Host Specificity Trials Research Articles

Time-dependent behavioural plasticity influences oviposition latency in no-choice host-specificity trials for a candidate weed biocontrol agent

Laboratory no-choice trials are standard bioassays used to evaluate the host range of arthropods under investigation as potential biological control agents against weeds. In these trials false positives can occur when oviposition and development occur on test plants that are not utilised as a host in the field. Deprivation from preferred hosts can lead to insects accepting a wider range of plant species for oviposition. Observations of what appears to be undiscriminating behaviour in host-specificity trials can result in a decision to not release what would otherwise be a safe, specialised agent.In conducting host specificity testing of the weevil, Listronotus sordidus (Gyllenhal) against the aquatic macrophyte, Sagittaria platyphylla (Engelm.) J.G.Sm. (Alismataceae) we observed oviposition on non-target species in no-choice quarantine laboratory trials ranging from only a few eggs being laid on test species to an abundance of eggs on others. While data showing high and very low (or no) oviposition provides a clear indication of the weevil’s likely acceptance of these plant species in the field, interpretation of the results becomes more difficult when intermediate levels of oviposition occur. In this study we sought to gain further insights into oviposition behaviour on ‘intermediate’ hosts.Oviposition by L. sordidus was observed over a range of time periods in no-choice host trials to determine if lower oviposition counts represented consistently low oviposition over time, or whether this was due to oviposition latency. Test species that were accepted for oviposition at intermediate levels at the conclusion of the trial were less likely to be oviposited on than the host during earlier time periods. Oviposition latency may indicate these species are not utilised as hosts in the field as we predict that the agent would initially reject these species and continue searching for a more suitable host. We recommend measures of time-dependent changes in oviposition be recorded in host-specificity trials, and that post-release host range studies could be used to validate the utility of oviposition latency as a predictor of host use in the field. In time, this information could be incorporated into risk assessment processes to reduce the risk of rejecting safe agents.

Insect Herbivores Associated with Nymphaea mexicana (Nymphaeaceae) in Southern United States: Potential Biological Control Agents for South Africa

Nymphaea mexicana Zuccarini (Nymphaeaceae) (Mexican waterlily) is an emergent floating-leaved aquatic plant from the southeastern USA that is invasive in South Africa. In invaded waterbodies this plant restricts water movement, increases siltation, decreases recreational activities, and can deplete water oxygen levels, which in turn negatively impacts aquatic fauna. Currently there are no chemical, mechanical, or biological control programs in place for N. mexicana in South Africa, but the sustainability of biological control makes this the most desirable option. Field surveys for potential biological control agents were conducted in the native range of N. mexicana in Florida, Louisiana, and Texas from Aug to Oct 2018. Leaves, stems, flowers, and roots of N. mexicana were searched for insect herbivores by hand and using Berlese funnels. Insects were prioritized for use as biological control agents by considering the extent and type of feeding damage, field host range, and incidence (percentage of sites in which each species was found). In total, 15 confirmed species were found feeding on N. mexicana, and some taxa were identified only to family level. Incidence coverage estimator mean, MMRuns, Chao 2 mean, and Chao 2 upper 95% CI species accumulation estimators predicted that between 2 and 5 species were missed during the surveys. Based on field observations, Bagous americanus LeConte (Coleoptera: Curculionidae), and Megamelus toddi Beamer (Hemiptera: Delphacidae) were prioritized. Host specificity trials will be conducted to determine whether these insects may be used as biological control agents of N. mexicana.

Pereskiophaga brasiliensis, a natural enemy of the invasive alien cactus Pereskia aculeata, is not suitably host specific for biological control in South Africa

ABSTRACTThe stem-mining weevil, Pereskiophaga brasiliensis, was a candidate biological control agent for the invasive cactus Pereskia aculeata in South Africa. In host specificity trials, it developed on two indigenous test plant species under choice and no-choice conditions. Pereskiophaga brasiliensis is therefore not suitably host specific for release in South Africa.

Host specificity of Puccinia spegazzinii (Pucciniales: pucciniaceae), a biological control agent for Mikania micrantha (Asteraceae) in Australia

ABSTRACTThe biology and host specificity of the rust fungus Puccinia spegazzinii, a biological control agent for Mikania micrantha, was studied in a quarantine laboratory in Brisbane, Australia. Nineteen plant species, all from the family Asteraceae were inoculated during host specificity trials. Spore formation occurred only on M. micrantha and no other plant was infected. The first signs of infection (white spots on leaves, petioles and stems) occurred 7 days after exposure to inoculum, becoming yellow by day 11. Mature pustules containing teliospores had developed by 20 days. Results obtained here support testing in other countries showing that P. spegazzinii is host specific to M. micrantha and no that other plant in Australia would be at risk if it were to be released in the environment. Recommendations on assessing pathogens to improve trials were proposed. Counting infected plants, plant parts and pustules formed can demonstrate that all test plants would have been adequately exposed to inoculum and thus increase the confidence in host specificity of pathogens in weed biological control.

The host specificity of Anomalococcus indicus Ayyar (Hemiptera: Lecanodiaspididae), a potential biological control agent for prickly acacia (Vachellia nilotica ssp. indica) in Australia

ABSTRACTPrickly acacia, Vachellia nilotica ssp. indica (Benth.) Kyal. & Boatwr, is a significant weed of northern Australia and has been a target of weed biological control in Australia since the 1980s. Following native range surveys in India, the scale insect Anomalococcus indicus Ayyar was identified as the most promising agent and was imported into Australia for further research. A. indicus is a major pest of prickly acacia on the Indian subcontinent, where it causes shoot tip dieback and plant death. Despite field observations suggesting the species was specific to V. nilotica, A. indicus completed development on 17 of the 84 non-target plant species tested during no-choice host specificity trials under quarantine conditions. Of these, Acacia falcata, V. bidwillii, V. sutherlandii and Neptunia major supported high numbers of mature females in all replicates. All of these species were utilised in choice trials. Combined risk scores indicate that V. sutherlandii, N. major and A. falcata may be attacked in the field. Due to the limited ability of scale insects to disperse, only those non-target species that occur on the Mitchell grass downs (i.e. V. sutherlandii) are considered to be at risk. Nevertheless, in view of the disparity between quarantine test results and the observed field host specificity of A. indicus in India, field trials are currently being conducted in India using Australian native species on which complete development has occurred. The future of A. indicus as a biological control agent for prickly acacia in Australia will be determined once results from these field trials are known.

Molecular identification of Azolla invasions in Africa: The Azolla specialist, Stenopelmus rufinasus proves to be an excellent taxonomist

Biological control of Azolla filiculoides in South Africa with the Azolla specialist Stenopelmus rufinasus has been highly successful. However, field surveys showed that the agent utilized another Azolla species, thought to be the native Azolla pinnata subsp. africana, which contradicted host specificity trials. It is notoriously difficult to determine Azolla species based on morphology so genetic analyses were required to confirm the identity of the Azolla used by the agent. Extensive sampling was conducted and samples were sequenced at the trnL-trnF and trnG-trnR chloroplastic regions and the nuclear ITS1 region. Current literature reported A. filiculoides as the only Section Azolla species in southern Africa but 24 samples were identified as Azolla cristata, an introduced species within Section Azolla that was not used during host specificity trials. A. pinnata subsp. africana was only located at one site in southern Africa, while the alien A. pinnata subsp. asiatica was located at three. What was thought to be A. pinnata subsp. africana was in fact A. cristata, a closer relative of A. filiculoides and a suitable host according to specificity trials. This study confirms that S. rufinasus is a proficient Azolla taxonomist but also supports the use of molecular techniques for resolving taxonomic conundrums.

Phylogeny of Cirsium spp. in North America: Host Specificity Does Not Follow Phylogeny.

Weedy invasive Cirsium spp. are widespread in temperate regions of North America and some of their biological control agents have attacked native Cirsium spp. A phylogenetic tree was developed from DNA sequences for the internal transcribed spacer and external transcribed spacer regions from native and non-native Great Plains Cirsium spp. and other thistles to determine if host specificity follows phylogeny. The monophyly of Cirsium spp. and Carduus within the tribe Cardinae was confirmed with native North American and European lineages of the Cirsium spp. examined. We did not detect interspecific hybridization between the introduced invasive and the native North American Cirsium spp. Selected host-biological control agent interactions were mapped onto the phylogenic tree derived by maximum likelihood analysis to examine the co-occurrence of known hosts with biological control agents. Within Cirsium-Cardueae, the insect biological control agents do not associate with host phylogenetic lines. Thus, more comprehensive testing of species in host-specificity trials, rather than relying on a single representative of a given clade may be necessary; because the assumption that host-specificity follows phylogeny does not necessarily hold. Since the assumption does not always hold, it will also be important to evaluate ecological factors to provide better cues for host specificity.

Manipulation of parasitoid state influences host exploitation by Diadromus pulchellus Wesmael (Hymenoptera: Ichneumonidae)

In biological control programs, it should be possible to manipulate agents during host specificity trials so that they exhibit a lower host acceptance threshold. This would help to ensure that any non-target species that could be attacked in the field will also be attacked in the laboratory and flagged as potential hosts. The question is how to encourage agents to express the widest possible host range during trials. This study examined the effects of variation in parasitoid state (mating status, nutritional status, age, host experience) and the test environment (competition, arena size, presence of host-damaged leek leaves) on the degree of host exploitation by Diadromus pulchellus Wesmael (Hymenoptera: Ichneumonidae). As a first step, this study looked for changes in oviposition by the parasitoid on its normal host, the leek moth, Acrolepiopsis assectella (Zeller) (Lepidoptera: Acrolepiidae). Each of the seven factors was tested independently to assess its effect on the number of parasitoid offspring produced and host mortality. Previous host experience led to a marginal increase, while competition led to a per-capita decrease, in the degree of host exploitation. In contrast, mating status, arena size and the presence of host-damaged leek leaves had no measurable effects. Finally, parasitoid age and nutritional status had strong effects in the opposite direction to predictions, with younger and sugar-fed individuals exhibiting higher host exploitation than older and sugar-starved parasitoids, respectively. This study suggests that a sound understanding of species-specific reproductive biology and parasitoid–host interactions is likely required before oviposition-enhancing parasitoid states can confidently be identified.

Prospective Agents for the Biological Control ofTithonia rotundifolia(Mill.) S.F.Blake andTithonia diversifolia(Hemsl.) A.Gray (Asteraceae) in South Africa

Starting in 2007, two weedy sunflower species, Tithonia rotundifolia (Mill.) S.F.Blake and Tithonia diversifolia (Hemsl.) A.Gray (Asteraceae: Heliantheae), were targeted for biological control in South Africa. Surveys conducted in their native range (Mexico) revealed that there were five potential biological control agents for T. rotundifolia, and three of these are currently undergoing host-specificity and performance evaluations in South Africa. Two leaf-feeding beetles, Zygogramma signatipennis (Stål) and Zygogramma piceicollis (Stål) (Coleoptera: Chrysomelidae), are the most promising biological control agents for T. rotundifolia: preliminary host-specificity trials suggest that they are adequately host-specific. The stem-boring beetle, Lixus fimbriolatus Boheman (Coleoptera: Curculionidae), is also highly damaging to T. rotundifolia, but its host range is yet to be determined. Two other stem-boring beetles, Canidia mexicana Thomson (Coleoptera: Cerambycidae) and Rhodobaenus auctus Chevrolat (Coleoptera: Curculionidae), have also been recorded on T. rotundifolia, and these will be considered for further testing if L. fimbriolatus is found to be unsuitable for release in South Africa. Only two insect species were imported as candidate agents on T. diversifolia, the leaf-feeding butterfly Chlosyne sp. (Lepidoptera: Nymphalidae), and an unidentified stem-boring moth (Lepidoptera: Tortricidae): the latter was tested in quarantine but rejected because it attacked several sunflower cultivars. Only one pathogen, Puccinia enceliae Dietel & Holw. (Uredinales: Pucciniaceae), was found that could potentially have been used as a biological control agent against the Tithonia species, but attempts to culture this rust were unsuccessful.

Potential impact and non-target effects of Gallerucida bifasciata (Coleoptera: Chrysomelidae), a candidate biological control agent for Fallopia japonica

A pre-release evaluation of a potential biological control agent is designed to estimate the impact the agent will have on the target in the introduced range, and whether the agent poses an acceptable level of risk to biodiversity. Here, we present an evaluation of the Asian leaf-beetle, Gallerucida bifasciata, a potential biological control agent for Japanese knotweed, Fallopia japonica. First we tested the impact of different larval and adult densities on the plant under laboratory conditions . After six weeks, the biomass of F. japonica was 15% or 28% that of control plants if larvae or adults were added, with levels of damage plateauing quickly with increasing insects densities. High fecundity and larval density dependence means that this agent could substantially and quickly suppress plant growth, although this damage only occurs early in the season, allowing F. japonica time to compensate. Second, following on from general host-specificity trials, we investigated potential non-target effects on the commercially important plant buckwheat, Fagopyrum esculentum, in more depth. The leaf-beetle showed some minor adult feeding on F. esculentum in choice trials, and in no-choice trials the leaf-beetle was able to complete its life cycle and sustain a population on F. esculentum, albeit causing lower levels of damage than on F. japonica. This suggests that if the beetle dispersed to areas where F. japonica is not present, it might colonise F. esculentum. Therefore, despite potentially being an effective agent, that the risk of non-target feeding is unacceptably high.

Should the mirid, Eccritotarsus catarinensis (Heteroptera: Miridae), be considered for release against water hyacinth in the United States of America?

Between one and seven biological control agents have been released against water hyacinth (Eichhornia crassipes (Mart.) Solms) in at least 30 countries, with varied success. A mirid, Eccritotarsus catarinensis (Carvalho) (Heteroptera: Miridae), the most recent agent released, is damaging to the plant on the African continent. It could be useful in the USA where water hyacinth remains a problem, but its introduction remains in doubt because during host specificity trials, it developed on Pontederia cordata L. (pickerelweed), indigenous to the USA. However, it did not establish on pickerelweed monocultures during South African field trials, and only light spillover feeding occurred where the two plants coexisted suggesting that the use of P. cordata as a host is a laboratory artefact and it may be suitable for use in the USA, if its thermal physiology allows establishment. We reran models developed for South Africa using CLIMEX to predict whether the mirid will establish where water hyacinth and pickerelweed co-occur, but not where pickerelweed occurs in the absence of water hyacinth. The models suggest that the mirid's distribution will be limited by cold winter temperatures and insufficient thermal accumulation to the southern states of the USA, within the main distribution of water hyacinth. Even though some spillover feeding on pickerelweed might result where the two plants co-occur, the risk of population level effects seems minimal and the risk to more northern pickerelweed negligible. The benefits, including improved habitat for pickerelweed, associated with further suppression of water hyacinth, outweigh the minimal risk of collateral damage to pickerelweed.

Biology, host range and varietal preference of the leaf-feeding geometrid, Leptostales ignifera, a potential biocontrol agent for Lantana camara in South Africa, under laboratory conditions

Lantana camara L. (Verbenaceae) is a weed of major importance in South Africa. It invades indigenous forests and veld, valuable commercial and agricultural forests, plantations and orchards, as well as river-catchment areas. Several natural enemies, including insect and pathogen species, have been released in South Africa, some of which established successfully. These do not exert sufficient control and additional natural enemies are required. Leptostales ignifera Warren (Lepidoptera: Geometridae), one of several new species being investigated as potential biocontrol agents, was collected in the subtropical parts of Florida, USA and Mexico. Host specificity trials indicated L. camara to be the preferred host plant of this quick-developing, voracious leaf-feeder, with some of the African indigenous Lippia species qualifying as very marginal hosts. The possible preference that L. ignifera might have for different South African naturalized L. camara varieties was studied during quarantine laboratory preference trials. Variety 029WP was the most suitable host for L. ignifera, although the other four tested varieties were able to support viable populations of the insect. Not taking other abiotic and biotic factors such as climate and predation into consideration, once released, L. ignifera should be able to establish on all five of the tested varieties in the field and contribute to the biological control of the complex of L. camara as a whole. Based on the above studies, permission has been granted for the release of L. ignifera in South Africa.

Biology, host range, and risk assessment supporting release in Africa of Falconia intermedia (Heteroptera: Miridae), a new biocontrol agent for Lantana camara

Abstract The ornamental hybrid shrub, Lantana camara L. (lantana), is a serious environmental weed and has been targeted for biological control in South Africa since 1961. The established biocontrol agents cause insufficient levels of damage and additional natural enemies are required to reduce the invasiveness of this weed. The lantana mirid, Falconia intermedia (Distant), is a promising new agent that was imported from the Caribbean for life history and host-range studies. The nymphs and adults are leaf-suckers that cause chlorotic speckling, which reduces the photosynthetic capacity of the plant. Biological studies indicate that F. intermedia has considerable biocontrol potential, in that it has a high intrinsic rate of increase, the potential for multiple generations a year, highly mobile adults, and a high level of damage per individual. Host-specificity trials indicated that the lantana mirid has a narrow host range, with L. camara being the most suitable host, but several indigenous African species in the closely related genus Lippia are suitable alternative host plants. Under multiple-choice conditions, adults showed a significant and strong oviposition preference for L. camara over the Lippia species. A risk assessment of potential nontarget effects indicated that three Lippia species could sustain damage levels in the field. The relatively low probability of damage to indigenous species was considered a justified trade-off for the potentially marked impact on L. camara. The regulatory authorities accepted the results of this study and F. intermedia was released against L. camara in South Africa in April 1999.

Failure of Eccritotarsus catarinensis, a biological control agent of waterhyacinth, to persist on pickerelweed, a non-target host in South Africa, after forced establishment

Abstract Waterhyacinth (Eichhornia crassipes) is the most damaging aquatic weed in South Africa, where five arthropod biological control agents have been released to date. The most recent introduction, a South American mirid, Eccritotarsus catarinensis, was shown to utilize Pontederia cordata (pickerelweed) during laboratory host-specificity trials. This did not influence the decision to release the mirid in South Africa, where the exotic pickerelweed is a declared invader. However, in the United States of America where waterhyacinth is also a problem, the possibility of spillover damage on indigenous pickerelweed may preclude the release of the mirid. In this study, E. catarinensis did not establish on pickerelweed in the field in South Africa, following either confined releases, where the mirids were initially released into sleeve cages, or unconfined releases where mirid-infested pickerelweed plants were placed amongst pickerelweed stands. We thus recommend that E. catarinensis be considered as an additional control agent of waterhyacinth in North America.

Laboratory Host Range Testing of the Flea Beetle, PSEUDOLAMPSIS GUTTATA (LeConte) (Coleoptera: Chrysomelidae), A Potential Natural Enemy for Red Water Fern, AZOLLA FILICULOIDES Lamarck (Pteridophyta: Azollaceae) in South Africa

The flea beetle, Pseudolampsis guttata (LeConte) (Coleoptera: Chrysomelidae) was assessed for release on the South American fern, Azolla filiculoides Lamarck (Pteridophyta: Azollaceae) which is invasive in aquatic ecosystems in South Africa. Favourable biological characteristics of the beetle include: long-lived and mobile adults, short generation time, high rate of increase and high per capita feeding rates. However, laboratory host specificity trials indicated that this beetle is an oligophagous species capable of utilizing several species in the genus Azolla and could pose a threat to native southern African Azolla species. Pseudolampsis guttata was therefore considered unsuitable for release in South Africa.

Life history and host specificity ofAltica cyanea [Coleoptera: Chrysomelidae], a potential biological control agent for water primrose,Ludwigia adscendens

The life history and host-specificity of the flea beetle,Altica cyanea (Weber) [Coleoptera: Chrysomelidae], were studied to determine its potential value as a biological control agent for water primrose,Ludwigia adscendens (Onagraceae). Females laid a mean of 146 yellowish eggs in masses on this weed and larvae passed through 3 instars. This multivoltine insect completed a generation in 80–84 days. In preliminary host-specificity trials, larvae and adults fed slightly on 2 varieties ofTrapanatans, but they caused considerable damage toLudwigia spp.; no feeding occurred on rice. In host plant choice tests, larvae and adults preferred to feed onL. adscendens andL. prostata toT. natans. In these same tests, onlyLudwigia spp. supported complete development, but it was fastest onL. adscendens. Adults that were reared onL. adscendens lived longer and laid more eggs than those reared onL. prostata, suggesting thatA. cyanea may be a promising biological control agent forL. adscendens growing in rice-paddies. AdultZicrona coerulea L. [Hemiptera: Pentatomidae] preyed upon 3rd instarA. cyanea larvae in the laboratory.