A recent study conducted by Stellenbosch University, aimed to contribute to the knowledge on biological control agents, such as entomopathogenic nematodes (EPNs) and entomopathogenic fungi (EPF), present in South Africa, which could be used against the Mediterranean fruit fly and be incorporated into current management practices.
The Mediterranean fruit fly, or Medfly, Ceratitis capitata (Wiedemann), is a notorious pest which originates from sub-Saharan Africa, but has also invaded and established throughout many tropical and warm temperate regions of the world. It is a common pest in many parts of South Africa and is the dominant fruit fly species in the Western Cape Province. Medflies utilise fruits as host for their offspring, which feed and develop in the fruit eventually exiting as third instar larvae to pupate in the soil. This life cycle results in the Medfly causing extensive crop damage and loss, requiring expensive controls and post-harvest treatments and has also resulted in restrictions to the export market. Current control methods mainly aim at controlling the adult life stage, but generally involve harmful chemicals which are hazardous to the environment and peoples' health. More environmentally-friendly, sustainable and efficient alternatives are thus required for use against this pest in an Integrated Pest Management (IPM) strategy.
Entomopathogens are organisms which are pathogenic to insects and EPNs and EPF both make use of suitable insect hosts to feed and breed. The free-living life stages of EPNs are known as infective juveniles, or IJs, and these organisms carry symbiotic bacteria in their gut, which is crucial for killing their host. EPF spores are abundant in the soil, and although non-mobile, are able to penetrate the skin of a suitable host when they come into contact, eventually killing the host. The soil life stages of the Medfly (third instar larvae, pupae and emerging adults) could thus be targeted by EPNs and EPF and utilization of these natural enemies could offer a biological Medfly control method.
In order to find local EPN and EPF species, soil was sampled from different fruit orchards in known fruit fly problem areas. Entomopathogens were isolated from the soil by adding highly susceptible wax moth larvae and meal worms, as well as Medfly larvae, to each soil container as bait. Baiting with the target host was done to try and directly isolate Medfly-specific EPNs or EPF that may have been present in the soil. Species were later identified using morphological and molecular tools.
In order to identify possible control agents, the ability of different entomopathogenic isolates to infect and kill Medfly was tested. Five different local EPNs (Heterorhabditis zealandica, H. noenieputensis, H. bacteriophora, H. indica, Steinernema yirgalemense) were tested in the laboratory by exposing individual third instar larvae to 50 and 100 IJs in a confined well and assessing infectivity and death after two weeks. Soil collected EPF (Metarhizium robertsii and Beauveria bassiana) and commercial EPF products (EcoBb®, Broadband® and Meta69) were similarly tested by dipping larvae into the fungal suspension (concentration = 1 x 107 conidia/ml) and then confining each larva to a well. After two weeks, any dead insects were placed on nutrient agar plates to facilitate fungal growth in order to confirm if the death was caused by a fungal infection (mycosis). Pathogenicity of selected entomopathogens were further tested in a more natural environment by adding third instar larvae to containers containing sterile soil inoculated with the EPNs or EPF.
All EPN species tested were able to infect and kill the exposed Medfly larvae. In the soil bioassays, the native EPN, H. noenieputensis, caused >92% mortality. Most exposed larvae died as pupae, showing that H. noenieputensis was able to find and infect the final instar larvae soon after they entered the soil, thus preventing them from emerging as adults. Another species, S. yirgalemense, which is currently in the process of being formulated into a commercial product, caused mortality of 34-53%. Its ability to offer some control against Medfly as well as other important pests such as false codling moth and codling moth, is likely to enhance the acceptance and uptake of the commercialised product by growers.
Both the commercial EPF products and soil isolates were able to infect and kill Medfly. The pathogenicity of the commercially available EPF products against Medfly observed should provide encouragement for the increased use of such alternatives for the control of agricultural pests, including the Medfly. This study found that the local soil-sampled Metarhizium robertsii strain MJ06, which was isolated using a Medfly larva, was able to provide a high level of control of both pupae and adults. When larvae were exposed to sand inoculated with the two non-commercial isolates, high adult mycosis was caused. Mycosis of adults showed that soil treatments can assist in fruit fly control, but also highlights the potential of using these two non-commercial isolates for control of the adults. They could be tested for use in bait sprays, or autoinoculation traps where adults are attracted, infected and killed by the EPF.
In conclusion, the ability of entomopathogenic nematodes and fungi to naturally infect and kill soil-dwelling stages of Medfly makes them useful natural enemies. The integration of such EPNs and EPF individually, and possibly together, into an IPM system could ultimately provide sustainable and effective control of this target pest and future research efforts should focus on the candidates highlighted in this study. This research has thus provided necessary groundwork and information on potential biological control agents for the control of Mediterranean fruit fly in South Africa.
The study was presented as an MSc thesis of the student, Monique James, carried out at the Department of Conservation Ecology and Entomology of the Stellenbosch University. The study and was supervised by Prof. Antoinette P. Malan, Dr Pia Addison and Dr Minette Karsten. HortGro and the National Research Foundation provided funding (NRF reference: SFH150706123538).