Aphalara Itadori Research Articles

Microbiome of psyllids of the family Aphalaridae, including Aphalara itadori, a biocontrol agent against Reynoutria spp.

AbstractSeveral European and North American countries have started releasing the Japanese knotweed psyllid, Aphalara itadori (Shinji) (Hemiptera: Aphalaridae), to control the Japanese knotweed, Reynoutria japonica Houtt. (Polygonaceae), and its relatives, which are among the worst invasive exotic plants. However, establishing populations of the currently released strains in the field has not been successful, desiring newly collected lineages. Moreover, little is known about the microbiome of the current strains, which potentially impacts properties as biocontrol agents. Hence, this study analyzed the microbiota of an A. itadori strain newly collected on Honshu Island, Japan, along with related species of the family Aphalaridae, using amplicon sequencing of 16S rRNA genes. The localization of symbionts identified in A. itadori was further analyzed using fluorescence in situ hybridization. The results demonstrated that the A. itadori bacteriome, a specialized organ for microbial symbiosis, maintains a dual symbiotic system with the primary symbiont “Candidatus Carsonella ruddii” (Gammaproteobacteria: Oceanospirillales: Halomonadaceae) and the secondary symbiont Sodalis sp. (Gammaproteobacteria: Enterobacterales: Pectobacteriaceae), suggesting that they are evolutionarily stable obligate mutualists for A. itadori. The central area of the bacteriome containing Sodalis comprised uninucleate bacteriocytes with nuclei larger than those of bacteriocytes harboring Carsonella. This observation contrasted previous reports on various psyllid lineages in which secondary symbionts are housed in a central syncytium with nuclei smaller than those of bacteriocytes for Carsonella. No known plant pathogens or parasitic manipulators of insect reproduction were identified in the analyzed A. itadori strain, indicating its suitability as a biocontrol agent, posing a minimum risk to the ecosystem. Besides distinct Carsonella lineages, Sodalis independently acquired by Craspedolepta miyatakeai Klimaszewski and an ambiguous Enterobacterales symbiont in Epheloscyta kalopanacis Loginova were identified. Only Carsonella was found in Togepsylla matsumurana Kuwayama. These results indicate repeated infections and replacements of bacterial symbionts during the evolution of Psylloidea, providing deeper insights into the microbe‐psyllid interactions.

Could hybridization increase the establishment success of the biological control agent Aphalara itadori (Hemiptera: Aphalaridae) against invasive knotweeds?

Intraspecific hybridization between distinct populations could increase the fitness and adaptive potential of biological control agents that often have low genetic diversity and can be inbred due to long-term laboratory rearing often at small population sizes. Hybridization can also alter host preference and performance when the parental insect populations are adapted to different host plants. We investigated the effects of hybridization between two populations (Northern and Southern) of the psyllid, Aphalara itadori, that have different fitness on three invasive knotweed species (Japanese, giant, and Bohemian). Fecundity, host choice, and developmental success of second-generation reciprocal hybrids and the parental psyllid populations were compared on the three knotweed species in multiple-choice tests. Hybridization did not increase fecundity. All three knotweed species were accepted for oviposition without preference by the Southern and the two hybrid psyllid populations. The northern psyllid population laid the most eggs on Bohemian knotweeds but those were maladaptive choices since almost all eggs failed to develop. The developmental success of the parental psyllid populations was highest on the knotweed species they were originally collected from, on Japanese knotweed of the Southern psyllids and giant knotweed of the Northern psyllids. Hybrids had intermediate or higher survival on given knotweed hosts compared to their parents. These results can inform release tactics of A. itadori in different regions especially where there appear to be climatic and/or host mismatches such as in Michigan. In southern Michigan, based on climate the Northern psyllid population should be released. However, the most common knotweed species in the region are Bohemian and Japanese knotweeds that do not support the development of the Northern psyllids. In this case, hybrids that may carry cold adaptations of the Northern psyllids but have better developmental success on the prevailing knotweed species may be considered for release to increase establishment success.

Leaf‐roll gall formation in Reynoutria × bohemica and its implications for biological control with Aphalara itadori

AbstractThe psyllid Aphalara itadori Shinji (Hemiptera: Psyllidae) is a biocontrol agent against invasive knotweed species (Polygonaceae) in Europe and North America. Despite an extensive release program, successful establishment of the agent has never been confirmed. The slow establishment of A. itadori in its introduced range appears to be predominantly the result of nymphal mortality, either through desiccation, predation, or the adaptation of captive populations to low‐stress environments. In 2016, CABI UK collected a new line of psyllids from Murakami, Niigata Prefecture, in Japan, where it was observed causing severe leaf‐roll galling on one of the many varieties of native knotweeds. The induction of leaf‐roll galls by Murakami psyllids on invasive Bohemian knotweed, Reynoutria × bohemica, has the potential to alleviate some or all of the observed nymphal mortality factors. We conducted a series of growth chamber experiments to explore the psyllid behaviors that initiate leaf‐roll galls, and the ways in which the galls might benefit the psyllids and inhibit invasive knotweed. Reynoutria × bohemica exposed to A. itadori for 4 weeks suffered reduced stem elongation and leaf area compared to control plants. Artificial leaf‐roll galls were consistently utilized by developing A. itadori nymphs, and contributed to their survival. Leaf‐roll galling in R. × bohemica was predominantly initiated by early instar nymphs of A. itadori, and only occurred when leaves were attacked at a very early stage in their development. Our results can inform the knotweed biocontrol program in several key ways. Releases of A. itadori should be timed so that developing nymphs have access to newly emerging leaves. Monitoring efforts at release sites should focus on the growing tips of R. × bohemica, where the highly mobile nymphs appear to migrate. Finally, our results provide motivation to seek additional lines of A. itadori that may affect other invasive knotweed species in similar ways.

Climate suitability analyses compare the distributions of invasive knotweeds in Europe and North America with the source localities of their introduced biological control agents.

Climate suitability analyses based on ecological niche modeling provide a powerful tool for biological control practitioners to assess the likelihood of establishment of different candidate agents prior to their introduction in the field. These same analyses could also be performed to understand why some agents establish more easily than others. The release of three strains of Aphalara itadori (Shinji) (Hemiptera: Pysllidae), each from a different source locality in Japan, for the biological control of invasive knotweed species, Reynoutria spp. Houtt. (Caryophyllales: Polygonaceae), provides an important opportunity to compare the utility of climate suitability analyses for identifying potential climate-based limitations for successful biological control introductions. Here, we predict climate suitability envelopes for three target species of knotweed in Europe and two target species of knotweed in North America and compare these suitability estimates for each of these species to the source localities of each A. itadori strain. We find that source locality of one strain, the Kyushu strain, has little-to-no suitability compared to other locations in Japan based on knotweed records from Europe, supporting an earlier study based on North American Japanese knotweed records. The source locality of a second strain, the Murakami strain, was predicted to have medium-to-high suitability based on records of knotweeds from North America. In contrast, European records of Reynoutria × bohemica Chrtek & Chrtková and Reynoutria sachalinensis (F. Schmidt) Nakai predicted no suitability for this locality compared to other locations in Japan, while European records for Reynoutria japonica Houtt. predicted low suitability. The source locality of the final strain, the Hokkaido strain, was predicted as having medium-to-high suitability based on knotweed records of all examined species from both North America and Europe.

Effects of simulated psyllid herbivory on the root development of Japanese knotweed.

Japanese knotweed (Reynoutria japonica) is a notoriously invasive riparian plant species which spreads primarily through detachment and dispersal of rhizomes. Currently management options are limited, and there are no biological counters to its spread across the country besides one species. Aphalara itadori, a psyllid native to East Asia has recently been approved by the USDA to be released in the United States to control the spread of Japanese knotweed. This psyllid feeds by piercing aboveground tissues and sucking phloem from the stems and leaves. We investigated the potential role of aboveground herbivory by A. itdori on root system development and rhizome mass in a simulated herbivory experiment. We hypothesized that simulated herbivory would induce the plants to allocate more resources to aboveground tissues for defense or regrowth, potentially limiting root development and reducing rhizome mass. We conducted an experiment with 84 young Japanese knotweed plants divided into two treatment groups: control and simulated herbivory. The simulated herbivory treatment was applied weekly, and plants were harvested weekly to assess changes in rhizome mass and root system development via high resolution root scanning using WinRhizo software. We found that simulated herbivory does affect the root system, though results were inconsistent between weeks. Simulated herbivory decreased root length in week 5 and reduced rhizome mass in weeks 3 and 6. While results were idiosyncratic, they highlight that simulated herbivory may negatively impact root system development in Japanese knotweed, with potential benefits for limiting the spread of knotweed invasion.

A new population of the biocontrol agent Aphalara itadori performs best on the hybrid host Reynoutria x bohemica

Aphalara itadori is a biological control agent of the invasive Asian knotweeds Reynoutria japonica, Reynoutria sachalinensis and their hybrid Reynoutria × bohemica, which emerged across different areas of their introduced range, including Europe, North America and Oceania. The performance of A. itadori on these three target plants differs between geographically distant psyllid populations. A petition to release a population of this psyllid freshly collected in Murakami (Japan) to control the three target species in the Netherlands was approved in 2020. In order to optimize a biocontrol program using this A. itadori population, we assessed the effect of the three knotweed species on its performance, impact on plant growth and oviposition preference. The results of no-choice experiments indicated that the Murakami population performed best on R. × bohemica, where juveniles developed the fastest and the number of emerged adults was twice as high as that recorded on the other two Reynoutria hosts. These differences in performance between hosts were associated with a lower acceptance of R. sachalinensis for oviposition and a higher juvenile mortality on R. japonica. Infestation with the Murakami population had an overall negative impact on final stem length, so that infested plants were around 8 % shorter than control plants, and it reduced final rhizome fresh biomass by circa 50 % in R. sachalinensis and 35 % in R. × bohemica. When subjected to two-choice tests, females of the Murakami population did not show an oviposition preference for any of the Reynoutria species. These results suggest that in the Netherlands R. × bohemica is the best host to optimize the rearing of the Murakami population, and field releases should target this host species to promote establishment in the field. Based on these results, the Murakami population is expected to have the largest impact in the field on R. × bohemica and R. sachalinensis.

Predation and Climate Limit Establishment Success of the Kyushu Strain of the Biological Control Agent Aphalara itadori (Hemiptera: Aphalaridae) in the Northeastern United States.

Species of knotweeds, Reynoutria spp. Houtt. (Caryophyllales: Polygonaceae), including Japanese knotweed (R. japonica Houtt.), are among the most invasive and ecologically destructive plant species introduced to North America and Europe. The Kyushu strain of the psyllid Aphalara itadori Shinji (Hemiptera: Aphalaridae) has been approved as a biological control agent for release against Japanese knotweed in the United Kingdom, Canada, and the United States. However, recent reports from Canada suggest that both biotic and abiotic factors may limit its establishment. Therefore, we examined the potential role of predation by comparing open- versus closed-sleeve treatments, and climate mis-matches by collecting temperature data from release sites and performing climate suitability analyses using MaxEnt. Our results indicated that populations of the Kyushu strain could only be maintained in the field in closed-sleeve treatments, suggesting that predation is likely limiting the establishment success of this strain. In addition, we noted that daily maximum temperatures at our field sites might exceed documented developmental thresholds for this strain, and that MaxEnt species distribution modeling indicates no climate similarities between locations in eastern North America and Kyushu. Combined with previous results, our study suggests that the establishment of the Kyushu strain of A. itadori as a biological control agent for Japanese knotweed may be limited in eastern North America. We suggest that one strategy to increase the probability of establishment of the Kyushu strain could be to increase the number of release sites in an effort to find a more optimal niche with predator-free space.

Combining photoperiod and thermal responses to predict phenological mismatch for introduced insects.

A wide variety of organisms use the regular seasonal changes in photoperiod as a cue to align their life cycles with favorable conditions. Yet the phenological consequences of photoperiodism for organisms exposed to new climates are often overlooked. We present a conceptual approach and phenology model that maps voltinism (generations per year) and the degree of phenological mismatch that can arise when organisms with a short-day diapause response are introduced to new regions or are otherwise exposed to new climates. Our degree-day-based model combines continent-wide spatialized daily climate data, calculated date-specific and latitude-specific day lengths, and experimentally determined developmental responses to both photoperiod and temperature. Using the case of the knotweed psyllid Aphalara itadori, a new biological control agent being introduced from Japan to North America and Europe to control an invasive weed, we show how incorporating a short-day diapause response will result in geographic patterns of attempted voltinism that are strikingly different from the potential number of generations based on degree-days alone. The difference between the attempted and potential generations represents a quantitative measure of phenological mismatch between diapause timing and the end of the growing season. We conclude that insects moved from lower to higher latitudes (or to cooler climates) will tend to diapause too late, potentially resulting in high mortality from inclement weather, and those moved from higher to lower latitude (to warmer climates) may be prone to diapausing too early, therefore not fully exploiting the growing season and/or suffering from insufficient reserves for the longer duration in diapause. Mapped output reveals a central region with good phenology match that shifts north or south depending on the geographic source of the insect and its corresponding critical photoperiod for diapause. These results have direct relevance for efforts to establish populations of classical biocontrol agents. More generally, our approach and model could be applied to a wide variety of photoperiod- and temperature-sensitive organisms that are exposed to changes in climate, including resident and invasive agricultural pests and species of conservation concern.

Long-term captive-rearing affects oviposition behavior and nymphal survival of a weed biological control agent

Adaptation to artificial rearing conditions is a well-documented phenomenon in insects and has been shown to reduce the efficacy of biological control agents. The knotweed psyllid, Aphalara itadori Shinji (Hemiptera: Psyllidae), is a biological control agent for Fallopia japonica (Houtt.) Ronse Decr. (Caryophyllales: Polygonaceae) that has been released in North America and Europe, however, to date successful establishment of the insect over multiple field seasons has not been observed. We conducted a series of experiments to explore the possibility that long-term rearing in laboratory conditions has impaired the psyllid’s ability to survive in the field. We used light intensity to manipulate leaf toughness in F. japonica, and used the resulting plants in laboratory bioassays to assess the effects of foliage toughness on the survival of the psyllids. We conducted laboratory-choice tests to compare the effects of leaf age on oviposition behavior by A. itadori. Finally, we conducted a field experiment, comparing the effects of foliage age on adult oviposition and nymphal survival. All experiments were conducted using a line of psyllids that have been reared in captivity since their collection from Japan, in 2004 (K1 psyllids). The final two experiments also utilized a line of psyllids collected from the same location more recently in 2019 (K2 psyllids). We observed a reduction in K1 A. itadori nymphal survival on tough F. japonica plants compared to soft plants. Aphalara itadori from the K1 line laid significantly more eggs on young versus old leaves, and survived best on young F. japonica regrowth in the field. Conversely, K2 A. itadori showed no oviposition preference between young and old leaves, and no preference between cut and uncut F. japonica foliage in the field. Our results indicate that recently collected psyllids (K2) may be better able to establish on field-conditioned knotweed in North America than older colonies and highlights the possible effects of long-term rearing on biological control agent performance.

Establishment of the biological control agent Aphalara itadori is limited by native predators and foliage age

AbstractThe knotweed psyllid, Aphalara itadori, is a biological control agent for invasive knotweed species in North America and Europe. Initial releases were conducted in Canada in 2014 but establishment has been slow, seemingly as a result of low nymphal survival. We conducted two field experiments in Ontario, Canada, to explore the effects of native predators and the age of knotweed (Fallopia japonica) foliage on nymphal survival in A. itadori. Survival of A. itadori nymphs was significantly reduced on potted plants that were exposed to native predators in the field, compared to plants from which predators were excluded. The number of surviving nymphs was also significantly reduced on older F. japonica foliage, compared to recent regrowth after a summer cutting treatment. We discuss our findings in the context of biological invasion theory and emphasize the potential for increased overlap between the fields of invasion ecology and biological control. Finally, we advocate the use of A. itadori in combination with other control measures as part of an integrated pest management programme, rather than as a solitary measure. Specifically, we recommend that future releases of A. itadori be concentrated shortly after cutting or herbicide treatments in order to maximize the availability of young tender foliage.

Effect of humidity and temperature on the performance of three strains of Aphalara itadori, a biocontrol agent for Japanese Knotweed

Japanese knotweed (Fallopia japonica) is a highly damaging invasive species affecting UK infrastructure and biodiversity. Under laboratory conditions, the psyllid Aphalara itadori has demonstrated its potential to be a successful biocontrol agent for F. japonica. However, this potential has not materialised in the field where long-term establishment of A. itadori has been unsuccessful and faces the added challenge of climate change. Intraspecific variation (variation among individuals of a species) has been shown to support establishment in alien species and improve resilience to changing environmental conditions. Here we propose it could improve the performance of biocontrols. To test this possibility we compared the performance and impact on F. japonica of three strains of A. itadori with different genetic backgrounds, including a newly created hybrid. We hypothesize that genetic variability would be increased in hybrids resulting in greater biocontrol effectiveness (greater impact on plant growth). We also explored the potential influence of changing climate on performance, testing all strains under two humidity conditions (with the same temperature). Contrary to our expectation, the hybrid strain had the worst performance (slowest development rate and lower survival from egg to adult emergence) under both environmental conditions. Exposure to different strains of A. itadori did not result in consistent differences in plant growth, suggesting similar biocontrol effectiveness among strains. Under the drier, more stressful, conditions plants exposed to A. itadori had fewer leaves and accumulated less above-ground biomass. Overall, our results suggest that genetic variability may not be the key to improve A. itadori biocontrol effectiveness, but that predicted climate change, which anticipates drier and hotter summers in the UK, could reduce the growth potential of F. japonica when exposed to A. itadori.

DNA barcoding for biosecurity: case studies from the UK plant protection program.

Since its conception, DNA barcoding has seen a rapid uptake within the research community. Nevertheless, as with many new scientific tools, progression towards the point of routine deployment within diagnostic laboratories has been slow. In this paper, we discuss the application of DNA barcoding in the Defra plant health diagnostic laboratories, where DNA barcoding is used primarily for the identification of invertebrate pests. We present a series of case studies that demonstrate the successful application of DNA barcoding but also reveal some potential limitations to expanded use. The regulated plant pest, Bursephalenchus xylophilus, and one of its vectors, Monochamus alternatus, were found in dining chairs. Some traded wood products are potentially high risk, allowing the movement of longhorn beetles; Trichoferus campestris, Leptura quadrifasciata, and Trichoferus holosericeus were found in a wooden cutlery tray, a railway sleeper, and a dining chair, respectively. An outbreak of Meloidogyne fallax was identified in Allium ampeloprasum and in three weed species. Reference sequences for UK native psyllids were generated to enable the development of rapid diagnostics to be used for monitoring following the release of Aphalara itadori as a biological control agent for Fallopia japonica.

A Mathematical Model for Biocontrol of the Invasive Weed Fallopia japonica.

We propose a mathematical model for biocontrol of the invasive weed Fallopia japonica using one of its co-evolved natural enemies, the Japanese sap-sucking psyllid Aphalara itadori. This insect sucks the sap from the stems of the plant thereby weakening it. Its diet is highly specific to F. japonica. We consider a single isolated knotweed stand, the plant’s size being described by time-dependent variables for total stem and rhizome biomass. It is the larvae of A. itadori that damage the plant most, so the insect population is described in terms of variables for the numbers of larvae and adults, using a stage-structured modelling approach. The dynamics of the model depends mainly on a parameter h, which measures how long it takes for an insect to handle (digest) one unit of F. japonica stem biomass. If h is too large, then the model does not have a positive equilibrium and the plant biomass and insect numbers both grow together without bound, though at a lower rate than if the insects were absent. If h is sufficiently small, then the model possesses a positive equilibrium which appears to be locally stable. The results based on our model imply that satisfactory long-term control of the knotweed F. japonica using the insect A. itadori is only possible if the insect is able to consume and digest knotweed biomass sufficiently quickly; if it cannot, then the insect can only slow down the growth which is still unbounded.

The regulation of biological control of weeds in Europe – an evolving landscape

After a difficult start, classical biological control of weeds is becoming recognized as an option for management of invasive plants in European Union (EU) Member States with intentional releases in three countries over the past 5 years. Many European countries are benefitting from the presence of Stenopelmus rufinasus, the azolla weevil, which has been accidentally introduced to the region. However, the UK experience with the official release of the psyllid Aphalara itadori against Japanese knotweed (Fallopia japonica) and the subsequent release of the rust Puccinia komarovii var glanduliferae against Impatiens glandulifera showed the regulatory framework that could be followed by EU Member States. This process was followed in advance of the subsequent release of Trichilogaster acaciaelongifoliae, a specific Australian bud‐galling wasp, against the invasive Acacia longifolia in Portugal. Soon the case of Ophraella communa, another accidental introduction that is severely limiting Ambrosia artemisiifolia populations, will influence affected countries, some of which have been uninterested in this technique until now, to consider the advantages and disadvantages of classical biocontrol. The future looks bright for classical weed biocontrol and the EU Regulation on Invasive Species should further aid this situation.

Development and verification of SNP arrays to monitor hybridization between two host-associated strains of knotweed psyllid, Aphalara itadori

Three species of invasive knotweeds (Fallopia japonica, Fallopia sachalinensis, and Fallopia×bohemica) cause extensive damage to riparian and roadside habitats in North America. Currently, two strains of the psyllid Aphalara itadori are being evaluated for introduction into the United States and Canada for the biological control of these knotweeds following the introduction of A. itadori into the United Kingdom. If approved and released, hybridization between individuals from these two strains is likely and understanding whether barriers to hybridization exist could have an important impact on the sustainability of this biological control program. Here we developed two single nucleotide polymorphism (SNP) arrays and examined their utility for identifying individuals of known pure strains (Hokkaido and Kyushu) and hybrid origins. Using an array of 141 SNPs we correctly identified all individuals to pure and hybrid classes, whereas using a smaller array of 29 SNPs we were able to correctly identify pure line individuals, but not hybrids.

Winter hosts of Aphalara itadori (Hemiptera: Psyllidae), a classical biological control agent of Fallopia japonica (Polygonaceae), in the UK

This study suggests that lack of suitable overwintering sites will not be a limiting factor to the establishment of Aphalara itadori, a classical biological control agent of Fallopia japonica, since it was able to overwinter on a common conifer species as well as non-conifers hosts.

Efficacy and host specificity compared between two populations of the psyllid Aphalara itadori, candidates for biological control of invasive knotweeds in North America

Invasive knotweeds are large perennial herbs in the Polygonaceae in the genus Fallopia that are native to Asia and invasive in North America. They include Fallopia japonica (Japanese knotweed), F. sachalinensis (giant knotweed), and a hybrid species F. x bohemica (Bohemian knotweed). Widespread throughout the continent and difficult to control by mechanical or chemical methods, these plants are good targets for classical biological control. We examined the suitability of two populations of the psyllid Aphalara itadori from Japan as biological control agents by comparing their impact on the target weeds and assessing their fundamental host ranges. Both populations were capable of halting knotweed plant growth and reducing both above and below ground biomass by more than 50% in just 50days. Moreover, the psyllids caused mortality of several of the plants during this period. The two populations differed markedly in their reproductive potential on the different knotweed species. The Kyushu psyllid performed best on F. japonica and F. bohemica and the Hokkaido psyllid performed best on F. sachalinensis. Both were found to be specialized to knotweeds, with only very low occurrence of development on a small number of related non-target plant species. For the few non-target plant species that supported development, choice tests and multi-generational tests were used to further evaluate the likelihood of non-target host use. We conclude that A. itadori would be both effective and low risk as a biological control agent for invasive knotweeds and that both the Kyushu and Hokkaido populations may be needed to effectively control the entire knotweed species complex.

Using life-history parameters and a degree-day model to predict climate suitability in England for the Japanese knotweed psyllid Aphalara itadori Shinji (Hemiptera: Psyllidae)

Japanese knotweed, Fallopia japonica, is an invasive weed that is spreading rapidly in large parts of Europe and North America. To reduce knotweed density, the knotweed psyllid Aphalara itadori was recently introduced from Mt. Aso, Kyushu Island of Japan into southeastern England as a classical weed biological control agent. To determine whether climate differences between the areas of origin and introduction might impede A. itadori, the effects of temperature on the development, survival, longevity and fecundity of Kyushu Island population of A. itadori were investigated to predict what the phenology and voltinism of the insects should be in southeastern England. The intrinsic rates of natural increase, survival rates and fecundity rates of A. itadori were significantly higher at 20 and 25°C than at 15 and 30°C, indicating that the psyllid is adapted for July and August temperatures found in both Mt. Aso and a representative site of southeastern England. The survival rate of A. itadori was significantly lower at 15°C than at 20 and 25°C, but there was still positive population growth at 15°C. The estimated numbers of generation per year were also similar between Mt. Aso and the southeastern England for A. itadori. For these reasons, we believe that A. itadori population from Mt. Aso should be able to thrive in at least some parts of southeastern England.

The life history and host range of the Japanese knotweed psyllid, Aphalara itadori Shinji: Potentially the first classical biological weed control agent for the European Union

Japanese knotweed is a serious invasive weed in the UK, North America and large parts of Europe. Current control measures are difficult to implement, unreliable and expensive. In 2003, a classical biological control programme was initiated, one that could lead to the first ever authorised release of a biocontrol agent against a weed in the European Union. Literature studies and surveys in the Japanese native range revealed over 180 species of arthropod natural enemies but only a psyllid, Aphalara itadori, has reached the point of official assessment for release. Aphalara itadori passes from egg to adult through five nymph stages in just under 33 days at 23 oC and the timing and physical appearance of these stages is presented. Multiple-choice oviposition studies using 87 species/varieties of test plants showed that only 1.52% of 146,885 eggs were laid outside what we call the invasive knotweed group. None of these eggs laid on non-targets plants were able to develop to adult, however, subsequent nymph transfer experiments revealed limited development on some closely related members of the Polygonaceae and the development to adult on Meuhlenbeckia complexa (7% of cases). Adult survival, used as a surrogate for feeding tests, further revealed that survival on non-targets was severely compromised. These results coupled with an assessment of likely overwintering habits show A. itadori to have a narrow physiological and behavioural host range making it an ideal candidate for the first official release of a classical biological control agent against an alien invasive weed in the European Union.