Spread of Huanglongbing, population dynamics and control of Diaphorina citri Kuwayama, the asiatic citrus psyllid in Sarawak

Leong,, Stephen Chan Teck (2006) Spread of Huanglongbing, population dynamics and control of Diaphorina citri Kuwayama, the asiatic citrus psyllid in Sarawak. PhD thesis, Universiti Malaysia Sarawak.

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Abstract

Aspects of the biology and ecology of the Asiatic citrus psyllid (Diaphorina citri Kuwayama) [Hemiptera: syllidae] were studied in the laboratory, screen houses and orchards. These were the first such studies to be undertaken in Sarawak as to determine the influences of host phenology (flushing cycles) on oviposition, seasonal population dynamics, flight activity and spatial distributions in relation to the incidence and spread of the disease huanglongbing (HLB) caused by the phloem-limited gram-negative bacterium Canditatus Liberibacter asiaticus vectored by D. citri. The effects of an nC24 horticultural mineral oils (HMOs) and conventional pesticides on the psyllid and its primary parasitoids were also determined. The screen house studies confirmed that D. citri can colonise and breed on citrus, jasmine orange and curry leaf plant, and that jasmine orange is the preferred host) It was also confirmed that females prefer to oviposit on immature flush 3-10 mm in length. In growth chamber experiments the life cycle of the psyllid was 18.5 days on jasmine orange, 19.0 days on citrus and 23.0 days on curry leaf. Field studies on citrus trees showed that the D. citri population fluctuates throughout the year in Sarawak. Generations overlapped but adult and egg population peaks for a short period generally coincided with three annual flushing cycles, in August-September, February-March and June-July. Populations were adversely affected by weather conditions and parasitoids. Adult psyllid populations increased exponentially during periods of flush growth, and migration and dispersal of the adults was also related to flushing cycles. The relative degree of aggregation (RDA) for adults increased during these cycles as females aggregated to lay eggs. Dispersal and colonisation of new trees was greatest in September-October, at the onset of the rainy season. Adults were trapped throughout the year and higher catches were recorded during September-October. Yellow traps provided an indication of adult abundance and flight activity. Rates of HLB transmission were related to high vector populations and spread was related to dispersing adults. HMO treated plots produced a lower percentage (11.43%) of diseased plants as compared with 42.20% in untreated control plots. The distributions of eggs and nymphs in naturally occurring psyllid populations in orchards were aggregated or contagious, following initially aggregated migrations of adults. A sequential sampling plan, based on the relationship between percent infested flushes and the population density was developed for use in control programs for the psyllid. Field studies showed that sprays of the nC24 horticultural mineral oils (HMOs) were as effective, or more effective, than sprays of conventional pesticides for control of the psyllid. These extensive studies were quite novel, although they were linked to concurrent studies elsewhere in Asia, the approach was radically different and the results influenced subsequent studies and recommendations for using mineral oils in citrus integrated pest management (IPM) programs. Suitability of HMOs for use in citrus IPM programs was also demonstrated by the fact that primary parasitoids of the psyllid were always higher in HMO-sprayed plots.

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