Abstract
The aim of this research was to study spatial distribution of flower thrips on French beans (Phaseolus vulgaris L.) in Kenya. Their build up and seasonal population dynamics was monitored using sticky blue colour traps and sampling of leaves and flowers in two seasons in 2002. Thrips infested French beans from the second week after crop emergence. Their population peaked at peak flowering. The sticky trap catches were linearly related to the actual presence of thrips on the crop and could estimate population build up of adult thrips on leaves and flowers. On the plants, most adults were on flowers. Larvae mainly inhabited leaves, buds and pods. The two thrips species, Frankliniella occidentalis (Pergande) and Megalurothrips sjostedti Trybom were spatially separated. The former colonized lower-canopy leaves and early flowers while the latter inhabited middle-canopy leaves and mature flowers. Overall, M. sjostedti was less than 5% of the total thrips population, implying that F. occidentalis was the main thrips pest of French beans. This study suggests that French bean growers should monitor thrips population before initiating any control measure. In addition, they should commence thrips control early, at pre-flowering, using larvicides to reduce the thrips pool and their migration to flowers. A combination of monitoring with sticky traps and proper sampling would contribute to sustainable thrips management.
Highlights
Thrips currently rank as primary pests of French beans (Phaseolus vulgaris L.) in Kenya (MOA, 2006)
The sticky trap catches were linearly related to the actual presence of thrips on the crop and could estimate population build up of adult thrips on leaves and flowers
M. sjostedti was less than 5% of the total thrips population, implying that F. occidentalis was the main thrips pest of French beans
Summary
Thrips currently rank as primary pests of French beans (Phaseolus vulgaris L.) in Kenya (MOA, 2006). Frankliniella occidentalis (Pergande) and Megalurothrips sjostedti Trybom are known as the main thrips species that cause 40-60% yield losses at farm level, mainly through abscission of buds, flower abortion and pod malformation making them unfit for the export market (Seif et al, 2001). Their punching and sucking feeding behaviour blemishes and causes silvery lesions on pods, resulting in a further 20% loss at harvest sites (Kibata and Anyango, 1996; Lohr, 1996). French beans are the most important horticultural crop in Kenya and they are grown mainly for export They contribute more than 55% of the value of vegetable exports and rank second, after cut flowers, in volume and value among export crops (MOA, 2006). The French bean industry supports the livelihood of many small-scale farmers, who contribute more than 80% of bean production, with an average gross margin of US $2,250 ha-1 season-1 (Minot and Ngigi, 2003)
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