Current control methods for diamondback moth and other brassica insect pests and the prospects for improved management with lepidopteran-resistant Bt vegetable brassicas in Asia and Africa

The diamondback moth (DBM), Plutella xylostella (L.), remains a major pest of brassica crops worldwide. DBM has been estimated globally to cost US$ 1 billion in direct losses and control costs. Chemical control of this pest remains difficult due to the rapid development of resistance to insecticides and to their effect on natural enemies. These problems are especially severe in South Asia and Africa where lack of knowledge, limited access to newer and safer insecticides, and a favourable climate result in DBM remaining a serious year-round pest which substantially increases the cost and uncertainty of crop production. Despite these problems, application of synthetic insecticides remains overwhelmingly the most common control strategy. Biologically-based efforts to control DBM in Africa and Asia have focused strongly on parasitoid introductions. However, despite the identification and deployment of promising parasitoids in many regions, these efforts have had limited impact, often because farmers continue early-season spraying of broad-spectrum insecticides that are lethal to parasitoids and thus exacerbate DBM outbreaks. A significant driver for this pattern of insecticide use is the presence of aphids and other pests whose appearance initiates inappropriate spraying. Despite often extensive training of producers in farmer field schools, many growers seem loath to discard calendar or prophylactic spraying of insecticides. The introduction of an IPM technology that could replace the use of broad-spectrum insecticides for DBM and other key Lepidoptera is crucial if the benefits of parasitoid introduction are to be fully realised. The deployment of DBM-resistant brassicas expressing proteins from Bacillus thuringiensis could help to break this cycle of insecticide misuse and crop loss, but their deployment should be part of an integrated pest management (IPM) package, which recognises the constraints of farmers while addressing the requirement to control other Lepidoptera, aphids and other secondary pests.     

The impact of indigenous soil and water conservation practices on soil productivity: examples from Kenya,Tanzania and Uganda

Farmers in many parts of Africa use indigenous soil and water conserving (ISWC) practices as an integral part of their farming systems. Farmers have developed such methods which have maintained productivity and contributed to long‐term sustainability, while introduced measures have often been rejected or simply failed to achieve their technical objectives. This paper examines the strengths and weaknesses of some ISWC practices in Kenya, Uganda and Tanzania. An evaluation has been carried out based on farmers' evaluation criteria as well as criteria identified by researchers. Farmers' criteria are often based on the quality of their natural resources (farmers are more likely to conserve those soils that will give the highest return on their investment), the resource level of the household (particularly income levels and labour availability), cropping intensity as well as cultural traditions related to age, education and gender. Of great importance, however, is the need to maintain or increase soil productivity. Methods that conserve moisture, reduce soil erosion, maintain soil fertility and increase productivity, which are socially acceptable and economically viable, are those which farmers favour. ISWC practices often have these qualities but farmers have not always been able to adjust the techniques to rapid changes in farming systems and increasing intensity of land use. Soil productivity and economic modelling indicate that although yields declines are lower when traditional technologies are used, long‐term productivity remains a problem. There is an urgent need to work closely with farmers to improve and develop traditional SWC techniques. Copyright © 2000 John Wiley & Sons, Ltd.     

An evaluation of gains in breeding for resistance to the cocoa swollen shoot virus disease in Ghana

The present study examines the gains in resistance to cocoa swollen shoot virus (CSSV) infection from investments in breeding over the past seven decades. The general susceptibility to CSSV infection of the West African Amelonado that dominated plantings prior to the start of formal research in 1938 necessitated the introduction of germplasm of Upper Amazon origin to better contain the disease spread. Included in this study are findings of two recent experiments. In the first, the genetic basis for resistance in the clone mvT85, developed from gamma radiation of clone T85/799 and putatively resistant to CSSV disease was investigated. In the second experiment, the comparative levels of resistance in sets of old, current and new cocoa varieties were tested following inoculations with the severe CSSV strain 1A. Absence of nucleotide differences at 29 single nucleotide polymorphism (SNP) loci between mvT85 and T85/799, and lack of segregation for resistance in the full-sib and backcross populations derived from mvT85 indicated that mvT85 did not carry novel genes for improving cocoa for CSSV disease resistance. Moreover, there were no differences in resistance to CSSV disease between mvT85 and T85/799. These observations conflict with the previous report that mvT85 is immune to CSSV disease, and distinct from T85/799. Between variety groups, disease severity scores based on three successive leaf flushes after inoculation were not effective in discriminating among them. Disease severity assessed eight months after inoculation was the most important criterion for separating varieties for resistance to CSSV disease. As expected, the older varieties were the most sensitive to infection. No differences were found between current varieties derived exclusively from Upper Amazon clones and new varieties. Contrary to the generally held opinion of a higher level of resistance in existing inter Upper Amazon cultivars, varieties derived from crosses using Catongo, RB 49 and C-SUL 7 (all of Lower Amazon origin) as males with specific Upper Amazon varieties were among the most resistant. A re-appraisal of variety recommendations for areas of mass infection and for less affected areas is advocated.     

QTLs for phosphorus deficiency tolerance detected in upland NERICA varieties

Phosphorous (P) deficiency is a major yield limiting factor in rice (Oryza sativa L.) production. The interspecific New Rice for Africa (NERICA) varieties combine general stress tolerance from African cultivated rice (Oryza glaberrima Steud) with characteristics associated with high yield from O. sativa. However, little is known about their ability to tolerate P deficiency. Here, we examined the variation for tolerance to P deficiency among the 18 upland NERICAs and their parents in multi‐year field experiments. The good performance under P deficiency of the O. glaberrima parent CG 14 and some NERICAs suggested that these tolerant NERICAs contain loci associated with P deficiency tolerance inherited from CG 14. Additionally, four QTL clusters for P deficiency tolerance were detected on chromosomes 4, 6 and 11 using F₃ lines derived from the cross between the P deficiency tolerant variety NERICA10 and a Japonica‐type sensitive variety ‘Hitomebore’. These QTLs represent the first step in identifying stress tolerance genes from O. glaberrima that could subsequently be used to enhance P deficiency tolerance in O. sativa.     

Agricultural nematology in East and Southern Africa: problems,management strategies and stakeholder linkages

By 2050, Africa's population is projected to exceed 2 billion. Africa will have to increase food production more than 50% in the coming 50 years to meet the nutritional requirements of its growing population. Nowhere is the need to increase agricultural productivity more pertinent than in much of Sub‐Saharan Africa, where it is currently static or declining. Optimal pest management will be essential, because intensification of any system creates heightened selection pressures for pests. Plant‐parasitic nematodes and their damage potential are intertwined with intensified systems and can be an indicator of unsustainable practices. As soil pests, nematodes are commonly overlooked or misdiagnosed, particularly where appropriate expertise and knowledge transfer systems are meager or inadequately funded. Nematode damage to roots results in less efficient root systems that are less able to access nutrients and water, which can produce symptoms typical of water or nutrient deficiency, leading to misdiagnosis of the underlying cause. Damage in subsistence agriculture is exacerbated by growing crops on degraded soils and in areas of low water retention where strong root growth is vital. This review focuses on the current knowledge of economically important nematode pests affecting key crops, nematode control methods and the research and development needs for sustainable management, stakeholder involvement and capacity building in the context of crop security in East and Southern Africa, especially Kenya, Tanzania, Uganda and Zimbabwe. © 2015 Society of Chemical Industry     

Determinants and rates of land degradation: Application of stationary time-series model to data from a semi-arid environment in Kenya

The causes of land degradation in the African drylands have been shown to vary. Some researchers consider climate to be the major contributor to degradation, with anthropogenic factors playing a minor role. Others reverse the significance of these two factors. A third group attributes land degradation to climate and anthropogenic factors equally. This study was undertaken to establish the factors influencing land degradation in a semi-arid environment in southeastern Kenya and the rate of change in vegetation types for a period of 35 years(1973–2007). The reduction in grassland cover was used as an indicator of land degradation. Causes of land degradation were determined by a multiple regression analysis. A log-linear regression analysis was used to establish the rate of vegetation change. The multiple and log-linear regression analyses showed:(1) woody vegetation, livestock population and cultivated area to be the main contributors of reduction in grassland cover in the area, and (2) an increase in undesirable woody species, livestock population and cultivated area had a significant(P0.05) negative effect on grassland vegetation. Increased human population, low amounts of rainfall and drought showed no significant negative effect on grassland vegetation cover. In conclusion, human and livestock population growth and increased agricultural land have contributed to intensive crop cultivation and overgrazing in the semi-arid lands. This overuse of the semi-arid rangelands has worsened the deterioration of the natural grassland vegetation.     

Rhamphicarpa fistulosa,a widespread facultative hemi‐parasitic weed,threatening rice production in Africa

Rhamphicarpa fistulosa is a facultative hemi‐parasitic plant of the Orobanchaceae family, adapted to wet soils. Apart from tropical Australia, it is only found in sub‐Saharan Africa, where it is considered a minor weed in cereal crops such as rice. Due to this status, the species has received only sporadic attention. Recent field observations and encounters with rice farmers in several African countries showed that R. fistulosa is, however, a more serious and increasing production constraint than previously thought. Results from a systematic literature review and a global herbarium study support this. The species has a broad distribution over Africa (at least 35 countries from Madagascar to Senegal and from Sudan to South Africa) and a wide range in altitude (0–2150 m a.s.l.) and environment (waterlogged swamps to moist free‐draining uplands). Rhamphicarpa fistulosa is relatively independent and persistent because of the presumably wide host range, the facultative nature of its parasitism and its prolific seed (estimated 100 000 seeds m^?2 under moderate infestation levels). Finally, R. fistulosa causes severe yield losses (average 60%) and high regional annual economic losses (estimated US $175 million), while effective control options are scant and awareness of the species among important R&D stakeholders is almost absent. An integrated approach is advocated to assist the rice sector to reduce current R. fistulosa‐inflicted losses and to prevent further spread of the species into new areas.     

The political economy of timber taxation: The case of Ghana

We analyze the political economy of timber taxation in Ghana. Our results show that politicians maintain control over allocation of timber rights, that taxation constitutes an insignificant share of the value of the timber resource, and that the distribution of timber revenues hardly contributes towards the official forest policy justifications. Our analysis suggests that politicians wield control over rent-seeking opportunities that are exchanged for political support through patron–client networks. This speaks to a larger literature on why governments waste resources and constitutes an argument for increased attention to the political economy underlying natural resource policies.     

Studies of the Pink Bollworm,Pectinophora gossypiella,on Sea Island Cotton in Barbados

Abstract

Barbados once grew large amounts of Sea Island cotton but the accidental introduction of pink bollworm (Pectinophora gossypiella) in the 1920s, falling prices and rising costs led to the collapse of the industry. Falling sugar prices subsequently led to a revival of interest in the crop but pink bollworm still presented a problem and warranted investigation.

The synthetic sex pheromone of pink bollworm, gossyplure, was shown to attract far more males than hexalure or virgin females. Sticky traps needed stirring daily and were only able to catch about 50 males a night. Water, with a drop of liquid detergent, proved to be a suitable trapping medium and was cheaper than liquid paraffin although not quite as effective. The best type of trap from materials readily available was one made from a plastic half gallon ice cream container with lid and holes cut around the upper sides. Traps without the attractant caught very few moths. Gossyplure adsorbed onto rubber caps was attractive to pink bollworm males for over 56 days but the half-life appeared to be about 28 days. Best catches were obtained in cotton when the traps were suspended just below the tops of the plants and, within fields, traps at the leeward end caught more than traps elsewhere. Peak catches of males occurred before midnight in a black light trap whereas pheromone traps caught no males before midnight but peaked at 03.00h.

A close season during which no cotton is allowed to grow is a recognised means of reducing pink bollworm carryover between crops. This was practised in Barbados but island-wide pheromone trapping showed that moths were caught throughout the close season. This continuous trapping of moths indicated that pink bollworm must be breeding (short-cycle) in order to survive. Trapping and laboratory studies confirmed the presence of long-cycle pink bollworm which entered diapause towards the end of the cotton season as daylength increased. This diapause terminated with shortening days after the new season's rains commenced. The case for a four month close season is discussed.

Pheromone trap catches were shown to be very closely correlated with boll damage. Catches of 8–9 moths in a night represented a 10% level of boll damage some ten days later and this is the trapping level at which cotton growers are advised to begin insecticidal treatment.

Thorough residue destruction is the key to a good close season and a suitable method was needed for use under Barbadian conditions. The method finally chosen and proved on an estate scale was to cut down the standing cotton with a horizontal-rotary chain slasher, driving across the rows of cotton, the debris was then raked along the rows, stacked and burnt and finally the stumps were ploughed in, again across the rows.

The parasite Perisierola nigrifemur is very common in pink bollworm larvae towards the end of the season but too late to be of economic value. Predation of diapausing larvae by the mite Pyemotes ventricosus is very common at the end of the cotton crop and during the close season. Insecticidal application in Sea Island cotton which usually grows very tall (> 1.75m) and rank is best achieved with hand-held, battery-operated, u.l.v. sprayers. Control of pink bollworm can be obtained with carbaryl S formulations applied through these sprayers with the addition of a little water and molasses. Synthetic pyrethroids in u.l.v. formulations are also expected to provide control.