Impact des foreurs de tiges et identification des biotypes de Orseolia oryzivora Harris et Gagné, 1982 (Diptera : Cecidomyiidae) dans les bassins rizicoles du Sud - Cameroun

Abstract

Data on rice stem borers remains embryonic in our country. The level of susceptibility of crop varieties to stem borers is unknown. African rice gall midge has been reported as a major pest in more than 20 African countries where more than one biotype exists. In Cameroon, little information is available on endemic areas, on the different biotypes of this pest. The present study focused on the impact of stem borers and the identification of African rice midge biotypes in the rice-growing areas in the south of the 6th parallel of Central Cameroon. In addition, the influence of the physicochemical characteristics of soils as well as the polyphenol and total sugars levels of the plant stems sensitivity were analyzed at two study sites. The aim was to assess the damage of stem borers, to study their diversity and dynamics, and to analyze the possible criteria for selecting rice varieties grown in southern Cameroon. It was also a question of knowing the most devastating species of driller, to identify its principal biotypes by the technique of the PCR (Polymerase Chain Reaction) and to catalog the possible parasitoids. Four NERICA varieties were sown (N 3, N 8, N 9 and N 13). The Japanese varieties (Oryza sativa) were sown to know if their parasites were present in the study site. Screening of stem borers resistant varieties was based on analyzes of yield parameters of cultivated rice varieties as well as the determination of total sugars and polyphenols in the rice stem. Japanese varieties «Akitachomachi» and «Fukuhibiki» had short development cycle (02 months) and their total sugar were low. The polyphenol level was high for the «Akitachomachi» variety and low for the «Fukuhibiki» variety. Japanese varieties were not attacked by the stem borers. So far, the presence of the Dipteran species of stem borers was unsuspected in the white panicles. Previous research showed that the Orseolia oryzivora and Diopsis apicalis species colonized the rice stem only at the vegetative stage of the plant. After rearing larvae contained in the white panicles, adults of the O. oryzivora, Chilo zacconius and D. apicalis species emerged from the rice stem. However, the list is not exhaustive. O. oryzivora and D. apicalis were present in the rice stem throughout the rice growing cycle (from vegetative to flowering stage), although the O. oryzivora species was dominant. The Chilo zacconius species was present in the stem during the flowering stage. The NERICA 3 variety was the most sensitive, it was sensitive to the three species of borers identified on the study site, the observation we made in the Ebolowa site did not permit us to say that this variety was resistant. The micro-Hymenoptera, parasitoids to the gall midge were: Platygaster diplosisae(Hymenoptera, Platygasteridae) in Yaoundé, andAprostocetus procerae (Hymenoptera, Eulophidae) in Santchou. The analyses of the soil samples of the study site revealed that parameters such as moisture content, the P and the N rates in the soil, could increase the levels of stem borers attacks. During the second year of study, the data collected in Yaoundé and Ntui sites were compared. The analysis of the physicochemical parameters of the two study site revealed that the moisture, Nitrogen, Potassium, clay and sand levels could justify the stem borers damages observed in the Yaoundé site. However, the damage levels were relatively low. Local varieties of Oryza sativa L., Indica varietal group (Tonga and FKR 60) were sown to know their level of sensitivity. The bio molecular identification of O. oryzivora individuals have been made, larval collections were carried out in the rice growing areas of South Cameroon. These were Santchou, Ndop, Tonga, Ébolowa, Baïgom and Yaoundé sites. O. oryzivora was found in the Ébolowa and Yaoundé sites. The primers used were those selected from sixty others in Nigeria by some researchers. After the PCR reaction, the interpretation of the bands observed with ultraviolet rays allowed us to say that there could be more than one biotype of Orseolia oryzivora in the South Cameroon.