In the battle against crop pests, scientists must take advantage of all available resources. In the case of a novel piece of research by IITA against the whitefly Bemisia tabaci, this means learning about the way insects use their sense of smell.
B. tabaci is one of the major agricultural pests of vegetable crops in the tropical and subtropical regions. It attacks cassava, which is a staple root crop in sub-Saharan Africa that provides food and income for millions of people. B. tabaci is the vector of viruses that cause Cassava Mosaic Disease (CMD) and Cassava Brown Streak Disease (CBSD), which can lead to total crop failure and result in overall crop losses valued at more than $1 billion. Most of the management and control practices for B. tabaci are not practical and cost-effective for smallholder farmers. Therefore, new approaches are needed, which require understanding the relationship between the whitefly vector and its host plants, particularly how B. tabaci identifies and locates its target plants.
Seeing by smelling
Plant-feeding insects such as B. tabaci use color, smell, and taste to detect and identify their host plants for food and egg laying. Smell is thought to be the first signal that plant-feeding insects detect as an indicator of the presence of a nearby host plant. It cues into chemicals emanating from the plant that can be dispersed by wind for long distances. Several scientists have investigated the behavior of the whitefly towards plant-produced chemical odors and have been able to identify some of the chemical compounds that attract insects to plants or repel them.
Studies that have demonstrated the repellent action of some of the plant-produced chemicals towards B. tabaci prompted the exploration of the use of such chemicals to manage the whiteflies. A diverse set of plants and plant products have been shown to produce volatile chemicals that repel whiteflies. These include several tomato varieties, extracts from neem, Aframomum and cinnamon spices, as well as the herbs geranium, dill, citronella, litsea, lemongrass, and savory. Several Large population of B. tabaci adults feeding on the underside of a young cassava leaf. Photo by IITA. Close-up of whitefly adults. 43 chemicals thought to be responsible for the attraction or repulsion have been identified but their mechanism of action is not yet known. Understanding this mechanism will enable researchers to identify the active chemical compounds that can be used to make cassava less appealing to whiteflies.
To attract or not to attract, that is the question
An innovative study being undertaken by IITA-Tanzania aims to generate an understanding of the odor-guided response of the cassava whitefly B. tabaci by identifying chemicals emitted by different plants and investigating the behavioral response of the whitefly towards these chemicals. This research also aims to determine the mechanism by which whiteflies can detect and identify the plant-produced chemical signals.
Preliminary results on the behavioral response of cassava whiteflies towards chemical odors produced by cassava (the host plant for feeding and laying eggs), tomato (host plant for feeding), and lemongrass (nonhost plant) showed that the whitefly can indeed identify its host plant based on plant-produced and emitted chemical odorants. The whiteflies were shown to be repelled by the chemical odorants from both lemongrass and tomato but not those from cassava. They were also not attracted by the chemical odorants from cassava. A variety of chemical compounds were identified from the chemical odors produced by cassava, tomato, and lemongrass, and these differed in number, type, and concentration. Of the identified chemical compounds, cymene, terpinene, and phellandrene, as well as citral and geraniol, were obtained from tomato and lemongrass. These compounds have been reported to have repulsive effects towards whiteflies; however, it is not known how the whiteflies can detect and identify these chemicals from among the wide range of chemicals being produced by the plants.
Further studies using different varieties of cassava and other plants are being done to confirm the observed behavior of the whiteflies. In addition, the mechanism by which the whitefly can detect and smell the variety of chemicals it encounters will also be determined. Once these volatile chemicals have been identified, they will be used in field studies to determine their effectiveness in attracting or repelling whiteflies in the environmental conditions used to cultivate cassava plants. This will generate a baseline for developing management strategies to prevent whitefly colonization of cassava plants either through intercropping with plants that repel the whiteflies or developing organic insecticides and traps for whiteflies. The chemicals that repel the whiteflies can also be used with transgenic approaches to produce whitefly-repelling cassava; thereby, reducing its likelihood of being infected by the viruses that cause CMD and CBSD.