Ecologie de Liriomyza trifolii Burgess 1880, principal bioagresseur du céleri (Apium graveolens L. 1857) dans le bassin maraîchaire de Nkolondom (Yaoundé - Cameroun)

Abstract

The celery, Apium graveolens is one of the main market-gardening cultivated in Nkolondom and others peri-urban areas of Yaoundé. This vegetable is unfortunately damaged by larvaes of Liriomyza trifolii. The larvaes of this parasite dig galleries on the celery’s leafs which become improper to consummation and commercialization. In order to find strategies for an integrated pest management of celery, the present study was realized from 2008 to 2011 in to sites: Nkolondom (periurban village of Yaoundé) and the Campus of the University of Yaoundé I. The research of the means about the celery’s integrated pest management depended to the study of the bio ecology of L. trifolii and its natural enemies. This study was based on (1) the determination of the agronomic statute of insects present in agro system based on the cultivation of celery, (2) the study of some biological and ecological features responsible of the numerical fluctuations of L. trifolii and its natural enemy’s populations. With the view to satisfy those objectives, many weekly prospections have been done in celery’s gardens where the sampling consist to: - cut infested leaves with a cutter; - collect adults and larvae of insect with soft holders or hoover. Infested leaves have been brought to the laboratory and incubated until the emergency of adult insects. Then adults were kept in 70 % alcohol, excepted Lepidoptera which have been conserved in dry. All those insects identified and counted have contributed to know the celery’s entomofauna and the numerical fluctuations of insects associated with the plant which are L. trifolii and its parasitoids Opius agromyzicola and Neochrysocharis agromyzae. The study of the biology and the ecology of L. trifolii and its parasitoids needed experimentations such as (1) the infestation of some celery plants by females of L. trifolii, (2) the infestation of larvae of L. trifolii by females of parasitoids (3) and field observations made on parasitoids. During these experimentations conducted in the garden and in the laboratory, the consideration of the impact of climatic factors, non imaginal development durations of L. trifolii and its parasitoids, impact of intra and interspecific competitions on the multiplication of parasitoids, foraging capacity of each specie of parasitoids and the impact of the use of insecticid has allowed to observed that those are the factors responsible of the fluctuations of the populations of L. trifolii and its parasitoids.; The present study showed that the entomofauna of celery (N = 7629 individuals) was constituted with 55.91 % associated to celery and 44.08 % for the others insects. The rate of proliferation of L. trifolii, O. agromyzicola and N. agromyzae have been respectively 28.05 %, 31.35 % and 40.59 %. The duration of the development of leafminers decreased when the larvaes have been reared during the dry period, in cut leaves inside the laboratory (16.53±0.26 days) compared to the duration obtained with larvaes reared during the raining period, in whole plants inside the garden (21.98±0.3 days). There was not significant differences between the reproductive capacity of L. trifolii reared in cut leaves and those reared in whole plants. The sanitary harvest is not and effective mean to protect celery against L. trifolii. Many intraspecific and interspecific competitions have been observed among females of parasitoids during the exploitation of the host larvaes. In such conditions, O. agromyzicola is koinobiont parasitoid who used only polyparasitoidism (many eggs-laying in the same host with the same parasitoid or many fellow creatures) has a less proliferation than N. agromyzae, an idiobiont parasitoid which in addition to the polyparasitoidism used successfully multiparasitoïdism (eggslaying by a specie of parasitoid in a host previously exploited by different specie of parasitoid). Koinobiont parasitoid is characterize by the fact that adult don’t kill the host during the egg-laying and her larvae kill the host at its pupal stage. On the other hand, idiobiont parasitoid is characterize by the fact that adult paralyse and kill the host during the egg-laying. The number of adults emerged from hosts larvaes of 3 days old were high with N. agromyzae and O. agromyzicola, that is the reason why these larvaes were the most appropriate for the reproduction of both parasitoids. O. agromyzicola had the highest proliferation rate in gardens during the raining period when host larvaes where scarce and generally alone on the infested leaves. About N. agromyzae, the abundance period was dry season when host larvaes were numerous in the garden. The use of insecticide treatment which hasn’t noticeable effect on the development of N. agromyzae has been unfavorable on the leafminer and particularly harmful on O. agromyzicola. .