bank. Plant density did not affect reproduction or plant size. The impacts of cutting were consistent across years and sites with distinct biotic and abiotic conditions, and A. petiolata densities. We anticipate that herbivore damage to A. petiolata populations by introduced biological control agents will likewise remain consistent under varying biotic and abiotic conditions if the agents are equally adapted to these.