Influence of insecticide treatment on ant diversity in tropical agroforest system: The case of Cameroonian cocoa farms

In order to understand the influence of anthropogenic disturbance on ant communities in tropical region; we conducted a study in southern Cameroonian cocoa farm. The survey aim to assess the influence of phytosanitary treatment on ant diversity and recolonization process of cocoa farm by ant species. During two consecutive years 2006-2007 and 2007-2008 ants sampling was performed using Chemical Knock-down technique in six traditional cocoa farms located in tree localities under different agro ecological environmental condition. Ant diversity was assessing using Shannon index of diversity and Sorensen index of similarity. After treatment: (1) the most abundant and most diverse sub-families were Formicinae and Myrmicinae; (2) at a local scale, with a small exception, specific richness of ant increase in plantations whereas diversity decrease; (3) at a regional scale, the specific richness increase and remain high both in forest (S= 46) and transitional (S= 46) areas compare to savannah (S= 42); diversity increase in transitional area whereas in forest and savannah diversities slide; (4) we have collected 10, 13 and 26 news ant species respectively in Bokito, Obala and Ngomedzap. Among the news establish species, Dorylus (Anoma) nigricans (Illiger) workers collected at Ngomedzap were the most abundant (36,36 %) and most frequent (53 %) on the tree; whereas among formerly establish species Oecophylla longinoda (Latreille) and Crematogaster gabonensis (Mary) were the most abundant respectively in Bokito (55,63 %) and Obala (31,48 %). Our result suggests formally that Phytosanitary treatment may affect ant diversity and favorise the establishment of some species like army ant D. (Anoma) nigricans (I.). In spite of the treatment certain ant species like O. longinoda (L.) and some species belonged to Crematogaster genus according to their nesting behavior may resist against treatment. This Knowledge may help to develop an integrate pest management program combine benefic ant species and selective insecticide spraying to protect tropical crops against scale insect. (Texte intégral

Diversity and mosaic structure of ant (Hymenoptera: Formicidae) community in traditional cocoa agroforestry systems in southern Cameroon

In cocoa agrosforestry systems, ants are the major arthropod component of the canopy. Since dominant ant species are usually active predators of insects. Ant community may contribute to protect cocoa against insect pests, and especially from cocoa mirids. On the other hand, ant activities may be involved in disease epidemics, and then lead to significant crop losses. In Cameroon, little information is available on ant community in cocoa agroforests and on their contribution to cocoa health. Therefore, the main objective of this study was to characterize the ant mosaic in cocoa farms by assessing the diversity of ant species, the nature of ant associations and their ecological statute. For two consecutive years, we collected ants using a chemical knock-down sampling method, in 21 cocoa plantations, situated in three different agro ecological areas of the center region of Cameroon. The ant diversity was assessed using the species richness, Shannon`s index of diversity and the Sorensen`s similarity index. Kendall`s correlation coefficient was used to determine the association nature between ant species. The robustness of associations was appreciated using chi-square test with Yates corrected test. Fifty one ant species were collected the first year of experimentation and sixty five the second year. Ants of the subfamilies Formicinae and Myrmicinae were the most abundant and the most diverse in cocoa plantations. According to the Shannon`s index of diversity, ant community was less diversified in the plantations located in the savannah compared to those of the forest areas. Oecophylla longinoda, Crematogaster clariventris, Crematogaster striatula and Camponotus acvapimensis were the most abundant species for the first year, with 50.9%, 35.2%, 21.1% and 14.1% individual respectively. The second year, O. longinoda, Tetramorium aculeatum, the army ant Dorylus (Anoma) nigricans and C. striatula were the most abundant, with rates of 82.2%, 44.1%, 39.2% and 29.4% respectively. Among the dominant species, O. longinoda and T. aculeatum are known to be profitable for cocoa. On the other hand, Crematogaster spp. are usually positively associated with aphids, mealybugs and scale insects, and may be involved in epidemics of the black pod rot of cocoa. The use of our results for integrated crop management strategies is discussed. (Texte intégral

Diversity and mosaic structure of ant (Hymenoptera: Formicidae) communities in traditional cocoa agroforestry systems in Southern Cameroon

In cocoa agroforestry systems, ants are the major arthropod component of the canopy. Since dominant ant species are usually active predators of insects, ant communities may help to protect cocoa against insect pests, and especially from cocoa mirids. On the other hand, ant activities may be implicated in disease epidemics, which can lead to significant crop losses. In Cameroon, little information is available on ant communities in cocoa agroforests and on their contribution to cocoa health. Therefore, the main objective of this study was to characterize the ant mosaic in cocoa farms by assessing the diversity of ant species, the nature of ant associations and their ecological statute. For two consecutive years, we collected ants using a chemical knock-down sampling method, in 16 cocoa plantations, situated in three different agro-ecological areas of the Centre region of Cameroon. Ant diversity was assessed using Shannon`s index of diversity and the Sorensen`s similarity index. Kendall`s correlation coefficient was used to determine the nature of the associations between ant species. The robustness of associations was appreciated using chi-square test followed by a Yates corrected test. Fifty one ant species were collected the first year of observation and sixty five the second year. Ants of the subfamilies Formicinae and Myrmicinae were the most abundant and the most diverse in cocoa plantations. According to the Shannon`s diversity index, ant communities were less diversified in the plantations located in savannah compared with forest areas. Oecophylla longinoda, Crematogaster clariventris, Crematogaster striatula and Camponotus acvapimensis were the most abundant species the first year, with 50.9%, 35.2%, 21.1% and 14.1% of trees colonized respectively. The second year, O. longinoda, Tetramorium aculeatum, the army ant Dorylus (Anoma) nigricans and C. striatula were the most abundant, with rates of 82.2%, 44.1%, 39.2% and 29.4%, respectively. Among the dominant species, O. longinoda and T. aculeatum are known to be beneficial to cocoa. On the other hand, Crematogaster spp are usually positively associated with aphids, mealybugs and scale insects, and may be involved in epidemics of the black pod rot of cocoa. The use of our results for integrated crop management strategies is discussed. (Texte intégral

Volatile compounds from coffee berries elicit responses in the coffee stink bug, Antestiopsis thunbergii (Heteroptera: Pentatomidae)

International trade of coffee from the great lakes region of Africa is threatened by the coffee stink bug, Antestiopsis thunbergii and its association with poor quality liquor, often called the “potato taste defect”. Previous studies have reported that immature coffee berries are strongly preferred by the pest unlike other coffee parts in the field. To test whether olfactory cues mediate the coffee bug's host selection process, behavioral and electrophysiological assays were conducted using headspace volatiles from coffee berries at two different stages of maturity. Additionally, coupled gas chromatography/mass spectrometry (GC/MS) was used to analyse the chemical composition of the volatiles emitted by the two coffee berry stages. In behavioural assays, whereas volatiles of immature berries attracted the coffee bug, volatiles of mature berries repelled it. Various chemical groups including; aldehydes, esters, hydrocarbons and terpenes which contribute to the divergent behavior of A. thunbergii towards coffee berries of different maturity stages and their possible use in pest management will be discussed. (Texte intégral

Field optimization of pheromone traps for monitoring and controlling cocoa mirids, Sahlbergella singularis

In Cameroon, pheromone traps appear interesting to monitor or even control Sahlbergella singularis populations on cocoa farms. The efficiency of traps baited with pheromones relies on a good knowledge of both visual and olfactory stimuli that attract cocoa mirids and on appropriate trap deployment in cocoa plantations. The aim of this preliminary study is first to compare the attractiveness of traps of different colors and baited with different pheromone blends. To achieve this purpose, we monitored a total of 90 traps of 3 different colors (30 red, 30 yellow and 30 white) and baited with 2 different pheromone blends, deployed in ten 1-ha plots (100 x 100m) PVC tubular traps were monitored from March on a weekly basis. Simultaneously, experiments in flight tunnel were conducted in the laboratory to confirm trends observed in the field. Secondly, to better understand how trap deployment influences mirid capture rate, we tested 3 trap densities in a total of twenty one 1-ha plots (100 x 100m) which received different treatments: 7 plots with a high trap density (16 traps ha-1), 7 plots with a medium trap density (9 traps ha-1) and 7 plots with a low trap density (4 traps ha-1). Traps were monitored on a weekly basis. Also, mirid populations and damage were assessed in each plot twice a year. Seven plantations with no trap were sprayed with insecticide according to spraying recommendations and were also monitored to evaluate the productivity gain or loss between the two control methods. Understanding the factors involved in oriented movements of mirids in the field, will help to formulate relevant recommendations to improve integrated management of S. singularis and potentially to reduce the economical cost of control strategies. (Résumé d'auteur

Spatial relationships between dominant ants and the cocoa mirid Sahlbergella singularis in traditional cocoa-based agroforestry systems

Manipulating ant communities to control pests of cocoa has proven to be a promising strategy, especially in Asia. However, concerning African cocoa mirids, the main pests of cocoa in Africa, basic knowledge on mirid-ant relationships is still incomplete. Our study aimed to characterize the spatial relationships between dominant ant species and the mirid Sahlbergella singularis (Hemiptera: Miridae) in traditional cocoa-based agroforestry systems of Cameroon. Over two consecutive years, mirid and ant populations were assessed by a chemical knock-down sampling method in four plots of 100 cocoa trees, located in three different agroecological zones in the Centre region of Cameroon. Mapping procedures were used to display spatial distribution of mirid and ant populations. Also, we adapted spatial statistics methodologies of point pattern analysis to consider the regular tree position effects on insect positions. These techniques allow testing the statistical significance of Poisson null models, leading to the classification of the spatial patterns of insects into association vs. segregation. Our results clearly demonstrated spatial segregation between mirid and the dominant weaver ant Oecophylla longinoda, known as a key-predator in natural ecosystems. Weaver ant trumps, especially its dominant status in ant communities of cocoa ecosystems and its ecological preferences for open habitats, make it a promising candidate for conservation biocontrol against cocoa mirids in Africa. (Résumé d'auteur

Abondance et diversité des peuplements d'Hémiptères des cacaoyères de la région du centre du Cameroun

Parmi les insectes retrouvés sur les cacaoyers, l'ordre des hémiptères est le groupe d'insectes le mieux représenté et le plus préjudiciable à la culture du cacaoyer au Cameroun. Pourtant, à l'exception des mirides, peu d'information a été collectée à leur sujet. Cette étude se propose de caractériser les peuplements d'hémiptères des cacaoyères de la région du centre du Cameroun en évaluant leur abondance et leur diversité. Un inventaire des hémiptères a été mené dans les plantations cacaoyères de 3 sites en zone forestière, en zone de savane et dans la zone de transition forêt-savane. Environ six mille cacaoyers ont été échantillonnés entre 2006 et 2008, selon deux méthodes: une collecte à vue des hémiptères et un traitement de lessivage par atomisation d'un produit insecticide. Les hémiptères collectés ont été identifiés et dénombrés au laboratoire. La diversité, l'abondance et les différences entre les sites ont été évaluées par l'indice de diversité de Shannon , l'équitabilité et l'indice de similarité de Sorensen. Cent cinquante espèces d'hémiptères ont été collectées dans les plantations, dont 10% sont connues pour être nuisibles aux cacaoyers. Sahlbergella singularis (Hemiptera, Miridae) et Atelocera serrata (Hemiptera, Pentatomidae) étaient de loin les espèces d'hétéroptères les plus communes. Chez les homoptères, Mesohomotoma tessmanni (Hemiptera, Psyllidae) et Stictococcus spp. (Hemiptera, Stictococcidae) étaient les espèces les mieux représentées. L'abondance et la diversité des hémiptères ont montré une variabilité importante entre les parcelles. Toutefois, les trois types d'agrosystèmes ont présenté une vingtaine d'espèces en commun. Les résultats obtenus nous amènent à discuter de l'influence de certains facteurs agro-écologiques, tels que la protection phytosanitaire, ou la richesse floristique des plantations, sur la diversité de l'entomofaune du cacaoyer. (Texte intégral