Un problème régional devenu mondial : l'aleurode Bemisia tabaci
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The little creature might not look much of a threat, but appearances can be deceptive, very deceptive. What looks like an innocent bug is, in fact, Bemisia tabaci (tobacco whitefly) ranked as one of the world’s worst 100 invasive species* and a real problem for European tomato and cucumber growers.
Once a concern only in sub-tropical regions, B. tabaci have spread around the world, trailing crop protection problems in their wake. B. Tabaci , which come in several biotypes (they share the same genotype) have been linked with the spread of more than 110 viruses.
For European growers, crops such as tomatoes, cucumber, melon and watermelon are particularly vulnerable to the viruses B. tabaci transmit. These include tomato yellow leaf curl virus (TYLCV) and the similar tomato yellow leaf curl Sardinia virus (TYLCSV), which reduce yields and can cause large economic losses in tomatoes; cucurbit yellow stunting disorder virus (CYSDV), which can cause severe losses for cucumber and melon growers, and tomato chlorosis virus (ToCV), which can also hit tomato yields, as it reduces fruit growth and delays ripening.
Charting the impacts of B. tabaci and, more generally, the make-up of the European tomato growing industry, has been a key task for ENDURE’s tomato case study team, part of the Network of Excellence’s Research Activity 1 group, which has been using the case study approach on a number of important crops. The team surveyed nine different countries (six in the European Union and three around the Mediterranean Sea), and conducted a review of current literature on the growing of one of Europe’s most popular foods.
In fact, tomatoes are an essential part of the European diet. According to Fruittoday.com, even in chilly northern Europe the average Briton eats more than 7kg of fresh tomatoes each year, while in southern Europe annual consumption soars to more than 17kg per Spaniard and 30kg per Italian.
ENDURE’s case study team identified Italy (more than 6 million tonnes annually) and Spain (around 4 million tonnes annually) as Europe’s largest tomato producing countries, with the EU as a whole producing some 17 million tonnes. The only country outside Europe to export tomatoes to the EU in any great quantity is Morocco.
The study team found that the whiteflies Bemisia tabaci and Trialeurodes vaporariorum are the key pests in most of the growing areas surveyed. The main B. tabaci -transmitted tomato diseases are TLYCV and TYLCSV, which have been recorded in countries all around the Mediterranean Basin and also on the Atlantic coast of Morocco, the Canary Islands and Reunion, the French Indian Ocean island and one of Europe’s furthest outposts.
The survey discovered that insecticides are used to control whiteflies in all the countries surveyed. It also found that biological controls are used in many countries, especially for greenhouse crops. In areas with very intensive vegetable production, such as southern Spain, Sicily, Greece and Israel and also in some areas with less intensive production, such as southern France, nets are used a physical barrier to stop whiteflies.
The team notes that several biotypes of B. tabaci are found in Europe, with biotype Q the most widespread, followed by biotype B. Understanding the different biotypes is important because previous research work has discovered, for example, that the Q biotype is more resistant to insecticides known as neonicotinoids (they act on the central nervous system of insects) than the B type. Follow an intensive chemical control programme and you may trigger biotype selection, and produce more survivors of B. tabaci biotype Q. B. tabaci have been reported as far north as the Netherlands but have not, for example, established themselves in the UK or Denmark, where you are more likely to find Trialeurodes vaporariorum, also known as the glasshouse or greenhouse whitefly.
How do tobacco whiteflies move around the world?
According to the Global Invasive Species Database, B. tabaci have been found on every continent on the planet, bar Antartica, but are thought to have originated from warmer climes, maybe India.
Poinsettia have been identified as a major source of B. tabaci in the UK. Copyright: Scott Bauer, USDA Agricultural Research Service, Bugwood.org.
B. tabaci , also known as sweet potato whiteflies, are happy to live on a range of more than 900 plants and not only transmit viruses, but also cause direct damage to plants when feeding and indirect damage through sooty moulds which develop on the honeydew they excrete on host plants.
Given B. tabaci’s ability to live on such a wide range of plants, controlling its spread has proved difficult. The Department for Environment, Food and Rural Affairs (DEFRA) in the UK, for example, has issued a leaflet available to all plant nurseries warning of what to look for and what measures to take.
This follows the first outbreak of B. tabaci in the UK in 1987 when it was discovered on cuttings from the popular Christmas plant poinsettia. Poinsettia have continued to be the major source of outbreaks, says DEFRA, but it has also intercepted B. tabaci on Ajuga , Ficus and Hibiscusplants, on cut flowers and on herbs such as thyme and rosemary. The pest has been found on plant material coming from other European countries, North and South America, Asia and Africa, adds DEFRA.
Source : ENDURE website