Impact of Spodoptera frugiperda (Lepidoptera: Noctuidae), on maize yield in humid tropical zones of Central Africa.

Fall armyworm, Spodoptera frugiperda Smith, became the most important maize pest in Africa in 2016, with management based on chemical pesticides. High yield losses across the continent were predicted based on farmers' perceptions, but existing agroecological differences were not considered. In the Democratic Republic of Congo, experiments were conducted to assess fall armyworm damage and yield losses in maize farms with and without treatment. The study included 2 seasons in the Kipopo wetland in 2020 and 2021, one rainy season in Kanyameshi in 2021, 2 rainy seasons in Mulungu in 2020 and 2021, and one season in a wetland on the Bishibiru site in 2020. In addition, the research was also conducted at 4 sites in Cameroon from September to December 2020 and from March to July 2021. High levels of damage incidences were recorded, but the density of larvae per plant was low, with low to moderate levels of damage severities in different seasons and sites. Treatment significantly reduced the number of fall armyworm larvae and their damage severity. However, the high infestation levels did not significantly reduce yield. Cost-benefit ratios were either negative or, in most cases, less than 1. In the best case, the use of pesticides only allowed the recovery of the amount used for the intervention. We discussed the implications of these findings for fall armyworm management in humid tropical agroecology.

Natural Enemies of Fall Armyworm Spodoptera frugiperda (Lepidoptera: Noctuidae) in Different Agro-Ecologies.

Fall armyworm (FAW) Spodoptera frugiperda (J.E. Smith) and southern armyworm (SAW) Spodoptera eridania (Stoll) have become major threats to crops in Africa since 2016. African governments adopted emergency actions around chemical insecticides, with limited efforts to assess the richness or roles of indigenous natural enemies. Field surveys and laboratory studies were conducted to identify and assess the performance of parasitoids associated with spodopterans in Cameroon. FAW was the most abundant spodopteran pest. Telenomus remus (Nixon), Trichogramma chilonis (Ishi), Charops sp. (Szépligeti), Coccygidium luteum (Cameron), Cotesia icipe (Fernandez & Fiaboe), and Cotesia sesamiae (Cameron) are the first records in the country on spodopterans. Telenomus remus, T. chilonis, C. icipe, and Charops sp. were obtained from both FAW and SAW; C. luteum and C. sesamiae from FAW. The distribution of spodopterans, their endoparasitoids, and parasitism rates varied with host, season and location. In the laboratory, T. remus showed significantly higher parasitism on FAW than SAW, and significant differences in the development parameters between the two host eggs, with shorter development time on FAW. It induced significant non-reproductive mortality on FAW but not on SAW. Developmental parameters showed that C. icipe has a shorter development time compared to other larval parasitoids. Implications for conservative and augmentative biocontrol are discussed.

Integrated Management of Aphis craccivora in Cowpea Using Intercropping and Entomopathogenic Fungi under Field Conditions.

Cowpea aphid, Aphis craccivora, is a major cowpea pest. Cowpea-cereal intercrop alone does not effectively manage the pest. Use of pesticides in intercrop leads to health and environmental risks. Fungal-based biopesticides offer a better option because they are environment- and consumer-friendly. This study assessed the combined effect of Metarhizium anisopliae ICIPE 62 and cowpea-maize intercrop against A. craccivora under six treatments: (1) untreated cowpea monocrop, (2) untreated cowpea-maize intercrop, (3) cowpea monocrop + ICIPE 62, (4) cowpea-maize intercrop + ICIPE 62, (5) cowpea monocrop + Duduthrin insecticide, and (6) cowpea-maize intercrop + Duduthrin during three seasons (long rainy/cold and dry/short rainy). In the cold and dry season, cowpea-maize intercrop treated with ICIPE 62 recorded the lowest infestation/cowpea damage, whereas the leaf yield was comparable to cowpea monocrop treated with ICIPE 62. In the short rainy season, the cowpea-maize intercrop treated with ICIPE 62 recorded the lowest infestation/damage, whereas leaf yield was similar to cowpea-maize intercrop treated with ICIPE 62 in the cold and dry season. Duduthrin in monocrop and intercrop did not reduce aphid infestation/cowpea damage levels in all the seasons. Although the efficacy of M. anisopliae ICIPE 62-based biopesticide could be affected by seasons, it successfully controlled aphid population in cowpea-maize intercrop under field conditions without affecting aphid-associated natural enemies.

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

Maize 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.

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

[This corrects the article DOI: 10.1371/journal.pone.0215749.].

Evaluation of the Entomopathogenic Fungi Metarhizium anisopliae, Beauveria bassiana and Isaria sp. for the Management of Aphis craccivora (Hemiptera: Aphididdae).

Cowpea, Vigna unguiculata ((L.) Walp; Fabales: Fabaceae), is an important indigenous vegetable and grain legume in the tropics where it represents a major diet component. Cowpea aphid, Aphis craccivora (Koch; Hemiptera: Aphididdae) is a major pest causing up to 100% yield losses. Aiming at establishing alternative approach to synthetic insecticides, we evaluated the pathogenicity of 23 fungal isolates including Metarhizium anisopliae ((Metschn.) Sorokin; Hypocreales: Clavicipitaceae), Beauveria bassiana ((Bals.) Vuill.; Hypocreales: Cordycipitaceae), and Isaria sp. (Hypocreales: Cordycipitaceae) against adult A. craccivora in the laboratory. Adult apterous aphids were sprayed with conidial suspensions titred at 1 × 108 conidia ml-1 for pathogenicity tests while 1 × 104, 1 × 105, 1 × 106, 1 × 107 and 1 × 108 conidia ml-1 were used in dose response bioassays. All the fungal isolates were found pathogenic to A. craccivora, causing mortality of between 34.5 and 90%. The lethal 50% mortality time (LT50) values varied between 3.3 and 6.3 d, with the best isolates being ICIPE 62, ICIPE 41 and ICIPE 644. The lethal concentration mortality (LC50) values were 2.3 × 106, 1.3 × 108 and 1.3 × 109 for ICIPE 62, ICIPE 41, and ICIPE 644, respectively. M. anisopliae isolate ICIPE 62 produced more conidia on aphid cadavers (4.5 × 107) than ICIPE 41 (2.7 × 107) and ICIPE 644 (2.1 × 107) 6 d post-treatment. Relative potency comparison showed that ICIPE 62 was more potent than the other two isolates. In the screenhouse, conidia of ICIPE 62 significantly reduced A. craccivora population compared to control but there was no significant difference between emulsifiable and aqueous formulations. Small-holder leafy vegetable producers could gain more profits using fungal-based biopesticides in Aphid-IPM strategies, leading to reduction of pre-harvest intervals after their application compared to synthetic insecticides.