Régis Babin

Field evaluation of synthetic sex pheromone traps for the cocoa mirid Sahlbergella singularis (Hemiptera: Miridae)

BACKGROUND Field trapping experiments were conducted to evaluate the attractiveness of five different pheromone blends as well as the impact of trap design on attractiveness towards cocoa mirids, Sahlbergella singularis Hagl. and Distantiella theobroma (Dist.), in Cameroon. RESULTS A total of 361 adults of S. singularis (359 males and two females) were caught. The highest numbers of mirids were found in traps with pheromone blends that combined a monoester and a diester, compared with traps with the diester or the monoester individually and control traps with no pheromone. Rectangular traps caught significantly more mirids compared with delta traps. The mean number of 5.1 mirids trap⁻¹ year⁻¹ caught in rectangular traps was significantly higher compared with the 1.8 mirids trap⁻¹ year⁻¹ for the delta traps. CONCLUSION The data revealed that rectangular traps containing pheromone blends combining both the monoester and the diester have a good potential to lure and trap adult males of S. singularis on cocoa farms. The pheromone blends used were specific for S. singularis, and the use of pheromone traps appears to be a promising strategy for incorporation into integrated pest management strategies for the monitoring or even the control of S. singularis in cocoa plantations.

Tree Spatial Structure, Host Composition and Resource Availability Influence Mirid Density or Black Pod Prevalence in Cacao Agroforests in Cameroon

Combining crop plants with other plant species in agro-ecosystems is one way to enhance ecological pest and disease regulation mechanisms. Resource availability and microclimatic variation mechanisms affect processes related to pest and pathogen life cycles. These mechanisms are supported both by empirical research and by epidemiological models, yet their relative importance in a real complex agro-ecosystem is still not known. Our aim was thus to assess the independent effects and the relative importance of different variables related to resource availability and microclimatic variation that explain pest and disease occurrence at the plot scale in real complex agro-ecosystems. The study was conducted in cacao (Theobroma cacao) agroforests in Cameroon, where cocoa production is mainly impacted by the mirid bug, Sahlbergella singularis, and black pod disease, caused by Phytophthora megakarya. Vegetation composition and spatial structure, resource availability and pest and disease occurrence were characterized in 20 real agroforest plots. Hierarchical partitioning was used to identify the causal variables that explain mirid density and black pod prevalence. The results of this study show that cacao agroforests can be differentiated on the basis of vegetation composition and spatial structure. This original approach revealed that mirid density decreased when a minimum number of randomly distributed forest trees were present compared with the aggregated distribution of forest trees, or when forest tree density was low. Moreover, a decrease in mirid density was also related to decreased availability of sensitive tissue, independently of the effect of forest tree structure. Contrary to expectations, black pod prevalence decreased with increasing cacao tree abundance. By revealing the effects of vegetation composition and spatial structure on mirids and black pod, this study opens new perspectives for the joint agro-ecological management of cacao pests and diseases at the plot scale, through the optimization of the spatial structure and composition of the vegetation.

Modeling Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) damages on cotton.

Abstract We studied and modeled damage caused by Helicoverpa armigera larvae on cotton with the aim of developing a coupled crop pest model. Two damage components were studied: the voracity (quantity of fresh matter and number of organs consumed) and feeding preferences (type of organ infested). The laboratory no-choice study of voracity on excised squares and bolls revealed that an H. armigera larva consumes 2,856 mg of fresh matter throughout its larval life, with the sixth instar consuming 86% of this quantity. This consumption rate corresponded to 23.6 squares, or 7.8 bolls. We developed equations to predict the quantity of fresh matter uptake from an individual plant organ, according to the organ mass and the larval instar. The field study of feeding preference confirmed previous findings that larvae prefer squares to bolls, with this preference decreasing as the larval age increases. However, no significant relationship was noted between the age of larvae and the size of infested organs within each organ class (square or boll). We developed a logistic model to predict the probability of a larva infesting a boll rather than a square. According to this model, the relative organ availability in the field and the larval instar were found to be significant factors.

Ant mosaics in cocoa agroforestry systems of Southern Cameroon: influence of shade on the occurrence and spatial distribution of dominant ants

In African cocoa agroforestry systems, ants represent the most important part of arboreal arthropod biomass and play a crucial role in ecosystem functioning. The more abundant are supposed to be involved in crop pest regulation. However, the lack of knowledge about ecological interactions between ants and their habitat, including other ant species of the community, leaded to contradictory views on their efficiency as potential biological control agents. In the present study, we studied the impact of shade trees on the occurrence and spatial distribution of the most abundant ants of cocoa agroforestry systems of Southern Cameroon. We characterized shade of four traditional cocoa farms and described distribution patterns of numerically dominant ants through spatial analysis. Our results showed that the populations of the most abundant arboreal species, in our study Oecophylla longinoda, Tetramorium aculeatum, Crematogaster spp., Camponotus spp. were generally aggregated in plantations. However, the distribution structure of ant populations was variable between samples, suggesting that the normal development of ant colonies may interfere with ecological constraints like interspecific competition and abiotic factors. Distribution mapping as well as statistical analysis showed that highest densities of O. longinoda were sheltered by cocoa trees in the sunniest areas of plots. On the other hand, Crematogaster species were usually strongly aggregated in the most shaded areas. Mechanisms involved in ant/shade relationships were discussed and we suggested that a good management of shade may take into account ant communities, which could play a significant role in conservation biological control against cocoa pests.

True Bug (Heteroptera) Impact on Cocoa Fruit Mortality and Productivity

ABSTRACT The real impact of true bug damage on cocoa pods has never been assessed precisely. We conducted a 2-yr study on 1,080 cocoa trees on 36 farms in Cameroon to assess the contribution of true bugs to fruit mortality and production loss. The cocoa fruiting cycle, fruit mortality, and damage caused by true bugs as well as other pests and diseases were monitored on a weekly basis. True bug damage also was described on 2,500 ripe pods per year. Pod weight, bean number, and bean weight were measured and compared for different degrees and types of damage on the ripe pods. Our results showed that true bugs were the main external cause of young fruit abortion. They reduced the abundance of young fruit by up to 10%. In contrast, although one-third of the ripe pods sampled had true bug lesions, only 4% were moderately to heavily damaged. The mean weight of ripe pods was reduced by 12% when there was medium to heavy damage. While the mean weight of wet beans was reduced significantly (by 3–10%), the number of beans per pod was not changed by damage. Despite the reduction in mean weight, the overall weight of beans for the pods sampled was reduced by <2%. Therefore, our study confirmed the common assumption that the economic impact of true bug damage on mature pods is negligible on cocoa farms in Cameroon. However, true bugs have a significant impact on young fruit mortality.

Developmental biology and demographic parameters of antestia bug Antestiopsis thunbergii (Hemiptera: Pentatomidae), on Coffea arabica (Rubiaceae) at different constant temperatures

The antestia bug Antestiopsis thunbergii (Gmelin 1790) is a major coffee pest in East Africa. Nymphs and adults feed on all vegetative and fruiting parts of the coffee tree leading to yield reduction and poor quality of coffee beans. Our study aimed to characterize the developmental biology and provide life table parameters for A. thunbergii reared in the laboratory. The biology of A. thunbergii was studied at four constant temperatures of 20,25, 30 and 35°C with 80 ± 5% RH and a photoperiod of L:D 12:12. Complete development of A. thunbergii from egg to adult occurred between 20 and 30°C, while eggs did not hatch at 35°C. Immature stage development time decreased significantly with an increase in temperature, with the exception of fifth nymphal stage, duration of which was similar for all temperatures. The fecundity was maximal at 20°C with an average of 132.8 eggs per female and 1.7 egg per female per day. The gross reproductive rate (GRR) was the highest at 20°C with 75.79 daughters per female compared to 19.56 and 2.69 daughters per female at 25 and 30°C, respectively. The intrinsic rate of increase r was maximal at 20°C with 0.013 and negative at 30°C. The time required for the reared population to double (doubling time Td) was shorter at 20°C with 53.31 days compared to 115.52 days at 25°C. Our study provides basic information on A. thunbergii biology that will contribute to a better understanding of the pest distribution and dynamics on arabica coffee in East Africa.

Effects of cocoa tree phenology and canopy microclimate on the performance of the mirid bug Sahlbergella singularis

Populations of the cocoa mirid, Sahlbergella singularis Haglund (Hemiptera: Miridae), on cocoa, Theobroma cacao L. (Malvaceae), display sharp seasonal fluctuations in population size. To better understand factors and mechanisms involved in seasonal variation, we studied the life history traits of 10 successive generations of a caged population under field conditions. Nymphal survival and development were assessed on pods stored under cocoa canopy, whereas female survival and fecundity were evaluated on sleeved pods in cocoa trees. At the same time, cocoa phenology and cocoa canopy microclimate were characterized. A life table analysis was conducted with survival and fecundity temporal data. Reproductive parameters showed far more variation than nymphal development parameters, both within and among generations. We obtained low intrinsic rates of natural increase (about 0.04), the variation of which was mainly linked to the decrease in fecundity. Fecundity as well as life table parameters varied in accordance with natural population fluctuations, suggesting that S. singularis seasonal variation might be mainly linked to variation in female fecundity. A multiple regression analysis showed that variation in female fecundity was first correlated with the presence of young fruits on cocoa trees. Fecundity was also weakly affected by daily minimum r.h. and daily mean temperature. The usefulness of our results for modeling of S. singularis population dynamics and for the improvement of IPM strategies is discussed.

Effect of genotype of cocoa (Theobroma cacao) on attractiveness to the mirid Sahlbergella singularis (Hemiptera: Miridae) in the laboratory.

BACKGROUND Mirids are a major constraint to cocoa growing in Africa. Cocoa breeding for tolerance/resistance could be effective in an integrated pest management system. Attractiveness is one aspect of tolerance/resistance, and decreasing the attractiveness of cocoa trees should be a good way of reducing damage. RESULTS Small-scale laboratory tests were carried out in Cameroon to assess differences in the attractiveness to mirids of eleven cocoa genotypes. The genotypes were ranked according to their attractiveness score and a distance from a mean value. An analysis of variance was performed and revealed significant differences between cocoa genotypes (F=3.15, P<0.001). The cocoa genotype groupings revealed three major categories, with BE10 and SNK413 proving to be the most attractive. In contrast, five genotypes, IMC60, the Catongo Trinitario genotypes, Playa Alta2 (from Venezuela), SIC5 and SNK614, proved to be less attractive than the mean. Four genotypes (PA107, SNK619, UPA134 and T60/887) displayed similar attractiveness to the mean. CONCLUSION The circular microtest offered the advantage of not needing a reference cocoa genotype. The least attractive clones, such as IMC60, were also the most tolerant in the field. Comparisons with the results of other studies are proposed.

Within-plant distribution of cotton boll-infesting lepidoptera: Application to Sampling

Abstract Sampling of Lepidoptera-infesting cotton bolls [Helicoverpa armigera (Hübner), Diparopsis watersi (Rothschild), Spodoptera littoralis (Boisduval), and Earias spp.] in Cameroon is based on examination of whole plants. To reduce the time taken by sampling procedures, we studied the suitability of a subsampling plan based on examination of plant terminals. The development of such a subsampling plan requires that the proportion of larvae located on plant terminals should remain constant. However, our study of the within-plant distribution of larvae revealed that their proportion on the five upper fruiting branches was too variable to allow the development of a sampling plan. Examination of the 10 upper branches led to less variable results, but the proportion was still significantly influenced by the pest species and time after crop emergence. We designed a conservative sampling plan in which the proportion was constant and equal to the lower bound of the 90% confidence interval for its predicted minimum (0.594). With this underestimation, the probability distribution of the number of larvae on the ten upper branches of a n plant sample was a compound of a negative binomial and a binomial distribution. These results enabled the design of a sampling plan that reduces sampling time by up to 60%, but the cost of this reduction is an increase in the risk in deciding on intervention when the mean infestation is lower than the critical density.

Rearing method and developmental biology of the African coffee white stem borer, Monochamus leuconotus (Coleoptera: Cerambycidae)

Abstract Although the African coffee white stem borer, Monochamus leuconotus (Pascoe) (Coleoptera: Cerambycidae), is a major insect pest of coffee in Africa, there is much to learn about its biology. The present paper describes an original rearing method for M. leuconotus and provides unpublished biological data for the pest. The coffee white stem borer colony was started with larvae extracted from infested coffee stems collected in the field. Larvae were reared on artificial diet while the adults were kept in reproduction cages with coffee sticks as feeding and oviposition substrate. Under our rearing conditions, mean adult longevity was 89.5 d for females and 81.8 d for males. Fecundity was 40.7 eggs per female on average, with an oviposition period of 53 d (one female laid 151 eggs for a period of 145 d). Egg viability was low (39%) and the incubation period was 26.6 d. Larval and pupal survival was high (80.9 and 88.5%, respectively). Larval development duration was variable, with a mean of 251.4 d, whereas pupal development was less variable, with a mean of 26.6 d. The rearing method described here is easily replicable and allowed a colony of coffee white stem borer to be continuously maintained in the laboratory for three years. Useful data on coffee white stem borers life history, including reproductive and feeding behaviors, are reported here and compared with previous reports in order to fill the knowledge gaps of this important but neglected pest of coffee.