Effect of Abiotic Factors on the Incidence of African Rice Gall Midge, Orseolia oryzivora and its Parasitism by Platygaster diplosisae and Aprostocetus procerae

African rice gall midge (AfRGM), Orseolia oryzivora is one of the most damaging insect pests of lowland rice in Nigeria in the recent time, and could result in total crop failure in endemic areas. Of all the control measures adopted so far biological control is the most promising. Two parasitoids (Platygaster diplosisae, Hym: Platygastridae and Aprostocetus procerae, Hym: Eulophidae) have been identified with the potential to suppressed AfRGM population. In the current study, we evaluated the effect of abiotic factors (rainfall, relative humidity and temperature) in two eco-sites for two years on the incidence of AfRGM, and on the efficiency of the two parasotioids in the management of AfRGM. The incidence of AfRGM was significantly influenced by abiotic factors (rainfall, relative humidity and temperature). The percentage infestation increased with increase in rainfall and relative humidity. The trend was similar at both locations with the highest infestation recorded in October, and decreased in November. At both locations, percentage tiller infestation was greater in the rainy season of 2006 with more frequent rains than in 2007, with highest infestation recorded at Edozhigi in both years. Percentage parasitisms by the two parasitoids were found to have significant correlation with the abiotic factors. Heavy rains and high humidity significantly reduced the efficiency of the two parasitoids. On the contrast, the population and efficiency of the parasitoids were observed to increase with reducing rainfall, RH, but increasing temperature thus increasing the efficiency of the parasitoids later in the season with up to 70% parasitism when combined, with P. diplosisae dominating the field. The results suggest that abiotic factors played significant role on the incidence of AfRGM and the efficiency of these parasitoids and should be considered in the adoption of these parasitoids as bio-control agents of AfRGM. Keywords: Orseolia oryzivora, Platygaster diplosisae, Aprostocetus procerae, abiotic factor

A simple head cage technique for monitoring sorghum midge (Diptera: Cecidomyiidae)

A head cage technique for monitoring populations of Stenodiplosis sorghicola, a variant of the type previously described for screening sorghum genotypes for resistance, was developed. The technique was effective and efficient in collecting adults from flowering sorghum panicles under field conditions in Andhra Pradesh, India. Adults emerged over 2-3 weeks during the 1992-93 post-rainy and 1-2 weeks during the 1993 rainy season. The activity (density) of S. sorghicola was higher during the rainy than during the post-rainy seaso

Tritrophic interactions in sorghum, midge (Stenodiplosis sorghicola) and its parasitoid (Aprostocetus spp.)

Studies were conducted on tritrophic interactions involving sorghum genotypes, Stenodiplosis sorghicola and the predominant parasitoids (Aprostocetus spp.) in Patancheru, Andhra Pradesh, India, using three midge resistant (ICSV 745, ICSV 89058 and IS 10712) and three susceptible (Swarna, CSH 9 and ICSV 112) genotypes during the post-rainy (1992/93) and rainy (1993) seasons. A. coimbatorensis, the predominant parasitoid during post-rainy season, preferred mid-late midge larvae for parasitization, while A. gala, which was predominant during the rainy season, preferred early-mid larval stages. Variations in the preference of A. coimbatorensis and A. gala for the developmental stage of their host larvae indicated good prospects for the biological control of sorghum midge populations. There were significant differences in parasitization level of midge by Aprostocetus spp. between resistant and susceptible sorghum genotypes, and season. Higher parasitization was observed on susceptible genotypes than on resistant ones during both post-rainy and rainy seasons. However, the level of parasitization was greater in post-rainy than in rainy seasons. There was also evidence of higher midge infestation in rainy than in post-rainy seasons. Susceptible genotypes attracted more parasitoids because of high levels of midge infestations. Low parasitoid density in midge resistant sorghum under glasshouse and field conditions was associated with low midge infestations in these genotypes. However, parasitoids were always associated with their host in spite of low midge densities in resistant genotypes. The present study revealed that the interaction between host plant resistance and parasitoids of sorghum midge would thus be synergistic and complementary and could result in successful integration of these two important pest management tactics

Emergence pattern of sorghum midge and its major parasitoids on midge-resistant and susceptible genotypes

Studies were conducted on the species composition of parasitoids of Stenodiplosis sorghicola, emergence pattern and level of parasitism. The studies were carried out in Andhra Pradesh, India, with three midge-resistant (ICSV 745, ICSV 89058 and IS 10712) and three susceptible (Swarna, CSH 9 and ICSV 112) sorghum genotypes during the 1992-93 post-rainy and 1993 rainy seasons. The species of parasitoids collected were Aprostocetus gala, A. coimbatorensis and Eupelmus spp. The species composition varied with the season, but was unaffected by varietal resistance and susceptibility to the midge. Although both species of Aprostocetus were present in rainy and post-rainy seasons, A. gala was predominant during the rainy season whereas A. Coimbatorensis was predominant in the post-rainy season. There was no significant difference in the pattern of parasitoid emergence or the level of midge parasitization between resistant and susceptible genotypes. These results indicate that resistance to midge in the genotypes studied was not antagonistic to parasitoid activity, and that there is potential to interface biological control with host-plant resistance in the management of this insect

Fecundity and Diurnal Oviposition Behaviour of Sorghum Shoot Fly, Atherigona soccata Rondani (Diptera: Muscidae)

The fecundity and diurnal oviposition behaviour of Atherigona soccata was studied on CSH 5, a susceptible sorghum genotype, under greenhouse conditions. The fecundity of females deprived of 10-day-old sorghum seedlings was drastically reduced, and no fertile eggs were laid after 11 days of host deprivation. However, when the females were provided with host plants, egg production was prolonged, and fell considerably after the flies reached 22 days of age. There were three distinct peaks (6-7, 13-14 and 18-19 days of age) in egg-laying activity. Further studies with flies of three age groups (7, 13 and 19 days old) did not show significant differences in diurnal oviposition pattern with age. However, most eggs (60%) were laid between 08.00 and 12.00 h. It was evident from the present study that the prolonged egg-laying capacity of female A. soccata when provided with a susceptible host may partly account for the dramatic increase in damage as the crop season advances, since there would be a geometric increase in active females with every new generation of flies

Evidence of shoot fly Atherigona soccata Rondani (Dipt., Muscidae) oviposition response to sorghum seedling volatiles

The effects of Atherigona soccata-resistant (IS 18551) and -susceptible (CSH 5) sorghum seedling volatiles on oviposition by the pest were investigated by exposing mated females to various treatments under greenhouse conditions. It is hypothesized that susceptible-genotype and younger (5-10 days after emergence, (DAE)) seedlings emit different quantities and/or blends of volatiles than resistant or older (14-21 DAE) ones. A. soccata response was greater to host plant odour alone than to host plant plus fishmeal. A transparent trap with 10-day-old susceptible seedlings was found to be more effective and efficient for adult attraction than the other two types of traps (live fly trap and black trap). The present study indicated that females of A. soccata are attracted both to the volatiles emitted by the susceptible seedlings, and to phototactic (optical) stimuli that may facilitate orientation to its host for oviposition, but volatile blends associated with adult attraction will be reported in due course to confirm these results

Potential of Insect Vector Screening Method for Development of Durable Resistance Cultivars to Rice yellow mottle virus Disease

The study aimed to investigate the potential of insect vector Rice yellow mottle virus (RYMV) cultivar screening method. Screening rice cultivars against RYMV under artificial conditions is usually carried out inside the screen house by mechanical inoculation of RYMV isolates. Such an approach may be highly criticized as not fully representative of how RYMV disease is spread or transmitted under field conditions. Consequently, the potential of three RYMV insect vectors, Oxya hyla, Locris rubra and chnootriba similes, was evaluated in comparing the cultivar screening method with mechanical transmission using eight differential rice genotypes against a highly virulent RYMV Nigerian isolate. The study revealed that each of the three insect vector methods is different from the mechanical transmission method and all methods screened rice cultivars in the same way. This study revealed the potential of the insect vector screening method to provide a basis not only for the development of durable resistant cultivars to RYMV disease but also for further investigation on vectors, virus and plants interaction

Multivariate analysis of diversity of landrace rice germplasm

Abstract Multivariate analysis involves observation and analysis of more than one statistical variable at a time. The variability of 434 accessions of rice (Oryzasativa L.) germplasm from Côte d'Ivoire was evaluated for 14 agromorphological traits in upland conditions at M'bé, Côte d'Ivoire (7°5´ N; 5°1´ W), using augmented experimental design, and analyzed with multivariate methods. The unweighted variable pair group method of the average linkage cluster analysis (UPGMA), canonical discriminant analysis (CAN) and principal component analysis (PCA) were used to analyze the data obtained. This enabled assessment of the extent and pattern of variation of the germplasm, and identification of the major traits contributing to the diversity. Seven cluster groups were obtained from the 14 agro-botanical traits using UPGMA. CAN showed the contribution of each trait to the classification of the rice accessions into different cluster groups. The first three principal components explained about 58.41% of the total variation among the 14 characters. The results of CAN and PCA suggested that traits such as plant height, leaf length, number of days to heading and maturity, tillering ability, panicle length and grain size (weight, length and width) were the principal discriminatory characteristics. It is concluded that variation exists in the germplasm, which provides opportunities for this collection to be useful for genetic improvement