Source and identity of insect contaminants in export consignments of table grapes
Thesis (MScAgric (Conservation Ecology and Entomology)--University of Stellenbosch, 2005.
The South African table grape industry exports approximately 60% of the table grapes produced. A major threat to the export of these grapes is the phytosanitary risk that insect pests pose. This study was conducted in the Hex River Valley, South Africa’s main table grape producing area. The aim of this study was to reduce the number of phytosanitary rejections from insects on table grapes from the Hex River Valley. Thus the main objectives of the study were to identify the most important phytosanitary pests in the Hex River Valley; the determination of their presence in the vineyards with possible means to control them; and to assess the possibility of using postharvest quarantine treatments in the Western Cape. Further aims were to determine the effect of different colour harvesting crates on the phytosanitary pests and whether the phytosanitary pests infested the grapes via packhouses. The most important phytosanitary pests of table grapes of the Hex River Valley are in order of importance: Phlyctinus callosus (Schonherr) (Coleoptera: Curculionidae), Epichoristodes acerbella Walker (Lepidoptera: Tortricidae), Planococcus ficus (Signoret) (Hemiptera: Pseudococcidae), Ceratitis capitata (Wiedemann) (Diptera: Tephritidae), Gonocephalum simplex Fabricius (Coleoptera: Tenebrionidae) and Dysdercus fasciatus Signoret (Hemiptera: Pyrrhocoridae). 12.71% of rejections were from species that were not identified, while a further 33% of the rejections were possibly identified incorrectly. Phytosanitary control of P. callosus appeared to be far more effective using Plantex® than pesticides. Weather conditions appeared to affect the abundance of P. callosus, especially warm weather, while bunches harboured less P. callosus later in the day. Control of E. acerbella with DiPel® (Bacillus thuringiensis var. kurstaki) appeared to at least reduce the population within the vineyards, and so its use is recommended. P. ficus is a non-actionable species for the USA market and is not listed as a phytosanitary pest for the Israeli market and so should not be causing any phytosanitary rejections. C. capitata appeared to be successfully controlled by the fruit fly sterile release program and the cold sterilisation it currently undergoes. G. simplex caused few rejections. It is still unclear where this pest infests the grapes, as it was found in both the field and in the packhouses. D. fasciatus occurrence on grapes was probably accidental. It was shown that picking during the early and late parts of the day, when this species was less active, reduced its occurrence in bunches. Gryllus bimaculatus (De Geer) (Orthoptera: Gryllidae), although not reported as a reason for rejections in table grapes for the past two years, was an actionable species that was present in large numbers in the Hex River Valley. There was a strong correlation between increasing quantities of pesticides and higher abundances of G. bimaculatus. It appeared to be an indicator of the overuse of pesticides. Results of this study showed that infestation by the phytosanitary pests came from neighbouring vineyards. The creation of barriers to prevent the movement of these pests between vineyards is suggested. Methyl bromide is the most commonly used postharvest quarantine treatment. Owing to the ozone-depleting properties of methyl bromide, it is scheduled to be outlawed in many countries from 2005. Alternative postharvest treatments are irradiation, extreme temperatures, forced air, vapour-heat treatments and the use of controlled atmospheres. Irradiation treatments appeared to control the pests at doses that do not damage the grapes. Controlled atmosphere treatments also have a high probability of success, although more research is required on this treatment. Low temperature treatments are relatively cheap as most exported fruit already undergoes cold storage, and appears to control species in the families Pseudococcidae and Tephritidae, although further research is required for the other pest. Colour or location of the harvesting crates in the vineyards appeared not to influence the number of phytosanitary pests collected, as they were not attracted to these crates.