Investigating biological control agents for the management of Ceratitis capitata (Wiedemann)

Persistent crop damage and loss caused by fruit flies (Diptera: Tephritidae) forces the reliance on chemical control methods by the fruit industry. However, social, environmental and economic consequences associated with such controls are forcing the exploration of alternative, more sustainable options. This study investigates the use of entomopathogenic nematodes (EPNs), entomopathogenic fungi (EPF) and parasitoid wasps, as biological control agents against one of the most widespread and dominant fruit flies in South Africa, the Mediterranean fruit fly or Medfly (Ceratitis capitata (Wiedemann)).

Different methods were used in order to (i) isolate and identify local EPNs and EPF from deciduous fruit orchard soils; (ii) evaluate the pathogenicity of local EPN and EPF isolates against the third larval stage of Medfly under controlled laboratory conditions, and the most virulent of each in a more natural (sand) environment; (iii) estimate the lethal concentration/dose needed to cause 50% C. capitata mortality (LD50) using selected EPN isolates; and (iv) survey for and identify fruit fly parasitoid species occurring in fruit-producing areas of the Western Cape, Mpumalanga and Limpopo Provinces.

Soil sampling yielded many local entomopathogenic isolates, including EPNs with new bacterial associations and an EPF, Metarhizium robertsii (MJ06), which was trapped using Medfly larvae as bait. Initial EPN screenings (100 IJs / 50 μl) showed all tested EPNs to be highly pathogenic against the third instar Medfly larvae, while the lower concentration (50 IJs/ 50 μl) highlighted H. noenieputensis as the most virulent EPN species. This species, as well as S. yirgalemense, which is currently in the process of being mass cultured and formulated for commercial use, was further tested in sand bioassays. Heterorhabditis noenieputensis caused significantly greater mortality (94-100%) and most Medfly infected as larvae pupated and then died within the puparium. Steinernema yirgalemense also offered good control, with 58-79% of exposed larvae dying as adults. The LD50 of H. noenieputensis was 37 IJs / insect, which was 14 times more effective than those of S. yirgalemense.

Local EPF isolates and commercial products tested against third instar larvae, using the dipping method, were pathogenic and caused visible fungal infection (mycosis) of 57-74%. A replicate experiment with reduced humidity, also reduced overall mycosis, with the highest mycosis of 55% caused by the local isolate, MJ06. Third instar Medfly larvae added to sand, sprayed with the non-commercial EPF, caused most exposed larvae to die and mycose as adults (62-86%).

Parasitoid wasps were obtained during fruit sampling, but difficulties with low DNA extraction, amplification and limited available barcodes of local fruit fly parasitoids, restricted their species identification. The use of sentinel traps – setting out apples infested with Medfly eggs and larvae as well as exposed Medfly pupae – did not trap any wasps during this study, but provides a simple and inexpensive method to be used in future studies.

This study highlights an EPN (H. noenieputensis SF669) and EPF (M. robertsii MJ06), virulent against the soil-life stages of Medfly, which should be the focus of future studies as potential biocontrol agents. Moreover, this study provides valuable groundwork for the future incorporation of local biological control agents, into an IPM system, to sustainably and effectively manage the Mediterranean fruit fly.