Weed Biocontrol Agents Research Articles

Application of a Standardized Impact Monitoring Protocol (SIMP) to assess biological weed control projects in the State of Idaho, USA

In the USA, the lack of systematic post-release assessments of weed biological control projects demonstrating quantitative effects of biological control agents on target weed densities, population biology, and/or vegetation responses remain a concern. While there are numerous quantitative assessments, overall conclusions are limited in scope by the spatial scale covered, duration of study, potentially confounding factors, or weed parameters measured. The United States Department of Agriculture’s Animal Plant Health Inspection Service is the regulatory agency responsible for biological control agent releases. USDA APHIS requires post-release impact monitoring of target weed populations as part of release permits issued for new biological control agents. However, large scale, long-term impact assessments are rarely conducted for a variety of reasons. To address this problem, biocontrol practitioners developed a standardized impact monitoring protocol (SIMP) for the State of Idaho in 2007. The intent was to collect ecological data which are sufficiently robust for quantitative analysis, while minimizing time spent in the field to make data collection appealing to citizen scientists and land managers. SIMP has since been implemented for ten weed biocontrol systems, with more than fifteen years of monitoring data for some systems. SIMP data are collected along permanent transects and include weed density and size parameters, categorical vegetation community and biocontrol agent abundance data. Data are repeated measures, which are well suited for dynamic population modeling and can be coupled with environmental factors (e.g., climate or soil data) to analyze how processes like climate (and climate change) can drive variation in weed biocontrol agent interactions. SIMP data can be combined with detailed weed, nontarget plant and/or biocontrol agent surveys to add analytical data. Finally, SIMP has also been implemented for two weeds for which biological control agents have not yet been introduced to provide weed and community vegetation data prior to releases.

Importance of pathobiomes to the success of microbial weed biocontrol agents

The role of microbial communities in the establishment of plant pathogens introduced to new environments as weed biocontrol agents is relatively unexplored. Substantial evidence from crop protection research shows how bulk microbial communities influence the establishment, survival, and success of pathogens. Just as mismatches between biocontrol agents, host genotypes, and climate can lead to establishment failure, we propose that mismatches between microbial biocontrol agents and resident microbiomes may be a cause of apparent weed biocontrol failures. From this perspective, we discuss recent progress in research on plant-associated microbial communities whose interactions influence disease development, or “pathobiomes,” and consider the implications for biocontrol agent evaluation and release. Open questions about how weed pathobiomes develop and influence agent success rates provide ample opportunity for researchers to adapt new methods and analytical frameworks that will enhance biocontrol programs. We conclude by prioritizing the development of standard methods to include microbiomes as a component of release studies. Developing a pathobiome-aware agent selection and release process will improve agent establishment rates and biocontrol outcomes.

Modeling Study of Factors Determining Efficacy of Biological Control of Adventive Weeds

We model the spatiotemporal dynamics of a community consisting of competing weed and cultivated plant species and a population of specialized phytophagous insects used as the weed biocontrol agent. The model is formulated as a PDE system of taxis–diffusion–reaction type and computer-implemented for one-dimensional and two-dimensional cases of spatial habitat for the Neumann zero-flux boundary condition. In order to discretize the original continuous system, we applied the method of lines. The obtained system of ODEs is integrated using the Runge–Kutta method with a variable time step and control of the integration accuracy. The numerical simulations provide insights into the mechanism of formation of solitary population waves (SPWs) of the phytophage, revealing the factors that determine the efficacy of combined application of the phytophagous insect (classical biological method) and cultivated plant (phytocenotic method) to suppress weed foci. In particular, the presented results illustrate the stabilizing action of cultivated plants, which fix the SPW effect by occupying the free area behind the wave front so that the weed remains suppressed in the absence of a phytophage.

Prioritizing candidate agents for the biological control of weeds

A dataset of weed biocontrol agents released in New Zealand was used to review the importance of several factors that have been hypothesized to be predictors of agent impact. Agents that were observed to be highly damaging (i.e., completely defoliating or killing plants or reducing populations in the field) in the native range were almost invariably highly damaging in the introduced range. In contrast, species that were subject to apparent competition mediated by parasitoids shared with closely related “native analogue” species failed to have an impact on the target weed. Agent guild also helped predict agent impact. In particular, the use of agents that only attack reproductive parts of the plant (e.g., seed and flower-feeders) is unlikely to result in reduced weed populations. A simple framework was developed to score candidate agents based on potential host specificity and predicted impact that could be used to assist the ranking of candidate agents for further investigations at the start of a biocontrol program. Nevertheless, further refinement from incorporating larger datasets is desirable as current parameter estimates are likely to be unreliable.

Inhibition of broomrape germination by 2,4-diacetylphloroglucinol produced by environmental Pseudomonas.

Parasitic weeds such as broomrapes (Phelipanche ramosa and Orobanche cumana) cause severe damage to crops and their development must be controlled. Given that phloroglucinol compounds (PGCs) produced by environmental Pseudomonas could be toxic towards certain plants, we assessed the potential herbicidal effect of the bacterial model Pseudomonas ogarae F113, a PGCs-producing bacterium, on parasitic weed. By combining the use of a mutagenesis approach and of pure PGCs, we evaluated the in vitro effect of PGC-produced by P. ogarae F113 on broomrape germination and assessed the protective activity of a PGC-producing bacteria on oilseed rape (Brassica napus) against P. ramosa in non-sterile soils. We showed that the inhibition of the germination depends on the PGCs molecular structure and their concentrations as well as the broomrape species and pathovars. This inhibition caused by the PGCs is irreversible, causing a brown coloration of the broomrape seeds. The inoculation of PGCs-producing bacteria limited the broomrape infection of P. ramosa, without affecting the host growth. Moreover, elemental profiling analysis of oilseed rape revealed that neither F113 nor applied PGCs affected the nutrition capacity of the oilseed rape host. Our study expands the knowledge on plant-beneficial Pseudomonas as weed biocontrol agents and opens new avenues for the development of natural bioherbicides to enhance crop yield.

The potential of invasive species Praxelis clematidea extract as a bioherbicide for Asystasia gangetica

Abstract. Wardini TH, Afifa IN, Esyanti RR, Astutiningsih NT, Pujisiswanto AH. 2023. The potential of invasive species Praxelis clematidea extract as a bioherbicide for Asystasia gangetica. Biodiversitas 24: 4738-4746. The occurrence of weeds in agriculture directly impacts crop productivity, and the reliance on synthetic herbicides to solve the problem has raised concerns about the increasing environmental damage. Using bioherbicides to control weeds is necessary to reduce the negative impact of synthetic herbicides. Bioactive compounds derived from plants can be utilized to develop eco-friendly herbicides. Praxelis clematidea extract was reported to exhibit allelopathic activity against weeds. This study aims to evaluate the potential of P. clematidea extract as a bioherbicide to control seed germination and seedling growth of Asystasia gangetica weed. P. clematidea water extract was screened for its chemical compounds and determined its phytotoxic effects on Asystasia gangetica. The results showed that P. clematidea water extract contained phenolics, alkaloids, and terpenoids. P. clematidea extract suppressed seed germination, the time needed for seed germination, sprout length, number of leaves, biomass, and total chlorophyll contents. LC50values for the seed germination, seedling, root, and shoot growth were calculated at 9%, 25%, 11%,and 37% (v/v), respectively. Thus, P. clematidea could be used as an A. gangetica weed biocontrol agent and might be integrated into weed management programs.

Impact of the leaf‐mining jewel beetle Hedwigiella jureceki on two forms of cat's claw creeper, Dolichandra unguis‐cati

AbstractThe impact assessment of a weed biocontrol agent offers insight into the potential of the agent and weed responses in long‐term weed management. The impact of the jewel beetle Hedwigiella jureceki Obenberger (Coleoptera: Buprestidae) and its first‐generation larvae on the long‐pod (LP) and short‐pod (SP) forms of cat's claw creeper, Dolichandra unguis‐cati (L.) L.G. Lohmann (Bignoniaceae) was studied by initiating one, two, or five pairs (male and female) of adults in a glasshouse experiment. With the higher density of adults, the number of leaves increased significantly, as did the numbers of dead leaves and larval mines, and the percentages of affected leaves (up to 40%), and damaged leaf areas (up to 60%). Plants with the five pairs of adults had fewer leaves and reduced dry weights (e.g., tuber biomass was reduced by 50.4–62.5%). Regardless of adult density, the dry weights of the leaves and tubers were lower in the SP form. Conversely, the LP form had fewer leaves (ca. 17% fewer) and slower shoot growth (mean ± SE = 46.3 ± 2.6 cm per week) than the SP form (66.5 ± 3.8 cm per week). However, the number of tubers was not affected by the plant form and adult density. Larval mines (262 ± 30), pupae (186 ± 17), and adults (144 ± 14) in LP form were higher than in SP form (243 ± 20, 167 ± 14, and 115 ± 11, respectively). Likewise, the average weight of adults that emerged on the LP form was higher. Therefore, H. jureceki impacted both LP and SP forms. The SP form showed a higher impact in terms of reduced tuber growth, and the LP form is anticipated to be harmed more in the long term as a higher number of H. jureceki offspring developed on it when competition for food was increased.

Application of nitrogen, phosphorus, potassium + carbon fertilizer improves the growth of Tecoma stans and the performance of its root-feeding biological control agent, Heikertingerella sp.

ABSTRACT The invasive tree Tecoma stans (L.) Juss. ex Kunth var stans (Bignoniaceae) has been targeted for biological control in South Africa since 2003. Plant nutrients contained in fertilizers typically improve host-plant quality, thereby enhancing the fitness of insect herbivores deployed as weed biocontrol agents. We investigated the effect of increasing nutrient levels (fertilizer treatments) on the growth of potted T. stans plants and on the subsequent performance of the root-feeding beetle Heikertingerella sp. (Chrysomelidae). Enhanced performance of Heikertingerella sp. in culture will improve mass-rearing initiatives for releases in South Africa. Ten newly emerged mating pairs of Heikertingerella sp. were exposed under glasshouse conditions to caged plants that were treated with four regimes of nitrogen: phosphorus: potassium 2:3:2 (14%) + carbon (8%) fertilizer, namely zero (control), low (5.6 g/m2), medium (9.4 g/m2) and high (13.2 g/m2). Increasing nutrient levels significantly increased leaf production and above – and below-ground biomass accumulation in T. stans plants. Similarly, the performance of Heikertingerella sp. was improved substantially, with significantly and progressively higher levels of P1 adult feeding and F1 progeny production, and significantly and progressively reduced F1 developmental times, under increasing nutrient levels. While F1 adult size was significantly increased by fertilizer application relative to the controls, there were no significant differences between the three nutrient levels. Although the high fertilizer application rates were the most suitable for the culturing and mass-rearing of Heikertingerella sp. adults, medium fertilizer applications may prove more economical to improve host-plant quality and boost beetle numbers for release.

Performance and field host range of the life stages of Cornops aquaticum, a biological control agent of water hyacinth

AbstractHost specificity determination of weed biocontrol agents has historically relied on evidence generated through quarantine trials in the region of introduction. These trials could give ‘false positive’ results due to a maximum type I error probability, and where possible, more research under field conditions should be conducted in the region of origin. The oligophagous, semiaquatic grasshopper, Cornops aquaticum Bruner (Orthoptera: Acrididae, Tetrataeniini), was released in South Africa for the biological control of Pontederia crassipes Pellegrini and Horn (Pontederiaceae). The aim of this study was to assess how the performance and field host range of C. aquaticum varies according to its stages of development, and how this contributes to the understanding of the relationship between the fundamental (laboratory‐based) and the ecological (field‐based) host range of this grasshopper, and its implications for water hyacinth biocontrol. We conducted post‐release laboratory no‐choice trials, confining early instars (instars 1 and 2), later instars (instars 3–6), and adult females and males in mesh cages, to determine insect performance on wetland plants growing in sympatry with P. crassipes. Also, gut analysis from field‐collected C. aquaticum was done to determine the ecological host range of this insect, identifying epidermal tissue of consumed plants. In no‐choice trials, survival rates of the later instars and adult C. aquaticum were similar on Pistia stratiotes L. (Araceae), Oxycaryum cubense (Poepp. & Kunth) Lye (Cyperaceae), and P. crassipes. However, under field conditions, P. crassipes and the congeneric Pontederia azurea Sw. were the only plant contents in the guts of early instars and the most abundant species in later instars and adults. The results support the hypothesis that C. aquaticum is an oligophagous insect on the genus Pontederia, and that different life stages should be considered when conducting host‐specificity trials in externally feeding mobile herbivore species. Diet composition of field‐collected insects thus could help detect false positives in laboratory trials, being an additional and realistic approach in understanding and predicting the selection processes of the insect in the new environment. Retrospective analysis of potential agents that were rejected due to lack of host‐specificity, using the methods from this study, could add a suite of additional agents to programs where invasive weeds remain unmanaged.

Sex-specific foraging response to interspecific interactions in carabid beetles

Carabids beetles are considered important biocontrol agents of weeds, but predicting levels of weed seed consumption and regulation is difficult. Olfactory cues from predators or potential competitors alter the selection and consumption of particular food resources by foraging individuals. Whether this leads to changes in consumption levels or simply to changes in the rate at which consumption takes place over time is not yet known. Identifying and understanding the factors that drive the seed foraging behaviour of carabid beetles, in the context of interspecific interaction, is essential for predicting consumption levels in different carabid communities and hence improve the ecosystem service of weed regulation by carabid beetles.We tested the response of 119 Harpalus affinis individuals foraging for Viola arvensis or Taraxacum officinale seeds, to encounters with individuals of another carabid species, Pterostichus melanarius. Their foraging behaviour (i.e. total consumption, latency to first seed acceptance and position in the arena) was recorded for 72 h and the consumption rates were reported in detail for the first 7 h of the experiment for all treatments.Even though the total number of seeds consumed after 24 h did not differ, the dynamic of seed consumption by H. affinis changed significantly in the presence of P. melanarius. The attraction of individuals to the seeds diminished over time, indicating a probable state of satiety being reached. This suggests that looking at the total number of seeds consumed after a long period of time may be misleading and that a greater focus on of the temporal dynamics of consumption is needed if we are to understand carabids interest in specific weed species in a specific foraging context. We found that only females seemed to react to the presence of P. melanarius individuals, indicating that the perception of the level of risk may be sex-specific in carabid beetles.

Understanding the host finding behavior of a biological weed control candidate specialist as a contribution to pre‐release risk assessments

AbstractPre‐release host range assessments of weed biocontrol agent (BCA) candidates typically rely on no‐choice and choice feeding, oviposition, and development tests. However, these tests may exclude potentially environmentally safe BCA candidates from consideration if they develop on nontarget plant species in no‐choice tests that they would not colonize in the field because of behavioral barriers during host selection. Here, we examined the behavioral responses of Ceutorhynchus rusticus Gyllenhal (Coleoptera: Curculionidae) to olfactory and visual plant cues of nine native North American and two Eurasian confamilial plant species of invasive Eurasian mustard dyer's woad, Isatis tinctoria L. (Brassicaceae). In behavioral bioassays with olfactory, visual, and combined plant cues of one plant species vs. purified air and/or empty arms (control treatments), weevils responded with attraction to all offered I. tinctoria plant cues but showed no preference for most nontarget plant species relative to control treatments. The weevil also responded faster in these bioassays to I. tinctoria cues when compared to most nontarget species. In bioassays with olfactory, visual, or combined cues of nontarget species vs. those of I. tinctoria, C. rusticus preferred I. tinctoria over most nontarget plant species (olfactory cues: eight of 11, visual cues: two of 11, and combined cues: 11 out of 11 nontarget plant species). The results also suggest that C. rusticus mainly uses olfactory or combined olfactory and visual plant cues rather than visual cues alone in the early stages of host finding. Additionally, the findings suggest that during host finding, C. rusticus prefers cues of its Eurasian field host over those of nontarget species, reducing risks of post‐release nontarget attack in the field. Assessing behavioral responses to multimodal cues can advance our understanding of BCA candidates' host discrimination and facilitate the accurate interpretation of conventional no‐choice and choice feeding and development data during pre‐release environmental safety assessments.

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.

Is Biological Control of Weeds Conservation’s Blind Spot?

Invasive alien species are among the most important threats to biodiversity, with invasive plants ranking among the highest. Classical weed biological control—or biocontrol—reunites exotic plants with host-specific natural enemies from their native range with the aim of controlling the invasive plant. We reviewed the attention classical weed biocontrol has received from scientific publications for the last 30 years, classified according to the area of academia and applied sciences, as well as the region of the world. Biological control journals were excluded from the analyses to avoid bias. This process allowed us to evaluate the support classical weed biocontrol has among the scientific community. We also recorded the number of weed biocontrol agents released from 1900 to date, where they were collected, and where they were released as a way to analyze the evolution of classical weed biocontrol policies in different parts of the world. Classical weed biocontrol releases peaked between 1990 and 1999, but have declined since, probably due to funding issues, increases in regulations, and bad publicity. Researchers in theoretical ecology appear to be more skeptical toward weed biocontrol than scientists in applied and experimental biology. Our synthesis also suggests that despite resistance to classical weed biocontrol in some quarters of the scientific community, the general scientific perception of the discipline has been consistently favorable. This means that the general scientific perception of classical weed biocontrol contradicts its level of current application. The five main objections against classical weed biocontrol—direct nontarget effects, indirect and hidden nontarget effects, evolution of host shifts in biocontrol agents, dispersion to unwanted areas, and disagreements on its level of success in the field—are summarized and analyzed in terms of their relevance and probability of occurrence. We also describe the way classical weed biocontrol practitioners deal with them at present to ensure safety and sustainability. Our analysis suggests the potential of classical weed biocontrol is undervalued in some areas of science and management due to objections that are plausible in theory, although their likelihood is very low and on-ground evidence scant.

Exploring plant volatile-mediated interactions between native and introduced plants and insects

In invasion scenarios, native and introduced species co-occur creating new interactions and modifying existing ones. Many plant–plant and plant–insect interactions are mediated by volatile organic compounds (VOCs), however, these have seldom been studied in an invasion context. To fill this knowledge gap, we explored some interactions mediated by VOCs between native and introduced plants and insects in a New Zealand system. We investigated whether a native plant, Leptospermum scoparium (mānuka), changes its volatile profile when grown adjacent to two European introduced plants, Calluna vulgaris (heather) and Cytisus scoparius (Scotch broom), in a semi-field trial using potted plants without above- or below-ground physical contact. We also investigated the influence of plant cues on the host-searching behaviour of two beetles, the native Pyronota festiva (mānuka beetle), and the introduced biocontrol agent Lochmaea suturalis (heather beetle), by offering them their host-plant and non-host volatiles versus clean air, and their combination in a Y-tube olfactometer. As a follow-up, we performed preference/feeding tests in Petri dishes with fresh plant material. Results of the semi-field experiment show a significant reduction in green leaf volatiles, sesquiterpenes and total volatile emissions by mānuka plants neighbouring heather. In the Y-tube assays, the native beetle P. festiva performed poorly in discriminating between host and non-host plants based on plant volatile cues only. However, it performed relatively well in the Petri dish tests, where other cues (i.e., visual, gustatory or tactile) were present. In contrast, the introduced beetle L. suturalis showed high host-specificity in both Y-tube and Petri dish assays. This study illustrates the importance of VOCs in mediating interactions between introduced and native species, suggesting that invasive plants can disrupt native plants’ communication and affect the host-searching behaviour of native insects. It also reinforces the relevance of regular host testing on introduced weed biocontrol agents to avoid unwanted host shifts or host-range expansion.

The benefits to sub-Saharan Africa of the biological control of weeds: already considerable, but could be far greater.

Sub-Saharan Africa (SSA) is climatically diverse, with many biomes. Many species of invasive alien plants (IAPs) are present, with severe negative impacts that largely remain unquantified. Importation biological weed control (IBWC) has been practiced on the continent since the early 20th century, with some notable successes. Weed biocontrol agents (WBA) (141 species) have been released in 30 countries on 69 weed species, and spread to a further eight countries. South Africa has the most active IBWC program, while several projects are underway in other countries, involving either the release of WBA or monitoring of their spread and impact. However, given the large and increasing footprint of IAPs, and the poor prospects for their successful management using other control methods, we discuss reasons for therelatively low uptake of IBWC in SSA, and suggest ways to increase this.

Plant pathogens as introduced weed biological control agents: Could antagonistic fungi be important factors determining agent success or failure?

Mycoparasitic interactions are common in nature, form part of the microbiota of plants, and are considered significant contributors to fungus-fungus antagonism. Mycoparasites kill plant pathogens, protect the plant from abiotic and biotic stressors, and reduce disease incidence and severity at the plant population level. Their exploitation as biocontrol agents in agriculture is well documented. However, mycoparasites may potentially affect classical fungal biocontrol agents of invasive weed species. Classical biological control, or biocontrol, of invasive weeds involves the intentional introduction of exotic, usually co-evolved plant pathogens and insects, for permanent establishment and long-term control of the target plant. Agent establishment, effectiveness, and safety are the critical elements for a successful weed biocontrol programme. Establishment and effectiveness of agents on the invasive plant often vary throughout the invaded range with about two-thirds of weed biocontrol agents failing to suppress their target weed. There are many documented reasons why weed biocontrol agents do not establish or are ineffective when they do, and the presence and accumulation of natural enemies in the invaded range is one of them. Endophyte-enriched, invasive weeds and those forming mutualistic associations with indigenous, native endophytes could explain the lack of consistency of some classical biological control introductions. However, another variable could be factored into the mix: mycoparasitism, where one fungus parasitises another, the natural enemies of the plant's natural enemies. In this review article, we introduce the concept of invasive weed biocontrol and the history of using plant pathogens as biocontrol agents. We discuss the success and failure of fungal agent programmes and delve into the patterns of success or failure, with a focus on the potential antagonistic role of endophytes and mycoparasites.

Relative risk scores generated from laboratory specificity tests predict non-target impacts of Microctonus spp. parasitoids in the field

A ‘relative risk score’ approach to help determine the potential impact of biological control agents on non-target species, previously proposed for weed biocontrol agents and aphid parasitoids, was examined in a retrospective study for Microctonus spp. parasitoids introduced to control pest weevils in New Zealand. ‘Relative risk scores’ (% parasitism for non-target weevil species/% parasitism for target weevil species) were calculated for 20 weevil (New Zealand adventive and indigenous) species and 3 introduced (New Zealand) Microctonus biocontrol taxa (Microctonus aethiopoides Moroccan biotype, Microctonus aethiopoides Irish biotype,and Microctonus hyperodae), based on previously published no-choice host range laboratory testing. Field hosts for each parasitoid and host combination were also examined through an extensive literature review and targeted field surveys. Strong correlations between ‘relative risk scores’ for no-choice laboratory assays and field host binary values (host: yes or no) were obtained for each Microctonus/weevil dataset, but there was no threshold score indicating that the approach cannot be relied upon to predict parasitoid field host ranges with a high degree of certainty. A second analysis that categorized weevil hosts as ‘major field hosts’, if literature records indicated high (≥10%) levels of parasitism in the field, showed a clearer relationship between relative risk score and the probability of a weevil being a major field host. There were no observations of a weevil being a major field host when the relative risk score value was <0.26. There was only one observation of a weevil not being a major field host where the relative risk score was >0.33. This nevertheless, indicates that the approach can mistakenly exclude some acceptable agents that would be host specific in the field. We conclude that this approach could potentially be used to assist risk assessment when introducing Microctonus spp. biocontrol agents, but other factors need to be assessed concurrently to reliably assess risk.

The thermal physiology of Lysathia sp. (Coleoptera: Chrysomelidae), a biocontrol agent of parrot’s feather in South Africa, supports its success

ABSTRACT The establishment success of biocontrol agents originating from tropical regions is often limited by climate when introduced in temperate regions. However, the flea beetle, Lysathia sp. (Coleoptera: Chrysomelidae), a biocontrol agent of Myriophyllum aquaticum (Vell.) Verdc. (Haloragaceae) in South Africa, is an effective agent in regions where other biocontrol agents of tropical aquatic weeds have failed due to winter-induced mortality. The development (degree-day model) and thermal tolerance (critical thermal minimum/maximum [CTmin/max] and lower/upper lethal limits [LLT/ULT50]) of Lysathia sp. were investigated to explain this success. The model predicted that Lysathia sp. could complete 6 to 12 generations per year in the colder regions of the country. The lower threshold for development (t0) was 13.0 °C and thermal constant (K) was 222.4 days, which is considerably lower than the K values of other biocontrol agents of aquatic weeds in South Africa. This suggests that above the temperature threshold, Lysathia sp. can develop faster than those other species and complete multiple life cycles in the cooler winter months, allowing for rapid population growth and thus improving M. aquaticum control. Furthermore, the CTmin of Lysathia sp. was 2.3 ± 0.2 °C and the CTmax was 49.0 ± 0.5 °C. The LLT50 was calculated as ∼ −7.0 °C and the ULT50 as ∼ 43.0 °C. These wide tolerance ranges and survival below freezing show why Lysathia sp. has established at cool sites and suggest that it may be a suitable agent for other cold countries invaded by M. aquaticum.

The Role of Mass-Rearing in Weed Biological Control Projects in South Africa

It has been documented that the continual release of high numbers of biological control (biocontrol) agents for weeds increases the likelihood of agent establishment and has been shown to reduce the time between the first release and subsequent control of the target weed. Here we review the mass-rearing activities for weed biocontrol agents in South Africa between 2011 and 2020. Some 4.7 million individual insects from 40 species of biocontrol agent have been released on 31 weed species at over 2000 sites throughout South Africa during the last decade. These insects were produced at mass-rearing facilities at eight research institutions, five schools and 10 Non-Governmental Organizations. These mass-rearing activities have created employment for 41 fulltime, fixed contract staff, of which 11 are people living with physical disabilities. To improve the uptake of mass-rearing through community engagement, appropriate protocols are required to ensure that agents are produced in high numbers to suppress invasive alien plant populations in South Africa.

Using Chemical Ecology to Enhance Weed Biological Control.

Simple SummarySignaling chemicals produced by one organism that bring about a behavioral response in a recipient organism are known as semiochemicals, with pheromones being a well-known example. Semiochemicals have been widely used to monitor and control insect pests in agricultural and forestry settings, but they have not been widely used in weed biological control. Here, we list the few examples of semiochemical use in the practice of classical weed biological control, where a natural enemy (biocontrol agent) from the native range of the plant is introduced into the new invaded range. Uses of semiochemicals include monitoring of biocontrol agents (sex pheromones), keeping biocontrol agents together long enough for them to become well established (aggregation pheromones) and repelling agents from areas where they may be unwanted (host or non-host plant volatile organic deterrents). We make the case that given the vast potential of biological control in suppressing invasive plants it is well worth developing and utilizing semiochemicals to enhance biocontrol programs.In agricultural systems, chemical ecology and the use of semiochemicals have become critical components of integrated pest management. The categories of semiochemicals that have been used include sex pheromones, aggregation pheromones, and plant volatile compounds used as attractants as well as repellents. In contrast, semiochemicals are rarely utilized for management of insects used in weed biological control. Here, we advocate for the benefit of chemical ecology principles in the implementation of weed biocontrol by describing successful utilization of semiochemicals for release, monitoring and manipulation of weed biocontrol agent populations. The potential for more widespread adoption and successful implementation of semiochemicals justifies multidisciplinary collaborations and increased research on how semiochemicals and chemical ecology can enhance weed biocontrol programs.