Whiteflies are one of the top 10 most serious pest threats to agriculture. Although whiteflies, in the taxonomic family Aleyrodidae, are a diverse group of insects of more than 1,200 species, only a few of these are economically important. Among this small group, Bemisia tabaci (Genn.) is by far the most important single species.
B. tabaci was first described from tobacco in Greece, towards the end of the 19th century. Its progress has closely matched developments seen in agriculture in subsequent years, and it now occurs virtually throughout the crop-growing parts of the globe. Its preference for warm weather means that it is particularly prevalent in the tropics, although it has also been able to exploit protected agricultural environments in temperate regions.
If B. tabaci contented itself with doing its own thing and sucking small quantities of sap from the plants that it feeds on, it would probably have fallen under the radar of those whose job it is to protect crops. But it did not. Over time, it evolved a relationship with plant viruses, a relationship that allowed the whitefly to pick up viruses when feeding on plants, harbor them for some time, before introducing them to another plant during feeding, thereby giving rise to a new infection. This enabled the viruses transmitted to expand their ranges as B. tabaci populations grew and spread. These deadly partnerships thus gave rise to plant disease epidemics that had devastating impacts on the crops affected, and on the people growing them.
B. tabaci transmits many hundreds of virus species, a number that keep rising as more viruses are described and research efforts on the B. tabaci vector are also broadened. The viruses transmitted fall into four virus genera: Begomovirus (family Geminiviridae), Ipomovirus(Potyviridae), Crinivirus, and Carlavirus (Closteroviridae). More than 90% of the more than 100 species transmitted, however, are in the Begomovirus group. One of Africa's most economically destructive diseases, cassava mosaic disease (CMD), is caused by a group of viruses in the Begomovirus genus. Collectively, these are usually referred to as the cassava mosaic geminiviruses. Evidence also points to B. tabaci being the vector of cassava's other major expanding disease threat, cassava brown streak disease (CBSD) caused by the Ipomovirus, cassava brown streak virus.
[Ed:this text has been taken from the excellent blogpost by Dr James Legg of IITA. See original here and more information on IITA's efforts]
Whiteflies are small, white, fly-like insects in their adult stage. The nymphal stages are tiny, flattened, oval scales that have no obvious legs, do not crawl (except immediately after egg hatch for a day or so), and with no obvious head, thorax, or abdomen, do not look like a "typical" insect. Because of their appearance and their location on the undersurfaces of leaves, the nymphal stages may go unnoticed.
When cassava plants are young, start scouting for whiteflies on the undersides of leaves in the mornings or evenings. Scouting is important because early transmission of viruses by whiteflies can cause damage. Regular scouting is essential to detect whitefly incidence and avoid economic damage. Crops must be inspected at weekly intervals to find infestations early. Monitor whitefly population levels by trapping winged adults on sticky cards or inspecting leaves for the presence of adults and immatures. If available place yellow sticky cards throughout the cassava field or around borders to provide information about the presence and movement of whiteflies. Detect whiteflies on plants by randomly selecting 10 plants per field and thoroughly examining these plants on the underside of leaves for the presence of whitefly adults, nymphs, and eggs. Whitefly eggs are generally concentrated on upper new leaves of the host, and nymphs are usually found on the lower (old) leaves, so a good population estimation of whiteflies can be made by sampling leaves from different parts of the plants. A 10x hand lens may be needed to see eggs or small nymphs. Because the recommended management practices for the two biotypes may vary, it is important to determine the whitefly biotype before applying any chemical in the affected region. Density levels requiring treatments vary depending on factors including the crop, source of infestation, history of disease transmission, and environmental conditions.
Look for plants (especially at the margins of fields) with sooty moulds and/or diseased sumptoms (from Cassava Mosaic Disease or Cassava Brown Streak Disease). Look for whiteflies taking flight when foliage is shaken and immediately settling on the undersurfaces of the leaves.
There are a number of control strategies to be consiered.
Intercropping and Planting Strategies
Intercropping is an environmentally benign method to control whiteflies and is associated with lower whitefly populations, CMD incidence, and disease severity (Night, 2011). This planting strategy has the potential to decrease the need for insecticide use and is already commonly practiced by many smallholder farmers. Evidence shows that higher density intercropping is most effective in controlling whitefly populations (Fargette & Fauquet, 1990). High density planting on cassava plots is more important than height barriers of intercropped or edge crops such as maize to control whitefly populations (Fargette & Fauquet, 1988). However, the precise planting densities at research stations may differ from what smallholder farmers practice, so results may vary when replicated on farms.
Different crops intercropped with cassava produce varying results in reducing whitefly populations. Cowpeas were shown to be more effective than maize in reducing whitefly egg densities on cassava leaves (Gold, 1990). The type of intercropping also has an impact on whether intercropped cassava has greater or lesser yields when compared to monocropped plots. Intercropping with maize has been shown to reduce cassava yield (Olasantan et al., 1996) while intercropping with legumes has been shown to increase cassava yield (Islami et al., 2011; Njoku & Muoneke, 2008). Thus, cowpeas generally are beneficial for reducing whitefly populations and increasing cassava yields while maize may not have the same overall positive benefits. Some crops should not be planted near cassava. Bellotti (2012b) recommends not planting jatropha in proximity to cassava due to whiteflies’ ability to adapt to previously resistant cassava via other plant hosts.
Plant architecture is also an important factor influencing whitefly populations. Intercropping with certain crops can reduce cassava plant and leaf size since whitefly egg densities can be less on smaller leaves (Gold, 1990). However, different studies find varying impacts of intercropping on cassava leaf size. Gold (1990) found that intercropping cassava can reduce leaf size due to competition from maize and cowpeas, and Olasantan (1996) found the Leaf Area Index (LAI) 2 to be lower in 1F cassava intercropped with maize. However, Njoku & Muoneke (2008) found that the LAI was higher or similar in plots intercropped with cowpeas. While intercropping can influence the LAI (Njoku, 2008), it is also heavily influenced by genotype, plant age, and environment. (IITA, n.d.).
Intercropping may be effective at controlling whiteflies because it changes the microclimate or field ecosystem, altering their movement and behavior (Fondong, 2002) or because competition for nutrients in an intercropped setting alters cassava plants (Olasantan, 1996; Gold, 1990). Impacts of intercropping to reduce whitefly populations on cassava leaves varies at different times of the growing cycle with the greatest reductions occurring post-harvest of the intercropped crop (Fondong, 2002; Gold, 1990).
Most studies rely on counting eggs, nymphs or adult whiteflies on cassava plants to determine if intercropping is an effective method for controlling populations. However counting methods are not consistent. Since whiteflies have a restless behavior and the numbers seen on leaves depend on the time of day and weather conditions, the measured effectiveness of intercropping on those populations will vary depending on how they are counted (Sseruwago et al., 2004; Abisgold & Fishpool, 1990). While intercropping can reduce whitefly populations on cassava plants, it does not eliminate the flies completely. The linkage between whiteflies and CMD remains strong: among intercropped plots, those with higher populations of whiteflies also have greater incidence of CMD (Fondong, 2002).
Insecticides and Insecticide Resistance Management (IRM)
Widespread insecticide use has historically been viewed as an ineffective and environmentally damaging strategy to control whitefly populations (Horowitz et al., 2011). Insecticide application on cassava is particularly challenging due to the location of flies (under leaves), their highly polyphagous nature, and their easy dispersion by the wind (Horowitz et al.; Navas-Castillo et al., 2011). While applying insecticide can reduce whitefly populations, the CRS Great Lakes Cassava Initiative report found that insecticides did not stop the spread of CBSV, and plots treated with insecticide were more susceptible to CBSD (Catholic Relief Services, n.d.).
According to Castle et al. (2010), over-reaching insecticide use has resulted in heightened resistance among whiteflies, “tipping the balance between a manageable infestation and uncontrolled outbreak.” The majority of the literature agrees with the limited efficacy of insecticide-based control strategies due to environmental concerns and resistance (Horowitz et al., 2011).
In addition to environmental and health consequences, Thresh & Cooter (2005) advocate against insecticide use to avoid harming natural predators. Evidence from cotton (Eveleens, 1983; Dittrich et al., 1985) suggests that insecticides are more effective against natural enemies than against whiteflies, which can lead to population resurgence after insecticide use. Bellotti further agrees that farmers using insecticides to control whiteflies will also reduce the effectiveness of biological control (Bellotti in Anderson, 2005).
Despite the general consensus that widespread insecticide use is an ineffective control strategy, more recent research indicates that newer, more targeted insecticides and insect growth regulators (IRGs) are preferable because of their ability to target specific pests, their effectiveness at low application rates, and their non-persistent characteristics in the environment. Further, their selectivity renders many of them suitable for IPM programs (Casida & Quistad, 1998).
Newer, more selective IGRs have not been mentioned for use on cassava specifically (Horowitz et al., 2011). Producers in the U.S. have had the greatest success with novel insecticide chemistries such as Nicotinoids, Imadacloprid soil treatments, second-generation nicotinoids, and non-neurotoxic IGRs such as buprofezin and pyriproxyfen (Palumbo, et al., 2001). Insecticide resistance management strategies based on the structured and restricted use of non-neurotoxic IGRs, coupled
with the use of cultural and biological pest management tactics, are presently held to provide the best model for combating insecticide resistance in B. tabaci (Ellsworth et al., 2001).
In combination with IRGs, refuge strategies can be effective in preventing resistance to pyriproxyfen. Spring melon, alfalfa, and cotton not treated with insecticides provide refuge for B. tabaci and promote their survival. Results may be useful to predict the spatial determination of a refuge strategy. Cotton refuges delayed pest resistance while treated cotton fields accelerated it (Carriere et al., 2012). Insecticide resistance management (IRM) strategies are important in addition to insecticides to “incorporate newer chemistries into viable control programs that emphasize conservation of natural enemies and active ingredients” (Castle et al., 2010).
While natural enemies alone do not typically solve B. tabaci problems (Horowitz et al., 2011), introducing enemies and biological control as part of an integrated pest management system (IPM) may prove more effective. Biological control methods can be combined and used with other pest management techniques such as intercropping (Legg et al., 2003). Other whitefly control methods (breeding for resistance, insecticide) can negatively affect natural enemies, making whitefly control more challenging over the long-term. Various biological control methods may also be incompatible. For example, some fungi that suppress whitefly also affect whitefly predators and parasitoids. Biological control mechanisms shown to be successful against whitefly include parasitoids, predators, and fungal control. Exotic parasitoids or predators have been used successfully in other crops (Gerling et al, 2001) and may be effective for cassava in some cases after careful suitability studies (Aiisime et al, 2007).
Biological control was initially dismissed as a control mechanism on cassava because until the 1990’s whitefly was not recognized as causing substantial direct damage. Interest was renewed when direct damage was noted in Uganda and elsewhere (Thresh & Cooter, 2005). Introducing exotic enemies poses risks, but may be more effective at controlling than local natural enemies. Natural enemies can be introduced, conserved, or augmented (Bellotti et al., 2012a). Conservation or augmentation of local natural enemies may be an effective strategy, particularly in areas where insecticide use has changed natural balances between pests and enemies (Legg et al., 2003).
Research is ongoing to identify natural enemies of whitefly and design interventions to use them for controlling whiteflies (Legg et al., 2006). Gerling et al. (2001) and Arno et al. (2010) identify 38 spider species and 123 insect species that are predators of B. tabaci (Horowitx et al., 2011). Predators are used primarily in greenhouse conditions and several are available commercially (Horowitz et al., 2011). However, predators may be specific to the host plant (Horowitz et al., 2011), so results from studies of other plants may not be applicable to cassava. Bellotti et al. (2012a) identifies Delphastus pusillus, and Condylostylus as whitefly predators on cassava. Aiisime et al. (2007) recommends conserving and/or enhancing parasitism to control whiteflies by developing cassava varieties that resist B. tabaci, but encourage survival of parasitoid species. They also recommend introducing exotic parasitoids (after careful suitability studies).
While parasitoids have been used most commonly in greenhouses, exotic parasitoids have also been introduced to control whiteflies on outdoor crops and nurseries (Gerling et al., 2001), including Eretmoccerus in Australia (De Barro & Coombs, 2008) and in Arizona, USA (Gould et al., 2008). Bellotti et al. (2012), identifies six parasitoids of B. tabaci on cassava: Ecarsia Sophia, E. lutea, E. Formosa, E. mineaoi, Encarsia sp., and Eretmocerus mundus. Appendix 2 provides Bellotti’s table of enemies for all species of whitefly that feed on cassava. Introduction of an exotic parasitoid to Africa successfully and economically controlled cassava mealybug and green mite, suggesting biological control of whiteflies could be feasible (Bellotti et al., 2012a; Bellotti et al., 2012b).
Products based on fungi, (Verticillium lecanii, Paecilomyces fumosoroseus and Beauveria bassiana) have the capacity to suppress whitefly (Faria & Wraight, 2001). Horowitz et al. (2011) also notes Aschersonia and Metarhisum as infectious to whitefly. Beauveria bassiana is sold as Eco-Bb by Plant Health Products for the control of whiteflies in South Africa and Zambia on beans, tomatoes, cucumbers, and eggplant. Constraints to effective use of fungal whitefly control include “slow action, poor adulticidal activity, potentially negative interactions with commonly used fungicides, relatively high cost, limited shelf life, and dependence on favorable environmental conditions” (Faria & Wraight, 2001). Bellotti et al. (2012b) notes these products appear to only be successful when applied when whitefly populations are low. Fungal pathogens can be delivered by spraying the underside of the crop leaves, but Faria & Wraight (2001), note in a paper geared towards commercial agriculture that cost is prohibitive. This suggests fungal products are unlikely to be economically feasible for small-scale cassava farmers. While fungi with potential for whitefly control do not pose risks for vertebrates, some types infect whitefly predators and parasites, potentially limiting these other mechanisms of whitefly control (Faria & Wraight, 2001).
[Ed: this whole section taken from Slackie et a 2013 EPAR Technical Report No. 233. You can find the references in that publication]
In 2016 Sri Lankan cassava mosaic virus (SLCMV) was reported for the first time infecting cassava in Southeast Asia. This report consisted of positive virus detection from a single commercial plantation with symptomatic plants in Ratanakiri province in Eastern Cambodia in May, 2015. Prior to this positive identification, Southeast Asia had been considered free of cassava mosaic disease (CMD). Given the negative effects on production and economic returns of CMD in other settings, an alert to notify the presence of the disease in the region was warranted . At the time a window of opportunity for effective disease control through eradication or quarantine seemed apparent, as presence of the disease was assumed to be restricted to a limited geographic area.
Like other cassava mosaic geminiviruses (CMGs), such as African cassava mosaic virus (ACMV) and Indian cassava mosaic virus (ICMV), Sri Lankan cassava mosaic virus (family Geminiviridae, genus Begomovirus) is a causative agent of CMD. In Asia, the occurrence of CMGs has historically been restricted to South Asia, with the exception of a report of ICMV on Jatropha curcas in Singapore . The recent report of SLCMV in Cambodia expands on previous identifications in Sri Lanka and India [1, 9–11]. Like other CMGs, SLCMV is transmitted by the whitefly Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae), and through the movement of infected planting materials . With ACMV, plants grown from infected cuttings are more seriously affected than those infected by the whitefly vector . Although whitefly vectoring has contributed greatly to CMD outbreaks across Africa, an epidemiological study in India demonstrated that primary spread in that context occurred through the use of infected planting materials, with whitefly vectoring playing a secondary role . The evidence for virus-induced quality degeneration of planting materials, associated yield decline, and consequent economic effects is abundant for CMD in Africa [2,15]. The level of yield decline experienced depends on several factors, including varietal responses, symptom severity, and means of propagation [13,16–18]. Experiences in Africa showed yield loss from CMD to be greater in cassava grown from infected cuttings (55–77%) than in plants infected later through whitefly vectoring (35–60%) .
[Ed:The previous two parapgraphs are from a paper by Minato et al 2019 in PloS One. The refs directly link to that paper and are left in]
The relative importance of the whitefly vector in SE Asia is unknown.
This list will expand through time