Bemisia tabaci (MED)
(silverleaf whitefly)

The exact origin of the MED species of Bemisia tabaci, and the reasons why it became such an important pest are still not fully known. MED species has been identified as a distinct member within the B. tabaci species complex (Tay et al., 2012). MED species is also an effective vector of many different plant viruses which, in conjunction with its high level of polyphagy, make it extremely problematic within agricultural regions where crops may be susceptible to viruses acquired from indigenous plants. Despite Bemisia in general being a tropical/sub-tropical whitefly species, MED species can easily be transported on plant species to temperate regions of the world (Cuthbertson and Vänninen, 2015). Within these cooler regions, MED species can survive within a protected environment and could feasibly spread virus diseases to new locations. It is for this reason that B. tabaci and members of its species complex, including MED species are on EPPO A2 Alert list.

MED species continues to be detected around the globe (Chu Dong et al., 2006).

Certain areas within Europe are still MED species free, e.g. Finland, Sweden, Republic of Ireland and the UK (Cuthbertson and Vänninen, 2015).

The risk to the EPPO region is primarily to the glasshouse industry in northern countries, and mainly concerns MED species and also MEAM1 species. Present legislation relates to B. tabaci, so reference to B. argentifolii should be avoided to prevent confusion in these areas. Since the recent introduction of this whitefly to several of these countries, the pest has proved particularly difficult to combat because of its polyphagy, its resistance to many insecticides and its disruption of biological control programmes (Prabhaker et al., 1985; Della Giustina et al., 1989; Mushtaq Ahmad et al., 2002).

Countries that already contain many begomoviruses that infect indigenous plants and weeds, and are associated with localized populations of B. tabaci that exhibit narrow host ranges may be particularly at risk from MED species. Should MED species become established in these countries its polyphagous feeding activities could enable viruses to move into new susceptible host plants, causing new crop protection problems.

MED species is documented as being polyphagous, having a host range of around 600 different plant species. This includes many glasshouse and field crops, as well as weeds. However, a study by de Courcy Williams et al. (1996) indicated that only a small number of individuals within a population can readily change host plants. It is the progeny of these particular individuals that lead to the species as a whole being highly polyphagous.

Eggs are usually laid in circular groups on the underside of leaves, with the broad end touching the surface and the long axis perpendicular to the leaf. They are anchored by a pedicel which is inserted into a fine slit made by the female in the plant tissue, and not into stomata as is the case with many other members of the Aleyrodidae. Eggs are whitish in colour when first laid, but gradually turn brown. Hatching occurs after 5-9 days at 30°C but this depends very much on host species, temperature and humidity.

On hatching, the first instar, or 'crawler', is flat, oval and scale-like in shape. The first instar is the only larval stage of this whitefly which is mobile. It moves from the egg site to a suitable feeding location on the lower surface of the leaf, after which its legs are lost in the next moult and the larva becomes sessile. It does not move again throughout the remaining nymphal stages. The first three nymphal stages each last 2-4 days, according to temperature. The fourth nymphal stage is termed the puparium, and is approximately 0.7 mm long. True pupation within the whitefly life-cycle does not occur, although the last (fourth) nymphal instar is typically referred to as a pupa after apolysis has been completed. Metamorphosis to adult occurs over about 6 days.

The adult emerges through a 'T'-shaped rupture in the skin of the puparium and spreads its wings for several minutes before beginning to powder itself with a waxy secretion from glands on the abdomen. Copulation begins 12-20 hours after emergence and takes place several times throughout the life of the adult. The lifespan of the female can extend to 60 days. The life of the male is generally much shorter, being between 9 and 17 days. Each female can oviposit over 300 eggs during her lifetime, these are often arranged in an arc around the female as she rotates on her stylet. Some 11 to 15 generations can occur within 1 year.

Virus Transmission

MED species is also responsible for vectoring many plant viruses in the genera Geminivirus, Closterovirus, Nepovirus, Carlavirus and Potyvirus. Whitefly-transmitted geminiviruses, now designated begomoviruses (Mayo and Pringle, 1998), are the most important of these agriculturally, causing yield losses to crops of between 20 and 100% (Brown and Bird, 1992; Cathrin and Ghanim, 2014). Begomoviruses cause a range of different symptoms which include yellow mosaics, yellow veining, leaf curling, stunting and vein thickening. Mansoor et al. (1993) reported that 1 million hectares of cotton were decimated in Pakistan by Cotton leaf curl virus (CLCuV) and important African subsistence crops such as cassava are affected by begomoviruses such as African cassava mosaic virus (ACMV) (see datasheet on African cassava mosaic disease). Tomato crops throughout the world are particularly susceptible to many different begomoviruses, and in most cases exhibit yellow leaf curl symptoms. This has caused their initial characterization as Tomato yellow leaf curl virus (TYLCV). A number of different species of TYCLV have now been recorded from within both the New World and Old World where MED occur (Jones, 2003). MED species has also been associated with transmission of several other begomoviruses such as Tomato mottle virus, Tobacco leaf curl virus (TLCV), Sida golden mosaic virus (SiGMV), Squash leaf curl virus (SLCV), Cotton leaf crumple virus (CLCV), Bean golden mosaic virus (BGMV) and Cotton leaf curl virus (Simon et al., 2003), some of which cause heavy yield losses in their respective hosts. Dual infections have also been shown to occur (Bedford et al., 1994c).

Natural enemies of whiteflies will, in most cases, have co-evolved with their prey and may, in some regions, be more efficient at controlling their native prey than an introduced one.

Various species of predatory mites have also been shown to be effective in feeding upon Mediterranean species of B. tabaci populations, including; Amblyseius limonicus, A. swirskii and Transeius montdorensis (Cuthbertson, 2014). No doubt they will also feed on other Bemisia species. A large range of natural enemies of B. tabaci have been recorded in China (Li et al., 2011). Their specificity to individual members of the B. tabaci species complex is unknown.

Entomopathogenic nematodes and fungi have been shown to offer much potential in controlling what has now been determined as MEAM1 species Bemisia populations (Cuthbertson et al., 2003a, 2005a, 2007a,b; Cuthbertson and Walters, 2005). The impact of these agents upon MED species has yet to be determined.

Numerous chlorotic spots develop on the leaves of affected plants, which may also be disfigured by honeydew and associated sooty moulds. Leaf curling, yellowing, mosaics or yellow veining could indicate the presence of whitefly-transmitted viruses, and phytotoxic responses such as a severe silvering of courgette and melon leaves indicate the presence of MED species, the immature stages being mainly responsible for this symptom (Costa et al., 1993). Other phytotoxic responses to themed species include mis-ripening of tomato fruits (Maynard and Cantliffe, 1989), white streaking of Brassica leaves (Brown et al., 1992) and yellow veining of some solanaceous plants (Bedford et al., 1998).

Close observation of the undersides of the leaves will show the tiny, yellow/white larval scales and in severe infestations, when the plant is shaken, numerous small, white adult whiteflies will flutter out and quickly resettle.

B. tabaci is now widely regarded to be a multi-species complex. Consisting of as many as 34 species that are morphologically indistinguishable from each other. They can however, be distinguished molecularly (De Barro et al., 2011; Tay et al., 2012; Boykin and De Barro, 2014).

Differentiation of MED species from other whitefly species on the basis of adult morphology is often difficult, although close observation of adult eye morphology may often show differences in ommatidial arrangements between some species. At rest, MED species has wings more closely pressed to the body than Trialeurodes vaporariorum, which is a larger whitefly and more triangular in appearance.

The fourth instar or puparium can also be used to distinguish MED species from T. vaporariorum as a glasshouse pest. T. vaporariorum is 'pork-pie shaped', regularly ovoid, has straight sides (viewed laterally) resulting from the vertical wax pallisade surrounding each puparium, and in most instances, 12 large setae. MED species has an irregular, 'pancake-like' oval shape, oblique sides and shorter, finer setae. Although the number of enlarged setae in the MED species and wax rods in T. vaporariorum can vary according to host plant morphology, the two caudal setae are always stout and nearly always as long as the vasiform orifice in the B biotype. The length of caudal setae can be used to distinguish some Bemisia species.

For more information on the identification of B. tabaci from slide-mounted pupae, see Martin (1987).

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Cultural Control

Intercropping practices using non-hosts have been used in many countries aiming to reduce numbers of whiteflies on specific crops. However, intercropping with susceptible crops can promote whitefly populations, by offering a greater leaf area for feeding.

Weed species can play an important role in harbouring whiteflies between crop plantings and attention should be paid to removing these in advance of planting susceptible crops. Weeds also often harbour whitefly-transmitted viruses (Bedford et al., 1998) and may be a major source of crop virus epidemics, especially where MED species is present, due to its polyphagous nature.

Cultural control is generally much more effective where whiteflies are physical pests rather than virus vectors.

Host-Plant Resistance

The development of transgenic resistant plant and crop species through genetic engineering must be considered and accepted as a future method of control where whitefly-transmitted viruses are already endemic and causing severe crop losses (Wilson, 1993; Raman and Altman, 1994). Traditional sources of resistance have been used successfully for the control of other whitefly species.

Chemical Control

The following active ingredients have been reported as effective in controlling MED species worldwide: bifenthrin, buprofezin, imidacloprid, fenpropathrin, amitraz, fenoxycarb, deltamethrin, azadirachtin, pymetrozine. Chemical resistance is an ever increasing concern within the B. tabaci species complex. The MED species is widely considered to evolve stable resistance to the neonicotinoid insecticides commonly used for Bemisia control more rapidly than MEAM1 (Nauen, 2005). Hence, MED species neonicotinoid resistance is becoming increasingly widespread and problematic, with numerous cases being reported worldwide (Horowitz et al., 2005; Fernandez et al., 2009; Dennehy et al., 2010; Luo et al., 2010; Wang et al., 2010). However sequential chemical treatments have been shown under laboratory conditions to offer excellent potential in eradicating MED species on poinsettia plants (Cuthbertson et al., 2012).

A rotation of insecticides that offer no cross-resistance must therefore be used to control infestations.

A new group of environmentally safer insecticides that effectively kill whitefly by a physical mode of action are appearing on the market in many countries (Cuthbertson and Collins, 2015). These products do not have a specific active ingredient, but appear to utilise surfactant-like properties to overcome the protective waxes on whitefly larvae and adults.

IPM

The increased fecundity and polyphagous habit of MED species has exacerbated many control problems in field and glasshouse crops worldwide, compounded by insecticide resistance. It appears that no single control treatment can be used on a long-term basis against this pest, and that approaches should be integrated to achieve an effective level of control.

IPM appears to offer the best option for controlling MED species infestations without causing contamination of the environment. Beneficial insects are used alongside chemicals that offer a high level of selectivity, such as insect growth regulators. However, if whitefly-transmitted viruses are present, it is unlikely that the threshold of whitefly vectors would ever be reduced to a level where virus transmission would cease by using these methods, because MEAM1 is such an efficient viral vector. Plant and crop species that exhibit a high level of resistance to both vector and virus must also be considered when designing an IPM system.

Entomopathogenic nematodes and fungi have been shown to be successfully tank-mixed with several chemical products for use in eradication programmes against what has now been deemed as MEAM1 species in the UK (Cuthbertson et al., 2003b, 2005b, 2007b). Their impact upon MED species populations has not been fully determined. Cuthbertson et al. (2012) devised a sequential treatment programme that successfully eradicated MED species on poinsettia plants under laboratory conditions using both chemicals and entomopathogenic fungi.

Phytosanitary Measures

In countries where MED species is not already present, the enforcement of strict phytosanitary regulations as required for B. tabaci in general, may help to reduce the risk of this whitefly becoming established (Cuthbertson and Vänninen, 2015).

Because of the difficulty of detecting low levels of infestation in consignments, it is best to ensure that the place of production is free from the pest (OEPP/EPPO, 1990). Particular attention is needed for consignments from countries where certain B. tabaci-listed viruses, now on the EPPO A1 or A2 quarantine lists, are present. These viruses can also be transmitted by MED species.

25/07/15 Original text by:

Andrew Cuthbertson, Food and Environment Research Agency, Sand Hutton, York, UK