D. Obeng-Ofori

Toxicity and protectant potential of camphor, a major component of essential oil of Ocimum kilimandscharicum, against four stored product beetles.

The biological activity of camphor, a major component of essential oil of the basil shrub, Ocimum kilimandscharicum, against the beetles, Sitophilus granarius, S. zeamais, Tribolium castaneum and Prostephanus truncatus, was investigated in the laboratory using contact toxicity, grain treatment and repellency assays. Camphor applied either topically, impregnated on filter papers or whole wheat and maize grains was highly toxic to all the four species. Beetle mortality was dosage-dependent with the highest doses of 100 mg/ filter paper and 100 mug/insect evoking over 93% and 100% mortalities, respectively, in S. granarius, S. zeamais and P. truncatus after 24 h exposure. Similar doses induced 70% and 100% mortality in T. castaneum. Camphor impregnated on the grain surface was more effective than on filter paper. There was, however, highly significant reduction in toxicity in grain after only 24 h following treatment. Development of eggs and immature stages within grain kernels, as well as progeny emergence,...

Host preference and suitability of some selected crops for two biotypes of Bemisia tabaci in Ghana

The economic importance of Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae) is mainly due to its wide host range, variable kinds of damage, and great intraspecific variation. The delineation of two African biotypes of this pest has been carried by molecular, ecological, and host‐affiliation approaches, with largely consistent results. However, an understanding of its intricate host–pest interaction is necessary as a basis of its sustainable integrated control. This study investigated the host preference and suitability of cassava and okra biotypes of B. tabaci, based on multiple‐choice landing and oviposition preference assays and stage‐specific survival on eight common whitefly hosts. The cassava biotype significantly preferred cassava, Manihot esculenta, for landing and oviposition, but did not oviposit on okra, Abelmoschus esculentus. The okra biotype preferred okra, oviposited on eggplant, Solanum melongena, tomato, Lycopersicon esculentum, garden egg, Solanum integrifolium, and cowpea, Vigna unguiculata, but did not oviposit on cassava. The okra biotype developed on all hosts except cassava, but only survived marginally on cabbage, Brassica oleracea, and pepper, Capsicum annum var. grosum, while the cassava biotype did not develop on okra, cabbage, or pepper. Thus the observed host acceptance of the two biotypes is wider than earlier reported by host transfer experiments and molecular genetic surveys. Mortality was highest in the first instar nymphal stage, during which total mortality occurred on non‐hosts. Development time was slightly longer on marginal hosts than on the preferred hosts. Cowpea, garden egg, and tomato are additional common hosts of the two biotypes, whose role as reservoir hosts and biotype interbreeding grounds should be investigated further.

Facilitating the use of alternative capsid control methods towards sustainable production of organic cocoa in Ghana

Cocoa (Theobroma cacao L.) is an important foreign exchange earner for Ghana. However, production is constrained by a high incidence of pests and diseases. Based on farmers’ needs, this study focused on the control of capsids, mainly Sahlbergella singularis Haglund and Distantiella theobroma (Distant) (both Hemiptera: Miridae). Annual crop loss caused by capsids is estimated at 25-30%. To control capsids, formal research recommends application of synthetic insecticides four times between August and December. However, farmers hardly adopt this recommendation, which they consider unsuitable for their conditions and context. Three alternative control methods were tested with farmers: mass trapping, using sex pheromones; applying crude aqueous neem Azadirachta indica A. Juss. (Meliaceae) seed extract (ANSE) and using the predatory ant Oecophylla longinoda Latreille (Hymenoptera: Formicidae) as a biological control agent. Contrary to most previous reports, studies on temporal distribution of cocoa capsids indicated that the population peaked in March. ANSE was effective against capsids and other cocoa insect pests and did not affect the predatory ant. When O. longinoda occurred in high numbers, capsid incidence was low. Shade did not influence ant or capsid abundance significantly. ANSE caused 100% mortality of capsids in cage and 79-88% in field experiments. The sex pheromone was as effective as ANSE or ants in suppressing capsids. All the three methods were effective and compatible; hence, they can be used in an integrated pest management strategy for cocoa, including organic production in Ghana.

Intra- and Interspecific Aggregation Responses of Locusta migratoria migratorioides and Schistocerca gregaria and a Comparison of Their Pheromone Emissions

Gregarious nymphs of Schistocerca gregaria and Locusta migratoria migratorioides are often seen to march together in common bands in the field. In the present study, the intraspecific aggregation responses of nymphal and older stages of the two insects were compared with their interspecific responses. Unlike S. gregaria, L. m. migratorioides shows a lesser pattern of stage and sex differentiation in its aggregation pheromone biology. Thus, although fifth-instar nymphs did not respond significantly to the adult pheromone, adults responded significantly to the nymph pheromone. No cross-stage aggregative responses occur in S. gregaria. In the adults of S. gregaria, production of the pheromone is male-specific; in L. m. migratorioides both sexes induced a significant level of aggregation from conspecifics. Aggregation assays between corresponding stages of the two species showed stronger interaction between the nymphal stages than between the adults and account for the frequent occurrence of mixed hopper bands in the field. GC-EAD studies of volatiles of nymphal and adult stages by using antennal preparations from both species also showed significant interspecies reactivities. GC-MS analyses showed that of S. gregaria nymphal pheromone components (C6 and C8 to C10 straight-chained aldehydes and acids and the fecal phenols, guaiacol and phenol), the acids and phenols constitute the common components of nymphal stages. Phenylacetonitrile, the major component of S. gregaria adult aggregation pheromone, is present in L. m. migratorioides nymphal volatiles, albeit at a lower level. Unlike S. gregaria adult pheromone blend, which is made up wholly of benzene derivatives, the volatile emissions of L. m. migratorioides adults is dominated by aliphatic aldehydes and alcohols. The possible role of cross-aggregation effects in mutually facilitating phase transformation of the two species is discussed.

Toxicity, growth regulatory and repellent activities of medicinal plant extracts on Musca domestica L. (Diptera: Muscidea)

Housefly, Musca domestica, is a major vector for many medical and veterinary pathogenic organisms. The development of naturally occurring insecticides, represent one of the most promising approaches for their ecochemical control. Petroleum-ether extracts of Griffonia simplicifolia and Zanthoxylum xanthoxyloides were assessed for their toxicity, growth regulatory and repellency to the housefly. Percent mortality and index of repellency induced by the extracts against the insects were found to be dose-dependent. Seed extracts of G. simplicifolia and root extracts of Z. xanthoxyloides were the most effective as toxicants and repellents against the fly. The LD50 in 24 h topical application of seed extracts of G. simplicifolia and root extracts of Z. xanthoxyloides were 0.28 and 0.35 ig, respectively. Seed extracts of G. simplicifolia evoked a very strong regulatory effect against the second larval instar of the housefly. The RD50 of crude extracts of G. simplicifolia and Z. xanthoxyloides against housefly ranged from 1.0 to 6.8 and 1.3 to 1.7 ig cm–2, respectively. Extracts of the two plant species may be useful as insecticides for controlling the housefly and should be exploited as a component of integrated vector control strategies or could be useful in the search of new larvicidal natural compounds.

Abundance and distribution of the Mediterranean fruit fly Ceratitis capitata (Diptera: Tephritidae), in Late Valencia citrus orchards in Ghana

The abundance and distribution of Ceratitis capitata (Wiedemann) in Late Valencia citrus orchards and other alternative host crops was investigated using trimedlure-baited traps at the University of Ghana’s Agricultural Research Centre in the Eastern region of Ghana. The effect of some climatic factors on the population dynamics of the pest was also studied. The population of C. capitata increased from September to October 2006 and from February to March 2007, when Late Valencia citrus fruits were ripening or ripened. The fruit fly population was the lowest from November 2006 to January 2007 when citrus fruits were in the immature green stage. Ceratitis capitata was not trapped in mango, pawpaw and pepper fields and did not emerge from fruits of these crops. Multiple regression analyses also revealed that rainfall and temperature significantly influenced the pest population. The results of this study have important implications on the decision-making process for the safe, effective monitoring and management of C. capitata in Late Valencia citrus orchards in the study area in Ghana and probably in other areas as well.

Assessing the effectiveness of a Local Agricultural Research Committee in diffusing sustainable cocoa production practices: the case of capsid control in Ghana

The conventional method of ‘delivering’ technologies recommended by researchers to farmers through extension has proved ineffective, resulting in a persistent low (3.5% over ten years) adoption of research-based cocoa technologies. The present study was conducted in the Eastern Region of Ghana and assessed the impact of the Local Agricultural Research Committee (LARC) approach on the diffusion of capsid management knowledge and practices, developed with the LARC, to others in the community. Capsids (Sahlbergella singularis and Distantiella theobroma) were diagnosed as the most serious production constraint. LARC members engaged in intensive interactive learning and experimentation to control them. The interactive approach developed by the International Centre for Tropical Agriculture was used to link the LARC with community farmers, a majority of whom aspired to produce organic cocoa for a premium. The LARC acquired vital agro-ecological knowledge on capsid management, including skills in scouting for capsids to determine their temporal distribution and systematic experimentation with control methods, before presenting its results to the community. This article reports on a survey comparing three categories of farmers: LARC members, exposed and non-exposed community farmers, so as to assess the diffusion and impact of LARC knowledge co-production. The results show that the LARC approach significantly influenced acquisition and diffusion of knowledge and practices.

Host suitability of two Ghanaian biotypes of Bemisia tabaci (Homoptera: Aleyrodidae) on five common tropical weeds

The sweet potato whitefly Bemisia tabaci (Gennadius) is a pest of diverse agricultural systems worldwide, owing to its capacity to transmit viruses, both intraspecific and host-associated variations. In Ghana, it is the only known arthropod vector for cassava-infecting begomoviruses (Geminiviridae). Two genetic variants of B. tabaci, previously characterized by RAPD–PCR and subsequently reared on cassava and okra, were allowed access to five weed species in order to examine host preference and suitability: Amaranthus retroflexus, Chromolaena odorata, Desmodium tortuosum, Euphorbia heterophylla and Malvastrum coromandelianum. Overall, the duration of the life cycle ranged between 16.5 and 26.3 days and survival rates were 0–27.5%. The population from fieldinfested okra reproduced and developed to maturity on C. odorata, D. tortuosum and E. heterophylla. Both populations produced male and female progeny. Amaranthus retroflexus and M. coromandelianum were considered poor hosts for both populations, due to the low fecundity and survival of eggs, pupae and adults. The rearing host did not significantly (P. 0.05) influence survival on subsequent hosts in either case. The host-restriction of the population collected from cassava was not corroborated here, based on the demonstrated ability of the Ghanaian cassava population to complete its life cycle on D. tortuosum and E. heterophylla. It was estimated that B. tabaci could achieve 13–22 generations per year in Ghana. Removal of C. odorata, D. tortuosum and E. heterophylla from the vicinity of susceptible crops is recommended.RésuméLa mouche blanche de la patate douce Bemisia tabaci (Gennadius) est un ravageur de divers systèmes culturaux á travers le monde, du fait de ses capacités de transmission des virus liées aussi bien á la variabilité de ses associations avec les plantes hô tes qu’á ses variations intraspécifiques. Au Ghana, c’est le seul arthropode vecteur connu du begomovirus du manioc (Geminiviridae). Deux variants génétiques de B. tabaci, d’abord caractérisés par RAPD–PCR puis élevés sur manioc et okra, ont été placés sur cinq mauvaises herbes afin d’étudier la préférence d’hô te et leur convenance: Amaranthus retroflexus, Chromolaena odorata, Desmodium tortuosum, Euphorbia heterophylla et Malvastrum coromandelianum. La durée du cycle de développement est comprise entre 16,5–26,3 jours et le taux de survie entre 0–27,5%. La population provenant de l’okra se reproduit et se développe jusqu’à maturité sur C. odorata, D. tortuosum et E. heterophylla. Les deux populations donnent des descendants mâles et femelles. Amaranthus retroflexus et M. coromandelianum sont des hô tes de faible qualité pour les deux populations, du fait d’une faible fécondité et d’un faible taux de survie des oeufs, des nymphes et des adultes. L’hô te d’origine n’influence pas significativement (P. 0.05) la survie sur les futurs hô tes. La spécificité d’hô te de la population récoltée sur manioc n’est pas confirmée ici, dans la mesure où elle est capable de boucler son cycle de développement sur D. tortuosum et E. heterophylla. On estime que B. tabaci pourrait développer 13–22 générations par an au Ghana. La suppression de C. odorata, D. tortuosum et E. heterophylla des friches présentes autour des cultures sensibles est recommandée.

Mating interactions between okra and cassava biotypes of Bemisia tabaci (Homoptera: Aleyrodidae) on eggplant

Genetic and biological implications from interbreeding the Ghanaian okra and cassava biotypes of Bemisia tabaci (Gennadius), reared on eggplant, were studied in reciprocal crossing plans and crosses compared with parental generations using RAPD-PCR. Interbreeding did not affect fecundity of mated females and survival of F1 progeny. However, the sex ratio of the hybrid progeny was significantly male dominated (by 70% males) compared to purebred progeny of each biotype (>60% females) (P < 0.01). Each biotype was characterized by its own RAPD profiles. Hybrid females had one locus identical to that of both parents, while the males resembled the mother biotype at two loci. The F1 females were oviposited normally and there was a possible restricted gene flow between insects from the two biotypes, sharing the same host plant. However, the biological isolation seems to buttress ecological isolation in nature, hence maintaining the genetic and biological identity of sympatric populations in both biotypes.RésuméLes conséquences des croisements entre deux biotypes de Bemisia tabaci (Gennadius), l’un du gombo et l’autre du manioc, sur le statut génétique et biologique de cet insecte ont été évaluées sur l’aubergine. Les croisements sont comparés aux générations parentales en utilisant la méthode RAPD-PCR. Le croisement n’a pas affecté la fécondité des femelles et la survie de la descendance. Par contre, le sex-ratio des lignées hybrides comprend 70% de mâles contre plus de 60% de femelles chez la lignée pur-sang de chaque biotype (P < 0.01). Chaque biotype présente des profils RAPD-PCR spécifiques. Les descendants femelles présentent un seul locus identique à celui de deux parents, alors que les hybrides mâles ressemblent au biotype maternel pour les deux locus. Les femelles hybrides pondent normalement mais il semble y avoir une possibilité limitée de flux de gènes entre les deux biotypes se développant sur une même plante hôte. Cependant, l’isolement biologique semble conforter l’existence de deux niches écologiques dans la nature, favorisant ainsi le maintient de l’identité génétique et biologique des populations sympatriques des deux biotypes.

Did the price-related reforms in Ghana's cocoa sector favour farmers?

It is generally hypothesized in the innovation systems literature that institutions can create production incentives for farmers. This paper examines whether the introduction in 1984 of the Producer Price Review Committee (PPRC) in Ghanas cocoa sector has improved the transmission of world prices to farmers. We test how fast and to what extent world prices have been transmitted, and also address the stability of the prices received by cocoa farmers. For the period 1960–2011, the results were as follows: (1) the production of cocoa beans depended positively on the prices farmers received and negatively on price variance; (2) the establishment of the PPRC provided higher prices for farmers; and (3) the PPRCs use of the flexible freight on board (FOB) price-setting rule resulted in a better price transmission than the employed cost-plus-margin approach. However, under the FOB price-setting rule, producer price variance rose sharply. We conclude that, although FOB pricing mechanisms are often recommended for markets where prices are institutionally determined, stabilization policies should be factored in to protect farmers against international price fluctuations.