Incredible facts about whiteflies - VIRTIGATION brings you all you need to know!
Tomato and cucurbits (i.e. cucumber, melon, pumpkin, zucchini and gourds) are among the most widely produced vegetables in the world, but are increasingly under threat from emerging viral diseases caused by begomoviruses (whitefly-transmitted) and tobamoviruses (mechanically transmitted). This blog brings you all you need to know about the begomovirus-transmitting pest, whiteflies!
The silverleaf whitefly (Bemisia tabaci, which is also informally sometimes called the sweet potato whitefly) is one of a number of species of whiteflies that are recognized as agricultural pests. As result of many studies carried out worldwide over the last decades, it is presently considered as a species complex. The whitefly species complex contains over 40 species which are morphologically indistinguishable although different in many biological traits, such as: host plants preference; fecundity; reproductive interactions; geographical distribution; virus transmission capacity; insecticide resistance; and, eco-ethological response to climate change. The B. tabaci species complex is included in a list of ‘100 of the world’s worst invasive alien species’; this is mostly due to the fact that this insect is not only a direct plant pest, but it is also a “supervector”. This means it can transmit viruses belonging to almost 500 species, including begomoviruses. B. tabaci species thrive around the world in tropical, subtropical, as well as temperate habitats, though they are less frequent in open field crops outside the tropics and subtropics, due to the fact that cold temperatures can kill both whitefly adults and nymphs.
Bemisia tabaci is thought to have originated in India, but it has become common around the world. It is believed to have spread from tropical regions via decorative plants, which were imported as ornamental flowers and shrubs for gardens in temperate regions. It was first described in Greece in the late 1880s, where it caused a significant impact on crops of tobacco. It was first recorded in the United States in 1896-7 on a crop of sweet potato (hence the name sweet potato whitefly). However, in the 1980s, a particularly aggressive strain appeared in Florida and began spreading among poinsettia crops. In under a year, this new strain was found to have moved to tomatoes, as well as to other fruit and vegetable crops. To differentiate this new, more aggressive variant from the milder, earlier known strain, the new strain became known as strain B (biotype B) and was eventually recognised as a new species, Bemisia argentifolii. It soon began to be found in other states and rapidly began causing significant losses.
“Bemisia tabaci outbreaks on vegetables in Georgia resulted in significant economic losses of 132.3 and 161.2 million US dollars (USD) in 2016 and 2017, respectively.” – Li, Y. et al., Insects 2021, 12, 198
Plants known to be affected by whiteflies of the B. tabaci species complex include: tomato, squash, cucumber, eggplant, zucchini, watermelon, and pumpkin. In addition to damage caused by the removal of sap and excretion of honeydew, whiteflies are also notorious for spreading diseases, such as plant viruses including begomoviruses, like the ones of the tomato yellow leaf curl group. Due to their swift transmission rate in fields and greenhouses, these plant diseases are extremely hard to control and can cause complete crop loss in affected areas.
The whitefly life cycle
Whiteflies lay eggs in groups on the underside of leaves. The eggs are small in size (around 0.2 millimetres (1⁄128 in)) and are initially whitish in colour, changing to a brownish colour near hatching. This process is swift, taking only 5 to 7 days.
Once hatched, the nymphs progress through four stages of development (known as instars), shedding their skin each time. In the first stage after hatching, the nymphs, commonly known as ‘crawlers’, are mobile. In this stage, they are flat, about one third of a millimetre in length and appear greenish in colour. They are able to walk/crawl to find a suitable area on the leaf with adequate nutrients, where they then moult into an immobile stage.
The subsequent three instars will stay in the same location, feeding by injecting their mouth-parts into the plant’s phloem tissue to suck sap and remove nutrients.
After the fourth instar, the nymph transforms into a pupal-like stage in which their eyes become a dark red colour and their bodies turn yellow and thicken. This is, however, not a true pupal stage. Finally, the whiteflies moult one last time, becoming an adult with light yellow bodies and white wings and measuring in at up to 0.9 millimetres (5⁄128 in) in length.
Whitefly feeding and population control
In places where the whiteflies feed, they can act as the vector for begomoviruses. Whitefly feeding may cause the host plant to develop chlorotic spots (that is spots where insufficient chlorophyll is produced), or to wilt, or lose leaves. Additionally, as they feed, whiteflies excrete honeydew as a waste product – this viscous liquid is high in sugars and can lead to the growth of mould, compounding the lack of chlorophyll and further reducing the plant’s capacity to photosynthesise. All this results in reduced growth and lower yields. Begomoviruses are responsible for a wide variety of symptoms in the hosts, including mosaics, mottles, yellowing, leaf curling and/or deformation, reduction in plant growth, and deformation in a decreased number of fruits. They can cause total loss of crop production. Furthermore, where crops do survive, additional costs to the growers are incurred through the increased need for washing after harvesting.
For all the reasons outlined above, it is desirable to reduce the numbers of whitefly in tomato and cucurbit growing areas. There are various ways of achieving this: discouraging whiteflies from settling on the tomato or cucurbits plants (a so-called ‘push’ technique) or trying to attract the whiteflies to other plants instead of the tomato or cucurbit (a so-called ‘pull’ technique); discouraging oviposition (laying eggs); and lowering the survivability of eggs which are laid or the adults that lay them.
Just as there are different strategies available, there are different methods of achieving each outcome. A natural mechanism for the control of whitefly populations is the use of insecticides of natural origin, such as ones extracted from plants. The VIRTIGATION project is investigating such substances to be used as a potential basis for innovative biopesticides. Various natural oils, such as olive oil, sunflower oil, and peanut oil, have also shown some promise in controlling whitefly populations.
Another method of combatting the whitefly is the introduction of natural predators, such as the mirid bug Macrolophus pygmaeus, which has been demonstrated to reduce whitefly damage to crops. As a whole, species of the B. tabaci group are attacked by predatory species belonging to eight arthropod orders.
Yet another way to reduce whitefly populations is the use of parasitoids. However, the control of whiteflies with natural enemies alone, either predators or parasitoids is currently more expensive than using insecticides. This is due to the fact that it is less efficient as a method of combatting virus propagation, as a result of the very low threshold of pest presence which is required for the spread of the viruses.
The role of VIRTIGATION
The world needs to increase its food production by at least half by 2050 to feed its growing population. This requires the mitigation of the devastating impact of the plant diseases spread by invasive pests like the whiteflies. VIRTIGATION will use the data it generates to identify factors driving the emergence of new viral diseases and virus outbreaks. Understanding plant-virus-vector interactions is a vital part of this work.
If you are interesting in finding out more about the VIRTIGATION project, please follow VIRTIGATION on Twitter, LinkedIn, Facebook, YouTube and Zenodo. You can also sign up to the VIRTIGATION newsletter to receive regular updates directly to your Inbox!
