Megamelus Scutellaris Research Articles

Monitoring and assessing the effectiveness of the biological control implemented to address the invasion of water hyacinth (Eichhornia crassipes) in Hartbeespoort Dam, South Africa

Water hyacinth is one of the most aggressive alien invasive plants, which invades freshwater resources and destroys native biodiversity. The plant proliferates rapidly over a short space of time, forming thick dense layer on the surface of freshwater bodies. Monitoring and management of water hyacinth is essential to protect water resources affected by the presence of this plant. The study assessed the effectiveness of biological agent (Megamelus scutellaris) applied in the Hartbeespoort Dam from pre (2016–2017) and post (2018–2023) biological control to manage water hyacinth spread and proliferation. In achieving this main goal, the study used advanced cloud-computing machine learning techniques and multi date Sentinel-2 Multispectral Instrument (MSI) data to monitor the effectiveness of such biological control. During this analysis, remote sensing data was acquired for two time periods namely: pre-intervention (2016–2017) and post intervention (2018–2023) to establish variation in the spatio-temporal distribution of water hyacinth in the Hartbeespoort Dam using various machine learning techniques (Support Vector Machine (SVM), Classification and Regression Tree (CART), Random Forest (RF) and Naïve Bayes (NB)) in Google Earth Engine cloud computing platform, and assessed the spectral separability of water hyacinth from numerous land cover types, within and around the Hartbeespoort Dam using the Sentinel-2 derived spectral reflectance curves. The results indicated that the extent of water hyacinth area coverage decreased from 15% to below 6% between the period of 2018 and 2021, however, a significant increase was noted between November 2022 and April 2023, after the biological control was introduced. The significant increase observed during the time period of November 2022 and April 2023 can be attributed to nutrient rich water discharging into the dam from the Crocodile River during the time of flooding reported in November 2022. The result further indicate that RF produced the highest overall accuracies ranging between 93.42% and 98.70%. While NB produced the lowest accuracies ranging between 87.76% and 92.08%. These findings underscore the relevance of new generation satellite dataset and machine learning algorithms in monitoring the effectiveness of the biological controls of alien invasive spread provide information regarding alien plant invasion. Therefore, aligning with Sustainable Development Goals (SDG 6) emphasizing on the importance of implementing effective control measures to control invasive species and their impact on water resources thus ensuring the sustainability of freshwater ecosystems and the availability of clean water resources.

Bridging boundaries: Six years of community engagement with biological control implementation and monitoring of water hyacinth on Hartbeespoort Dam, South Africa

Water hyacinth has been present on Hartbeespoort Dam since the 1960s. Historical weed management consisted of herbicide and mechanical/manual removal. However, due to funding constraints, biological control was implemented as a cost-effective and environmentally friendly alternative. The impoundment receives excessive phosphorus and nitrogen due to its location downstream from densely populated cities, which includes one of the capital cities of South Africa, Pretoria. Considering that Hartbeespoort Dam is in a temperate region, with cool winters, we embarked on an augmentative biological control approach. To increase our biological control efforts, we had to bridge the research-implementation gap. We fostered a community of practice, by creating a space for the affected community to actively participate in the programme. This would give us the opportunity to share scientific knowledge to affected stakeholders, gauge the public perception, and align potential goals. Social media, in-person meetings and training sessions were used to engage with the public. To increase the availability of the water hyacinth biological control agent, Megamelus scutellaris, community members (i.e. private stakeholders) were offered the opportunity to manage satellite rearing stations. Between 2018 and 2023, nearly half a million Megamelus scutellaris individuals, sourced from both the Centre for Biological Control’s mass-rearing facility and satellite rearing stations around Hartbeespoort Dam, have been released into the dam. Water hyacinth cover on the dam drastically declined every year following a build-up of Megamelus scutellaris. Over the course of the programme, private stakeholders that were actively involved grew from two (2018) to 16 (2023). Analysing the participation of private stakeholders using a framework that looks at the level of value gained from our interactions, two partners reached a value chain phase that led to an expansion of biological control implementation. Longer-term private stakeholders generated increased interest for biological control implementation in their community, which resulted in recruitment of new private stakeholders. We also evaluated the knowledge of the affected community in 2023 through an online questionnaire. Out of 132 respondents, 51 % had knowledge of what biological control entails, with 56 % acknowledging that biological control has had a positive impact on the management of water hyacinth. Bridging the research-implementation boundary has strengthened biological control efforts, enabling a more sustainable approach to the management of water hyacinth on Hartbeespoort Dam. However, continued community engagement is necessary to increase the public’s knowledge about the practice of biological control.

Effects of elevated CO2 on the water hyacinth-biocontrol agent Megamelus scutellaris (Hemiptera: Delphacidae) and its yeast-like symbiotes

Water hyacinth, Pontederia crassipes, is a highly invasive plant native to South America and one of the most invasive aquatic plants in the world. For its control, the planthopper Megamelus scutellaris Berg (Hemiptera: Delphacidae), a phloem feeder also native to South America has been introduced to the USA and South Africa. Considering predicted climate change scenarios, understanding their impacts on biological control agents is crucial. An intriguing yet scarcely explored subject, is the effect of climatic changes on the obligate endosymbionts associated with sap-sucking feeders. Planthoppers establish an obligate relationship with yeast-like symbiotes (YLS), unicellular fungal microorganisms that play an important role in their development, providing missing nutrients in their diet. Considering that increased atmospheric CO2 affects plant chemical composition, this might have a direct impact on their insect host and on their number of YLS. We evaluated the effect of two different CO2 environments: current (cCO2 − 400 ppm) and elevated (eCO2 − 800 ppm) on the abundance of YLS (number of YLS cells/insect) of M. scutellaris, as well as the, age structure, sex, and weight of insects. Heavier females harbored more YLS under eCO2 which underscores the importance of the interaction of CO2 levels and insects’ weight in shaping the abundance of YLS. Additionally, there was a significant increase in the total abundance of insects for instars III to V and adults under eCO2 conditions. However, male number significantly exceeded that of females under both CO2 conditions. Our results suggest a potential positive impact of eCO2 on M. scutellaris populations, which could, in turn, enhance the control of P. crassipes.

Evaluating the establishment of a new water hyacinth biological control agent in South Africa

Megamelus scutellaris Berg (Hemiptera: Delphacidae) is the most recent of nine biological control agents developed to manage invasive water hyacinth, Pontederia (=Eichhornia) crassipes Mart. (Pontederiaceae), in South Africa. More than a million M. scutellaris have been mass-reared and released since the first introduction of the agent into South Africa in 2013, successfully establishing overwintering populations at 32 sites in seven of the nine provinces. Establishment has also been recorded at seven of these sites through natural dispersal from sites where they had established. Inundative releases, where large numbers of M. scutellaris are released regularly, have resulted in excellent establishment, and caused a significant reduction in water hyacinth cover in areas where, historically, biological control seemed unlikely due to excessive eutrophication. Although M. scutellaris has established well throughout South Africa through classical biological control methods, this study also showed that inundative releases of biological control agents over multiple seasons results in the most effective control of the weed, especially at cool temperate and eutrophic sites.

Toxicity of herbicides used for control of waterhyacinth in the California Delta towards the planthopper Megamelus scutellaris released for biological control

ABSTRACT The water hyacinth planthopper Megamelus scutellaris has been released for biological control of waterhyacinth (Eichhornia crassipes or Pontederia crassipes) in Florida and is widely established there, with additional releases in Louisiana and California, USA, and in South Africa. Management still often requires the use of herbicides. Knowledge of their direct toxicity to M. scutellaris would aid in integrated management. We tested the contact toxicity of 2,4-D, diquat, glyphosate, imazamox and penoxsulam as well as a paraffinic oil-based and a vegetable oil-based adjuvant, by exposing planthoppers to dipped leaves in the laboratory, and by applying chemicals to infested mesocosms in the field. Exposure to 2,4-D, glyphosate, imazamox and penoxsulam did not cause significant mortality to M. scutellaris compared to exposure to water-dipped leaves in the laboratory, and did not consistently reduce final counts of M. scutellaris compared to counts in water-sprayed mesocosms in field trials. Laboratory exposure to diquat-dipped leaves caused significantly (45%–69%) higher mortality than did exposure to water-dipped leaves, and exposure to leaves dipped in paraffinic-oil based adjuvant also increased mortality (40–50%). In field mesocosms, application of diquat reduced recovery of live M. scutellaris by 64% in one trial, and the same adjuvant reduced recovery by 36%–49% in two trials. The results suggest that biological and chemical control of water hyacinth using M. scutellaris can be safely integrated using most of the herbicides applied for control of this weed in California.

Effects of elevated CO2 on feeding responses of biological control agents of Pontederia crassipes

AbstractElevated carbon dioxide (eCO2) and rising temperatures will have far‐reaching effects on global plant‐insect interactions, yet their implications for future biological control programs are not fully understood. Studies have shown that elevated CO2 will affect insect feeding guilds differently and these responses can be predicted with some confidence. Water hyacinth, Pontederia crassipes Mart. (Pontederiaceae), is a native and representative species of the Del Plata wetlands (Argentina) that invades outside of its native environment. It is considered one of the world's worst aquatic weeds and a target for biological control. In this study, water hyacinth plants were grown under two CO2 concentrations – current (400 p.p.m.) or elevated (800 p.p.m.) –, with and without two biocontrol agents representing different feeding guilds, the leaf‐chewing Cornops aquaticum Brüner (Orthoptera: Acrididae) and the phloem‐feeding Megamelus scutellaris Berg (Hemiptera: Delphacidae). Under eCO2 concentration, photosynthetic rate, total dry weight, and relative growth rate of P. crassipes acclimated to eCO2 conditions and plants showed very little CO2 fertilization response in eutrophic water. Insect herbivory varied depending on feeding guilds at eCO2; however, P. crassipes growth responses increased when exposed to insect herbivory. Chewing herbivory by C. aquaticum was consistent across CO2 conditions, whereas the feeding by M. scutellaris increased substantially at eCO2. These results indicate that successful biological control of P. crassipes under conditions of elevated CO2 might rely on phloem‐feeding insects, with chewers playing a lesser role.

It's a numbers game: inundative biological control of water hyacinth (Pontederia crassipes), using Megamelus scutellaris (Hemiptera: Delphacidae) yields success at a high elevation, hypertrophic reservoir in South Africa.

ABSTRACT Classical biological control of water hyacinth in South Africa has been constrained by cool winter temperatures that limit population growth of the biological control agents, and highly eutrophic waters which enhance plant growth. However, inundative releases of the control agent, Megamelus scutellaris (Hemiptera: Delphacidae), at the Hartbeespoort Dam, South Africa, suggest that water hyacinth can be managed successfully using biological control as a standalone intervention for the first time in the absence of herbicide operations, despite eutrophication and a temperate climate. Sentinel-2 satellite images were used to measure the reduction in water hyacinth cover from over 37% to less than 6% over two consecutive years since M. scutellaris was first released on the dam in 2018, while site surveys confirmed a corresponding increase in M. scutellaris population density from fewer than 500 insects/m2 in October 2019, to more than 6000 insects/m2 by March 2020. Inundative release strategies are recommended for the control of water hyacinth in South Africa at key stages of its invasion, particularly after winter, and flooding events.

Integrating herbicide rates, coverage, and classical biological control insects (Megamelus scutellaris, Neochetina eichhorniae,and Neochetina bruchi) to manage Pontederia (Eichhornia) crassipes

The invasive floating aquatic plant Pontederia crassipes once monopolized large areas of open water in Florida, USA. Following the implementation of wide-scale herbicide applications in the mid-1950s, coverage declined but the plant was not eliminated. Herbicides are costly, requiring regular re-application and sustained funding to maintain suppression. Although classical biological control using host-specific insects can augment the impact of herbicides on weed populations, current herbicidal management of P. crassipes in Florida does not intentionally utilize the biological control insects already present (Megamelus scutellaris, Neochetina eichhorniae, and Neochetina bruchi) despite strong evidence they can improve outcomes. A completely randomized mesocosm experiment was conducted to examine the outcomes of an integrated plant management design using combinations of herbicide rates, spray coverage, and biological control agents to control P. crassipes. The analysis indicated that treatments with both moderate levels of herbicidal control (treating 40–80% of the mat) and biological control agents provided similar levels of control to those that treated 100% of the mat with herbicides. Additionally, leaving untreated refuges increased the local density of some species of biological control insects. This experiment shows that biological control insects can reduce the rate and frequency of herbicide applications, an outcome that would likely garner increased public support for continued operations.

A Review of the Biocontrol Programmes Against Aquatic Weeds in South Africa

Biological control (biocontrol) against invasive macrophytes is one of the longest standing programmes in South Africa, initiated in the 1970s against water hyacinth, Pontederia crassipes Mart. (Pontederiaceae). Since then, 15 agent species (13 insects, one mite and one pathogen) have been released against six weeds, most of which are floating macrophytes, with excellent levels of success. The release of the water hyacinth planthopper Megamelus scutellaris Berg (Hemiptera: Delphacidae) in particular, has improved biocontrol prospects for water hyacinth since 2018. In the last decade, however, a new suite of submerged and rooted emergent invasive macrophytes has been targeted. The first release against a submerged macrophyte in South Africa, and the first release against Brazilian waterweed, Egeria densa Planch. (Hydrocharitaceae), anywhere in the world, was achieved with the release of a leafmining fly, Hydrellia egeriae Rodrigues-Júnior, Mathis & Hauser (Diptera: Ephydridae). Yellow flag, Iris pseudacorus L. (Iridaceae) and Mexican waterlily, Nymphaea mexicana Zucc. (Nymphaeaceae), have also been targeted for biocontrol for the first time worldwide, and are in the early stages of agent development. Post-release evaluations, long term monitoring and controlled experiments have highlighted the need for a more holistic approach to managing aquatic invasive plants in South Africa, whose presence is largely driven by eutrophication, resulting in regime shifts between floating and submerged invaded states.

Megamelus scutellaris (Berg) (Hemiptera: Delphacidae) biology and population dynamics in the highly variable landscape of southern Florida

Changes to the historical flow and nutrient levels of freshwater bodies in Florida have made control of waterhyacinth difficult. Biological control agents were introduced to augment herbicidal control of this plant. The newest insect agent, Megamelus scutellaris, was released in 2010, has established, and has been documented dispersing more than 6 kms away from release locations, across aquatic and terrestrial habitats. These insects were able to successfully disperse away from herbicide treated areas and across terrestrial habitats to reach new mats of waterhyacinth. Macropterous individuals were the first M. scutellaris documented at three non-release sites, indicating that the established wild populations are producing flighted individuals and are dispersing actively. Established populations appear to have reached an equilibrium density of 10.74 ± 19.74 M. scutellaris/m2 and this does not fluctuate with additional releases.

Chevroned Water Hyacinth weevil Neochetina bruchi Hustache (Insecta: Coleoptera: Curculionidae)

Neochetina bruchi Hustache is commonly referred to as the chevroned water hyacinth weevil and is a weed biological control agent used to manage water hyacinth, Pontederia crassipes Mart. [formely Eichhornia crassipes (Mart.) Solms (Pellegrini et al. 2018)], in more than 30 countries (Winston et al. 2014). Imported from Argentina, the insect was first introduced into the United States in Florida in 1974 and released in Louisiana later in 1974 (Manning 1979), Texas 1980, and California 1982 to 1983 (Winston et al. 2014). Now Neochetina bruchi occurs throughout the Gulf Coast States (Winston et al. 2014). The target weed of Neochetina bruchi, water hyacinth, is an invasive aquatic plant in the United States and is included on Florida’s list of prohibited aquatic plants.
 Neochetina bruchi is among four species of insect biological control agents that have been introduced into the United States to manage water hyacinth. The other three species include a weevil, Neochetina eichhorniae; a moth, Niphograptaalbiguttalis; and a planthopper, Megamelus scutellaris, which were introduced into the United States in 1972, 1977, and 2010, respectively (Tipping et al. 2014).
 Although the larvae and pupae of Neochetina bruchi and Neochetina eichhorniae have similar appearance and behavior and can be difficult to differentiate by casual observation (Deloach and Cordo 1976), the adult stages of the two species of water hyacinth weevils can be distinguished relatively easily based on the color patterns on their elytra (hardened fore wings).

Proximity to host plant of a congener determines parasitism of a waterhyacinth biological control agent by a native parasitoid

The biological control agent Megamelus scutellaris Berg (Hemiptera: Delphacidae) was released in Florida in 2010 to improve control of the invasive aquatic plant, Pontederia (Eichhornia) crassipes (Mart.) Solms (Commelinales: Pontederiaceae). A native egg parasitoid, Kalopolynema ema (Schauff and Grissell) (Hymenoptera: Mymaridae), was documented utilizing M. scutellaris, potentially limiting its effectiveness as a biological control agent. An experiment to determine if the distance from K. ema’s native host impacted parasitism rate of M. scutellaris was conducted in artificial ponds with naturally occurring populations of the native insect species. Models indicated that parasitism decreased with distance from the native host and with increasing M. scutellaris density. Although parasitism increased with K. ema density, rainfall, and temperature, parasitism was low with over half of the test plants indicating no signs of parasitism on M. scutellaris. The lack of a density dependent response by K. ema suggests it will not interfere with biological control of P. crassipes in Florida.

Mind the gap: the delayed recovery of a population of the biological control agent Megamelus scutellaris Berg. (Hemiptera: Delphacidae) on water hyacinth after winter.

Cold winter temperatures significantly affect the biological control effort against water hyacinth, Pontederia ( = Eichhornia) crassipes Mart. (Pontederiaceae), in more temperate regions around the world. The population dynamics of the planthopper Megamelus scutellaris Berg. (Hemiptera: Delphacidae), a newly released biological control agent of water hyacinth, were recorded on the Kubusi River in the Eastern Cape Province (South Africa) over 15 months to determine the population recovery post-winter. Megamelus scutellaris incurred a severe population decline at the onset of winter when the water hyacinth plants became frost damaged. The combined effect of a population bottleneck and low minimum winter temperatures (6.12°C) below the agent's lower developmental threshold (11.46°C) caused a post-winter lag in agent density increase. Subsequently, the maximum agent population density was only reached at the end of the following summer growing season which allowed the water hyacinth population to recover in the absence of any significant biological control immediately post-winter. Supplementary releases of agents from mass-reared cultures at the beginning of the growing season (spring) is suggested as a potential method of reducing the lag-period in field populations in colder areas where natural population recovery of agents is slower.

Interactions among biological control agents on waterhyacinth: impacts of herbivory on the oviposition and development of Megamelus scutellaris

ABSTRACT Multiple biological control agents have been developed and released to increase control and reduce treatment costs of Pontederia (Eichhornia) crassipes in Florida. Although two species of weevil (Neochetina eichhorniae and Neochetina bruchi) and the planthopper (Megamelus scutellaris) utilise P. crassipes for feeding and reproduction, little is known about their interactions including any negative interactions that might reduce their individual or overall effectiveness in suppressing the plant. Megamelus scutellaris was provided with the choice of weevil-damaged or undamaged plants and allowed to oviposit. Plants were then monitored individually for nymph emergence to determine the plant preference for adults. There were no differences in F1 emergence numbers among the treatments indicating that in this specific situation the potential for negative interactions between Neochetina spp. and M. scutellaris is minimal. Additional studies are required to quantify the potential for other types of interactions among these biological control agents.

Classical biological control and apparent competition: Evaluating a waterhyacinth invaded community module

Abstract The scope and complexity of interactions within community food webs necessitates their simplification to a community module scale for conducting empirical studies. An outdoor mesocosm study in the USA quantified the strengths of direct and indirect interactions between two herbivore congeners that fed on two aquatic plant species while sharing a parasitoid. Kalopolynema ema (Hymenoptera: Mymaridae) is a stenophagous native egg parasitoid that attacks the hemipteran species in this study, Megamelus davisi (Hemiptera: Delphacidae), a native herbivore that feeds on the native aquatic species Nuphar advena, and the introduced biological control agent Megamelus scutellaris (Hemiptera: Delphacidae) that feeds on the non‐native aquatic species waterhyacinth, Pontederia crassipes. The presence of M. scutellaris did not significantly increase parasitism of M. davisi indicating that apparent competition was not a factor in this community module. There was no evidence of any trophic cascades caused by these interactions based on the relative growth rates of biomass and leaf area for both plant species. Synthesis and applications. The relative strengths of community interactions varied with herbivore densities suggesting that should negative indirect effects with biological control agents occur they would likely be transient and closely linked to population dynamics as influenced by abiotic factors like temperature. Despite identifying potential negative interactions using path analysis, we saw no apparent competition following the insertion of a weed biological control agent into a community that contains an ecological analog. In this example, the primary negative interaction was biotic resistance to Megamelus scutellaris from attack by Kalopolynema ema. Adopting weed management tactics that emphasize efficacy, sustainability and environmental benevolence argue for the use of classical biological control when appropriate.

First record of an indigenous South African parasitoid wasp on an imported biological control agent, the water hyacinth hopper

ABSTRACTWater hyacinth, Pontederia crassipes (Martius) [≡Eichhornia crassipes (Martius) Solms-Laubach] (Pontederiaceae), is native to South America, but has expanded its range to many other regions of the world including South Africa. Megamelus scutellaris Berg (Hemiptera: Delphacidae) was released as a biological control agent and has established in several regions. Recently, the indigenous species Echthrodelphax migratorius Benoit, (Hymenoptera: Dryinidae) was discovered in South Africa parasitising M. scutellaris. This newly discovered relationship might have repercussions for the efficacy of biological control of water hyacinth by the delphacid. The wasp may negatively impact M. scutellaris populations making it difficult for the agent to successfully manage the invasive weed. Contrarily, the parasitoid may be beneficial by keeping the M. scutellaris populations stable, serving as a natural enemy.

Chlorophyll fluorometry as a method of determining the effectiveness of a biological control agent in post-release evaluations

ABSTRACTThe impact of the planthopper Megamelus scutellaris, a biocontrol agent of water hyacinth in South Africa, was assessed using chlorophyll fluorometry in a greenhouse study under two different eutrophic nutrient treatments and agent densities (high and low). The results indicated that plants grown in low nutrients with high densities of M. scutellaris showed the greatest reduction in the fluorescence parameters Fv/Fm and PIabs. The successful use of chlorophyll fluorometry for the detection of subtle insect damage to water hyacinth leaves could have future application in post-release studies to measure the impact of M. scutellaris in the field.

Small-scale dispersal of a biological control agent – Implications for more effective releases

Eichhornia crassipes (Martius) Solms Laubach (Liliales: Pontederiaceae) was introduced to Florida in the 1880s as an ornamental and it once infested thousands of square kilometers across the state. Megamelus scutellaris Berg (Hemiptera: Delphacidae) was developed as a classical biological control agent for this plant primarily because its free-living life stages allow it to better integrate with herbicides, which are currently used as the main control method for E. crassipes in Florida. Mass rearing and distribution programs can accelerate the benefits of biological control by augmenting natural dispersal, but an optimal release strategy must consider the entire system including the agent, the target weed, and the habitat. The effectiveness of various release strategies was evaluated using a tank experiment where single and multiple releases of either adult M. scutellaris only or E. crassipes infested with M. scutellaris eggs were compared to control treatments. The post-release dispersal capability of brachypterous M. scutellaris was evaluated using a linear transect of E. crassipes. Two density release treatments were tested and emerging nymphs were used as a proxy for female dispersal distances. All release treatments resulted in successful M. scutellaris population establishment and levels of M. scutellaris were not significantly different among them. The dispersal experiment indicated that adult females oviposit near the release point before dispersing. While the release experiment indicated that all treatments were similar, the continually fluctuating populations of E. crassipes makes establishment of populations difficult in the field. By releasing both adults and infested plants, additional propagule pressure can be attained from a single release event which can counter the tendency of adult M. scutellaris to disperse rapidly following release.

Spatial and temporal variation of biological control agents associated with Eichhornia crassipes in the Sacramento-San Joaquin River Delta, California

The invasive water hyacinth (Eichhornia crassipes) severely limits the ecosystem services provided by the Sacramento-San Joaquin River Delta in California, USA. As part of the biological control program in the Delta, two weevils, Neochetina bruchi and N. eichhorniae (Coleoptera: Curculionidae) and a moth, Niphograpta albiguttalis (Lepidoptera: Pyralidae), were released in the 1980s. Additionally, a planthopper, Megamelus scutellaris (Hemiptera: Delphacidae) was released in 2011. We conducted monthly surveys for one year at 16 sites throughout 1667km2 of the Delta to determine the resulting establishment, abundance and distribution of these introduced herbivores. Morphological identifications, and partial sequencing of the mitochondrial cytochrome oxidase subunit 1 gene determined that 96.6% of the examined weevils were N. bruchi. N. eichhorniae was only recovered from two sites in the southern Delta tributaries. Densities (larvae and adult weevils per destructively sampled plant) varied spatially and temporally. Peak mean densities (averaged across August–November) decreased with increasing distance from the original release sites. Peak mean densities ranged from 0.31 to 6.31 weevils per plant. Densities averaged across sites were the lowest in June 2015 (0.54 weevils), increasing in August to 5.35 weevils, and peaking in November at 6.22 weevils. The proportion of damaged leaf area from weevil feeding increased concomitantly with weevil densities. Although N. albiguttalis was not recovered, M. scutellaris remained established at its original release site but has not dispersed into the other surveyed regions. We propose hypotheses to explain patterns in species establishment and distribution, with potential mechanisms for improved future biological control.

Naturally occurring phytopathogens enhance biological control of water hyacinth (Eichhornia crassipes) by Megamelus scutellaris (Hemiptera: Delphacidae), even in eutrophic water

Insect biological control agents directly damage target weeds by removal of plant biomass, but herbivorous insects have both direct and indirect impacts on their host plants and can also facilitate pathogen infection. Megamelus scutellaris Berg (Hemiptera: Delphacidae) was recently released into South Africa to help control invasive water hyacinth (Eichhornia crassipes, Pontederiaceae). We compared the impact of fungicide surface-sterilised and unsterilised M. scutellaris individuals and water hyacinth leaves on growth of the weed at two nutrient levels. The survival and reproduction of adult M. scutellaris was not reduced by sterilisation. Under high nutrient conditions, unsterilised M. scutellaris with unsterilised leaves reduced water hyacinth daughter plant production by 32%, length of the second petiole by 15%, chlorophyll content by 27% and wet weight biomass by 48%, while also increasing leaf chlorosis 17-fold, in relation to control plants under the same nutrient regime. Surface sterilisation of the insect and/or plant surfaces led to a general reduction in these impacts on water hyacinth growth and health. This contrast was less evident under low nutrient conditions. Megamelus scutellaris facilitated infection by fungal and other pathogens, thus its biology is compatible with pathogens that could be developed into mycoherbicides. This integrated approach may be ideal for management of infestations of water hyacinth in eutrophic water systems where control has been problematic, both in South Africa and elsewhere.