There is a proverb – “You can lead a horse to the water, but you can’t make it drink”.  In the context of the narrative below, it roughly equates to “you can present people with indisputable evidence, but you can’t make them believe it”.  The events I describe here relate to the early days of SIT for Medfly in the Western Cape, South Africa, in the period 1997 to 2001.  Most of it is drawn from publications referenced at the end.  First, some background.

Fruit production in South Africa – ‘different strokes for different folks’

The Western Cape of South Africa is blessed with thousands of hectares of deciduous fruit and table and wine grapes, mostly grown in a number of valley systems with different soil types and climates.  Different areas are thus suited to different fruit types and cultivars, some of which are more or less susceptible to fruit fly infestation.  South Africa is also blessed with a great many excellent fruit growers who are dedicated to producing top quality fruit for the export and local markets.  They are generally up to date with modern trends, are open to new and research developments, and, mostly, readily apply them where appropriate.

The Western Cape fruit industry is further characterised by differing agricultural ‘cultures’ of the grower fraternities in the different fruit production areas.  Over the decades growers in different areas have in certain respects developed different perceptions and approaches to e.g. orchard and vineyard sanitation, host plant management, pest management practices, and fruit packing practices.  Growers in some areas are staunchly independent when it comes to their fruit growing operations, and, it could be said, are sometimes somewhat reluctant to take the advice from outsiders.  However, they all produce fruit of exceptional quality.

The birth of SIT in South Africa – a difficult childhood

SIT in South Africa was born in 1997 when a project largely funded by the International Atomic Energy Agency (IAEA) was initiated to investigate the feasibility of using SIT to eradicate or suppress Mediterranean fruit fly in Hex River Valley in the Western Cape Province.  As I at that time was Manager of the Pest Management Division at ARC Infruitec-Nietvoorbij in Stellenbosch, the administration and oversight of the pilot project fell on my shoulders, assisted by David Eyles, a researcher in the Pest Management Division, rearing facility staff, and an area coordinator in the Hex Valley.  During the events described here, project funding was through the IAEA, and an SIT Partnership between ARC Infruitec-Nietvoorbij and the deciduous fruit industry.  No direct government funding was made available at the time.

The Hex River Valley is surrounded by mountains with some 4500 ha under export table grape cultivation, and was selected for the pilot project because of its geographical isolation, largely single-crop cultivation, and good buy-in into the proposed SIT project by all of the growers in the valley.  

The Hex River Valley and a map of Western Cape showing the Hex River Valley (yellow ellipse)

The project was also supported by the Hex River Valley Table Grape Association, which oversaw the interests of all the growers in the valley, and they contributed financially to the project.  The fruit fly population comprised almost exclusively of Medfly, with very few numbers Cape fruit fly, Ceratitis quilici (then Natal fruit fly, C. rosa) occurring in isolated areas. 

In preparation for the release of sterile Medfly males, old insect breeding rooms at ARC Infruitec-Nietvoorbij in Stellenbosch were converted into a small pilot facility for the purpose of rearing a genetic sexing strain (GSS) of Medfly, which was obtained from the IAEA Laboratories at Seibersdorf, Austria.  Maximum production in the facility was estimated at 5 to 8 million sterile males per week, which was considered sufficient for the Hex Valley.  Male pupae were irradiated at 90 Gy with a Co⁶⁰ source on site. 

In those early days of SIT a Filter Rearing System to control the accumulation of genetic recombinants in the GSS was not set up owing to lack of space and funding.  As a result, quality control tests at the rearing facility showed that from time to time up to 30% of the sterile Medflies sent to the Hex River Valley were female recombinants.  However, as female Medfly pupae are rendered 100% sterile after irradiation at 80 Gy, any females sent to the Hex Valley would have been 100% sterile.

A 3-year fruit fly population reduction programme was initiated with the intention of reducing the Medfly population level to an acceptably low level prior to initiating SIT with release of sterile Medfly males.  The strategy involved coordinated ground applications of fruit fly bait, vineyard sanitation, and management of alternative host plants, in addition to two or three organophosphate cover sprays normally used at that time against fruit flies.

During that period David Eyles and I made many visits to the Hex Valley, often in the company of overseas SIT experts sponsored by the IAEA, and we attended a number of meetings of the Hex River Valley Table Grape Association.  During these visits we explained to growers the concept of SIT and the workings of such an area-wide programme, the benefits of SIT, and their role in helping to suppress wild Medfly populations.

The population reduction efforts from 1997 to 1999 did not yield the desired results, and wild Medfly populations remained higher than was desired.  Nevertheless, as the production of sterile male Medflies in the rearing facility was on target, a decision was made to begin aerial releases of sterile males in the spring (October) of 1999, a stage in the season when wild Medfly populations were very low.  

Aerial releases of 500 sterile males/ha/week were made from a Cessna 207 aircraft fitted with a chilled-fly release machine, from October to May (the growing season).  Between winter and early spring (June to September inclusive), when vineyards were totally inhospitable to fruit flies and weather conditions were often unsuitable for aerial releases, all releases were made on the ground and concentrated in farm and town gardens and other known hotspots.  The Cessna was on permanent hire for the duration of the aerial releases (winter period included), which was paid for by the Hex River Valley growers; as a result they had a significant say in aerial release operations.

Infestation outbreak – “they’re coming from the sky”

In December 2001, a critical time of the season in table grape production as grapes were being packed for export, an outbreak of Medfly larvae in table grapes in the SIT area was reported by the Hex Valley SIT coordinator.  Both table grape growers and the coordinator ascribed the infestation to the presence of unsterilised Medfly females in deliveries of sterile males from the rearing facility; the rearing facility was thus held accountable for the resulting crop losses.  

David Eryles and I personally inspected some of the infestation sites together with the Hex Valley coordinator and the growers.  As we were inspecting one infested vineyard, the disgruntled grower turned to us and said “I’m telling you, the infestation is coming from the sky”, alluding to there being fertile females amongst the released sterile flies.  Despite our best efforts he refused to believe any other possibility that we mentioned.

While the ARC Infruitec-Nietvoorbij team knew this to be highly unlikely, it became critical to determine the real origin of the fertile female flies causing the infestation.  All GSS flies used in the rearing facility contained two types of genetic ‘fingerprint’ (and only these fingerprints), collectively termed Type A, which had not by then ever been found in wild Medfly from sub-Saharan Africa.  It would thus be possible to identify flies originating in the facility from in any collection of medfly specimens collected in the release area.

Searching for the source of the culprit

A chain of events was set in motion to establish the real source of the infestation. Consignments of sterile males to the Hex Valley were stopped.  Fruit fly-infested grapes were collected from seven different sites in the Hex Valley release area; 137 pupae were recovered and couriered live to the Entomology Unit, IAEA Laboratories, Seibersdorf, Austria, together with a sample of GSS pupae from the facility. There, 88 wild fly adults emerged; these and 58 flies from the GSS from the rearing facility were subjected to mitochondrial DNA analysis (mtDNA) to determine the presence or absence of the GSS genetic fingerprints.

While awaiting the results from Seibersdorf, a series of laboratory and field-cage tests at ARC Infruitec-Nietvoorbij found no evidence of fertility amongst irradiated females that resulted from genetic recombination in the rearing facility. To rule out under-irradiation of Medfly males, the dosimetry of the irradiator was checked by an independent expert, and found to be correct. Further, an audit was carried out of the irradiation procedures of all suspect batches of Medfly irradiated at ARC Infruitec-Nietvoorbij, with no irregularities identified.

The proof of the mtDNA ‘pudding’

When the results of the mitochondrial DNA analyses became known at Seibersdorf, and considering the high stakes involved, one of the IAEA’s SIT experts from Vienna, Dr J-P Cayol, came to Stellenbosch to explain, using a PowerPoint presentation, the characteristics of the Medfly GSS strain used in the SIT programme, details of the mtDNA analysis carried out on the collected Medflies, and the results of the analysis results to both the ARC Infruitec-Nietvoorbij team as well as leading Hex Valley growers.

The results showed that none of the wild Medflies collected from the Hex Valley and analysed at Seibersdorf showed the Type A genetic fingerprints present in the GSS from the rearing facility.  The graphic below was used to illustrate the difference in genetic fingerprints of the two groups of flies involved, showing the fingerprints of a representative sample of 11 Hex Valley flies compared with that of the GSS.  The facility flies were Type A, and all flies in the Hex Valley sample (and all other Hex Valley flies collected) were Type B:

A representative example of the results from DNA analysis of 11 Medfly adults recovered from infested grapes from the Hex River Valley.  Band patterns (‘fingerprints’) from the Hex Valley wild flies are distinctly different to those from representative GSS flies from the rearing facility. All wild flies recovered from the Hex Valley were type B, and all facility flies were type A.  (From: Barnes et al., 2016.)

From this evidence, and that from the laboratory investigations at ARC Infruitec-Nietvoorbij, Dr Cayol concluded beyond doubt that the Hex Valley infestation did not originate from the rearing facility, but arose from poor vineyard sanitation and poor management of alternative host plants by growers. At the time this was a serious problem in the Hex Valley.

“I still don’t believe it”

While those at ARC Infruitec-Nietvoorbij were thus vindicated of any contribution to the Medfly infestation in the valley, it was clear from the faces at Dr Cayol’s presentation that this was not the message the growers wanted to hear. In the face of this overwhelming evidence I, for one, was nevertheless very surprised to hear one of the leading growers remark as he left the presentation “I still do not believe it”. Even more surprising was that this Hex Valley grower had, prior to this, accompanied me on two visits to overseas Medfly SIT programmes. “You can lead a horse to the water, but you can’t make it drink.”

In closing, after a difficult childhood, Medfly SIT in South Africa has slowly grown from strength to strength as more people began to understand the mechanisms and dynamics in such an area-wide programme. Since it was privatised in 2003, the Medfly SIT programme (now under the name FruitFly Africa) has become very successful with a state-of-the-art rearing facility, highly trained staff, a statutory fruit fly SIT levy, funding from Government through a 50:50 Partnership Agreement, and a weekly production of up to nearly 60 million sterile males per week. Weekly from October to April, helicopters release sterile males over about 38,000 ha in three separate deciduous fruit and table grape production areas.  Medfly suppression ranges from excellent to very good, mostly depending on the degree of susceptibility of the fruit kind to Medfly. Table grapes are a very good host for Medfly, and the specific production practices, vineyard sanitation and management of alternative host plants present challenges.

Bibliography

Barnes B.N., Targovska A. & Franz G. (2006). Origin of Mediterranean fruit fly, Ceratitis capitata (Wiedemann), outbreak determined by DNA analysis. Afr. Entomol. 14: 205–209.

Barnes B.N., Hofmeyr J.H., Groenewald S., Conlong D.E. & Wohlfarter M. (2015). The sterile insect technique in agricultural crops in South Africa: a metamorphosis… but will it fly? Afr. Entomol. 23: 1–18.

Barnes B.N., 2016. Sterile Insect Technique (SIT) for Fruit Fly Control – The South African Experience.  Ch. 19 in: S. Ekesi et al. (eds.), Fruit Fly Research and Development in Africa – Towards a Sustainable Management Strategy to Improve Horticulture, DOI 10.1007/978-3-319-43226-7_19.