Lepidopteran stemborers (Chilo zacconius Bleszynski, Chilo diffusilineus de Joannis and Maliarpha separatella Ragonot) and diopsid flies (Diopsis spp.) are economically significant insect pests of rice in Burkina Faso. The objective of this study was to monitor damage trends and the pre-imaginal populations of these insects on two host plants, cultivated (Oryza sativa L.) and wild rice (Oryza longistaminata Chev and Roehr). The study was conducted over four consecutive rice crop seasons from 1999 to 2001 in three irrigated rice-growing locations in western Burkina Faso. Sampling of cultivated rice was conducted monthly in 1 m2 quadrates in 100 randomly selected farmers' fields. Data on wild rice were collected monthly from each of the three locations using 500 tillers that were plucked at random and dissected. Results showed that in the absence of a rice crop lepidopteran stemborers and diopsid flies are able to survive on O. longistaminata, which can host the pre-imaginal populations of these insects throughout the year. Damage caused on cultivated rice varied across farmers' fields, seasons and phenological stages of the plants. With 8% the highest average extent of ‘whiteheads’ was recorded during the dry season. These results are important elements in the design of a sustainable management strategy for these insect pests.

The efficiency and profitability of an integrated pest management system consisting of a phytosanitary survey and threshold interventions (based on levels of insect damage) was tested at the Vallée du Kou irrigated rice scheme in Burkina Faso, for two consecutive crop seasons in 1987. There was a considerable advantage in cost, number of insecticidal applications and yield of a threshold intervention-based system over arbitrary or routine insecticidal application methods. The proposed method is considered to be environmentally friendly.

A study of the noxiousness of the African white rice stem borer, Maliarpha separatella Rag carried out under controlled field conditions has revealed that the impact of this pest on the yield and its components is assessable. For mean infestation rates up to 68% during the period of highest sensitivity of a paddy rice culture (second half of tillering to panicle initiation), no significant decrease could be observed in the number of panicles, nor any significant increase in the number of white heads and the number of empty grains per hill. However, with increasing infestation rates, the number of immature panicles increased and the number and weight of filled grains per hill decreased. An explorative analysis of the infestation rate-yield loss relationship, showed that a sigmoid model describes this interdependence appropriately. The proposed economic injury level (EIL), is here considered more the result of an approach, which had as its goal the re-definition of the status of the pest than as an instrument of a chemical control programme with treatments based on an action threshold.

Studies aimed at setting up a system of rational control of pests were carried out at the Vallée du Kou rice irrigation scheme near Bobo-Dioulasso in 1986. The first part of the study focuses on the phytosanitary survey of the above mentioned rice irrigation scheme of 1043 ha through a network of observation sites. This part has given prominence to the fact that the development and importance of the pest infestation were affected by the spacing out of the rice planting and has also allowed us to locate the critical periods of the infestations and their economic incidence. These pests consist mainly of lepidopterous stem borers: Chilo sp., Sesamia sp., and Maliarpha separatella.

The second part of the study focused on phytosanitary experiments. Using Decis (Deltamethrin) at 12.5 g a.i. per ha at the intervention thresholds of 5% dead heart and 1% whitehead, has proved to be the most interesting formula as it combines efficiency and profitability.

The system of rational control that has been set up based on phytosanitary survey and insecticide application at threshold levels, presents a double advantage, it reduces the number and cost of interventions, and limits the anarchical use of pesticides, hence their harmful effect on the environment. Such experience in insect pest management will be extended to other rice growing areas throughout the country.

Studies were carried out in 1983 to determine the relative abundance and species composition of three lepidopterous stem borers in upland and irrigated rice at Ibadan, southwestern Nigeria. The relationship between irrigated rice crop phenology and the population of the borers and stalk-eyed fly was also studied.

White stem borer, Maliarpha separatella was the predominant species in upland and irrigated rice, followed in upland rice by pink stem borer, Sesamia calamistis, and striped stem borer, Chilo zacconius and in irrigated rice by C. zacconius and S. calamistis. The proportion of M. separatella was higher in irrigated rice than upland. Three population peaks were observed annually in both upland and irrigated rice.

Studies on the relationship between irrigated rice phenology and field population of lepidopterous borers and a dipteran borer showed that the stalk-eyed fly, Diopsis spp., occurred before 20 days after transplanting (DAT) in ITA 212 and continued up to 90 DAT: the peak was observed at 40 DAT. The lepidopterous borers did not appear until 40 DAT and populations peaked at 80 DAT.

The relative resistance of rice varieties to Maliarpha separatella was studied during 1981 and 1982 long rain seasons. The observations covered the laying of egg masses, tiller infestation, incidence of “whiteheads” and, in some cases, the grain yield.

The results indicated that the varieties BG 34™8 and BG 90™2 received the highest egg masses per plot which apparently caused “whiteheads”. The remaining 10 varieties only indicated moderate response. The 1982 long rain season under no control and carbofuran 5G application regimes showed reduced tiller infestation only in BG 90™2 and Basmati 217. The five tested varieties did not show any marked difference in tiller infestation under the two regimes. Hence, BG 90™2 and Basmati 217 showed some susceptibility of M. separatella, while the rest of the varieties remained moderately resistant.

Moth population varied according to the growth stages of the rice crop in the field, with the peak occurring during the booting to maturation stages.

Greenhouse studies were conducted by artificially infesting potted rice plants with first instar larvae of Maliarpha separatella and Sesamia botanephaga Tarns and Bowden at two stages of plant growth, namely: early tillering stage and at the booting stage. Early infestation by M. separatella resulted in reduction of plant height and the number of filled grains. At the booting stage, the number of filled grains and their weight were higher in uninfested plants. Infestation by S. botanephaga at both stages of plant growth mainly affected the number of productive tillers. Grain weight was unaffected. Field studies on individual hills gave erratic results and there were instances when hills containing tillers bored by M. separatella produced higher yields than unattacked hills.

Preliminary observations were made in the attraction of moths of Chilo partellus Swin., Busseola fusca Fuller, Sesamia calamistis Hmps. and Maliarpha separatella Rag. to light traps and to traps baited with virgin females of these species. The observation was conducted from January 1981 to April 1982 at Mbita Point Field Station in western Kenya. M. separatella showed the highest attraction to light traps followed in descending order by C. partellus, E. saccharina, S. calamistis and B. fusca. The mean catches/trap/night in pheromone traps was highest for M. separatella, followed by C. partellus and B. fusca. The period of atttraction of the moths during the night and effect of moonlight on trap catches were also studied. Catches from light and pheromone traps indicated that attraction of C. partellus and M. separatella was greatest between 23:00 and 03:00hr. The effect of moonlight on light trap catches was greater than on pheromone trap catches.

Survey and experiments were conducted to investigate the distribution and bionomics of the rice stem-borers in Kenya. Stem-borers collected and identified included: Maliarpha separatella Rag., Diopsis thoracica West, Sesamia calamistis Hmps. and Chilo partellus Swin. Maliarpha separatella were most abundant and infested throughout the crop season, D. thoracica infested during vegetative phase. S. calamistis frequently occurred at the onset of flowering stage toward harvest. C. partellus occurred more frequently in upland than lowland rice and was at low population. Assessment of crop loss caused by M. separatella was carried out under field, caged condition. It was observed that the per cent of empty grains was affected by Maliarpha infestation. Calculated yield loss based on percentage empty grains and number of ‘whiteheads’ was higher than the actual differences between grain weight of uninvested and infested crops.

Maliarpha separatella (Rag) was found to infest the rice crop for the greater part of the crop's growth cycle. There was an extended peak of larval activity, and the build up of the larval population was correlated with stand establishment and stem development. The intensity of both tiller infestation and internode tunnelling was found to increase with the crop maturity. Some aspects of control measures of the pest as related to the insect-pest/host-plant relationship are discussed.

Surveys on the stem borer complex of rice were initiated at Mbita Point Field Station during 1980–1981. Stem borers collected and identified so far included Maliarpha separatella Rag; Diopsis thoracica West; Sesamia calamistis Hmps., and Chilo partellus Wlk. Among these four species, M. separatella was the most abundant and showed highest infestation during the reproductive and ripening phases of the rice crop. D. thoracica was found to cause greatest damage on the crop during the crop's tillering phase. It is suggested that there may be differences in Maliarpha infestation due to rice varieties.