No abstract

The recent discovery of Bogia coconut syndrome in Papua New Guinea (PNG) is the first report of a lethal yellowing disease (LYD) in Oceania. Numerous outbreaks of LYDs of coconut have been recorded in the Caribbean and Africa since the late Nineteenth century and have caused the death of millions of palms across several continents during the Twentieth century. Despite the severity of economic losses, it was only in the 1970s that the causes of LYDs were identified as phytoplasmas, a group of insect-transmitted bacteria associated with diseases in many other economically important crop species. Since the development of polymerase chain reaction (PCR) technology, knowledge of LYDs epidemiology, ecology and vectors has grown rapidly. There is no economically viable treatment for LYDs and vector-based management is hampered by the fact that vectors have been positively identified in very few cases despite many attempted transmission trials. Some varieties and hybrids of coconut palm are known to be less susceptible to LYD but none are completely resistant. Optimal and current management of LYD is through strict quarantine, prompt detection and destruction of symptomatic palms, and replanting with less susceptible varieties or crop species. Advances in technology such as loop mediated isothermal amplification (LAMP) for detection and tracking of phytoplasma DNA in plants and insects, remote sensing for identifying symptomatic palms, and the advent of clustered regularly interspaced short palindromic repeats (CRISPR)-based tools for gene editing and plant breeding are likely to allow rapid progress in taxonomy as well as understanding and managing LYD phytoplasma pathosystems.

Intensive agriculture, which is associated with heavy inputs of synthetic insecticides, has serious ecological impacts, leading to loss of vital ecosystem services including insect-mediated pest suppression. In recent years, efforts have been made towards obtaining safer options to chemical insecticides for sustainable pest management. Habitat manipulation is a part of conservation biological control which aims at providing floral resources, alternative prey and shelter to predators and parasitoids to enhance and sustain natural pest suppression. The use of plant extracts as botanical insecticides is also an important provisioning ecosystem service. Selection of plant species for habitat manipulation has focused mainly on plants with suitable floral qualities to support natural enemies. To increase the benefits, habitat manipulation plants that can provide multiple ecosystem services in addition to floral resources would be an ideal. In this review, we focus on the potential of achieving the dual ecosystem services of bioinsecticidal source plants in addition to the provision of floral resources from selected plant species. Our literature search found 283 plants species from 44 plant families that have been involved in habitat manipulation studies. Fifteen of these plant families have species that have been exploited for their insecticidal properties. Three families, Apiaceae, Asteraceae and Lamiaceae, have the largest number of species that have been used for both habitat manipulation and botanical insecticides. Of the four most popular habitat manipulation plants, alyssum Lobularia maritime (L.) Desv. (Brassicaceae), buck wheat Fagopyrum esculentum Moench (Polygonaceae), coriander Coriandrum sativum L. (Apiaceae) and phacelia Phacelia tanacetifolia Benth. (Boraginaceae), buckwheat and coriander have been used for insecticidal purposes whilst no records exist of phacelia and alyssum as botanical insecticide species. There is great potential for identifying plant species that can support natural enemies as well as providing potent plant extracts as botanical insecticides by selecting species from the Apiaceae, Asteraceae and Lamiaceae families.

Sweetpotato has been the subject of little research worldwide compared with other major crop staples, and this is especially so for less developed countries where sweetpotato is critical for food security. This review synthesises information on plant protection issues that affect smallholder sweetpotato farmers in less developed countries to identify major issues and suggest research priorities. Though the pests and diseases of sweetpotato in less developed countries are largely common to industrialised systems, their relative importance differs and losses tend to be more severe as a result of differing agronomic practices and relative unavailability of management options and technical support that are important in developed countries. Smallholders are heavily reliant on cultural practices such as traditional forms of biological control using ants and livestock, fallowing and composting (sometimes with plant materials having biocidal properties). Crop protection methods that have been developed for use in sweetpotato production in developed countries, such as pathogen-tested planting material, early maturing varieties, pheromone trapping and pesticides are less accessible to, and relevant for, smallholders. Smallholders also typically harvest a given crop progressively which extends the period over which storage roots are potentially vulnerable to attack but reduces the risk of post-harvest losses. Human population growth in developing countries is leading to an increase in cropping intensity with shorter fallow periods and more years of continuous crops. This has the dual effect of depleting soil nutrients and increasing the potential for pest and pathogen build-up. Associated with this, the adoption of strategies to manage crop nutrition, such as not burning crop residues, promote carryover of pests and pathogen inocula. As a consequence of these factors, sweetpotato yield losses from diseases, especially viruses, and pests, particularly weevils, can be high. Climate change is likely to result in more frequent drought and this will increase losses caused by sweetpotato weevils that are favoured by dry conditions. This review of sweetpotato pests and their management options concludes with suggestions for some future research priorities including the combination of traditional practices that have pest management outcomes with relevant practices from industrial production that are able to be transferred or modified for use in smallholder production. Increased technical support for decision making and diagnostics, including molecular approaches that have scope for field use, will be important in reducing the burden imposed by biotic threats to this important global crop.