Comparison of the predicted impact of a screwworm fly outbreak in Australia using a growth index model and a life-cycle model

Toward Implementation of Mosquito Sterile Insect Technique: The Effect of Storage Conditions on Survival of Male Aedes aegypti Mosquitoes (Diptera: Culicidae) During Transport

Sterile insect technique (SIT) is a promising, environmentally friendly alternative to the use of pesticides for insect pest control. However, implementing SIT with Aedes aegypti (Linnaeus) mosquitoes presents unique challenges. For example, during transport from the rearing facility to the release site and during the actual release in the field, damage to male mosquitoes should be minimized to preserve their reproductive competitiveness. The short flight range of male Ae. aegypti requires elaborate release strategies such as release via Unmanned Aircraft Systems, more commonly referred to as drones. Two key parameters during transport and release are storage temperature and compaction rate. We performed a set of laboratory experiments to identify the optimal temperatures and compaction rates for storage and transport of male Ae. aegypti. We then conducted shipping experiments to test our laboratory-derived results in a 'real-life' setting. The laboratory results indicate that male Ae. aegypti can survive at a broad range of storage temperatures ranging from 7 to 28°C, but storage time should not exceed 24 h. Male survival was high at all compaction rates we tested with a low at 40 males/cm3. Interestingly, results from our 'real-life' shipping experiment showed that high compaction rates were beneficial to survival. This study advances key understudied aspects of the practicalities of moving lab-reared insects into the field and lies the foundation for further studies on the effect of transport conditions on male reproductive fitness.

Spatial analysis of targeted surveillance for screw-worm fly (Chrysomya bezziana or Cochliomyia hominivorax) in Australia

OBJECTIVE

To assess Australia's targeted surveillance to detect an incursion of screw-worm fly (Chrysomya bezziana).

METHODS

A multi-criteria analysis shell was used to combine data on potential pathways of entry, availability of host species and environmental factors affecting survival of screw-worm fly in order to map spatial variation in the relative likelihood of a screw-worm fly incursion into Australia. Australia's current screw-worm fly surveillance activities were reviewed to determine whether they are located in the areas of highest likelihood of an incursion.

RESULTS

Under average environmental conditions, an incursion of screw-worm fly in Australia is relatively more likely to occur along the north coast, down the eastern seaboard or in the south-east. Cold winter temperatures would limit the environmental suitability for screw-worm fly survival to the north and north-east coast and adjacent inland areas. Australia's current targeted surveillance conducted by the Northern Australia Quarantine Strategy program of the Australian Department of Agriculture (adult screw-worm fly trapping and myiasis sampling) correlated well with areas considered to have a high relative likelihood of an incursion of screw-worm fly. Adult fly trapping conducted at sea ports was less well correlated.

DISCUSSION

Changes to surveillance at sea ports are proposed to better target areas considered to have a higher relative likelihood of screw-worm fly incursion. These include increasing the trapping intensity along the north and north-east coasts and shifting surveillance activity from the west coast to the south-east.

Applying GIS and population genetics for managing livestock insect pests: case studies of tsetse and screwworm flies

The Food and Agriculture Organization of the United Nations (FAO) and the International Atomic Energy Agency (IAEA) have supported a Co-ordinated Research Project (CRP) on 'Applying GIS and population genetics for managing livestock insect pests'. This six-year CRP (2008-2013) focused on research aimed at under-pinning the Area-Wide Integrated Pest Management (AW-IPM) of populations of tsetse and screwworm flies, and this introductory paper to the Special Issue integrates the findings of the CRP participants and discusses them in a broader context. The tools and techniques for mapping and modelling the distributions of genetically-characterised populations of tsetse and screwworm flies are increasingly used by researchers and managers for more effective decision-making in AW-IPM programmes, as illustrated by the reports in this Special Issue. Currently, the insect pests are often characterized only by neutral genetic markers suitable for recognizing spatially isolated populations that are sometimes associated with specific environments. Two challenges for those involved in AW-IPM are the standardization of best practice to permit the efficient application of GIS and genetic tools by regional teams, and the need to develop further the mapping and modelling of parasite and pest phenotypes that are epidemiologically important.

Applying GIS and high performance agent-based simulation for managing an Old World Screwworm fly invasion of Australia

Agent-based modelling has proven to be a promising approach for developing rich simulations for complex phenomena that provide decision support functions across a broad range of areas including biological, social and agricultural sciences. This paper demonstrates how high performance computing technologies, namely General-Purpose Computing on Graphics Processing Units (GPGPU), and commercial Geographic Information Systems (GIS) can be applied to develop a national scale, agent-based simulation of an incursion of Old World Screwworm fly (OWS fly) into the Australian mainland. The development of this simulation model leverages the combination of massively data-parallel processing capabilities supported by NVidia's Compute Unified Device Architecture (CUDA) and the advanced spatial visualisation capabilities of GIS. These technologies have enabled the implementation of an individual-based, stochastic lifecycle and dispersal algorithm for the OWS fly invasion. The simulation model draws upon a wide range of biological data as input to stochastically determine the reproduction and survival of the OWS fly through the different stages of its lifecycle and dispersal of gravid females. Through this model, a highly efficient computational platform has been developed for studying the effectiveness of control and mitigation strategies and their associated economic impact on livestock industries can be materialised.

Advances in biosecurity to 2010 and beyond: towards integrated detection, analysis and response to exotic pest invasions

In order to limit the number and impact of exotic pest invasions, leading-edge technologies must be embraced and embedded within integrated national and international biosecurity systems. Outlined here are recent advances in the detection of exotic pests, and prospects for the early recognition of disease. Applications of new tools are described, using our understanding of the genomes of pathogens and vectors. In addition, the role of mathematical and simulation models to aid both biosecurity planning, and decision making in the face of an epidemic, are discussed, and recent attempts to unify epidemiology and evolutionary dynamics are outlined. Given the importance of emerging diseases and zoonoses, the need to align human and veterinary surveillance within fully integrated systems is underlined.

Seasonality of Old World screwworm myiasis in the Mesopotamia valley in Iraq

Following the first recorded introduction of the Old World screwworm fly (OWS), Chrysomya bezziana Villeneuve (Diptera: Calliphoridae), into the Mesopotamia valley in Iraq in September 1996, cases of livestock myiasis caused by OWS developed a distinctly seasonal pattern. The annual cycle of clinical OWS cases is explained here on the basis of environmental variables that affect the different life-cycle stages of C. bezziana. This analysis suggests that low temperatures restricted pupal development during the winter, whereas the dispersal of adult flies was constrained by hot/dry summer conditions. A restricted number of OWS foci persisted throughout the year. In these foci, pupal development was fastest during the autumn months. In autumn, rapid multiplication, lasting several OWS generations, allowed subsequent adult fly dispersal across the valley floor during the winter. Hence, the monthly incidence of clinical OWS cases in livestock peaked during December-January and was lowest during July-August. In addition to temperature and humidity, vegetation cover played a role in OWS distribution. Hence the majority of OWS cases were clustered in the medium density type of vegetation [normalized difference vegetation index (NDVI) values of 0.2-0.4] along the main watercourses in the marshy Mesopotamia valley. Although sheep were the host most commonly infested by C. bezziana, local sheep density was not found to be a major factor in disease spread. Satellite imagery and the application of Geographical Information System (GIS) tools were found to be valuable in understanding the distribution of OWS in relation to vegetation and watercourses. The presence of screwworm in Iraq, at the perimeter of the intercontinental OWS distribution, may give rise to major seasonal flare-ups.

Efficacy of macrocyclic lactones for the control of larvae of the Old World screw-worm fly (Chrysomya bezziana)

OBJECTIVE

To assess the efficacy of four macrocyclic lactones for the control of larvae of the Old World Screw-worm Fly (OWS), Chrysomya bezziana, and to examine the effects of excreted residues on the dung fauna.

ANIMALS

100 heifers were divided into five groups of 20 animals. One group remained untreated, whereas the other groups were treated respectively with pour-on formulations of moxidectin, eprinomectin or doramectin, or a sustained-release bolus of ivermectin.

PROCEDURES

At intervals of 1 to 15 weeks after treatment, five cattle from each group were challenged with newly-laid eggs of OWS. The efficacy of each treatment was determined 48 h later by comparing the number of myiases in the treated and untreated groups. Abundance of fly larvae in naturally-voided dung pads and the survival of a species of dung beetle, Onthophagus sagittarius, were used to assess the effects of drug residues on the dung fauna.

RESULTS AND CONCLUSIONS

Moxidectin showed no activity against larvae of OWS during the first 14 days after treatment. Eprinomectin provided protection for 3 days after dosing, but failed at days 7 and 14, whereas doramectin was effective at day 7, but not at days 14 or 21. In contrast, no myiases were established on bolus-treated cattle from 14 to 102 days after treatment. Faecal residues of moxidectin had no effect on the survival of larvae of dung-feeding flies, whereas those of eprinomectin and doramectin reduced survival for 1 to 2 weeks. Dung voided by bolus-treated cattle inhibited fly breeding and had adverse effects on the development and survival of O sagittarius for up to 15 weeks after treatment.

Vaccination against the Old World screwworm fly (Chrysomya bezziana)

Chrysomya bezziana is an endemic pest of livestock or a threat to livestock production in large areas of Africa, the Middle East, southern and south-east Asia and Australia. Its control is difficult. The feasibility of vaccinating against this pest has now been explored. In-vitro and in-vivo assays have been established. Using these assays, it has been shown that first instar larvae, third instar peritrophic membrane and cardia are all sources of material able to induce immunological reactions in sheep which lead to significant reductions in larval growth. In-vitro assays following vaccination with peritrophic membrane also show larval mortality. Taken together, these effects lead to an 82% reduction in the weight of recovered larvae in vitro and 45% reduction in vivo. Preliminary evidence suggests that the mechanism of protection may be complex.

Mating competitiveness of irradiated flies for screwworm fly eradication campaigns

Should the screwworm fly invade Australia, the sterile insect technique (as used successfully overseas) is currently the only feasible method of eradication. Used in conjunction with chemical control methods, it relies on large numbers of factory-reared, sterilized males competing successfully with wild males for the wild females. However, laboratory and field studies have shown that the processes of mass rearing, irradiation and distribution seriously impair the competitiveness of the sterilized flies. This study collates and analyses the relatively sparse information on the relative mating competitiveness of sterilized screwworm flies, from both controlled experiments and large-scale field studies. A population dynamics example then demonstrates that competitiveness will be a key parameter in the effectiveness and economic feasibility of any future eradication campaign.

Evaluating the risk of the establishment of screwworm fly in Australia

OBJECTIVE

To investigate probabilities of establishment of screwworm fly throughout the year, for several locations around Australia's coastline.

METHODS

A simulation model that predicts the spread and economic impact of an established screwworm fly population was modified to include stochastic survival functions, to investigate the risks of the pest actually establishing in this country. The effects of time of year, climate, vegetation and the number of incoming flies or larvae were investigated for four locations around Australia.

RESULTS

Analysis of variance identified a dominant three-way interaction between site, time, and the number of introduced flies. These probabilities are graphed.

DISCUSSION

In southern areas, as exemplified by Fremantle, the cold winters limit survival. A high probability of establishment exists year round in tropical regions, except in areas around the Gulf of Carpentaria and in the Northern Territory where dry weather mid-year would limit survival. Despite these comparatively lower risks, there were no areas or times where reductions in quarantine efforts could be justified.

Trapping the flies that cause myiasis: their responses to host-stimuli

A critical phase in the life cycle of the fly species that cause myiasis is host location by the adult females, whether by direct location of and contact with the host, or by indirect location through the selection of sites frequented by hosts. The stimuli to which these flies respond in host location, principally visual and olfactory, are discussed here in the context of their use to enhance the performance of trapping systems used in research, monitoring and control. The three major families of flies encountered in myiasis can be divided into two groups for comparison of host-location strategies: (1) Oestridae; and (2) Calliphoridae and Sacrophagidae. The former are all obligate parasites, the latter include obligate and primary facultative parasites. In the Oestridae, the majority of Cuterebrinae deposit their eggs at sites likely to be visited by the host, whereas the majority of Oestrinae, Hypodermatinae and Gasterophilinae deposit their eggs or larvae directly onto the host. It is presumed that most host location is, like that of haematophagous insects, by response to olfactory and visual cues released by healthy hosts, but the subject is very under-researched. In the Calliphoridae and Sacrophagidae, most species that cause myiasis deposit their eggs or larvae directly onto the host at some predisposing site, such as those caused by wounding, necrosis and/or bacterial contamination. Healthy hosts are much less attractive, even for obligate species such as Wohlfahrtia magnifica, which can invade undamaged tissues. The last decade has seen a renewed interest in the potential for using traps in control of myiasis, leading to significant advances in trap and bait efficiency. However, there is great potential for further research and development, particularly on the general responses of Oestridae to hosts and in the precise identification of the olfactory cues for Calliphoridae and Sarcophagidae.