C. Borgemeister

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.

Trees or stores? The origin of migrating Prostephanus truncatus collected in different ecological habitats in southern Benin

Migrating Prostephanus truncatus (Horn) (Col.: Bostrichidae) were collected weekly with pheromone‐baited funnel traps at three different sites in southern Benin for 12 months. One site was located in a primary forest, one in a peri‐urban area, and one in a region with intensive agriculture. The sex of the trapped beetles was determined. The gut‐content of the specimens was analyzed for remains of lignin and starch, the former indicating recent feeding on woody, the latter on a starchy substrate, such as stored maize or dried cassava. At all locations, the sex ratio of migrating P. truncatus was significantly female‐biased, with the greatest proportion of females trapped at the peri‐urban site. At the forest site, most beetles had lignin in their guts, while the proportion of beetles containing starch was highest in the peri‐urban site. Approximately equal proportions of beetles with either starch and lignin were trapped in the region with intensive agriculture. The results are discussed with regard to the population dynamics of P. truncatus in different habitats and the flight activity of the beetles.

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.

EAG and behavioural responses of the larger grain borer, Prostephanus truncatus, and its predator, Teretriosoma nigrescens, to the borer-produced aggregation pheromone

Abstract.Electroantennogram (EAG) and behavioural studies were conducted with Prostephanus truncatus (Horn) (Col.: Bostrichidae) and the predatory beetle, Teretriosoma nigrescens Lewis (Col: Histeridae) in regard to their responses to the components of the prey‐produced aggregation pheromone. There were hardly any differences between species or sexes regarding perception thresholds. In field and olfactometer experiments, female P. truncatus were more responsive to the pheromone than males, and both sexes reacted more strongly to the minor pheromone component, T2, than to T1. Sex ratios among trap catches of T. nigrescens were slightly male‐biased. The predator did not differentiate behaviourally between the pheromone components.

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.

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.