William G. Meikle

Grain injury models for Prostephanus truncatus (Coleoptera: Bostrichidae) and Sitophilus zeamais (Coleoptera: Curculionidae) in rural maize stores in West Africa.

Abstract Prostephanus truncatus (Horn) and Sitophilus zeamais Motschulsky have been reported as the two most serious pests of stored maize in sub-Saharan Africa and smallholder farmers are in urgent need of guidelines for their proper management. In this article we investigate the injury rates attributable to these two species in terms of percentage weight loss and percentage grain damage, and we derive functional response models for the two species on maize. The models successfully described the progression of grain injury in an extensive data set compiled from previously published studies, comprising 46 time series of data relating maize injury and insect pest density. The grain injury models can be used in conjunction with predictive models of pest population dynamics to guide the development of integrated management strategies for postharvest maize pests in West Africa and comparable regions elsewhere.

Sensitivity of Prostephanus truncatus (Coleoptera: Bostrichidae) Flight Activity to Environmental Variables in Benin, West Africa

Abstract Based on pheromone trap catches, a model of weekly Prostephanus truncatus (Horn) flight activity was generated for southern Benin. Using response surface regression, the following environmental variables were examined: number of rainy days per week, precipitation, minimum and maximum temperatures, minimum relative air humidity, and daylength. A time-variable, year, was included to account for the variance between years. From step-wise exclusion of variables with the lowest contribution to the model fit, a model was generated which included three environmental variables (daylength, minimum relative air humidity, and minimum temperature) that explained 55% of the total variance, and the yearly variable explaining 8%. The response surface regression analysis of P. truncatus flight activity revealed the following: (1) it was positively correlated with daylength when daily minimum temperature and relative air humidity were low, (2) it was positively associated with minimum relative air humidity when lower than 75%, (3) it was negatively associated with minimum temperature, (4) unexplained yearly variation was important for the predictive strength of the model, (5) interactions of environmental variables contributed substantially to the model fit, and (6) precipitation, both as mm rain and as number of rainy days, had little influence on P. truncatus flight activity. Independent data showed that the model predicted P. truncatus flight activity well elsewhere in southern Benin, whereas in central Benin new coefficients for the same environmental variables were needed to produce an adequate prediction. The model did not fit pheromone baited trap catches from northern Benin.

The effects of maize variety on the density of Prostephanus truncatus (Coleoptera: Bostrichidae) and Sitophilus zeamais (Coleoptera: Curculionidae) in post-harvest stores in Benin Republic

Abstract Maize varietal characteristics were evaluated in the field and in the laboratory for their efficacy in providing resistance to storage pests, in particular Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae) the larger grain borer, and Sitophilus zeamais (Motsch.) (Coleoptera: Curculionidae) the maize weevil. Resistance appeared to be associated more with the husk cover than with the grain. Higher-yielding varieties, even with “hard” flinty kernels, tended to suffer high P. truncatus damage, possibly due to the quality of the husk cover. Varietal susceptibility to S. zeamais did not appear to be associated with husk cover. Most damage by storage pests occurred later in the season, and damage was most strongly associated with P. truncatus density. An ideal maize breeding programme should include the development of maize varieties able to resist insect attack for a long storage season, in addition to varieties with a high yield.

Integrated pest management in post-harvest maize: a case study from the Republic of Togo (West Africa)

Abstract A large-scale experiment on maize storage systems was carried out in Atakpame (Plateaux region of Togo), between autumn 1996 and spring 1997. An integrated pest management (IPM) approach based on research findings at the International Institute of Tropical Agriculture (IITA) and intended to control insect pests in rural maize stores was compared with (1) the locally prevalent methods of storage of farmers from the Plateaux region and (2) with the storage system recommended by the National Plant Protection Service in Togo (SPV). Two storage systems were designed based on IPM principles, one using a local variety (chosen mainly for its excellent husk cover characteristics), the second including an improved variety (with higher yield and moderately good husk cover characteristics), and both depending on visual selection of cobs at harvest to reduce initial insect infestation. The experiment was conducted in a collaborative manner, including representatives of the SPV, farmers from the Plateaux region, and researchers from IITA. Population dynamics of major insects and associated losses to stored maize were monitored monthly over an 8-month period, while an on-site evaluation of pest infestation and losses was conducted after 6 and 8 months of storage. The performance of the different storage systems was evaluated in terms of pest densities, grain weight losses and cost-efficiency. The IPM-based systems were as effective as the pesticide-based SPV system in reducing infestation and grain losses. When only the incremental net returns from the storage practices were considered, both the IPM approach and the recommendations of the SPV performed better than the farmers’ methods of storage. However, when the yields (per hectare) of the different maize varieties used in the respective storage systems were included in the cost and return analysis, the IPM storage systems using the local variety performed worse.

Seasonal and weather factors influencing the annual flight cycle of Prostephanus truncatus (Coleoptera: Bostrichidae) and its predator Teretriosoma nigrescens (Coleoptera: Histeridae) in Benin

Investigations were carried out in southern Benin on the annual flight cycle and the effects of weather variables on the flight activity of Prostephanus truncatus (Horn) and its natural enemy, Teretriosoma nigrescens Lewis. Two seasonal peaks in flight activity of P. truncatus were observed, one between the end of December and the beginning of January and a second one between May and June. Teretriosoma nigrescens showed a single delayed peak in June, approximately six weeks after the major peak of P. truncatus. Flight activity of P. truncatus was only weakly associated with weather characteristics (standardized regression coefficient for mean daily temperature b = 0.18, t = 2.87, P<0.05)), whereas for T. nigrescens it was associated with precipitation (standardized regression coefficient for accumulated rainfall during the trapping period b = 0.38, t = 4.76, P < 0.05). The possibility that one of the P. truncatus peaks was associated with dispersal from crowded maize stores and the other with the search for natural woody host plants is discussed.

Infestation of maize by Prostephanus truncatus initiated by male-produced pheromone

Delta traps baited with maize cobs, which were infested each with one male Prostephanus truncatus (Horn) (Col.: Bostrichidae), were distributed in southern Benin and collected after one, two, three and four weeks. The numbers of P. truncatus caught during the different trapping periods were not significantly different. Sixty‐four percent of the trapped P. truncatus were females. Females attracted during the one‐week trapping period produced a mean of 6.9 progeny during the seven days. The sex ratio of the progeny was 1:1. Trap catches with the infested cobs were on average 13 times lower than catches with 2 mg of the artificial pheromone. Estimation of P. truncatus densities in a maize store at the beginning of the storage period (based on laboratory data) revealed that small initial numbers of P. truncatus, possibly attracted by a single male, sufficed to initiate high infestation rates later in the storage season.

Successional sequence of forest types in a disturbed dry forest reserve in southern Benin, West Africa

Human pressure on forest resources in southern Benin has led to a dramatic reduction and disturbance of natural forest patches. The largest remaining forest reserve in southern Benin is the Lama forest, which is known to be a reserve for many endangered species and situated in the boundary between dry forest and savanna. From cluster analysis and hierarchical classification of tree species composition, five forest types were identified in the Lama forest, and ordination techniques were used to determine to what extent these forest types were associated with physiognomic tree characteristics. The geographical position of vegetation plots was used to examine the spatial distribution of the identified forest types. The forest types were interpreted as levels in a successional sequence ranging from either disturbed forest or formerly cultivated farmland to dense forest. Based on the outlined succession sequence it was suggested that reforestation should focus on establishment of Anogeissus leiocarpus, Albizia zygia, Lannea nigritana, Ficus exasperata and Lonchocarpus sericeus as these tree species are associated with the initial forest types.

Evaluation of Sequential Sampling Plans for the Larger Grain Borer (Coleoptera: Bostrichidae) and the Maize Weevil (Coleoptera: Curculionidae) and of Visual Grain Assessment in West Africa

Abstract Repeated sampling of rural maize stores in Benin was conducted toevaluate published parameters of a sequential sampling plan for anegative binomial distribution to determine pest status forProstephanus truncatus (Horn) and Sitophiluszeamais Motschulsky. A computer program was used to rerandomizethe data and evaluate the effects, in terms of average sample numberand error rates, of different sampling plan parameter values. Withrespect to P. truncatus, lower and upper thresholds of 0.2and 1.0 insects per ear and parameter values of k = 0.2and α = β = 0.1 were found to be adequate. With respectto S. zeamais, lower and upper thresholds of 10 and 20insects per ear and parameter values of k = 1.0 andα = β = 0.1 were found to be adequate. Simplifiedsampling rules were proposed in which 11 ears should be sampled and ifno P. truncatus are found, the population is low; otherwisethe Wald plan should be followed. Owing to the lower per capita rate ofdamage, effective simplified sampling rules for S. zeamaiswere difficult to construct. An evaluation of the visual assessmentscale using whole ears showed that a visual scale estimating percentagedamage rather than percentage loss, might be easy to construct andpreferable for traders. Further work is needed to improve theusefulness of the visual scale in pest management decision support.

Studies on predation of Prostephanus truncatus (Horn) (Col., Bostrichidae) and Sitophilus zeamais Mots. (Col., Curculionidae) at different densities on maize by Teretriosoma nigrescens Lewis (Col., Histeridae)

Laboratory experiments using whole cobs were conducted to examine the effect of varying densities of the larger grain borer, Prostephanus truncatus and the maize weevil, Sitophilus zeamais on rate of population increase by the histerid beetle, Teretriosoma nigrescens, a predator primarily of P. truncatus. Densities of all species of insects were determined at the end of the experiment, and an electrophoretic analysis of gut content was conducted on larval and adult T. nigrescens sampled during the experiments. Results indicated that T. nigrescens has a strong preference for P. truncatus and densities of T. nigrescens were associated only with densities of P. truncatus. The maize weevil played little role as an alternative prey or in interfering with T. nigrescens reproduction.

Intraspecific competition in larvae of the larger grain borer, Prostephanus truncatus (Horn) within maize grains

The effects of egg clutch size on development and survivorship of the larger grain borer, Prostephanus truncatus (Horn) (Col.: Bostrichidae) on maize were measured in the laboratory using single grains of the white maize variety, TZSR-W, at 30 ± 2°C and 70 ± 5% RH. Varying numbers of P. truncatus eggs (1, 2, 4, 8, or 16) were introduced into a hole drilled into a single maize grain. Destructive sampling was used at regular intervals to obtain data on immature developmental parameters, the location of immatures within the grain and the weight and sex of any emerged adults. First instars fed mainly on the floury endosperm tissue whereas the second and third instars preferred the germ tissue. Mortality due to competition was highest in first instars. At high initial densities (> 4 per kernel), some P. truncatus larvae reduced competition by moving out of the grain. A maximum of 6 adults emerged from a single grain. The mean number of adults that emerged per grain for initial egg densities of 8 and 16 were 3.3 and 3.5, respectively. Prostephanus truncatus adult weight at emergence was not significantly influenced by initial egg density except in the case of clutch size 16. The sex ratio of emerged adults was also unaffected by competition, and was always 1:1. Complete developmental period within grains ranged between 28 and 32 days.RésuméLes effets de la taille de la fournée d’oeufs déposés par le grand capucin, Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae), sur son développement et sa survie, ont été étudiés au laboratoire sur les graines isolées du maïs blanc, variété TZSR-W, à une température de 30 ± 2°C et une humidité relative de 70 ± 5%. Un nombre différent d’oeufs (1,2, 4,8 ou 16) de P. truncatus a été introduit dans un trou foré dans chaque graine de maïs. Une méthode destructive a été utilisée à des intervalles réguliers pour recueillir les données sur les paramètres de développement larvaire, la localisation des stades immatures à l’intérieur de la graine ainsi que le poids et le sexe des adultes émergés. Les premiers stades larvaires se nourrissaient principalement de l’endosperme tandis que les second et troisième stades larvaires préféraient se nourrir sur le germe. La mortalité due à la compétition nutritionnelle était très élevée chez les larves du premier stade. A des densités initiales élevées (> 4 individus par grain), certaines larves de P. truncatus évitaient cette compétition en sortant de la graine. Un maximum de 6 adultes a émergé d’une seule graine. Avec des densités initiales de 8 et 16 oeufs par graine, la moyenne d’adultes émergés étaient respectivement de 3,3 et 3,5. Le poids moyen des adultes à l’émergence n’était pas significativement influencé par la densité initiale d’oeufs, sauf dans le cas de 16 oeufs par graine. Le rapport de mâles et femelles adultes n’était pas affecté par la compétition et il est resté tout le temps de 1:1. Le développement complet de l’insecte à l’intérieur des graines se rangeait entre 28 et 32 jours.