Fritz Schulthess

A systems approach to assess crop health and maize yield losses due to pests and diseases in Cameroon

Abstract Three surveys of maize fields were undertaken in 1993 in Cameroon, ranging from low to high altitudes, and from forests in the south with bimodal rainfall (two cropping seasons) to the northern savanna with monomodal rainfall distribution (one cropping season). Field conditions, insects and diseases, plant growth and yield variables were assessed in 164 fields. Increased soil fertility (expressed as plant vigour) favoured Bipolaris maydis, Physoderma maydis , and Busseola fusca . There was a positive relationship between soil organic matter and B. fusca, Eldana saccharina and B. maydis . As organic matter decreased, percentage leaf area lost to pathogens, particularly Puccinia sorghi , increased. There was an inverse relationship between wild grasses around a field and stem borers in the field. Stem diameter, cob fill and weight increased with altitude reflecting the change from early maturing lowland cultivars to very late maturing mid- to high-altitude cultivars. Across ecological zones and surveys, B. fusca accounted for 95% of all the species found on maize, followed by E. saccharina . In the first cropping season, the mean percent borer infestation was similar in lowland and highland with a mean of 43%. Borer incidence was higher during the second cropping season. In both low and mid-altitude fields, 52–56% of the plants were infested, resulting in a calculated cob weight loss of 9 g per plant. At that time, the average plant loss from dead hearts across zones was 11%. B. maydis, Puccinia polysora , and Physoderma maydis predominated in the lowlands, and E. turcicum, P. sorghi , and Phaeosphaeria maydis occurred mainly in mid- to high-altitudes. Maize streak virus, stem diseases, B. maydis, Rhizoctonia sheath blight, and P. sorghi consistently had significant negative impacts on cob weight. The aetiology of stem lesions was not determined, but they significantly affected cob weight in the humid lowlands. Average reduction in cob weight from foliar and stem diseases in the different ecological zones and seasons ranged between 10 and 12 g per plant.

Diversity of lepidopteran stem borers on monocotyledonous plants in eastern Africa and the islands of Madagascar and Zanzibar revisited.

Surveys were completed in Eritrea, Ethiopia, Kenya, Madagascar, Mozambique, Tanzania, Uganda and Zanzibar to assess the lepidopteran stem borer species diversity on wild host plants. A total of 24,674 larvae belonging to 135 species were collected from 75 species of wild host plants belonging to the Poaceae, Cyperaceae and Typhaceae. Amongst them were 44 noctuid species belonging to at least nine genera, 33 crambids, 15 pyralids, 16 Pyraloidea species not yet identified, 25 tortricids and three cossids. The noctuid larvae represented 73.6% of the total number of larvae collected, with 66.3, 3.5 and 3.8% found on Poaceae, Cyperaceae and Typhaceae, respectively. The Crambidae, Pyralidae, Tortricidae and Cossidae represented 19.8, 1.9, 2.5 and 0.1% of the total larvae collected, respectively, with 90.4% of the Crambidae and Pyralidae collected from Poaceae, and 99.7% of the Tortricidae collected from Cyperaceae. The lepidopteran stem borer species diversity in the wild host plants was far more diverse than previously reported.

Exchange of natural enemies of lepidopteran cereal stemborers between African regions.

Difficulties of identification of natural enemies of cereal stemborers for ‘redistribution’ in Africa are discussed. Tritrophic level studies on wild and cultivated habitats of borers and beneficial species are needed to judge the impact of a natural enemy species in the cropping system. Areas with low pest pressure which are climatically favorable for pest development may be selected for study. Based on the results of various countrywide surveys to map the relative importance of Sesamia calamistis Hampson, Eldana saccharina (Walker) and Busseola fusca (Fuller) in western Africa, recommendations are given for sites for tritrophic level studies. It is hypothesised that because maize is not always present in the field and because of its high susceptibility, natural biological control has to come from wild habitats. This emphasises the importance of the knowledge on the wild host-plant range. Survey results complemented with oviposition and life-table studies in the laboratory showed that, rather than being reservoirs for pests, most wild grass species act as trap plants causing mortalities of 100%. A comparison of light trap catches with pupae found on wild hosts and the scarcity of known wild hosts in areas with high pest pressure suggest gaps in our knowledge of the range of host plant species. Based on comparison of known natural enemy complexes in East and West Africa, the scelionid egg parasitoid Telenomus isis and an East African strain of the braconid larval parasitoid Cotesia sesamiae (Cameron) are proposed for redistribution against B. fusca and S. calamistis, respectively. Telenomus isis has never been reported from East Africa whereas C. sesamiae is common in East and southern Africa and scarce in western Africa, suggesting that C. sesamiae is probably not adapted to S. calamistis and B. fusca in this region.RésuméLes difficultés d’identification des ennemis naturels des foreurs des tiges de céréales pour leur ’redistribution’ en Afrique sont discutées. On a besoin d’études du niveau tritrophique sur les habitats sauvages et cultivés des foreurs et des espèces utiles pour juger de l’impact des espèces d’ennemis naturels dans le système cultural. Des régions à basse pression de ravageurs qui sont climatiquement favorables au développement de ces derniers, pourraient être choisies pour l’étude. Sur la base des résultats des différentes enquêtes à travers le pays pour dresser une carte de l’importance relative de Sesamia calamistis Hampson, Eldana saccharina (Walker) et Busseola fusca (Fuller) en Afrique de l’Ouest, des recommandations ont été formulées quant aux sites pour des études du niveau tritrophique. Une hypothèse a été avancée selon laquelle, du fait que le maïs n’est pas toujours présent dans le champ et à cause de sa haute susceptibilité, le contrôle biologique naturel doit provenir des habitats sauvages. Ceci met l’accent sur l’importance de la connaissance sur la circonscription territoriale de la plante hôte sauvage. Les résultats de l’enquête complétés par des études sur la ponte et la table de survie au laboratoire ont montré qu’au lieu d’être des réservoirs pour ravageurs, la plupart des herbes sauvages agissent comme des pièges, causant des mortalités à 100%. Une comparaison des captures au piège lumineux avec les pupes trouvées sur les hôtes sauvages et la rareté des hôtes sauvages connus dans des régions à haute pression de ravageurs suggèrent qu’il y a des lacunes dans notre connaissance sur la circonscription territoriale des espèces de plantes hôtes. Sur la base de la comparaison des complexes d’ennemis naturels connus en Afrique Orientale et Occidentale, le scélionidé parasitoïde de oeufs Telenomus isis et un biotype Est Africain du braconidé parasitoïde des larves Cotesia sesamiae (Cameron) sont proposés pour redistribution respectivement contre B. fusca et S. calamistis. Le fait que Telenomus isis n’a jamais été rapporté en Afrique Orientale tandis que C. sesamiae est commun en Afrique Orientale et Australe et rare en Afrique Occidentale suggère que probablement C. sesamiae n’est pas adapté à S. calamistis et B. fusca dans cette region.

Effect of plant nitrogen and silica on the bionomics of Sesamia calamistis (Lepidoptera: Noctuidae)

The effects of nitrogen and silica application to maize plants on various aspects of the bionomics of the maize stem borer, Sesamia calamistis Hampson, were studied in the laboratory. Increasing nitrogen doses significantly increased larval survival (from 18.7% (control) to 37.3% at 2.25 g N/plant), larval weight (from 49.0 mg (control) to 99.5 mg at 2.25 g N/plant), and female fecundity (from 77 eggs per female (control) to 365 eggs per female at 1.69 g N/plant). A significant, positive relationship was found between moth fecundity and female pupal weight. Nitrogen had no effect on larval and pupal developmental time but increased adult longevity. Intrinsic rate of increase, r m , and the net reproductive rate, R o , were positively related to leaf and stem nitrogen, while generation time, G, was negatively related. Silica had the opposite effect from nitrogen on larval survival. Increasing silica supply reduced ultimate larval survival from 26.0% (control) to 4.0% at 0.56 g Si/plant. Immature developmental time, larval and pupal weight, pupal survival, female fecundity, egg viability and adult longevity of S. calamistis were not affected by silica application.

Geographic distribution and host plant ranges of East African noctuid stem borers

Abstract Surveys were carried out in Kenya, Tanzania, Uganda and Zanzibar to establish geographic distribution in the main vegetation mosaics and ecological (host plant range, feeding behaviour) characteristics of the East African noctuid stem borers. 49 wild plant species belonging to Poaceae, Cyperaceae and Typhaceae were found to harbour stem borers in the six vegetation mosaics surveyed. A total of 36 noctuid species belonging to nine genera were identified from 14,318 larvae collected, out of which 17 were new to science. The species diversity varied among vegetation mosaics and host plants. Most borer species appeared to be specialised feeders with 24 species being monophagous. Species belonging to the same types (named as the Busseola Thurau 1904 and the Sesamia Guenée 1852 types) or genus harboured common ecological characteristics such as pigmentation and feeding site. The Sciomesa Tams and Bowden 1953 genus was an exception as it had a mixture of these characters.

Pathogen incidence and their potential as microbial control agents in IPM of maize stem borers in West Africa

A review of the existing basis for maize stem borer IPM is given and the role of pathogens in the system is evaluated. Survey work outlining the major groups of insect pathogens is described; fungi (Beauveria bassiana and Metarhizium anisopliae), bacteria (Bacillus thuringiensis and Serratia marcesens), and viruses (granuloviruses and cytoplasmic polyhedroviruses) were identified. The presence of other unidentified protozoans, nematodes, fungi and viruses was noted. The virulence of some of the more promising known insect pathogens was explored in preliminary bioassays. Considering the cryptic habits of the insects, and the low input agriculture practiced by the majority of maize farmers in sub-Saharan Africa, Beauveria bassiana isolates possessing the capacity to grow systemically in the maize plant are considered one of the more interesting candidates for development as microbial control agents despite limited control in preliminary trials. Further work should also investigate the potential of pathogens of moderate virulence, such as the protozoans and CPVs.

Experimental evaluation of the efficiency of Epidinocarsis lopezi, a parasitoid introduced into Africa against the cassava mealybug Phenacoccus manihoti

The capability of Epidinocarsis lopezi (De Santis) (Hymenoptera: Encyrtidae) to control the cassava mealybug (CM) Phenacoccus manihoti Mat.‐Ferr. (Homoptera: Pseudococcidae) was investigated in Nigeria using physical and chemical exclusion experiments. In two sleeve cage experiments CM populations, about 2 months after artificial infestation, were 7.0 and 2.3 × lower on artificially infested cassava tips covered with open cages than on tips in closed cages which excluded most parasitoids. On similarly infested but uncovered tips, CM populations were 24.3 and 37.5 × lower, and parasitisation rates were higher. In an artificially infested field which was treated weekly with carbaryl, parasitisation rates were below 10% and CM populations exceeded 200 per tip. In the chemically untreated plot, parasitisation rates were up to 25% and CM densities were mostly below 10 per tip. This study demonstrates the efficiency of E. lopezi in controlling its host under the experimental conditions.

The effect of stem and cob borers on maize subjected to different nitrogen treatments

The influence of four nitrogen levels (0, 60, 90 and 120 kg N/ha) on growth of maize and development of lepidopterous pests was investigated in a field trial. Nitrogen had a positive effect on both plant growth variables (plant height, stem diameter and yield), and development and survival of Sesamia calamistis and Eldana saccharina, and thereby increased the incidence of dead hearts and stem tunneling. However, the percent yield loss due to artificial infestation decreased with increasing N application rate from 20% to 11% in the in the 0kg/ha and 120kg/ha treatment, respectively. Using a multiple regression analysis, plant height, plant diameter and stem tunneling were found to be the most important variables explaining the variability in maize yield.

Distribution, relative importance and effect of lepidopterous borers on maize yields in the forest zone and mid-altitude of Cameroon.

Abstract Geographic distribution, relative importance, and effect of lepidopterous stem and ear borers on maize, Zea mays L., were studied in the humid forest and the mid-altitude zones of Cameroon from 1994 to 1997. Six villages were chosen in the forest zone and grouped into three blocks representing gradients in human population density. A single block with three villages was chosen in the mid-altitude. Farmers’ maize fields were surveyed during the vegetative growth phase for Busseola fusca (Fuller) egg batches, and at harvest for number of larvae and pupae by species, plant damage, and ear weight. There was no significant block effect for any of the variables measured, and most of the overall variance (72–99%) was attributed to within-field variability. In the forest zone, the noctuid B. fusca and the pyralid Eldana saccharina (Walker) accounted for >80% of all species in almost all locations, followed by the pyralid Mussidia nigrivenella (Ragonot) and the tortricid Cryptophlebia leucotreta (Meyrick). The noctuid Sesamia calamistis (Hampson) was found in almost all locations during the first season but disappeared in most locations in the second season. B. fusca egg infestation was significantly higher during the second compared with the first season, whereas larval and pupal densities were much higher during the first season. E. saccharina was the predominant species during the second season, when densities increased fourfold. In the mid-altitude, B. fusca was the predominant species. No significant differences in pest densities, plant damage, and ear weight were found between years. Results of stepwise regression of stem and ear damage on pest densities verified the relative importance of the individual species. In the forest zone, ear and stem damage significantly reduced ear weight, whereas in the mid-altitude only stem tunneling was significant.

Lepidopteran cereal stemborers and associated natural enemies on maize and wild grass hosts in Cameroon

Abstract:  In Cameroon, the noctuid Busseola fusca is the most important pest of maize. The braconid Cotesia sesamiae, which is the most common larval parasitoid of noctuid stemborers in eastern Africa, was absent on B. fusca attacking maize. Thus, it is planned to introduce several strains of the parasitoid from Kenya. Pre‐release surveys were undertaken in major maize growing areas to catalogue stemborer species, and larval and pupal parasitoids on maize and four wild host plant species. On maize, B. fusca was the predominant borer in all ecozones except for the lowland coastal forest, usually accounting for 60–99%, followed by the pyralid Eldana saccharina in the forest zone and the crambid Chilo sp. in the mid‐altitudes. Contrary to what was reported before, the noctuid Poeonoma serrata– and not B. fusca– was the predominant borer on elephant grass, constituting 70–96% of all borers. On wild sorghum in the forest zone, the noctuid Sesamia poephaga was the most abundant species, while on Panicum sp., Chilo sp. predominated. On Setaria megaphylla in the forest zone, Chilo sp. was the most abundant species followed by Busseola quadrata. Busseola fusca was scarce on all wild grass species, indicating that previous reports on the predominance of this pest species on wild host plants were the result of misidentifications. Three tachinid and 16 hymenopteran parasitoids were obtained, most of them from B. fusca and P. serrata, on maize and Pennisetum purpureum respectively. C. sesamiae was scarce and never recovered from B. fusca on maize. In view of the new findings, acceptability and suitability studies involving the different stemborer species identified from wild plant hosts are required to determine if they will form a reproductive sink or perennate C. sesamiae populations during the off‐season when maize is scarce and B. fusca is diapausing.