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Variations of the fecundity of a rearing population of Sahlbergella singularis Hagl. (Hemiptera: Miridae) in relation with cocoa tree phenology and canopy microclimatic conditions

Babin R., Dibog L., Anikwe J.C., Lumaret J.P.. 2010. In : 16th International Cocoa Research Conference. Proceedings : towards rational cocoa production and efficient use ofr a sustainable world cocoa economy. Lagos : Cocoa Producers' Alliance, p. 1127-1133. Conférence Internationale sur la Recherche Cacaoyère. 16, 2009-11-16/2009-11-21, Bali (Indonésie).

Mirid bugs are regarded as the most damaging pests of cocoa in Africa. In Cameroon, Sahlbergella singularis is by far the most common mirid species on cocoa. In nature, S. singularis populations display sharp seasonal fluctuations. Yet, in absence of integrated studies, including cocoa tree growth and physiology, chemical ecology, as well as mirid life history, the origin of these seasonal variations has not been clarified. Therefore, we studied the impact of cocoa tree phenology and canopy microclimatic conditions on survival and fecundity of S. singularis, with the aim of contributing to a better understanding of the seasonal variations of S. singularis. Ten to 25 newly moulted females were sampled from ten successive generations of a rearing population, i.e. generations G1 to G5 in 2006 and generations G8 to G12 in 2007. After a premating period of 5 days, females were mated for 24 hours and transferred on sleeved pods, in an on-station plot. Because they were embedded in the plant tissues, eggs were difficult to detect. Therefore, we assessed fecundity by a daily monitoring of nymph emergence. Cocoa phenology was characterized by weekly counting of flowers, growing pods, adult pods and ripe pods, of one branch of the ten cocoa trees used for experimentation. Vegetative growth was assessed by grading the flush abundance on a notation scale. Temperature and relative humidity were recorded within the cocoa canopy with a Testo humidity/temperature logger. Fecundity displayed great variations both within and between generations. Mean fecundity per generation decreased significantly from G1 to G5 and from G8 to G12. In 2006, mean fecundity dropped from 49.3 ± 41.4 nymphs/female to 4.6 ± 12.1 nymphs/female. In 2007, mean fecundity dropped from 89.2 ± 94.0 nymphs/female to 16.7 ± 21.1 nymphs/female. The maximum fecundity was obtained for a female of the generation G8, with 236 nymphs. Fecundity was correlated with the abundance of the fruiting organs and especially with the abundance of growing pods. In the other hand, individual fecundity was negatively correlated with the mean flushing score and the maximum daily temperature. Since mean fecundity varied according to natural populations, we hypothesize that the drop of fecundity is an important parameter of the decrease in mirid densities in nature. The use of such results in pest management strategies against cocoa mirids is discussed. (Texte intégral)

Mots-clés : sahlbergella; theobroma cacao; cameroun; sahlbergella singularis

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