Aulacaspis yasumatsui Delivers a Blow to International Cycad Horticulture

The literature covering the biology, invasion chronology, host plant responses, and control efforts of the armored scale Aulacaspis yasumatsui Takagi (Hempitera: Diaspididae) is reviewed. The small size of this cycad pest and complex surface morphology of the host cycad organs combine to make visual detection of every cryptic infestation difficult or impossible to achieve. The international movement of Cycas revoluta Thunb. nursery plants and the presence of C. revoluta nursery industries in so many countries have enabled this pest to wreak havoc on the international cycad horticulture trade over the last 25 years. The short pre-oviposition period and considerable female fecundity lead to rapid population expansion on the plants initially infested in newly invaded regions. A depletion of non-structural carbohydrates accompanies long-term infestations and precedes plant death. Enemy escape within the invasive range allows the scale population growth to remain unchecked until anthropogenic efforts establish non-native biological control.

Interpreting Pattern in Plant-Soil Feedback Experiments with Co-occurring Invasive Species: A Graphical Framework and Case Study

Despite the ubiquity of multiple plant invasions, the underlying mechanisms of invasive-invasive interactions remain relatively unknown. Given the importance of plant–soil feedback (PSF) in contributing to single species invasions, it may be an important factor influencing invasive–invasive species interactions as well. PSF between multiple invaders has rarely been examined, but could inform the nature of invasive–invasive interactions and advance understanding of how multiple invaders impact plant communities. Alternative mechanisms of plant invasions include novel weapons and enemy escape. We develop graphical PSF predictions based on these mechanisms and other possible invasive–invasive dynamics. Comparing these predictions to observed results is a first step in interpreting PSF among co-occurring invasive species. We illustrate this with a case study of net pairwise PSF among three common invaders of tallgrass prairie: Lotus corniculatus (birdsfoot trefoil), Phalaris arundinacea (reed canarygrass), and Cirsium arvense (Canada thistle). We found that feedback among all pairwise combinations of these invasive species was neutral. Neutral feedback can arise from a mutual lack of soil borne pathogens, consistent with the enemy escape hypothesis, although we cannot rule out shared benefit from generalist mutualists. While both facilitative and competitive interactions among these three species have previously been shown, our data suggest that such interactions are unlikely to operate through a legacy effect of PSF. Our results inform follow-up PSF experiments that would help to confirm the existence and nature of PSF interactions among these species.

A climate migrant escapes its parasites

When a species colonizes a new range, it can escape enemies found in its original range. Examples of enemy escape abound for invasive species, but are rare for climate migrants, which are populations of a species that colonize a new range due to climate-driven range shifts or expansions. The fiddler crab Minuca (=Uca) pugnax is found in the intertidal salt marshes of the US east coast. It recently expanded its range north into the Gulf of Maine as a result of ocean warming. We tested the hypothesis that M. pugnax had escaped its parasite enemies. Parasite richness and trematode intensity were lower in populations in the expanded range than in populations in the historical range, but infection prevalence did not differ. Although M. pugnax escaped most of its historical parasites when it migrated northward, it was infected with black-gill lamellae (indicative of Synophrya hypertrophica), which was found in the historical range, and with the trematode Odhneria cf. odhneri, which was not found in the historical range. To our knowledge, this is the first time that O. cf. odhneri has been reported in fiddler crabs. These results demonstrate that although M. pugnax escaped some of its historical parasites when it expanded its range, it appears to have gained a new parasite (O. cf. odhneri) in the expanded range. Overall, our results demonstrate that climate migrants can escape their enemies despite colonizing habitats adjacent to their enemy-filled historical range.

Testing the Enemy Release Hypothesis on tall-statured grasses in South Africa, using Arundo donax, Phragmites australis, and Phragmites mauritianus as models

The Enemy Release Hypothesis (ERH) predicts that introduced plant species can escape herbivory and therefore have a competitive advantage over native plants, which are exposed to both generalist and specialist natural enemies. In this study, the ERH was explored using the invasive alien species, Arundo donax and two native tall-statured grasses, the cosmopolitan Phragmites australis and African endemic Phragmites mauritianus in South Africa. It was predicted that A. donax would have reduced species richness of herbivores compared with the native Phragmites spp., that it would be devoid of specialist herbivores and would thus be experiencing enemy escape in the adventive range. The herbivore assemblages were determined from both field surveys and a literature review. The assumptions of the ERH were for the most part not met; 13 herbivores were found on A. donax compared with 17 on P. australis and 20 on P. mauritianus. Arundo donax had two specialist herbivores from its native range, and shared native herbivores with Phragmites spp. Although A. donax had reduced species richness and diversity compared with that found in the native distribution, it has partially re-acquired a herbivore assemblage which is similar to that found on analogous native species. This suggests that enemy release may not fully explain the invasive success of A. donax in South Africa.

Enemy escape: A general phenomenon in a fragmented literature?

Many populations are thought to be regulated, in part, by their natural enemies. If so, disruption of this regulation should allow rapid population growth. Such “enemy escape” may occur in a variety of circumstances, including invasion, natural range expansion, range edges, suppression of enemy populations, host shifting, phenological changes, and defensive innovation. Periods of relaxed enemy pressure also occur in, and may drive, population oscillations and outbreaks. We draw attention to similarities among circumstances of enemy escape and build a general conceptual framework for the phenomenon. Although these circumstances share common mechanisms and depend on common assumptions, enemy escape can involve dynamics operating on very different temporal and spatial scales. In particular, the duration of enemy escape is rarely considered but will likely vary among circumstances. Enemy escape can have important evolutionary consequences including increasing competitive ability, spurring diversification, or triggering enemy counteradaptation. These evolutionary consequences have been considered for plant–herbivore interactions and invasions but largely neglected for other circumstances of enemy escape. We aim to unite the fragmented literature, which we argue has impeded progress in building a broader understanding of the eco-evolutionary dynamics of enemy escape.