African Crop Science Journal, Vol. 15, No. 4, Dec, 2007, pp. 211-222
Potential for Biological Control of Rice Yellow Mottle Virus Vectors
N. Woin, Djonmaïla1, Sadou Ismael, Sali Bourou And T. Bebom
IRAD, Box 33 Maroua, Cameroon
Code Number: cs07022
Insect pests and disease infestations are the primary constraints in rice (Oryza sativa) production systems in Africa and Asia. For Cameroon in particular, the rice yellow mottle virus (RYMV) is increasingly a serious problem to rice production. During the poulations of the various insect vectors of RMYV are not known, and hence the need for this study. Unfortunately, 2002 - 2004, two sampling methods were combined to assess the population of insects vectors of rice yellow mottle virus (RYMV) in the three major irrigated rice ecosystems in northern Cameroon (Lagdo, Maga and Yagoua), and in low land rice fields. Sampling was conducted using sweep net and D-Vac (sucking trap) fortnightly in 2002 and 2003 until mid October in low land rice fields, while in the irrigated fields, samplings occurred between December and April. Rearing using dead insects was conducted simultaneously in the laboratory to identify the parasitoid insect species. From samples obtained at different sites: (i) the dominant structure of the RYMV insect vectors was analysed according to the rice phenology; and (ii) the diversity and the occurrence of potential major groups of predators and parasitoids were assessed. Among the RYMV insect vectors sampled: Chaetocnema pulla Chapuis (Coleoptera: Chrysomelidae), Chnootriba similis Mulsant (Coleoptera: Coccinellidae), Trichispa sericea Guerin-Meneville (Coleoptera: Chrysomelidae), Locris rubra Fabricius (Hemiptera: Cicadellidae), Oxya hyla Stål (Orthoptera: Acrididae), and Conocephalus longipennis (de Haan) (Orthoptera: Tettigoniidae) were the most encountered insect species during the rice growing seasons. With regard to predator populations, spiders (Araneae) were the most abundant, with high concurrency of Pardosa spp (> 42 %) in all sites. In Maga and Yagoua sites, the carabid beetle species, Abacetus crenulatus Dejean and Abacetus foveolatus Chaudoir, were the most numerous whereas the Lagdo site was highly colonised by Clivina. erythropyga Putzeys. Paederus sabaeus Erichson, and Stenus ravus Puthz were the most abundant Staphylinid beetles. From reared dead RYMV insect vectors, Eurytoma spp., Pediobius spp., Tetrastichus spp. and Telenomus spp. emerged as parasitoids. Results of this study reveal a great potential of biological control against rice yellow mottle virus vectors using predators and parastoids. This potential should be developed to manage the yield losses caused by the virus infection in rice cropping systems.
Key Words: Cameroon, Oryza sativa, parasitoids, predators
Les infestations dues aux insectes et les maladies sont les contraintes primaires dans le système de production du riz (Oryza sativa) en Afrique et en Asie. Pour le Cameroun en particulier, le virus marbre jaune de riz (RYMV) est de plus en plus un problème sérieux à la production de riz. Cependant, les populations des divers vecteurs d’insecte de rmyv ne sont pas connu, et donc le besoin pour cette étude. Malheureusement entre 2002 et 2004, deux méthodes d’échantillonnages ont été combinées pour évaluer la population de vecteurs d’insectes de virus marbre jaune de riz (RYMV) dans les trois écosystèmes majeurs de riz irrigué au nord du Cameroun (Lagdo, Maga et Yagoua), et dans les champs de riz de terre bas. L’essai a été dirigé utilisant le coup de balai net et D- vac (suçant le piège) les nuits de 2002 et 2003 jusqu’ a mi-octobre dans les champs de riz de terre bas, pendant que dans les champs irrigué, les échantillons sont arrivés entre Décembre et Avril. Utilisation des insectes morts était simultanément dirigée dans le laboratoire pour identifier l’espèce d’insecte parasitaire. Des échantillons obtenus aux sites différents: (i) la structure dominante des vecteurs d’insecte de rymv a été analysée selon la phénologie de riz ; et (ii) la diversité et l’événement de groupes majeurs potentiels de prédateurs et de parasitoides ont été évaluées. Les vecteurs d’insecte de RYMV : Chaetocnema pulla Chapuis (Coleoptera : Chrysomelidae), Chnootriba similis Mulsant (Coleoptera : Coccinellidae), Trichispa sericea guerin meneville (Coleoptera : Chrysomelidae), Locris rubra Fabricius (Hemiptera : Cicadellidae), Oxya hyla stål (Orthoptera : Acrididae), et Conocephalus longipennis (de Haan) (Orthoptera : Tettigoniidae) étaient les espèces d’insecte le plus communes pendant les périodes de croissance de riz. En ce qui concerne les populations de prédateur, les araignées (Araneae) étaient le plus abondant, avec haute concurrence de Pardosa spp. (> 42 %) dans tous les sites. Sur les sites de Maga et Yagoua, l’espèce de coléoptère de carabid, abacetus crenulatus dejean et Abacetus foveolatus Chaudoir, étaient les plus nombreux tandis que sur le site de Lagdo a été extrêmement colonisé par Clivina. erythropyga Putzeys. Paederus sabaeus Erichson, et Stenus ravus Puthz étaient les coléoptères du Staphylin le plus abondant. Les vecteurs d’insecte de RYMV, Eurytoma spp., Pediobius spp., Tetrastichus spp., et Telenomus spp. ont émergé comme parasitologies. Les résultats de cette étude révèlent un grand potentiel de contrôle biologique contre les vecteurs de virus marbre jaune de riz utilisant des prédateurs et des parasitoses. Ce potentiel devrait être développer pour gérer les pertes de rendement causées par l’infection de virus dans le système de riz.
Mots Clés: Cameroun, Oryza sativa, parasito¿des, prédateurs
Africa produces only 2.7% of the world’s rice (Oryza sativa) and is the second largest rice importing region in the world (6.5 metric in 2003). Africa’s rice imports represent about 25% of the world rice importation (Méndez del Villar, 2003). With an average yield of 2 t ha-1 . With the exception of Egypt, rice production in Africa remains significantly below the world average and regional averages such as Asia (3.8 t ha-1 ), Latin America (3.0 t ha-1 ) and United States (7.0 t ha-1 ), according to FAO (published online). Compared to the other West African countries, the yearly amount of rice importation in Cameroon is at least 62 % of thequantity needed.
Insect pests and diseases are the major constraints limiting rice production in Africa and Asia. Of all the rice diseases, the one caused by the rice yellow mottle virus (RYMV), first reported in Kenya in 1966, is one of the most damaging in Africa. The RYMV has by far been reported in many countries in East and West Africa including Cameroon, where in some cases whole fields have been devastated. Based on available evidence, RYMV has only been reported from the African continent and is endemic in every country where it has been reported. Among the known vectors of the disease insects constitute one of the indubitable and important group. Since evidence of the existence of the disease in Cameroon in year 2 000 (Traoré et al., 2001), studies have never been carried out on RYMV nor on its vectors.
The aim of this work was to assess the species diversity and population abundance of RYMV insect vectors including those of their potential natural antagonists in lowland/irrigated rice fields in the Sudano-sahelian savannah of Cameroon with a view of establishing the potential of biological control strategies against RYMV vectors.
MATERIAL AND METHODS
Experiments were conducted from 2002 to 2004 in the Sudano-sahelian climate, with a relative humidity of 65 ± 25 %, temperature range of 13- 42°C, and mean annual rainfall of 1,000 mm. Three sites, at least 200 km apart, were sampled. These were (1) Lagdo: 9° 3' 0N; 13° 43' 60E - 232 m a.s.l; Maga: 10° 50' 12N; 14° 56' 37E – 305 m a.s.l, and Yagoua: 10° 20' 34N; 15° 14' 26E – 313 m a.s.l. The experimental fields consisted of a plot (0.5 ha) in each site. Sampling in lowland rice was carried out only in Lagdo and Yagoua where field plots were contiguous to farms cropped by maize (Zea mays L.).
The RYMV vectors and their natural enemies populating the aerial rice parts were sampled using 2 methods as described:
(i) sweeping with net: 200 doubled sweeps (50 random sweeps per plot along diagonal and median transects); and
Dead RYMV vector materials were reared to confirm emergence of parasitoids
All the experimental fields had not been treated with insecticides in the 10 years preceding the study. All arthropods sampled were labeled, stored at -4°C, sorted in the laboratory under a stereoscopic binocular microscope, and then transferred into 70% alcohol, pending identification.
As species diversity combines the area of ‘number of species’ (species richness) and the way in which the individuals are apportioned into those species (evenness) (Vandermeer, 1981), the heterogeneity of such a community is determined using the Shannon-Weiner index (Risch et al., 1983).
The frequency distribution was assessed considering dominance class according to Engelmann (1978). Representative specimens of rove and ground beetles were sent to the International Institute of Entomology (CAB International, London) for identification or confirmation.
To compare diversity values estimated with the Shannon–Weiner index (H’) on the basis of the frequency distribution of the beetles, parametric statistical techniques were questioned through a t-test (MacArthur, 1965; Poole, 1974) at 5%.
Population abundance of RYMV insect vectors.
In the 3 locations of the study, the RYMV insect vectors sampled were: Chaetocnema pulla Chapuis (Coleoptera: Chrysomelidae), Chnootriba similis Mulsant (Coleoptera: Coccinellidae) Trichispa sericea Guerin-Meneville (Coleoptera: Chrysomelidae), Locris rubra Fabricius (Hemiptera: Cicadellidae), Oxya hyla Stål (Orthoptera: Acrididae) and Conocephalus longipennis (de Haan) (Orthoptera: Tettigoniidae).
From 2002 to 2004 (Figs. 1, 2 and 3), C.similis was always the most dominant species in Lagdo rice fields, with an overall frequencies between 55 and 70% of the total number of vectors caught; followed by O. hyla. In the same period, C.longipennis occurred most frequently in Maga, followed by O. hyla and T.sericea. In Yagoua, where sampling was carried out only in irrigated rice fields during 2003 and 2004, the meadow grasshopper C.longipennis was highly dominant throughout both cropping seasons, followed by O. hyla, L.rubra, T. sericea and C.pulla.
Populations of predators colonising rice fields in the sites
a) Spiders. Throughout the sampling period 16,968 spider specimens belonging to 28 species and 10 families were collected from the 3 sites, (Table 1). The species richness was different at Lagdo and Yagoua during 2002. Values fluctuated from year to year at Lagdo where populations ranged between 1,862 and 2,394 individuals. The abundance was higher in Yagoua (2,874 specimens) than in Lagdo (2,394 individuals), during the same year. However, this fact was not concomitant with higher species richness in Lagdo. Shannon index marginally varied between sites and years (Table 2).
Ten species (82.40 %) were common to all sites (Table 3). Lycosidae specimens represented 55.08% of the total sample among which Pardosa spp. accounted for 42.03%, followed by Arctosa spp. (5.60%). In 2 sites, Pardosa spp. were the most dominant species throughout the sampling period. Its population abundance in Lagdo ranged from 38.39 to 51.63% of the total specimens collected. This was followed by Maga where 44.40% of specimens belonged to Pardosa spp. whereas in Yagoua the species came second (20.76%) after Langbiana vokrensis Bosmans and Van Hove (27.35%).
b) The carabids. The overall trend in cumulative frequency distribution (%) of species is presented in Table 4, whereas, Table 5 gives number of ground beetle species collected for the same period. Abacetus crenulatus Dejean were the dorminant ground beetle species at Lagdo and Yagoua (Table 4). Overall, Pterostichinae ground beetles were the most abundant followed by Scaritinae (Table 5).
c) The Staphylinids. The overall trend in frequency distribution (cumulative values) of rove beetle species is presented in Table 6 and 7. When considering the accumulation of different species encountered, one notices that the dominant species are limited to only a few: Paederus sabaeus, Stenus ravus and Stenus (Mendicus) S. fulgidus
d) Some dominant parasitoids. Among the potential parasitoids obtained by rearing dead insect vectors of RYMV, several hemepteran species observed in different localities (Table 8).
Based on existing literature, this study represents the first report of the insect vector species being present in Cameroon and their role as a vector of RYMV. The fact that the prevalence of the species investigated differs from one site to another, indicates that pest problems may vary from one area to another in view of the diverse and surrounding conditions under which rice is grown. Habitat preference of C. similis appears to be Lagdo. In Nigeria, Alam (1992) indicated that C. similis occurs in all climatic zones, from the humid tropical to the Sudanian savanna. However, populations of C. similis are lower than that of most other pests reported in Nigeria. Akinsola and Agyen-Sampong (1984) reported similar observations for West Africa as a whole. Various reports (e.g., Bakker, 1971; Reckhaus and Andriamasintseheno, 1997) observed a positive relationship between vector number and RYMV infection.
Conocephalus longipennis has been reported as being a transmission agent for RYMV in West Africa (Abo, 1998; Nwilene, 1999). Conocephalus populations were highest in the Guinea savanna zone (E.A. Heinrichs and C. Williams, WARDA, 1995, unpubl. data). The status of this pest in the Sudanian savanna has not been reported. Studies on the continuum toposequence on the WARDA M’bé Farm indicated a trend similar to that of Chaetocnema. Populations occurred in all ecologies but were highest in the uplands (Heinrichs and Barrion, 2004 ). In Latin America, intensification of crop management practices has been shown to result in decreasing importance of Chaetocnema spp. in lowland rice (Weber and Parada, 1994).
Many studies have been conducted on polyphagous beetles both in natural vegetation and various habitats in agro-ecosystems ( Fagel, 1970 and1973; Puthz, 1971; Thiele, 1977; Luff, 1987; Ishitani and Yano, 1994). However, in paddy rice fields, the population dynamics of these insects have not been documented, probably because of the inundated environment (Yano et al., 1995). In Africa and Latin America where upland rice represents, respectively, 60 and 73 % of total rice-growing area (Jacquot and Courtois, 1993), predatory soil-dwelling beetles may be considered in establishing any integrated insect pest management program.
Despite the early emphasis placed on specialist predators and their potential control of target prey populations, recent evidence indicates that generalist predators may also have important influences on prey. They are abundant polyphagous predators in most agricultural systems and are potentially important pest control organisms (Riechert and Lockley, 1984; Nyffeler and Benz, 1988; Woin, 1999). Spiller’s (1992) experimental work with two spider species, demonstrated the potential interference effects that generalist predators may have on each other’s predation effects on insect populations. Riechert and Bishop (1990), nevertheless, found that in conserving the spider assemblage by the application of mulch, one can significantly reduce phytophagous insect numbers and plant damage.
However, little is known about the population ecology of Afrotropical species. Among the spiders collected, Pardosa spp. were the most abundant as mentioned by Woin (1999).
In Maga and Yagoua, the species A. crenulatus and A. foveolatus the most numerous whereas the Lagdo site was highly colonized by C. erythropyga over the three sampling years. Investigations conducted in the south of Ethiopia (Gebre-Tsadik, 1998) showed that these species were mostly trapped at the aerial part of wheat crop. The most common species, Paederus sabaeus, belongs to a genus known as active predators in tropical agro-ecosystems (Ooi and Shepard, 1994; Gebre-Tsadik, 1998). The Stenus species were numerous and made up an important percentage of the population. Bernhauer (1939) emphasized the importance of this group of polyphagous predators in the Cameroon environment. From parasitoids identified on larvae and pupae, P. furvus was known as enemy of C. similis in the sudano-sahalian cropping systems in Cameroon.
We would like to thank the West African Rice Development Association (WARDA)/ROCARIZ under whose auspices the study was granted. We thank CAB International (UK) for their support in taronomic groupings and identification of arthropods.
©2007, African Crop Science Society
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