R. H. Markham

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.

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.

Resistance in Accessions of Cowpea to the Coreid Pod-Bug Clavigralla tomentosicollis (Hemiptera: Coreidae)

Abstract Laboratory and screenhouse experiments were conducted to identify antibiosis and tolerance in four wild accessions of cowpea, Vigna unguiculata subsp. dekindtiana, to Clavigralla tomentosicollis Stål, the most damaging pod-sucking bug on cultivated Vigna genotypes in West Africa. The wild accession TVnu 151 showed antibiosis resistance, causing >50% mortality of the nymphs within 3 d of placing them on pods. Nymphs died more quickly on TVnu 151 than on TVnu 72, the wild and resistant control of the V. vexillata species which affected the weights of surviving insects to a much greater degree than TVnu 151. The three other accessions of the subspecies dekindtiana (TVnu 369, TVnu 517, and TVnu 707) did not cause significant mortality to the bugs, but rather extended their developmental time, with surviving adults showing lower weights and slower oviposition rates than those on the susceptible control IT84S-2246. Wild accessions affected male and female bugs differently, resulting in differential survival. This was reflected by the sex ratio which was male biased on the wild accessions (1:0.3–1:0.9), and female biased on IT84S-2246 (1:1.5). No evidence of tolerance was found in the four accessions of V. unguiculata subsp. dekindtiana. Overall, seed traits seemed to be the major resistance component in these wild accessions.

Host-finding behaviour of Prostephanus truncatus (Horn) (Col., Bostrichidae): primary attraction or random attack?

Host‐finding behaviour of Prostephanus truncatus was evaluated in a four‐choice olfactometer for reactions to odours of maize, cassava chips, wheat, cowpea (a non‐host plant), and woody plant species in which reproduction has been observed, and of organisms associated with maize stores. P. truncatus reacted in general to odours from starchy commodities and to some of the woody plants. Beetles did not respond to volatiles from cowpea or organisms associated with maize stores, but did react to the aggregation pheromone produced by single male P. truncatus (secondary attraction). P. truncatus raised on cassava and emigrants from maize responded as strongly to maize odours as beetles raised on maize, whereas P. truncatus trapped with pheromone traps showed reduced or no reactions. Stored commodities seem to elicit short‐range arrestment, but not primary attraction at long range. However, stored products might only be attacked facultatively by P. truncatus, co‐evolved primary attraction could possibly exist to the native host‐plant complex.

Flight initiation in Prostephanus truncatus: influence of population density and aggregation pheromone

The influence of population density and aggregation pheromone were investigated as possible flight initiating factors for Prostephanus truncatus (Horn) (Col.: Bostrichidae). Maize grains (175 g in glass jars) were infested with 20, 50, 150 or 300 beetles, all of which were removed after 2 weeks. Flight initiation of the progeny was observed over a period of 4 weeks, with and without the synthetic aggregation pheromone of P. truncatus. Addition of the synthetic pheromone had no influence on the number of beetles taking off. The number of beetles dispersing increased with growing initial and progeny densities. Dispersal rates (% of total progeny dispersing) rose with declining rate till they reached a maximum at ca. 33%. Of the beetles dispersing, 74% left the cultures between 18.00 and 20.00 hours. Surviving progeny per female decreased with increasing initial density. Sex ratios in the initial cultures and in the F1 were ca. 1:1, whereas a higher proportion of females (67%) was found among the dispersing beetles.

Electrophoretic analysis of the prey spectrum of Teretrius nigrescens (Lewis) (Col., Histeridae), a predator of Prostephanus truncatus (Horn) (Col., Bostrichidae), in Mexico, Honduras, and Benin

Abstract: Extensive collections of Teretrius nigrescens in Mexico, Honduras, and Benin, were electrophoretically analysed to elucidate the prey spectrum of the predator. Both polyacrylamide gel electrophoresis and cellulose acetate electrophoresis were used. Beetles were sampled with pheromone traps, using the synthetic aggregation pheromone of Prostephanus truncatus, and directly from farmers’ maize stores. The proportion of electrophoretically detected prey protein from adult T. nigrescens in pheromone traps was low: of the 1108 specimens analysed, only in 34 cases, prey protein could be clearly identified. More frequently, prey protein was detected in adult T. nigrescens sampled in maize stores in Mexico, Honduras, and Benin, with 87 samples showing distinct prey bands of the total 1214 predators analysed. Of the 241 T. nigrescens larvae sampled in maize stores in Benin, 136 showed distinct bands of prey protein. In all samples, P. truncatus was the most frequently detected prey species. The second most often identified prey species was Sitophilus zeamais. The results are discussed with regard to various methods for prey spectrum analysis and specifically the biology of T. nigrescens.

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.