Mark Schwarzländer

Inference of allelopathy is complicated by effects of activated carbon on plant growth

Allelopathy can play an important role in structuring plant communities, but allelopathic effects are often difficult to detect because many methods used to test for allelopathy can be confounded by experimental artifacts. The use of activated carbon, a technique for neutralizing allelopathic compounds, is now employed in tests for allelopathy; however, this technique also could produce large experimental artifacts. In three independent experiments, it was shown that adding activated carbon to potting media affected nutrient availability and plant growth. For most species tested, activated carbon increased plant biomass, even in the absence of the potentially allelopathic agent. The increased growth corresponded to increased plant nitrogen content, likely resulting from greater nitrogen availability. Activated carbon also affected nitrogen and other nutrient concentrations in soil media in the absence of plants. The observed effects of activated carbon on plant growth can confound its use to test for allelopathy. The detection of allelopathy relies on the difference between plant growth in medium with carbon and that in medium without carbon in the presence of the potentially allelopathic competitor; however, this difference may be biased if activated carbon alters soil nutrient availability and plant growth even in the absence of the focal allelopathic agent.

Successes We May Not Have Had: A Retrospective Analysis of Selected Weed Biological Control Agents in the United States

Abstract In this paper, we describe five successful classical biological weed control agents released in the United States. For each of the five arthropod species, we compared data from prerelease studies that experimentally predicted the agents host range with data collected postrelease. In general, experimental host range data accurately predicted or overestimated risks to nontarget plants. We compare the five cases with insects recently denied for introduction in the United States and conclude that none of the discussed agents would likely be approved if they were petitioned today. Three agents would be rejected because they potentially could attack economic plants, and two because of potential attack on threatened or endangered plants. All five biocontrol agents have contributed significantly to the successful management of major weeds with no or minimal environmental risk. We believe that the United States may miss opportunities for sustainable and environmentally benign management of weeds using biological control if the regulatory framework only considers the risks of agents as potential plant pests and treats any host-range data regarding economic or threatened and endangered species as a binary decision (i.e., mandates rejection if there is any chance of feeding or development). As a way forward we propose the following: (1) the addition of risk and benefit analyses at the habitat level with a clear ranking of decision-making criteria as part of the U.S. Department of Agriculture Animal and Plant Health Inspection Service Technical Advisory Groups evaluation process of biocontrol agents; (2) recognition of the primacy of realized host range data for potential agents that considers the insects host selection behavior instead of emphasizing fundamental host range data during release evaluations, and (3) development of formalized postrelease monitoring of target and nontarget species as part of the release permit. These recommendations may initially be advanced through reassessment of current policies but may in the longer term require the implementation of dedicated biocontrol legislation.

Induced chemical defenses in invasive plants: a case study with Cynoglossum officinale L.

The ‘evolution of increased competitive ability’ (EICA) hypothesis is an extension of optimal defense theory and predicts that reduced pressure from insect herbivores in the introduced range results in evolution of reduced defenses in invading plant populations, allowing greater allocation of resources to competitive traits such as growth rate and reproduction. The EICA hypothesis considered levels of defensive chemistry to be fixed within a particular genotype. In this paper, we propose that if herbivory is reduced in the introduced range, but chemical defenses are inducible in response to herbivory, evolution of reduced defenses and any associated increase competitive ability should not occur. Rather, mean induced and constitutive levels of induced defenses should be similar in introduced and native ranges, but the variance about mean induced levels should be greater in the introduced range. This is predicted because induced levels will occur less frequently in the introduced range where herbivory is reduced, thereby insulating these levels from the stabilizing selection expected in the native range where induced levels occur more frequently. We conducted a preliminary study to examine this by comparing constitutive and induced concentrations of total pyrrolizidine alkaloids (PAs) from native (European) and introduced (western North America) populations of Cynoglossum officinale L. The mean constitutive and induced concentrations of PAs did not differ between continents, but the variability of the induced concentrations was significantly greater for plants from the introduced range. Although our study with C. officinale is provisional due to a small sample size, it supports our predictions for evolution of inducible defenses in introduced ranges where herbivore pressure is reduced. Most chemical defenses in plants have been found to be inducible, so similar patterns may occur widely. If so, this weakens the generality of EICA’s predictions concerning chemical defenses. The effects of inducible defenses should be considered in cross-continent comparisons of other invasive plant species.

Hybrid vigor in the biological control agent, Longitarsus jacobaeae

Hybridization is an important evolutionary mechanism that can increase the fitness and adaptive potential of populations. A growing body of evidence supports its importance as a key factor contributing to rapid evolution in invasive species, but the effects of hybridization have rarely been assessed in intentionally introduced biological control agents. We investigated hybrids between a Swiss and an Italian population of the beetle, Longitarsus jacobaeae, a biological control agent of Jacobaea vulgaris, by reciprocally crossing individuals in the laboratory. Phenological traits of F1 and F2 hybrid lineages showed intermediate values relative to their parental populations, with some maternal influence. Fitness of the F2 generation, measured as lifetime fecundity, was higher than that of the Italian parent in one of the lineages and higher than that of either parent in the other hybrid lineage. The increased fecundity of hybrids may benefit tansy ragwort biological control by increasing the establishment success and facilitating a more rapid population buildup in the early generations. Even though the long‐term consequences of hybridization in this and other systems are hard to predict, intentional hybridization may be a useful tool in biological control strategies as it would promote similar microevolutionary processes operating in numerous targeted invasive species.

Post-introduction evolution in the biological control agent Longitarsus jacobaeae (Coleoptera: Chrysomelidae).

Rapid evolution has rarely been assessed in biological control systems despite the similarity with biological invasions, which are widely used as model systems. We assessed post‐introduction climatic adaptation in a population of Longitarsus jacobaeae, a biological control agent of Jacobaea vulgaris, which originated from a low‐elevation site in Italy and was introduced in the USA to a high‐elevation site (Mt. Hood, Oregon) in the early 1980s. Life‐history characteristics of beetle populations from Mt. Hood, from two low‐elevation sites in Oregon (Italian origin) and from a high‐elevation site from Switzerland were compared in common gardens. The performance of low‐ and high‐elevation populations at a low‐ and a high‐elevation site was evaluated using reciprocal transplants. The results revealed significant changes in aestival diapause and shifts in phenology in the Mt. Hood population, compared with the low‐elevation populations. We found increased performance of the Mt. Hood population in its home environment compared with the low‐elevation populations that it originated from. The results indicate that the beetles at Mt. Hood have adapted to the cooler conditions by life‐history changes that conform to predictions based on theory and the phenology of the cold‐adapted Swiss beetles.

Density dependence, precipitation and biological control agent herbivory influence landscape‐scale dynamics of the invasive Eurasian plant Linaria dalmatica

Summary 1. Resource availability and natural enemies are among the most commonly cited mechanisms affecting competitive ability of invasive plants, but their simultaneous effects on plant dynamics are seldom evaluated in the field. Understanding how endogenous and exogenous factors affect invasive plant abundance is essential when evaluating the impact of classical weed biological control agents because misinterpretations of the mechanisms regulating plant demography may bias inference of herbivore impact. 2. In this study, we report results from a citizen-science monitoring programme initiated to evaluate the effects of the stem-mining weevil Mecinus janthiniformis on the Eurasian invasive weed Dalmatian toadflax Linaria dalmatica. We used a discrete model of population dynamics to evaluate the relative importance of endogenous and exogenous processes affecting changes in ramet density of L. dalmatica at 38 spatially replicated sites in Idaho, USA. 3. Analysis of per-capita population growth rates based on ramet density indicated the presence of direct density dependence, which was most likely due to intraspecific competition. Changes in ramet density were not influenced by the abundance of the resident plant community. 4. Precipitation had a strong, positive effect on ramet density of L. dalmatica, while the abundance of M. janthiniformis had a weaker, but significant, negative effect after accounting for the density dependence. There was no support for an interactive effect between precipitation and herbivory. 5. Synthesis and applications. Our results indicate that biological control is an important factor affecting weed population growth at the landscape scale, but they also suggest that biological control impact may vary considerably on local infestations due to site-specific variation in rainfall and density-dependent processes. We recommend that invasive plant management strategies integrate precipitation and biological control agent monitoring into their programmes to estimate expected biological control efficacy. Alternative control methods should be prioritized in areas where herbivore impact is expected to be low.

Propagule pressure, genetic structure, and geographic origins of Chondrilla juncea (Asteraceae): An apomictic invader on three continents

UNLABELLED PREMISE OF THE STUDY Assessing propagule pressure and geographic origins of invasive species provides insight into the invasion process. Rush skeletonweed (Chondrilla juncea; Asteraceae) is an apomictic, perennial plant that is invasive in Australia, South America (Argentina), and North America (Canada and the United States). This study comprehensively compares propagule pressure and geographic structure of genotypes to improve our understanding of a clonal invasion and enhance management strategies. • METHODS We analyzed 1056 native range plants from Eurasia and 1156 plants from three invaded continents using amplified fragment length polymorphism (AFLP) techniques. We used measures of diversity (Simpsons D) and evenness (E), analysis of molecular variance, and Mantel tests to compare invasions, and genotype similarity to determine origins of invasive genotypes. • KEY RESULTS We found 682 unique genotypes in the native range, but only 13 in the invaded regions. Each invaded region contained distinct AFLP genotypes, suggesting independent introduction events, probably with different geographic origins. Relatively low propagule pressure was associated with each introduction around the globe, but levels of among-population variation differed. We found exact AFLP genotype matches between the native and invaded ranges for five of the 13 invasive genotypes. • CONCLUSIONS Invasion dynamics can vary across invaded ranges within a species. Intensive sampling for molecular analyses can provide insight for understanding intraspecific invasion dynamics, which can hold significance for the management of plant species, especially by finding origins and distributions of invasive genotypes for classical biological control efforts.

Effect of generalist insect herbivores on introduced Lepidium draba (Brassicaceae): implications for the enemy release hypothesis

The enemy release hypothesis (ERH) states that decreased regulation by natural enemies allows plants to increase in distribution, abundance and vigour following their introduction into an exotic range. Invasive plants rarely escape herbivory entirely, and for hoary cress [Lepidium draba L. (Brassicaceae)] it has been demonstrated that generalist insect abundance is greater in its introduced North American range than in the native European range. We assessed the role of increased generalist herbivory on hoary cress using representatives of four important herbivore niches commonly found in the introduced range. We experimentally examined the density dependent impact of these herbivores individually and in combination on hoary cress in a series of greenhouse experiments. We found that defoliation of the oligophagous diamondback moth Plutella xylostella (L.) (Lep., Plutellidae) had the strongest and most consistent impact, while damage by the stem‐mining weevil Ceutorhynchus americanus Buchanan (Col., Curculionidae) tended to have the highest per capita effect. Plant response to feeding by the oligophagous crucifer flea beetle Phyllotreta cruciferae (Goeze) (Col., Chrysomelidae) was minor despite obvious feeding damage, and the impact of the polyphagous tarnished plant bug Lygus hesperus Knight (Het., Miridae) was negligible. In multiple‐species experiments, herbivore impacts were usually additive. In general, we found that hoary cress can tolerate high densities of oligophagous insect herbivory and effectively resisted attack by the polyphagous L. hesperus, but also the oligophagous C. americanus. Our results indicate that a combination of plant resistance and tolerance allows hoary cress to withstand increased generalist herbivore load in its introduced range, consistent with the predictions of the ERH.

Genetic Identity and Diversity of Perennial Pepperweed (Lepidium latifolium) in Its Native and Invaded Ranges

Abstract Perennial pepperweed is an invasive plant species in North America, native to temperate Eurasia and northern Africa. Effective biological control depends upon correct taxonomic identification. Therefore, we investigated morphological and genetic data (cpDNA sequences and amplified fragment length polymorphisms [AFLP]) in its native range, where the species is at times treated as multiple taxa (L. latifolium, L. affine and L. obtusum). We also analyzed genetic data to determine the number and distribution of haplotypes and genotypes in the invaded range. Using Bayesian analysis, we found three clusters of AFLP genotypes in the native range, but little correlation between these clusters and morphological characters used to distinguish taxa. Also, we found combinations of morphological character states within many native range plants that are incompatible with current species descriptions, offering no support for splitting L. latifolium sensu lato into three species. In North America 97% of the genetic variation was among populations and there were only eight AFLP genotypes in 288 plants, suggesting few introductions or a severe bottleneck, and little or no creation of new genotypes since introduction. We found plants in the native range that are genetically similar (88 to 99%) to six of the eight invasive AFLP genotypes, suggesting that Kazakhstan and China are origins for much of the North American invasion. Nomenclature: Perennial pepperweed, Lepidium latifolium L. LEPLA, Lepidium affine Ledeb., Lepidium obtusum Basiner Management Implications: Clear taxonomic identification of an invasive plant in its native and introduced ranges is a prerequisite for the success of any biological control project. Also, invasions can contain diverse assemblages of genotypes, which can affect herbivory. To verify the identity of perennial pepperweed populations from Eurasia that are being explored for biological control agent candidates we collected and analyzed morphological and genotypic data from populations in Eurasia and North America. In addition, we analyzed genotypic data to determine the diversity and distribution of perennial pepperweed genotypes in North America. Our results indicate that perennial pepperweed most probably consists of one taxon; not two or three as suggested in some recent floristic studies. We found that origins of common invasive genotypes are in Kazakhstan and China, suggesting that these regions should preferentially be searched in future foreign exploration for additional biological control agents. We found only eight invasive genotypes in North America, suggesting few introductions or a severe bottleneck, and little or no creation of new genotypes since introduction. The low diversity allowed us to provide seed representing all cpDNA sequence haplotypes for host-specificity studies, which should reduce the risk of any resistance to biological control agents being found after release.

Minimal genetic diversity in the facultatively outcrossing perennial pepperweed (Lepidium latifolium) invasion

Perennial pepperweed (Lepidium latifolium L.) is a Eurasian plant species that is invasive in North America. The invasion often forms large, dense monocultural stands. We investigated the genetic diversity along transects in dense populations in the western USA using Amplified Fragment Length Polymorphisms. We also analyzed transect collections from the native Eurasian range for comparison. In addition, we conducted crossing studies to determine possible modes of seed production (sexual outcrossing vs. self-fertilization vs. apomixis). In our study of seed production we determined that self-fertilization and outcrossing both produce germinable seed in perennial pepperweed. Genetic diversity in the USA was unexpectedly low, with only three genotypes in 388 plants, and those three had genetic similarity of ≥98%. Up to 97% of the plants from Turkey and Russia transects were unique genotypes, while <4% of USA plants in a transect were unique. This lack of diversity in the USA samples suggests that perennial pepperweed, despite its success as an invader, is not well-positioned to adapt to new selective pressures, or to recruit pre-adapted genotypes that may vary in resistance or tolerance to disease or herbivory. Because 99% of the USA plants were genetically identical, we were unable to determine if increases in stand size were due to spread by rhizomes or seed derived from outcrossing between genetically identical parents or self-fertilization, as each of these methods produces shoots genetically identical to parental plants.