Hamback, P.A., J. Agren, and L. Ericson, 2000. Associational Resistance: Insect Damage to Purple Loosestrife Reduced in Thickets of Sweet Gale, Umeå University.

Reprinted with Permission from Ecology (2000) 81(7): 1784-1794

Associational Resistance: Insect Damage to Purple Loosestrife Reduced in Thickets of Sweet Gale

Abstract
Associational resistance occurs when herbivore damage to a focal plant is reduced by the presence of other plant species. Neighboring plants can reduce herbivore damage (1) by their effects on the predator community, (2) by reducing the ability of herbivore to find their host plants, and (3) by reducing the time herbivores remain on their host plant. We examined how the presence of the aromatic low shrub Myrica gale and of predatory lady beetles affected herbivore damage and reproductive output in a population of the perennial herb Lythrum salicaria in northern Sweden. An observational study showed that L. salicaria growing in Myrica thickets were less damaged by herbivores, had a lower abundance of the monophagus, leaf-feeding, chrysomelid beetle Galerucella calmariensis, and had higher flower and seed production than L. salicaria outside Myrica thickens. To test whether these differences could be explained by (a) differences in some aspect of plant quality, or (b) differences in predator abundance, we placed potted L. salicaria within and outside Myrica thickets. To determine whether differences in the abundance of G. calmariensis were primarily the result of different rates of colonization or emigration, we marked adult beetles and placed them on a second set of potted plants in the two micro-habitats. The results show that differences in herbivore abundance, plant damage, and reproductive output between potted L. salicaria placed within and outside Myrica thickets were in the same direction and of the same magnitude as those observed for naturally occurring plants, indicating that the observed patterns were not an effect of differences in the chemical composition of the host plant. Moreover, we found no support for the hypothesis that a higher abundance of insect predators could explain the lower abundance of G. calmariensis on L. salicaria in Myrica thickets. On the contrary, lady beetles (Coccinella quinqempuncta and Cocinella septemunctata) were observed on a greater proportion of the plants placed outside the Myrica thickets. The monitoring of marked beetles indicated that differences in the abundance of G. calmariensis were the result of a markedly higher colonization rate and a somewhat lower emigration rate from L. salicaria outride Myrica thickets. Outside the Myrica thickets. the survival of G. calmariensis and the magnitude of herbivore damage were lower, and fruit and seed output were higher on plants with observations of lady beetles than on plants without lady beetles. The results indicate that the abundance of the specialist herbivore G. calmariensis, and the herbivore damage and reproductive output of its host plant, L. salicaria, are affected both by the presence of the nonhost Myrica and by predation from lady beetles. We suggest that the most likely mechanism causing decreased feeding on L. salicaria growing in Myrica thickets is that Myrica affects the ability of G. calmariensis to find its host, either through visual or olfactory interference.

Entire Paper
Contact: Lars Ericson, Umeå University, Department of Ecology and Environmental Science, Biology Building, SE - 901 87 Umeå, SWEDEN
Key Words: Purple_loosestrife, Predators, Population_dynamics
Product Type: Research, Control
User Type: Resource Management