Investigation into the apparent failure of chemical control for management of western flower thrips, Frankliniella occidentalis (Pergande), on plums in the Western Cape Province of South Africa

Western flower thrips (WFT), Frankliniella occidentalis (Pergande), causes russetting, pansy spot and silvering damage on plums. Despite routine insecticide applications for thrips control, some plum producers report economic losses due to pansy spot damage, pits and holes that render fruit unfit for export. Six commercial plum orchards in two climatic regions in the Western Cape Province, South Africa, were monitored to determine (1) why current management practices based on monitoring and insecticide applications failed to prevent damage in some orchards, and (2) whether WFT is responsible for pitting damage. Western flower thrips was the dominant thrips species in all orchards, though low numbers of Frankliniella schultzei Trybom also occurred. Blue sticky traps were more efficient for monitoring WFT and F. schultzei than yellow traps. Insecticide applications early in bloom and unfavourable weather conditions for thrips flight contributed to very low numbers of WFT on sticky traps during the flowering and fruit set periods. Flower dissections, however, revealed varying levels of oviposition damage to ovaries and fruitlets. Dissections confirmed that WFT enter flowers to lay eggs in the ovaries and other flower parts before petals are fully open. No consistent significant relationship between sticky trap counts and WFT oviposition damage to plums were found, therefore no treatment threshold level could be recommended. Insecticide applications during bloom limited thrips feeding damage, but were too late to prevent oviposition damage. Some oviposition sites developed into typical pansy spots, whilst others formed holes or pits that increased in size as the fruit matured. After insecticide applications ended, and as air temperature increased, WFT populations increased. Weeds and wild vegetation in and around orchards provide a continuous source of potential infestation for as long as the fruit remains attractive and vulnerable to WFT. As oviposition damage by WFT can occur before flowers open, and because of the risk to pollinators when insecticides are applied during flowering, a purely chemical control strategy does not appear to be feasible. A new approach to WFT management is required.