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Résumé :
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The influence of duration of infestation by cowpea aphid Aphis craccivora Koch on aphid-resistant (ICV-12) and aphid-susceptible (ICV-I) cultivars of cowpea Vign?a unguiculata (L.) Walp, were compared to uninfested controls. Five apterous adult aphids of similar age were infested on seedling plants for five durations of infestation: 3, 7, 14, 21 and 28 days, respectively. Treatments were removed after stipulated durations and extended leaf heights of plants (cm), counts of aphids per plant and samples for final estimates of crop growth rate (g/dm(2) land surface area/day) and relative leaf area growth rate (dm(2)/dm(2)/ day), were recorded. After harvest, the pods per plant and seeds per pod were recorded as plant yields. ANOVA indicated significant (P less than or equal to 0.05) interactions between cultivar and duration of infestation, and between treatment and durations, for plant yields. Extended leaf heights of ICV-1 plants were significantly reduced after 7 days duration. Aphid counts were significantly higher on ICV-1 than on ICV-12 for all durations of infestation. Compared to ICV-12 plants, pods produced by ICV-1 plants were significantly reduced at all durations, while seeds per pod were significantly reduced by infestations of 7 days or longer. Time-based action thresholds within which Aphis craccivora infestations significantly reduced the growth and yield of the susceptible ICV-1 cultivar were 7-14 days for crop growth in biomass accumulation and leaf expansion, and 3-7 days for economic yields and extended leaf heights.
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