Host Discrimination by the Green Peach Aphid (Myzus persicae Sulzer) Complex and Relative Importance of Non-Colonizing Aphid Species in the Transmission of Bushy-top and PVY in Tobacco
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This study focused on host preference by the tobacco-adapted form of Myzus persicae, suitability of different host plants to settling and reproduction of the aphid (M. persicae) and on monitoring aphid flight patterns using water traps. Focus was also on the ability to transmit PVY and Bushy-top virus of some non-colonising aphids observed on tobacco (Nicotiana tabacum) plants. Transmission efficiency of the non-colonising aphids was established through the real-time Polymerase Chain Reaction procedure on leaves of indicator plants. The non-colonising aphids evaluated were Brevicoryne brassicae, Aphis fabae and Aphis gossypii. Suitability tests were conducted on tobacco, Solanum tuberosum, Brassica rapa, Raphanus sativus, Nicandra physalodes, Solanum lycopersicum, Solanum nigrum, Solanum melongena, Galinsoga parviflora, Tagetes minuta and Prunus persicae. The tobacco-adapted M. persicae subspecies, Myzus persicae nicotianae, had a high preference for tobacco (which acted as the control), followed by S. tuberosum and B. rapa. No aphids were recorded on T. minuta, and a mean number of 0.33 aphid landings was recorded for N. physalodes. Tobacco also recorded the highest suitability index (2000). N. physalodes had a higher suitability (1666.7) compared to S. tuberosum (1466.7). Tagetes minuta was observed to be highly unsuitable for the development of M. persicae nicotianae. Aphids placed on T. minuta leaf discs failed to survive, and therefore no progeny were produced. Prunus persicae (peach tree), the primary host for M. persicae, was less preferred by the aphids, and was very unsuitable for their growth and reproduction. All the aphids placed on P. persicae discs did not survive. Aphid flight patterns were monitored at Kutsaga Research Station for a period of 14 months. The highest peak in M. persicae flight was observed in February 2011, and the lowest numbers were recorded in April 2011. All three non-colonising aphids that were tested for their ability to transmit PVY and bushy-top viruses to tobacco failed to do so. In conclusion, more replicated experiments using a large sample of plants as well as different aphid populations (Myzus and non-Myzus species) from iii the different tobacco-growing regions of the country are recommended before we can make concrete conclusions. Of particular importance would be to thoroughly investigate the role of residual tobacco plants in the seasonal carryover of M. persicae nicotianae and the associated virus diseases it transmits. A very important study would be aimed at exploring ways of analysing the gut contents of aphids so that the host plant origins of M. persicae alates caught in water traps during the tobacco off-season are known. If such a study exonerates residual tobacco plants, this may necessitate revisiting the legislation on tobacco ‘dead periods’. From a farmer’s perspective, such a result would point to the need to come up with an integrated pest management strategy which controls M. persicae on non-tobacco hosts during the tobacco off-season.