The invasive shrub Lonicera maackii receives significantly less herbivory in the field than two native relatives: evidence for enemy release

Background/Question/Methods

Some exotic invasive plants achieve increased vigor, abundance, and distribution in their novel habitat due to a release from regulation by herbivores, a concept known as the ‘Enemy Release Hypothesis.’ The introduced shrub Lonicera maackii has become one of the most important and prominent invasive plant species in the Midwestern United States. The distribution of L. maackii overlaps with that of several native congeners and other related species, allowing natural, yet phylogentically controlled, comparisons of herbivory in the field. At two sites in southwestern Ohio, we compared percent leaf damage from insect herbivores on L. maackii, Lonicera reticulata, and Viburnum prunifolium plants growing naturally in close proximity to each other.  We hypothesized that L. maackii would receive less damage than both native species due to enemy release.

Results/Conclusions

At the one site that was sampled in 2009, L. maackii received significantly lower amounts of folivory than the native L. reticulata throughout the season, and exhibited lower incidence of aphid attack.  In 2010, we resampled this site, added another site to the study, and added observations of the confamilial V. prunifolium.   The native L. reticulata received heavy damage (11.48% ± 25.22) from the specialist honeysuckle sawfly (Zarea inflata) early in the spring with low levels following through the rest of season; L. maackii received very low levels of damage (1.31% ± 6.07) that accumulated steadily through the season; and V. prunifolium received most damage (7.66% ± 14.63) in late summer. The low amount of damage on L. maackii compared to the native relatives and the presence of honeysuckle specialist herbivores feeding on natives and avoiding the invasive relative indicate that enemy release is occurring on L. maackii in North America. Additional research will include bioassays using Zarea inflata larvae to determine whether difference in herbivores rates in the field result from differential oviposition choices or larval performance.  Sampling is also currently being repeated with a third site with an attempt to characterize the insect community assemblage present on each shrub species.