First Record of Spiraling Whitefly in Coastal Kenya: Emergence, Host Range, Distribution and Association with Cassava Brown Streak Virus Disease.
Unconfirmed reports of a spiraling whitefly-like pest suspected to be contributing to high cassava brown streak disease (CBSD) incidences on cassava prompted a study to determine its presence and importance in coastal Kenya. Multistage sampling was conducted to determine occurrence and distribution of the whitefly, Aleurodicus dispersus (Homoptera, Aleyrodidae) in relation to CBSD incidence in popular cassava cultivars in four cassava growing districts of coastal Kenya. A. dispersus has a wide host range (56) of different food crops, fruit trees, shade trees, ornamentals, weeds and vegetables. CBSD was prevalent in all the districts surveyed with highest incidence being in Msambweni (79.4%) followed by Kwale (68.3%) and Kilifi 64.0%, whereas the lowest (42.8%) prevalence was reported in Kaloleni district. There was a significant positive correlation (r=+0.5189, p5 months) plants.
High A. dispersus populations were recorded in districts geographically located near Indian Ocean than those off the coastal belt (Table III). However there was significantly (P<0.05) low whitefly population in Shimba hills as compared to the rest of the areas sampled in Msambweni and Kwale districts, whereas presence of whitefly presence was not recorded in four fields sampled in Mbuguni location of Matuga division in Kwale district (Table III). In general, there was a low A. dispersus population in Kaloleni district than any other district surveyed.
CBSD was prevalent in all the districts surveyed with the highest (79.4%) incidence recorded in Msambweni and lowest (42.8%) in Kaloleni (Table III). CBSD incidence similar to the A. dispersus populations also followed a general trend of decline with distance away from coastal belt. For instance, Mtwapa (86.0%), Mavueni (68.3), Msambweni (86.4%) and Mlalani (76.0%) had higher CBSD incidences and were closer to the ocean than Mbuguni (42.0%), Chonyi (35.1%) and Kayafungo (45.0%) (Table III). Nevertheless there were certain locations, which did not fit in this trend such as Goloni, which had a CBSD incidence of 80.0%. The correlation between the number of adult A. dispersus and CBSD incidence was significant (r= 0.519, p<0.001) and positive.
This is the first report on the occurrence and distribution of the spiraling whitefly in coastal Kenya. The results of the biometric study of the adult spiraling whitely are similar to the findings in earlier studies regarding the size, egg laying pattern and host range (Waterhouse & Norris, 1989; Gill, 1990; Wijesekera & Kudagamage, 1990; Palaniswami et al., 1995; Geetha, 2000).
The list of spiraling whitefly hosts (56) is narrower as compared to the extensive host range of 481 plants already reported belonging to 295 genera from 90 families (Srinivasa, 2000). This may suggest that the whitefly has not exhaustively colonized all the available hosts in coastal Kenya. However, the list already points to the polyphagous nature of the spiraling whitefly.
A. dispersus was noticed in 2002 in Unguja Island Tanzania but there is uncertainty about its introduction and spread in Kenya. During a survey conducted in the Eastern Zone of Tanzania mainland in August 2004, the pest was found in Tanga, Coast and Morogoro regions, where twelve crops namely banana, cassava, cashew nut, a number of vegetables and fruit crops were reported to be infested (Pallangyo et al., 2007). This suggests a possible spread of A. dispersus from Tanzania along the coastal belt shared with Kenya. It is worth noting that the whitefly occurs mainly along the coastal region with the highest counts being observed in Msambweni, which borders coasta
Table III: CBSD Incidence (%) and Aleurodicus dispersus counts in selected cassava growing locations in coastal Kenya
District###Location###A. dispersus counts###CBSD incidence (%)
Tanzania. It is plausible that inadvertent introduction of A. dispersus into Kenya may have also happened through planting materials and through fruit and vegetable imports since the pest lays eggs both on fruits and leaves.
The widespread and large host range pose a difficult pest management puzzle in Kenya. In addition, several factors such as the wide host range, damage potential and rapid spread (migratory behavior) can enable the whitefly to maintain its status as a serious pest on a wide variety of plants. Hence for a successful management of this pest, a detailed study of its biology and host range is necessary so as to find out the vulnerable period during the lifecycle and also the perennial sources of infestation.
A positive correlation between the CBSD incidence and the number of adult whiteflies indicated a considerable contribution of the whiteflies in the spread of the CBSV. It had been observed in Tanzania that considerable spread takes place between plants through vector transmission (Robertson, 1987). All the districts and locations, where spiraling whitefly was observed are geographically located at low altitudes (0-300 m). This coincides with the general delimited distribution of CBSD along the coastal lowlands where high disease incidence occurs at altitudes below 300 m, less common between 300 to 700 m and rare at altitudes above 700 m (Hillocks et al., 1999 & 2002). For instance, in Msambweni the highest A. dispersus populations coincided with the highest CBSD incidence. Since this survey was limited in scope, there is need for wide coverage surveys for spiraling whitefly in the regions, where cassava is grown to ascertain its status in these regions.
In conclusion, this is the first record on the occurrence and distribution of the spiraling whitefly in coastal Kenya. Further studies to determine whether A. dispersus transmits CBSV will have a direct bearing in understanding the epidemiology of CBSV and hence its management.
Acknowledgement: The authors thank the Eastern African Regional Research Network for Biotechnology, Biosafety and Biotechnology Policy Development, BIO-EARN for financial support, Dr. Muinga, Dr. Saha, Dr. T. Munga (all of KARI-Mtwapa) for their support and availing green facility for the experiments. Many thanks to Dr. S. Ariga and Mr. Aura (both from University of Nairobi) for their help in the identification of hosts and spiraling whitefly.
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Department of Plant Science and Crop protection, Faculty of Agriculture, University of Nairobi, P.O. Box 30197-00200 Nairobi, Kenya
Kenya Agricultural Research Institute/National Agricultural Research Laboratories, P.O. Box 14733, 00800 Nairobi, Kenya
Department of Horticulture, Jomo Kenyatta University of Agriculture and Technology, P.O. Box 62000, 00200 Nairobi, Kenya
Faculty of Agriculture, Makerere University, P.O. Box 7062, Kampala, Uganda
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|Publication:||International Journal of Agriculture and Biology|
|Date:||May 31, 2010|
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