Sampling procedures and action threshold level of vectors of viruses that cause maize lethal necrosis disease in Kenya

Abstract

Maize lethal necrosis disease (MLND) has emerged as a major threat to maize production in East Africa. The disease is caused by a co-infection of maize by maize chlorotic mottle virus (MCMV) and sugarcane mosaic virus (SCMV) which are respectively vectored by corn thrips, Frakliniella williamsi Hood and corn leaf aphids, Rhopalosiphum maidis Fitch. Developing a reliable and practical sampling plan and action threshold for population monitoring of the two vectors is key for implementing a reliable integrated vector management strategy. A study was carried out to determine the sampling regime and action threshold levels for the vectors. Three maize stratas (upper, mid and lower foliage levels), three sampling intervals and five spray regimes using Thunder OD 145 (110 g/L Imidacloprid and 100 g/ L Betacyfluthrin 45 g/L) (were tested. The upper plant strata proved a preferred target for corn thrips (P < 0.001) while the lower section was preferred by corn leaf aphids during season 2(P = 0.03). Scouting at 8.30 to 10.30 am (P = 0.04) gave a better estimation of the vector infestation. Corn thrips infestation was significantly lower on maize sprayed on a weekly regime (P = 0.01). However, maize crop from thunder sprayed plots was MLND negative while the unsprayed maize crop was positive (P < 0.001). Although the yield from weekly sprayed plots was higher than any other treatments (P < 0.001), the net returns from monthly sprayed plots was higher. The action threshold level for corn thrips and corn leaf aphids was found to be six and three per plant respectively. These findings provide information on how, when and where to scout and spray for the two vectors. This may eventually lead to minimal wide spread of the virus by the vectors.