V. Hattingh

Cold Susceptibility and Disinfestation of Bactrocera invadens (Diptera: Tephritidae) in Oranges

ABSTRACT To develop a cold disinfestation treatment for the fruit fly Bactrocera invadens Drew, Tsuruta & White (Diptera: Tephritidae) that is rapidly spreading across Africa, research was conducted in Nairobi, Kenya, using flies from a laboratory culture and ‘Valencia’ orange (Citrus sinensis L. Osbeck) as the host. The developmental rate of B. invadens in Valencia oranges was determined at 28°C, and the third instar was found to be the least susceptible of the egg and larval life stages to cold treatment at 1.1°C in oranges. When 22,449 B. invadens third instars were exposed in oranges to a cold treatment with an approximate midpoint of 1.1 ± 0.5°C, the results suggested that a period of 16 d would be worthwhile verifying on a larger scale in oranges. Results from the first replicate of 16,617 larvae showed no survivors, but the second replicate of 23,536 larvae had three survivors. Because a longer cold treatment based on a mean temperature of 1.1°C would create logistical difficulties for some export markets, further replicates were conducted at an approximate midpoint of 0.5°C and at mean hourly maximum of 0.9 ± 0.5°C, for 16 d. After three replicates, in which 65,752 B. invadens third instars in total were treated with no survivors, the Japanese requirement of 99.99% mortality at the 95% confidence level was surpassed. The following treatment protocol for B. invadens larvae in oranges can therefore be recommended: fruit pulp to be maintained at temperatures of 0.9°C or lower for 16 consecutive days.

The potential distribution of Bactrocera dorsalis: considering phenology and irrigation patterns.

A species in the Bactrocera dorsalis (Hendel) complex was detected in Kenya during 2003 and classified as Bactrocera invadens Drew, Tsuruta & White. Having spread rapidly throughout Africa, it threatens agriculture due to crop damage and loss of market access. In a recent revision of the B. dorsalis complex, B. invadens was incorporated into the species B. dorsalis. The potential distribution of B. dorsalis has been previously modelled. However, previous models were based on presence data and did not incorporate information on the seasonal phenology of B. dorsalis, nor on the possible influence that irrigation may have on its distribution. Methyl eugenol-baited traps were used to collect B. dorsalis in Africa. Seasonal phenology data, measured as fly abundance throughout the year, was related to each locations climate to infer climatic growth response parameters. These functions were used along with African distribution records and development studies to fit the niche model for B. dorsalis, using independent global distribution records outside Africa for model validation. Areas at greatest risk of invasion by B. dorsalis are South and Central America, Mexico, southernmost USA, parts of the Mediterranean coast, parts of Southern and Eastern Australia and New Zealands North Island. Under irrigation, most of Africa and Australia appear climatically suitable.

Eradication of Bactrocera invadens (Diptera: Tephritidae) in Limpopo Province, South Africa

On 5 May 2010, Bactrocera invadens Drew, Tsuruta & White was detected in a methyl eugenol-baited surveillance trap in the northernmost part of the Limpopo Province in South Africa, an area adjacent to the Zimbabwe border. A delimiting survey was carried out to determine extent of spread in the area by trapping with both methyl eugenol and Biolure-3-component lures. A quarantine area of approximately 1100 km2 (surrounding the area of detection) was implemented to regulate movement of host fruits. Eradication of the pest was achieved in the quarantine area through male annihilation technique (MAT) using fibreboard blocks containing methyl eugenol and malathion in combination with protein bait sprays (application of GF-120 and LokLure mixed with malathion) and orchard sanitation. Eradication measures were carried out for a period of 12 weeks. Thereafter, MAT blocks were removed and trapping continued for a period of four weeks to confirm eradication. No B. invadens was caught in the area during the four weeks after control measures had stopped. No B. invadens was captured within a period of 12 weeks (approximately three generations) after the last fly find in the area. This constitutes the first successful eradication of B. invadens from an area of incursion.

The Distribution, Relative Abundance, and Seasonal Phenology of Ceratitis capitata, Ceratitis rosa, and Ceratitis cosyra (Diptera: Tephritidae) in South Africa

ABSTRACT Ceratitis capitata (Wiedemann), Ceratitis rosa Karsch, and Ceratitis cosyra (Walker) are fruit fly species (Diptera: Tephritidae) of economic importance in South Africa. These pests cause direct damage to a number of commercially produced fruit and are of phytosanitary concern. A study was conducted to determine the distribution, relative abundance, and seasonal occurrence of the three species in different climatic regions of South Africa. The relative abundance and seasonal phenology of C. capitata and C. rosa were also compared between production areas and home gardens in Stellenbosch, Western Cape. Yellow bucket traps baited with Biolure were used to trap the flies over a 2-yr period in the different sampling areas. Different fruit types were sampled in Stellenbosch to determine fruit fly infestation. C. capitata was found to have a widespread distribution in South Africa, whereas C. rosa were absent from or only present in low numbers in the drier regions. C. cosyra was restricted to the North East and East coast, following a similar pattern to the distribution of marula, Sclerocarrya hirrea, an important wild host. Fruit in home gardens provided a breeding ground for C. capitata and C. rosa and a source for infestation of orchards when fruit started to mature, highlighting the need for an area-wide strategy for the control of fruit flies.

Cold Treatment of Ceratitis capitata (Diptera: Tephritidae) in Oranges using a Larval Endpoint

ABSTRACT South Africa currently exports fresh citrus (Citrus spp.) fruit to Japan using an in-transit cold treatment protocol of 14 d or 12 d at temperatures <0°C for treatment of Ceratitis capitata (Wiedemann) (Diptera: Tephritidae) in ‘Clementine’ mandarins (Citrus reticulata Blanco) and other citrus types, respectively. To reduce the risk of chilling injury with this treatment, research was conducted with temperatures >0°C. Earlier South African research had shown that young (6-d-old) larvae were slightly more tolerant of cold treatment and that there were no significant differences between cold tolerance of these larvae in different citrus types [oranges, Citrus sinensis (L.) Osbeck; grapefruits, Citrus paradisi Macfad.; lemons, Citrus limon (L.) Burm.f; and mandarins). Due to their ready availability, ‘Valencia’ oranges were used in this study. When 62,492 larvae in total were treated in three replicates at a mean temperature of 1.5°C for 16 d, there were three larval survivors. The trial was therefore repeated with oranges using a 16-d period at a mean temperature of 1.0°C and a mean of 1.4°C for the hourly maximum probe readings. Three replicates were again conducted and the resultant mean mortality in the control was 8.1% of 21,801 larvae, whereas the cold treatment mortality was 100% of 71,756 larvae. This treatment at a mean temperature of 1°C exceeded the Japanese confidence level requirement and also exceeded the Probit-9 mortality level, but not at a confidence level of 95%. These data support the establishment of a treatment protocol of 16 d at temperatures <1.4°C, commencing once all fruit pulp probes reach a temperature of 1°C or lower.

Comparing the Use of Laboratory-Reared and Field-Collected Thaumatotibia leucotreta (Lepidoptera: Tortricidae) Larvae for Demonstrating Efficacy of Postharvest Cold Treatments in Citrus Fruit.

Abstract Some of South Africas export markets require postharvest cold treatment of citrus fruit for phytosanitary risk mitigation for Thaumatotibia leucotreta (Meyrick) (Lepidoptera: Tortricidae). An alternative to a standalone cold treatment may be a reduced intensity cold treatment as a step in a systems approach. For cold treatment trials, large numbers of larvae are required. Due to recent dramatic improvement of T. leucotreta control in the field, sufficient naturally infested citrus fruit are no longer available. Artificial infestation of fruit is not viable due to rapid decay of the fruit. Consequently, it is necessary to use laboratory-reared T. leucotreta larvae in artificial diet. In trials, field-collected larvae from the Eastern Cape were at least as cold-tolerant as those from other regions. Larvae in Navel oranges showed the median level of susceptibility in a range of citrus types evaluated at 6°C, and their use in trials was considered acceptable due to their greater natural susceptibility to T. leucotreta infestation. We demonstrated that larvae at high density in artificial diet were at least as cold-tolerant as larvae at lower densities. When exposed to 2°C for 18 d or longer, larvae in artificial diet as used in the trials were at least as cold-tolerant as larvae in fruit. Very few surviving larvae from fruit completed development, with no subsequent generation. Consequently, it is considered justifiable to conduct cold-treatment trials with laboratory-reared T. leucotreta larvae in artificial diet without risk of underestimating the effect of cold on feral larvae in citrus fruit.

Partial Cold Treatment of Citrus Fruit for Export Risk Mitigation for Thaumatotibia leucotreta (Lepidoptera: Tortricidae) as Part of a Systems Approach

Abstract Some of South Africas citrus export markets require mandatory postharvest cold treatment of citrus fruit as a phytosanitary risk mitigation treatment for Thaumatotibia leucotreta (Meyrick) (Lepidoptera: Tortricidae). An alternative to this may be partial cold treatment as one of the final steps in a systems approach to mitigate phytosanitary risk. Consequently, the efficacy of such partial cold treatments was evaluated. It was first determined that a 2°C cold treatment was significantly more effective against fourth and fifth instars (the most cold-tolerant instars) than treatments at 3°C and 4°C for a duration of 18 d. Secondly, it was determined that 2°C for 18 d and 1°C for 16 d were similarly effective, but both treatments were significantly more effective than 1°C for 14 d. Mean mortality of fourth and fifth instars treated with 2°C for 18 d in seven replicates from four trials was 99.94%. Finally, it was determined that the inability of the majority of surviving larvae to develop to adulthood would further increase the efficacy of a 2°C for 18 d treatment to 99.96%. Inclusion of reproductive nonviability of survivors increased mortality to 99.99%.

Invasive Pathogens in Plant Biosecurity. Case Study: Citrus Biosecurity

Most of the world’s major citrus production areas were developed outside the citrus centres of origin, separated from many co-evolved natural enemies (pests and pathogens), but progressive globalisation has reunited some pests with their citrus hosts. Additionally, some ‘new-encounter’ pathogens have not co-evolved with citrus. The movement of major citrus pathogens of biosecurity concern is discussed with particular emphasis on tristeza, leprosis, huanglongbing and citrus variegated chlorosis. The chapter details recent attempts to eradicate citrus canker in Florida (USA) and Emerald (Australia) and focusses on the processes and impediments encountered to achieve eradication under very different climatic, legislative and industry conditions. The impact of citrus black spot in areas climatically conducive to the disease and a discussion of fruit as a pathway for introduction of the disease to new areas are discussed. The experience and learning acquired from managing and eradicating these citrus pests will be of value to other countries and regions that are faced with similar pest incursions.

Scientific critique of the paper “Climatic distribution of citrus black spot caused by Phyllosticta citricarpa. A historical analysis of disease spread in South Africa” by Martínez-Minaya et al. (2015)

The global distribution of citrus black spot (CBS) disease, caused by Phyllosticta citricarpa, is climatically constrained, which is evident from its occurrence in citrus growing areas with warm, summer rainfall and its absence from areas with cooler, Mediterranean-type winter rainfall. Various epidemiological and modelling studies have supported this observation, predominantly estimating unsuitability for P. citricarpa in Mediterranean type climates, with no more than marginal suitability estimated at a few localities within some regions with Mediterranean type climates. The study by Martínez-Minaya et al. (European Journal of Plant Pathology, 143, 69–83, 2015), describes an historic sequence of recorded CBS occurrence in parts of South Africa, conducts an autocorrelation analysis and a correlative analysis with Köppen-Geiger climate zones and makes observations about the occurrence of certain Köppen-Geiger climate zones in the European Union. The study suggests that significant portions of the European Union and the broader Mediterranean basin are climatically similar to warm, summer rainfall areas in South Africa where P. citricarpa persists and causes CBS disease and concludes that the potential distribution of P. citricarpa is less constrained by climatic factors than spatial contagion. However, in this critique we expose methodological shortcomings in the Martínez-Minaya et al. (European Journal of Plant Pathology, 143, 69–83, 2015) study and conclude that the study grossly overestimated the extent of the geographical area that could support P. citricarpa, thereby rendering the findings scientifically unreliable.

Verification of Inspection Standards and Efficacy of a Systems Approach for Thaumatotibia leucotreta (Lepidoptera: Tortricidae) for Export Citrus From South Africa

Abstract A systems approach has been developed for mitigation of risk associated with Thaumatotibia leucotreta (Meyrick) (Lepidoptera: Tortricidae), in citrus fruit exported from South Africa, as an alternative to a stand-alone cold treatment. This study was undertaken to assess compliance with inspection standards applicable to various steps within the systems approach and to determine its overall efficacy. Larval infestation of fruit was monitored weekly in fruit from 33 orchards, until the time of harvest, postpicking, and postpacking into export cartons. Significant positive regressions were recorded between infestation of fruit during the full monitoring period in the orchard and the last 4 wk before harvest, between the last 4 wk before harvest and on delivery to the packinghouse, and on delivery to the packinghouse and in the packed carton. There was an improvement in the level of compliance with each of these successive steps in the system, thus verifying that the grading and inspection thresholds were appropriately sensitive and confirmed the effectiveness of the system. The overall risk mitigation efficacy of the systems approach was calculated. The calculation included several known compounding under estimations of efficacy. Nonetheless, the proportion of fruit that could be infested with T. leucotreta after application of the systems approach was between P ≤ 5.328 × 10−6 and P ≤ 8.380 × 10−7, 6–38 times less than the proportion associated with the probit 9 (P ≤ 3.2 × 10−5) standard for a stand-alone cold treatment, being three survivors in 100,000 at the 95% confidence level.