Spodoptera frugiperda Smith (Lepidoptera: Noctuidae) in Cameroon: case study on its distribution, damage, pesticide use, genetic differentiation and host plants

Open Access JournalMaize farmers in sub-Saharan Africa recently experienced unusual damage in their farms, attributed to the fall armyworm (FAW) Spodoptera frugiperda (J. E. Smith). This pest was first recorded in Africa in 2016, but detailed information on its distribution and damage and farmer's response in invaded areas are largely lacking. In this study, we determined FAW distribution, genetic diversity, host plants, crop damage, and farmers' responses. S. frugiperda was recorded in the 10 regions of Cameroon. Average percentage of infested plants and damage severity (on a scale of 1 to 5) were lowest-20.7 ± 7.4% and 2.1 ± 0.1 respectively-in the Sahelian regions and greatest-69.0 ± 4.3% and 3.1 ± 0.1 respectively-in the Western Highlands. Altitude did not influence FAW incidence and severity and its larvae infrequently co-occurred with maize stemborers on the same plants, suggesting possible direct and/or indirect competition between the two groups of maize pests. In response to this new threat to maize production, farmers have opted for the application of synthetic pesticides. Although our experiments were not designed to determine pesticide efficacy, as parameters such as time since application were not considered, our observations suggest lack of a drastic effect on S. frugiperda infestations on maize. There were two haplotypes of FAW co-occurring in Cameroon corresponding to the rice and corn strains and separated by 1.7% sequence divergence, which does not support the existence of cryptic species. S. frugiperda larvae were also recorded on Sorghum bicolor (L.) Moench (10.6%), Solanum tuberosum L. (2.8%), Ipomoea batatas (L.) Lam. (1.9%), Saccharum officinarum L (0.8%), Phaseolus vulgaris L. (0.4%) and Gossypium hirsutum L. (1.9%). This study show that two strains are present in all agroecological zones in Cameroon, and probably in neighboring countries of central Africa sharing the same agroecologies. Management options should therefore consider the use of specific natural enemies and an informed decision of intervention based on strain capture and damage threshold, to avoid pesticide resistance that may arise from inadequate use of chemicals. Further studies should also be undertaken to assess the response of the two S. frugiperda strains to biopesticides and botanical insecticides

Parasitism of Lepidopterous Stem Borers in Cultivated and Natural Habitats

Plant infestation, stem borer density, parasitism, and parasitoid abundance were assessed during two years in two host plants, Zea mays (L.) (Cyperales: Poaceae) and Sorghum bicolor (L.) (Cyperales: Poaceae), in cultivated habitats. The four major host plants (Cyperus spp., Panicum spp., Pennisetum spp., and Sorghum spp.) found in natural habitats were also assessed, and both the cultivated and natural habitat species occurred in four agroecological zones in Kenya. Across habitats, plant infestation (23.2%), stem borer density (2.2 per plant), and larval parasitism (15.0%) were highest in maize in cultivated habitats. Pupal parasitism was not higher than 4.7% in both habitats, and did not vary with locality during each season or with host plant between each season. Cotesia sesamiae (Cameron) and C. flavipes Cameron (Hymenoptera: Braconidae) were the key parasitoids in cultivated habitats (both species accounted for 76.4% of parasitized stem borers in cereal crops), but not in natural habitats (the two Cotesia species accounted for 14.5% of parasitized stem borers in wild host plants). No single parasitoid species exerted high parasitism rates on stem borer populations in wild host plants. Low stem borer densities across seasons in natural habitats indicate that cereal stem borer pests do not necessarily survive the non-cropping season feeding actively in wild host plants. Although natural habitats provided refuges for some parasitoid species, stem borer parasitism was generally low in wild host plants. Overall, because parasitoids contribute little in reducing cereal stem borer pest populations in cultivated habitats, there is need to further enhance their effectiveness in the field to regulate these pests

Species composition and seasonal dynamics of lepidopterous stemborers on maize and the elephant grass, Pennisetum purpureum (Moench)(Poaceae), at two forest margin sites in Cameroon

Lepidopterous stem borers were monitored on plots of maize, Zea mays, and elephant grass, Pennisetum purpureum (Moench), during two consecutive cropping seasons and the off-season at two forest margin locations, Nkolbisson and Minkomeyos, in Cameroon. Busseola fusca (Fuller), Sesamia calamistis (Hampson) (Lepidoptera: Noctuidae) and Eldana saccharina Walker (Lepidoptera: Pyralidae) were found on both hosts. In addition, Cryptophlebia leucotreta (Meyrick) (Lepidoptera: Tortricidae) and Mussidia nigrivenella Ragonot (Lepidoptera: Pyralidae) were collected from maize, and Poeonoma serrata (Hampson) (Lepidoptera: Noctuidae) from elephant grass. Busseola fusca was the most abundant species on both host plants. The numbers of eggs per square metre on different plant hosts was not significantly different, whereas numbers of larvae and pupae were 3-9 times higher on elephant grass than on maize. Analysis of numbers of B. fusca larvae according to size/age classes indicate that all sizes tended to be more abundant on elephant grass than on maize, but significant differences were found only during the second season at Minkomeyos, when densities were high. No significant differences were found between host plants for diapausing larvae and pupae. The implications for the possible function of the grass as a trap plant or reservoir for natural enemies in the management of B. fusca are discussed

Spatial dynamics of lepidopterous pests on Zea mays (Linnaeus) and Pennisetum purpureum (Moench) in the forest zone of Cameroon and their implications for sampling schemes

common lepidopterous borers attacking maize and/or the wild host Penni.l'etum purpureum in the forest zonc of Camcroon arc the noctuids Buss('ola.li/sca (Fuller), Sesamia calamistis (Hampson) and Poeol1oma serrata (Hampson), the pyralids Eldani! s{{cc/wrin{{ (Walker) and Mus.l'idia nigrivel1ella (Ragonot), and Cryptophlehia leucotreta (Tortricidae). The within-plant distribution on maize and elephant grass was studied for the predominant species B . .Iil.\·('(l, and on maize only for E. s{{ccharilla to determine the basic sampling unit. On both plant species, B . .Ii/sea showed a strong oviposition preference for young plant parts. By contrast, E. .I'acc/wrina larvae and pupae on maize were only found on older plant parts indicating that it does not oviposit on young plants. No part of the plant strata appeared to be a stable sampling unit and it is recommended to carry out whole plant or whole tiller sampling for maize and grass, respectively. For the development of sampling plans, dispersion was described for all species using Taylor's power law and a non-linear model which gives the relationship between the proportion of infested plants [P(J)] and mean density (/11). Busseo/{{ fi/sca egg batches as well as diapausing larvae and pupae on maize showed a random distribution whereas all the other cases were aggregated, with B. fu.l'c([ egg batches on elephant grass exhibiting the lowest and M. lligriv(,I1('lIa on maize the highest aggregation. Optimal sample size/mean density curves were developed for groups of insects with low and high aggregation

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

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

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

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