Prospective Biological Control Agent Research Articles

Life cycle, host specificity and potential impact of a gall-inducing thrips Acaciothrips ebneri, a biological control agent for prickly acacia (Vachellia nilotica subsp. indica) in Australia

AbstractPrickly acacia (Vachellia nilotica subsp. indica (Benth.) Kyal. & Boatwr.; Fabales: Fabaceae) is a Weed of National Significance and a target for biological control in Australia. Currently there are no effective biological control agents for the weed in Australia. Based on genetic and climate matching, a gall thrips (Acaciothrips ebneri Karny; Thysanoptera: Phlaeothripidae) inducing rosette galls resulting in shoot tip dieback, was identified as a prospective biological control agent from Ethiopia. No-choice host-specificity tests were conducted on 59 test plant species in a high security quarantine in Brisbane, Australia. Acaciothrips ebneri is host-specific, inducing galls and reproducing only on prickly acacia. Acaciothrips ebneri, as predicted by the CLIMEX model, is suited to hot and arid western Queensland where major prickly acacia infestations occur. The Australian Government approved A. ebneri for field release in October 2022. This is the first time a true gall-inducing thrips has ever been approved as a weed biological control agent. Field releases commenced in January 2023 and are in progress. There are early signs of field establishment resulting in shoot tip die back in the field, and field release and monitoring will continue.

Fungi associated with seeds of the invasive grass Nassella trichotoma (serrated tussock) in its native range as prospective biological control agents

Context The South American grass Nassella trichotoma (serrated tussock) is widely distributed in central Argentina and one of the most damaging invasive species in Australia, New Zealand, and South Africa. In Australia, it is a weed of national significance. Aims Our aim was to characterise the fungi able to colonise N. trichotoma seeds buried at a site in south-western Buenos Aires province, Argentina, and measure their impact on seed survival and germination. Methods We tested the germination of healthy seeds at the beginning of the experiment (control). We buried 10 mesh bags containing 100 disinfected N. trichotoma seeds each, during 3 months in spring and autumn. At the end of each experiment, the contents of five of the bags were counted and classified as germinated, healthy, damaged, or disintegrated. Germination tests were then conducted with seeds of the latter three categories. The remaining seeds were used for recording fungal signs/symptoms, and the isolation of fungi. Key results We identified and described a diverse group of fungi associated with the seeds and a seasonal variation in the specific composition. Seeds showing fungal signs and/or symptoms germinated less than the control. Conclusions This is the first study on fungi associated with seeds of this species in the native range, which can affect their survival and longevity. Implications It is expected that studies on the seed microbiome may help us understand the differences in behaviour of the plant between ranges, and test the enemy-release hypothesis.

A review of the biology, distribution, and management challenges posed by the invasive weed Ziziphus mauritianaL., with special reference to its invasion in Australia

AbstractZiziphus mauritiana is an economically detrimental and environmentally destructive plant in non‐native areas where it has escaped cultivation. It forms dense, impenetrable thickets that restrict the movement of livestock across the landscape and has the capacity to alter various ecological functions at the site of invasion, all of which contribute towards land degradation and the reduction of economic profitability. Although there are several management strategies implemented to control Z. mauritiana, it is clear that no single‐method approach will effectively control the species in the long‐term. Whilst chemical and mechanical methods appear to show promising results, they tend to be restricted to areas that are easily accessible and, even so, can be challenging and laborious to treat evenly across dense thicket areas. Several prospective biological control agents have been identified for Z. mauritiana, although further investigations are required to ascertain the host specificity, and to explore and identify their climatic and environmental suitability of host specific agents for release in non‐native regions. Ecological burning alone is not effective in controlling Z. mauritiana and will likely increase its emergence. As such, it could be adopted as part of an integrated management approach to assist other methods for long‐term control, but again the development of such an approach requires further investigation. To contribute towards the control of Z. mauritiana, this review explores its biology, distribution and management challenges whilst identifying areas of research that will assist in the long‐term and confident control of the species, with an emphasis on its invasion in Australia.

Selected Leaf Diseases of Rubber: Symptoms and Control – A Review

This paper is a review article with the objective of providing valuable, recent and solid information and critical understanding of symptoms and control of selected leaf diseases of rubber tree (Hevea brasiliensis Muell. Arg) using secondary information and data harvested from previously published studies found online. Data gathered showed that H. brasiliensis is subject to a plethora of economically important diseases mainly of fungal origin, classified as leaf, stem and branch, panel and root disease. Some fundamental causal organisms of leaf diseases in H. brasiliensis includes Pseudocercospora ulei, Oidium heveae, Colletotrichum gloeosporioides and Corynespora cassiicola. Corynespora leaf fall caused by Corynespora cassiicola is currently accepted as the leaf disease which causes about 45% of yield loss to rubber in mostly Asian and African continents. The symptoms of South American Leaf Blight (SALB) range from discoloration of lamina to death of tree when untreated. Leaf disease symptoms range from white powdery patches to shriveling and falling off of tender leaves and translucent brownish yellow spots in semi-matured leaves in Oidium leaf disease. Colletotrichum leaf disease symptoms range from necrosis of tender leaves of young plants to defoliation and death of entire plant. Disease control is achieved by the use of chemicals and planting of disease-resistant varieties. Some phylloplane fungi are prospective biological control agents. The best treatment to manage Corynespora leaf fall disease (CLFD) in nurseries is the introduction of overhead shading coupled with application of the fungicide mancozeb. Diseases affecting the leaves of H. brasiliensis can be devastating and can lead to loss of multiple rubber trees and reduction in the latex production of the trees. Therefore, adequate preventive, curative and control measures against the diseases should be taking to ensure maintenance of a healthy rubber tree plantation.

Exploration for native parasitoids of Asian longhorned beetle in China as prospective biological control agents

Abstract Native to China and the Korean Peninsula, the Asian longhorned beetle (ALB) Anoplophora glabripennis (Motschulsky) (Coleoptera: Cerambycidae) is a high‐risk invasive pest of hardwood trees. To explore the potential of biological control of ALB in the United States, we conducted surveys of ALB parasitoids using sentinel logs in five geographical regions (Beijing, Gansu, Guizhou, Shanghai, and Yunnan) in China from 2019 to 2022. Seven hymenopteran larval parasitoid species, Bracon planitibiae Yang, Eurytoma chinensis Yang, Heydenia sp., Oxysychus glabripennis Yang, Sclerodermus guani Xiao et Wu, Spathius anoplophorae Yang, and Zolotarewskya anoplophorae Yang, were collected in Beijing. Four, three and two of these species were also found in Shanghai, Guizhou, and Yunnan, respectively. No parasitoid was found in Gansu. Mean parasitism rates by all parasitoid species were 3.6%–15.8% in Beijing, 1.4%–16.4% in Guizhou, 1.6%–7.1% in Shanghai, and 5.8% in Yunnan. Spathius anoplophorae and O. glabripennis were the two most dominant parasitoids collected consistently in the four different regions and may be considered as prospective agents for biological control introduction against ALB in the United States. We discuss some underlying ecological mechanisms that might contribute to the rarity of egg parasitoids and future opportunities and challenges for biological control of ALB.

Studies on gall-inducing behaviour and life cycle to aid host specificity testing of Notomma mutilum (Diptera: Tephritidae) – a prospective biological control agent for prickly acacia (Vachellia nilotica subsp. indica)

ABSTRACT The gall-inducing tephritid fly Notomma mutilum (Bezzi) (Diptera: Tephritidae) from Senegal is a prospective biological control agent of prickly acacia, Vachellia nilotica subsp. indica (Benth.) Kyal. & Boatwr (Fabaceae) in Australia. Wide variation in the oviposition behaviour and life cycle of N. mutilum in quarantine resulted in difficulties with colony maintenance and host specificity testing. To identify the factors responsible, adults that emerged from galls cut from plants 100 days after oviposition (enforced population, EP) were compared with adults that emerged from galls on the plants (normal population, NP). Oviposition by females of different ages did not vary significantly. The EP females had more variable egg-laying (galls on 46.8% plants) and made fewer oviposition scars (≈37 per female) than NP females (galls on >90% plants and ≈95 scars per female). However, the life cycle duration of EP progeny was shorter (140.7 ± 1.7 days) and less variable than NP progeny (160.1 ± 12.8 days). Egg-laying on all the plants was achieved when a greater number of EP adults (three pairs) were used. Actively growing tender shoots in the upper crown of the older host plants were preferred sites for oviposition. Galls were initiated (3.5 ± 0.2 days) before egg hatching (11.3 ± 0.3 days), and bigger (longer and wider) galls were observed on the preferred oviposition sites. Therefore, plants with actively growing shoots in the upper crown should be selected for host specificity testing and three pairs of N. mutilum adults should be used.

Host Specificity and Preliminary Impact of Lepidapion argentatum (Coleoptera, Brentidae), a Biocontrol Candidate for French Broom (Genista monspessulana, Fabaceae).

Simple SummaryFrench broom is a leguminous shrub that is an internationally significant invasive alien weed in California and nearby US states that competes with native vegetation and increases the risk of wildfires. French broom originates from the Mediterranean region and is the target for classical biological control. Exploration for prospective biological control agents in the native region of Western Europe resulted in the collection of the weevil Lepidapion argentatum. We conducted preliminary field studies showing that this weevil can impact seed production in the native habitat. We also measured the specificity of this candidate biocontrol agent in laboratory conditions on 36 plant species growing in California and Australia. Our data showed that, in no-choice tests, seven species were directly negatively impacted by gall formation induced on plant stems by the weevil, but at different levels, including two lupine species commonly distributed in California. Further experiments are needed to ascertain the safety of this candidate biocontrol agent prior to release.French broom (Genista monspessulana) (Fabaceae) is a perennial species native to the Mediterranean basin. Introduced in the 19th century as an ornamental plant, it is currently invasive in California and Australia. The current research is focused on biocontrol with the use of the phytophagous weevil Lepidapion argentatum (Brentidae). Its capacity to develop both in the stem galls and pods of French broom makes it a promising candidate. The impact on the reproduction of French broom was studied in Southern France and revealed that it could effectively reduce the number of viable seeds by 18.8%, but also increased the number of aborted seeds by 10% within the attacked pods. To evaluate the specificity of L. argentatum, choice and no-choice tests were performed in 2012 and 2015 on a total of 36 non-target closely related species. Results revealed the presence of galls and larvae in the stems of seven species, including two endemic Californian lupines; i.e., Lupinus arboreus blue and Lupinus chamissonis. In the future, new tests will be conducted to determine if L. argentatum is able to complete its entire development lifecycle on the non-target species where galls have previously been observed.

Reproductive biology, abundance and control potential of Paramecops farinosus (Wiedemann) (Coleoptera: Curculionidae): a prospective biological control agent for Calotropis procera (Apocynaceae)

ABSTRACT Reproductive biology, abundance and control potential of Paramecops farinosus Wiedemann, (Aak weevil) a prospective biological control agent for Calotropis procera (Aiton) W.T. Aiton, (Calotrope) were studied under field conditions in Lahore, Pakistan, from spring to winter season. Newly emerged adult pairs were introduced inside insect-proof pouches with calotrope fruits, and the life cycle and reproductive biology of P. farinosus were studied. Pre-oviposition period was 4.8 ± 0.17 days and there were 6.1 ± 0.09 eggs per clutch. A female, laid eggs about 5.4 ± 0.32 times, producing 24 offspring during its lifespan. The mortality from egg to adult was 27% and the progeny sex ratio was 3:2 (female to male ratio). The developmental period of P. farinosus (egg to adult) was 59.9 ± 0.82 days and at least two generations were produced per year. The longevity of males and females recorded was 97 ± 1.98 and 69 ± 1.41 days, respectively and the pupae overwinter inside the infested fruits. The number of eggs and larvae inside fruits increased from March to July and declined in subsequent months. The highest number of eggs and larvae per fruit were recorded in July i.e. 8.27 ± 0.65 and 6.31 ± 1.77, respectively. The developing larvae destroyed 100% seeds and internal tissue of infested fruit pods of C. procera, resulting in significant reductions in fruit size (−12%) and biomass (−26%) compared to un-infested (healthy) fruits. The high reproductive capacity and control potential of P. farinosus indicate that this insect holds promise as a biological control agent of C. procera in its invaded ranges, including Australia, the Pacific, and South America.

Eriophyid Mites in Classical Biological Control of Weeds: Progress and Challenges.

Simple SummaryEriophyid mites are tiny creatures, no bigger than a speck of dust. All species feed on plants and some can cause considerable damage. These mites have an intimate relationship with the plants that they live on, and most of the known species have been collected only from a single plant species, which suggests they are very specific to their host. They reproduce extremely quickly and can build up populations of millions, if not billions, of individuals within a single season. In recent years, research to evaluate their potential for the biological control of invasive plants has increased. Working with these minuscule herbivores poses challenges and offers opportunities for researchers. We review the most updated information in the context of weed biocontrol, giving current information on the challenges already faced and possible opportunities and solutions. We cover topics on taxonomy, evaluation of safety as biological control agents, impact and efficacy on the targeted plant species, and release and post-release monitoring. By offering the lessons learned from past research in a single updated document, our goal is to equip researchers with a valuable tool to help deal with the challenges and opportunities offered by eriophyid mites for the management of invasive plants.A classical biological control agent is an exotic host-specific natural enemy, which is intentionally introduced to obtain long-term control of an alien invasive species. Among the arthropods considered for this role, eriophyid mites are likely to possess the main attributes required: host specificity, efficacy, and long-lasting effects. However, so far, only a few species have been approved for release. Due to their microscopic size and the general lack of knowledge regarding their biology and behavior, working with eriophyids is particularly challenging. Furthermore, mites disperse in wind, and little is known about biotic and abiotic constraints to their population growth. All these aspects pose challenges that, if not properly dealt with, can make it particularly difficult to evaluate eriophyids as prospective biological control agents and jeopardize the general success of control programs. We identified some of the critical aspects of working with eriophyids in classical biological control of weeds and focused on how they have been or may be addressed. In particular, we analyzed the importance of accurate mite identification, the difficulties faced in the evaluation of their host specificity, risk assessment of nontarget species, their impact on the weed, and the final steps of mite release and post-release monitoring.

Field‐based ecological studies to assess prospective biological control agents for invasive alien plants: An example from giant rat's tail grass

Abstract Biological control (biocontrol) of invasive alien plants is a widely utilised weed management tool. Prospective biocontrol agents are typically assessed through host specificity testing and pre‐release efficacy studies performed in quarantine. However, rearing of the potential biocontrol agents and/or test plants is often difficult or impossible under quarantine conditions. Moreover, practitioners may attain laboratory artefacts in quarantine, which may result in the potential agent being needlessly rejected. Field‐based studies in the weed's indigenous distribution could overcome these issues. Sporobolus pyramidalis and Sporobolus natalensis (giant rat's tail grass; Poaceae) are indigenous in Africa but have become problematic invasive alien plants in Australia. A previous biocontrol program was terminated because the candidate agent could not be reared and tested in quarantine. We performed field‐based host specificity and efficacy studies for prospective biocontrol agents in South Africa (indigenous distribution). Forty‐seven non‐target grass species were sampled during host specificity assessments. Candidate agent efficacy was estimated based on damage to the target weeds, for each host specific candidate individually and in combination with other host‐specific candidates. Three species of endophagous wasps were deemed host specific. Efficacy assessments identified an undescribed stem‐boring wasp (Tetramesa sp.) species as the most damaging candidate. A second Tetramesa species was much less damaging alone but had a cumulative impact on the plant in combination with the more damaging Tetramesa species. Both Tetramesa species are recommended for importation into quarantine in Australia for confirmatory host specificity testing with a significantly reduced test plant list. Synthesis and applications. Similar field‐based assessments in the indigenous distribution of weeds targeted for biocontrol could be included in the future programs. Where rearing of potential agents and/or test plants is difficult or impossible under quarantine conditions, our field‐based method provides an alternative. Where quarantine‐based testing is feasible, this method ensures that only candidates that have passed an ecologically realistic host‐specificity and potential efficacy screening are imported into quarantine. This may reduce the number of agents that are imported and the length of time each agent is kept in quarantine. This is advantageous because quarantine space is highly valuable and is usually a limiting factor in pre‐release assessments of biocontrol agents.

Studies on the fruit feeding weevil, Paramecops farinosa (Coleoptera: Curculionidae) in Sri Lanka as a prospective weed biological control agent of invasive weed, Calotropis spp.

Calotropis gigantea is a large shrub or a small tree native to Sri Lanka with an Ayurvedic medicinal value. The plant is considered as an invasive weed in countries where it has been introduced. Paramecops farinosa Schoenherr (Aak weevil) is a monophagous pest that feeds on C. gigantea . Present study was conducted to elucidate the life history and damage potential of P. farinosa , in order to assess its potential as a biological control agent against C. procera and C. gigantea in countries where the plants are invasive. The field sampling was done throughout Sri Lanka covering 120 sampling sites from December 2014 to October 2015, and C. gigantea fruits were examined for the incidence and intensity of damage by P. farinosa . It lays yellowish, oval and mostly one-clustered eggs in the inner-pericarp fibrous layer of the Calotropis fruit. Newly emerged larvae were apodous, pale yellowish-white with brown head capsule whereas developing larvae were creamy-white, curved and stout. Paramecops farinosa larvae voraciously feed on all Calotropis seeds (100%) and fifth larval instar pupated by forming silky cocoons within the seed chamber. The adults feed on leaves, buds and flowers and its damage is highly correlated with the amount of P. farinosa inhabit on trees. P. farinosa is a seed predator and highly damage reproductive structures of C. gigantea thus directly influences the reproductive ability of the plant. These results provide baseline information needed in adopting P. farinosa as potential biological control agent against C. procera and C. gigantea .  Â

Ranking the host range of biological control agents with quantitative metrics of taxonomic specificity

The need to prevent negative impacts of importation biological control on native biodiversity has made evaluating the specificity (host range) of natural enemies a central issue for the application of biological control programs against pest organisms. Thus, when there are a number of candidate species being considered for introduction as biological control agents against a particular target pest, it is important to compare their relative host ranges. These comparisons are usually made informally using categorical terminology (e.g., generalist, specialist, oligophagous). However, relative differences in natural enemy host range are better expressed quantitatively, with both ecological (how many host species a natural enemy is capable of exploiting) and evolutionary (how the host species are related phylogenetically) components. We propose using two previously developed quantitative taxonomic and phylogenetic metrics (taxonomic host range – STD; phylogenetic species variability – PSV), in combination with the number of associated host species (host species richness), as heuristic tools to rank the relative host specificity of candidate biological control agents. We first show that although STD and PSV were developed independently, they are directly related mathematically and convey exactly the same information except on different numerical scales. We then apply the metrics to case studies regarding the host range of candidate biological control agents for: (i) an invasive whitefly in greenhouse crops, Aleurotrachelus trachoides (Hemiptera: Aleyrodidae), and (ii) an invasive pest of grapes, Lobesia botrana (Lepidoptera: Tortricidae). We also provide open source code to easily allow the calculation of these metrics for any natural enemy for which taxonomic host range information is available. Finally, we describe potential uses of these metrics in applied biological control as well as important caveats and limitations that require further theoretical work to address. We conclude that while these metrics should not be considered as absolute or infallible measurements of host range, their application should encourage biological control practitioners to explicitly consider the phylogenetic component of host range when ranking prospective candidate biological control agents.

Bionomics and damage potential of fruit fly Dacus persicus (Diptera: Tephritidae): a prospective biological control agent of Calotropis procera (Apocynaceae)

ABSTRACT Life history and damage potential of the fruit fly, Dacus persicus Hendel, a prospective biological control agent for calotrope, Calotropis procera (Aiton) W.T. Aiton, are described. The duration of life cycle of D. persicus (egg to the adult) was 42.2 ± 0.38 days (range 38–50 days). Oviposition commenced 5.1 ± 0.2 days after emergence and each female infested 4.5 ± 0.2 fruits during its lifespan. D. persicus produced 76.9 ± 5.21 larvae from which 66.1 ± 3.7 adults emerged with a sex ratio of 4:3 female to male. The lifespan of adults was 16.3 ± 0.68 days. The fruit fly larvae destroyed 100% of immature seeds in infested pods and significantly reduced the biomass of infested fruits by 62%. The field host specificity, high reproductive capacity, and damage potential of D. persicus indicate this species as a potential biological control of C. procera in Australia.

Three prospective agents instead of one? Cryptic diversity of the biological control agent Psylliodes chalcomera

The flea beetle, Psylliodes chalcomera, was previously permitted as a biological control agent of Carduus nutans (musk thistle) in the USA. However, during subsequent exploration for prospective biological control agents of Centaurea solstitialis (yellow starthistle), we observed individuals attacking this plant in southern Russia. We also observed this insect on Onopordum spp. (Scotch thistle), which raised the question whether this beetle is polyphagous, or whether it is comprised of biotypes, host races or cryptic species that are specialized on different host plants. We analyzed a fragment of the cox1 mitochondrial gene of adult and larval specimens collected from these three plants in Bulgaria, Italy, Russia and Turkey to study the population structure of this beetle. The results indicate at least three different groups: 1) associated with C. solstitialis in Bulgaria, Russia and Turkey, 2) associated with Onopordum spp. in Bulgaria, Russia and Turkey, and 3) associated with Carduus spp., Onopordum spp., and possibly C. solstitialis, in Bulgaria, Italy, Russia and Turkey. Adult specimens indicated a fourth group associated with Onopordum spp. and C. solstitialis in Turkey, but we did not have any larval specimens to confirm the host plant of this group. Our results suggest that three of these biotypes could be interesting to evaluate as prospective candidates for classical biological control of these three weed targets.

The golden death bacillus Chryseobacterium nematophagum is a novel matrix digesting pathogen of nematodes

BackgroundNematodes represent important pathogens of humans and farmed animals and cause significant health and economic impacts. The control of nematodes is primarily carried out by applying a limited number of anthelmintic compounds, for which there is now widespread resistance being reported. There is a current unmet need to develop novel control measures including the identification and characterisation of natural pathogens of nematodes.ResultsNematode killing bacilli were isolated from a rotten fruit in association with wild free-living nematodes. These bacteria belong to the Chryseobacterium genus (golden bacteria) and represent a new species named Chryseobacterium nematophagum. These bacilli are oxidase-positive, flexirubin-pigmented, gram-negative rods that exhibit gelatinase activity. Caenorhabditis elegans are attracted to and eat these bacteria. Within 3 h of ingestion, however, the bacilli have degraded the anterior pharyngeal chitinous lining and entered the body cavity, ultimately killing the host. Within 24 h, the internal contents of the worms are digested followed by the final digestion of the remaining cuticle over a 2–3-day period. These bacteria will also infect and kill bacterivorous free-living (L1-L3) stages of all tested parasitic nematodes including the important veterinary Trichostrongylids such as Haemonchus contortus and Ostertagia ostertagi. The bacteria exhibit potent collagen-digesting properties, and genome sequencing has identified novel metalloprotease, collagenase and chitinase enzymes representing potential virulence factors.ConclusionsChryseobacterium nematophagum is a newly discovered pathogen of nematodes that rapidly kills environmental stages of a wide range of key nematode parasites. These bacilli exhibit a unique invasion process, entering the body via the anterior pharynx through the specific degradation of extracellular matrices. This bacterial pathogen represents a prospective biological control agent for important nematode parasites.

When taxonomy and biological control researchers unite: Species delimitation of Eadya parasitoids (Braconidae) and consequences for classical biological control of invasive paropsine pests of Eucalyptus

The invasive eucalyptus tortoise beetle, Paropsis charybdis, defoliates plantations of Eucalyptus nitens in New Zealand. Recent efforts to identify host specific biological control agents (parasitoids) from Tasmania, Australia, have focused on the larval parasitoid wasp, Eadya paropsidis (Braconidae), first described in 1978. In Tasmania, Eadya has been reared from Paropsisterna agricola (genus abbreviated Pst.), a smaller paropsine that feeds as a larva on juvenile rather than adult foliage of Eucalyptus nitens. To determine which of the many paropsine beetle hosts native to Tasmania are utilized by E. paropsidis, and to rule out the presence of cryptic species, a molecular phylogenetic approach was combined with host data from rearing experiments from multiple locations across six years. Sampling included 188 wasps and 94 beetles for molecular data alone. Two mitochondrial genes (COI and Cytb) and one nuclear gene (28S) were analyzed to assess the species limits in the parasitoid wasps. The mitochondrial genes were congruent in delimiting four separate phylogenetic species, all supported by morphological examinations of Eadya specimens collected throughout Tasmania. Eadya paropsidis was true to the type description, and was almost exclusively associated with P. tasmanica. A new cryptic species similar to E. paropsidis, Eadya sp. 3, was readily reared from Pst. agricola and P. charybdis from all sites and all years. Eadya sp. 3 represents the best candidate for biological control of P. charybdis and was determined as the species undergoing host range testing in New Zealand for its potential as a biological control agent. Another new species, Eadya sp. 1, was morphologically distinctive and attacked multiple hosts. The most common host was Pst. variicollis, but was also reared from Pst. nobilitata and Pst. selmani. Eadya sp. 1 may have potential for control against Pst. variicollis, a new incursion in New Zealand, and possibly Pst. selmani in Ireland. Our molecular data suggests that Pst. variicollis is in need of taxonomic revision and the geographic source of the beetle in New Zealand may not be Tasmania. Eadya sp. 2 was rarely collected and attacked P. aegrota elliotti and P. charybdis. Most species of Eadya present in Tasmania are not host specific to one beetle species alone, but demonstrate some host plasticity across the genera Paropsisterna and Paropsis. This study is an excellent example of collaborative phylogenetic and biological control research prior to the release of prospective biological control agents, and has important implications for the Eucalyptus industry worldwide.

The importance of cryptic species and subspecific populations in classic biological control of weeds: a North American perspective

Classical biological control of weeds depends on finding agents that are highly host-specific. This requires not only correctly understanding the identity of the target plant, sometimes to subspecific levels, in order to find suitable agents, but also identifying agents that are sufficiently specific to be safe and effective. Behavioral experiments and molecular genetic tools have revealed that some arthropod species previously thought to be polyphagous really consist of multiple cryptic species, host races or biotypes, some of which are more host-specific than others. Whereas true species are reproductively isolated, individuals from subspecific populations may potentially interbreed with those of other populations if they should encounter them. Furthermore, biotypes may consist of individuals sharing a genotype that is not fixed within a monophyletic group, and thus may not be evolutionarily stable. This raises the question of how such populations should be classified, and how to confirm the identity of live arthropods before releasing them as classical biological control agents. The existence of host races or cryptic species may greatly increase the number of prospective biological control agents available. However, it may also create new challenges for governmental regulation. These issues are discussed using pertinent examples, mainly from North America.

Biological control of Ziziphus mauritiana (Rhamnaceae): feasibility, prospective agents and research gaps

AbstractThe tropical fruit tree, Ziziphus mauritiana (Rhamnaceae), a native of the Indian subcontinent, is a pasture and environmental weed in northern Australia and Fiji. In their native range, Ziziphus spp., including commercially cultivated Z. mauritiana and Z. jujuba, are subjected to a wide range of pests and diseases. The feasibility of classical biological control of this weed has not been explored to date. Effective biological control could reduce plant vigour and seed output, thereby limiting the spread of Z. mauritiana in Australia. Two Ziziphus species are native to Australia, hence, any prospective biological control agent should be specific to Z. mauritiana. Opportunistic field surveys and literature searches identified 133 species of phytophagous insects, 9 species of phytophagous mites and 12 plant pathogens on Ziziphus spp. Host records suggest the following are possibly specific to Z. mauritiana and hence are prospective biological control agents in Australia: the seed‐feeding weevil Aubeus himalayanus; the leaf‐feeding gracillariid moth Phyllonorycter iochrysis; the leaf‐mining chrysomelid beetle Platypria erinaceus; the leaf‐folding crambid moth Synclera univocalis; the leaf‐galling midge Phyllodiplosis jujubae; and the gall‐mites Aceria cernuus and Larvacarus transitans. Host range of the rust Phakopsora zizyphi‐vulgaris includes many Ziziphus species, including the native Z. oenoplia and hence would not be a suitable biological control agent in Australia. The powdery mildew Pseudoidium ziziphi, with a host range restricted to Ziziphus species, has not been reported on Z. oenoplia. All available information on the pests and diseases of Z. mauritiana are from cultivated varieties. Hence, future surveys should focus on wild Z. mauritiana in the Indian subcontinent in areas that are climatically similar to the regions of northern Australia, where it is currently most abundant.

Duration of emission of volatile organic compounds from mechanically damaged plant leaves

Classical biological control of invasive alien weeds depends on the use of arthropod herbivores that are sufficiently host specific to avoid risk of injuring nontarget plants. Host plant specificity is usually evaluated by using a combination of behavioral and developmental experiments under choice, no-choice and field conditions. Secondary plant compounds are likely to have an important influence on host plant specificity. However, relatively little is known about the volatile organic compounds (VOCs) that are emitted by target and nontarget plants, and how environmental conditions may affect their emission. Previous studies have shown that mechanical damage of leaves increases the composition and content of VOCs emitted. In this study we measured the VOC emissions of five species of plants in the subtribe Centaureinae (Asteraceae) – Carthamus tinctorius, Centaurea cineraria, Centaurea melitensis, Centaurea rothrockii, and Centaurea solstitialis – that have previously been used in host specificity experiments for a prospective biological control agent of yellow starthistle (C. solstitialis). Leaves of each plant were punctured with a needle and the VOCs were collected by solid-phase microextraction (SPME) periodically over 48h and analyzed by GC–MS. A total of 49 compounds were detected. Damage caused an immediate increase of 200–600% in the composition of VOCs emitted from each plant species, and the amounts generally remained high for at least 48h. The results indicate that a very unspecific mechanical damage can cause a prolonged change in the VOC profile of plants.

Life cycle and host range ofPhycitasp. rejected for biological control of prickly acacia in Australia

Prickly acacia (Vachellia nilotica subsp. indica), a native of the Indian subcontinent, is a serious weed of the grazing areas of northern Australia and is a target for classical biological control. Native range surveys in India identified a leaf webber, Phycita sp. (Lepidoptera: Pyralidae) as a prospective biological control agent for prickly acacia. In this study, we report the life cycle and host-specificity test results Phycita sp. and highlight the contradictory results between the no-choice tests in India and Australia and the field host range in India. In no-choice tests in India and Australia, Phycita sp. completed development on two of 11 and 16 of 27 non-target test plant species, respectively. Although Phycita sp. fed and completed development on two non-target test plant species (Vachellia planifrons and V. leucophloea) in no-choice tests in India, there was no evidence of the insect on the two non-target test plant species in the field. Our contention is that oviposition behaviour could be the key mechanism in host selection of Phycita sp., resulting in its incidence only on prickly acacia in India. This is supported by paired oviposition choice tests involving three test plant species (Acacia baileyana, A. mearnsii and A. deanei) in quarantine in Australia, where eggs were laid only on prickly acacia. However, in paired oviposition choice trials, only few eggs were laid, making the results unreliable. Although oviposition choice tests suggest that prickly acacia is the most preferred and natural host, difficulties in conducting choice oviposition tests with fully grown trees under quarantine conditions in Australia and the logistic difficulties of conducting open-field tests with fully grown native Australian plants in India have led to rejection of Phycita sp. as a potential biological control agent for prickly acacia in Australia.