Author : Chou Cheythyrith

Summary : The current world population of 7.3 billion people is expected to increase to 9.6 billion people in 2050. Rice is the staple food of almost half of current world population, especially for people in Asia. Additionally, rice production is an important source of livelihood for approximately 140 million rice farming households. In Cambodia, rice was cultivated on 3,335,929 hectares in 2018, comprising 75% of the total agricultural land in the country. Moreover, rice production is the main source of income for 85% of rural households.

There are several major challenges in rice production such as decline in the biodiversity of rice ecosystems, soil degradation, climate change, and pest damages. The primary mean of rice pest control practiced by most rice farmers is the application of synthetic pesticides. Increasing rice yield has often been attained through the overuse of chemical pesticides and fertilizers. The indiscriminate use of these synthetic agrochemicals can cause environmental and human health problems which jeopardize the productivity and sustainability of rice agroecosystems in the long run.

Therefore, effective alternative methods to chemical pesticides need to be explored to reduce the environmental and human risks from pesticide overuse to improve ecosystem services, and to increase crop production profitability. By reducing reliance on synthetic pesticides, applications of BCA and resistant varieties hold a particular promise to manage pest populations while preserving the biodiversity in rice ecosystems and farmers’ health.

The main objective of this study "Biological Control Agent for the Management of Insect Pests and Diseases on Rice in Cambodia” was to establish field and laboratory evidence on the suitability of BCA as a major component of rice pest management in the country.

In order to pursue the foresaid objective, three studies, two in the fields and one in the laboratory, were conducted from 2016-2018. The following results were obtained:

The study described in Chapter II revealed how much formulation of T. harzianum, a commercially available BCA, could reduce the incidence of rice blast in Cambodian rice fields, and the effectiveness of the rice variety CAR14 resistant to rice blast to suppress the rice blast incidence under field conditions during wet and dry seasons. In total, five field experiments were conducted in Koktrap and Polors Research Station from 2016 to 2017 with four treatments in 2016 and five treatments in 2017. Those treatments in 2016 were: 1) BlPM + CAR14+ T. harzianum, 2) BIPM + IR504 (susceptible variety) + T. harzianum, 3) BIPM + CAR 14 without T. harzianum, and 4) BIPM + IR504 without T. harzianum. In 2017, another treatment (conventional practice) which represented farmers’ use of fungicides to manage diseases was added to the existing treatments. The results showed that the T. harzianum treatment reduced the incidences of leaf blast and neck blast on the susceptible variety IR504. The blast incidences on the resistant variety CAR14 were significantly lower even without T. harzianum than those on plants applied with fungicides. Therefore, when a susceptible variety such as IR504 is to be cultivated due to, for example, high market demands, a BCA such as T. harzianum should be used to prevent neck blast and leaf blast incidences and yield losses.

The study described in Chapter III demonstrated the effects or B. bassiana as BCA and orange oil extract (PREV-AM) as a botanical pesticide on insect pests and natural enemies in rice fields of Cambodia, the incidences of insect damage and the grain yields in the rice fields of Cambodia. The field experiments were conducted in farmers’ fields from 2017 to 2018 in two villages of Prey Veng and Kampong Thom province with four treatments and four replications. The four treatments were 1) BIPM with the orange oil. 2) BlPM with B. bassiana. 3) BIPM alone, and 4) the conventional practice which in this case represents farmers’ use of insecticides to manage insect pests. Our study suggested that the treatment of B. bassiana as a component of BIPM was consistently effective to suppress the collective populations of rice insect pests and the injuries from the insect pests. Application of B. bassiana was also shown to increase the collective populations of natural enemies and rice grain yields. There was virtually no significant difference in collective populations of insect pests and natural enemies between the plots treated with B. bassiana compared to those treated with orange oil.

The study described in Chapter IV revealed the presence of microorganisms that can degrade the infection-promoting factor 2DU of the rice blast fungus in the rice fields of Cambodia that may serve as a novel BCA against the rice blast fungus. A total of 151 samples of rice plants and soils from rice fields were collected from Battambang, Kam pong Thom, Prey Veng, Svay Rieng and Takeo provinces. Among 151 enrichment cultures of microorganisms originating from the soil and rice plant samples, 105 showed to contain 2DU degradation activity. Subsequently, 90 isolates of 2DU degrading bacteria (2DUDB) were obtained. Out of the 90 isolates, 81 were classified into four classes (a-proteobacteria, B-proteobacteria, y-proteobacteria, and Actinobacteria) and fifteen genera (Burkholderia, Sphingomonas, Acidovorax, Agrobacterium, Xanthomonas, Cupriavidus, Microbacterium, Ralstonia, Stenotrophomonas, Bosea, Caulobactor, Delftia, Enterobacter, Herbaspirillum, and Novosphingobium). Among the 2DUDB isolated from rice plants, those belonging to Burkholderia was the most abundant in Cambodia, which was also found to be the most abundant in Japan. The 2DUDB isolates belonging to Burkholderia in Japan were able to suppress the blast pathogen under laboratory conditions, implying that the 2DUDB isolates belonging to Burkholderia in Cambodia may have the same ability.

In summary, the studies conducted for this dissertation showed that the deployment of a resistant variety coupled with the applications of BCA are highly suitable to manage major pests and pathogen in the country. These also identified species of microorganisms with further biocontrol potentials from Cambodian soil. Furthermore, these also showed that the applications of BCA preserved or even increased biodiversity in rice ecosystems compared to conventional practice. Consequently, these IPM components are also effective in preserving yield from pests and diseases. Wherever possible, the use of resistant variety and BCA applications should then form the first line of defense against rice pests and diseases and should be strongly recommended to farmers as a way to reduce their reliance on synthetic pesticides. Further research to find and formulate microbial BCA from local strains should be encouraged to facilitate production of local BCAs in Cambodia.