Date : 1992
Type : Livre / Book
Type : Thèse / ThesisLangue / Language : français / French
Curculionidés -- Reproduction (biologie)
Résumé / Abstract : Baris coerulescens scop. est un nouveau ravageur du colza dans la region Poitou-Charentes. Les adultes envahissent les cultures a partir de fin février lorsque la température de l'air dépasse 15c. La ponte ne débute qu'à mi-avril. La vitellogénèse est sous l'influence de la température et du stade phénologique de la plante-hôte. Au laboratoire, une température de 20c avec une plante de stade d et une photophase de 16 h appliqués pendant 2 a 3 semaines sont en faveur d'une levée de la diapause chez les femelles qui ont hiverné pendant au moins 7 ou 8 semaines. La fécondite des baris, étudiée à 20c, est en moyenne de 315 ufs par femelle pendant 4 mois. 88% des ufs sont déposés dans le collet. La fécondité journalière est variable selon la qualite de la plante-hôte. Elle est reduite de moitié après 8 jours passés sur des plantes-hôtes présentant un fort déficit hydrique avant l'infestation. Le passage sur des plantes arrosées régulièrement permet un rétablissement de la production ovarienne. La fécondité de l'insecte est fortement dépendante de la température, elle est trois fois plus faible à 15c qu'à 20c. Lorsqu'elle a le choix, la femelle pond préférentiellement sur les plantes de fort diamètre au collet. La vitesse de développement des larves a été étudiée à 6 températures constantes : la croissance est presque inhibée à une température de 10c. Elle est optimale vers 30c. Une infestation précoce des plantes provoque une réduction de la production grainière de 19% par rapport à celle des plantes saines. Nous observons également une relation entre la précocite de l'infestation et l'importance du syndrome des pieds secs. Les fluctuations de population de b. Coerulescens sur colza semblent donc très dépendantes des conditions climatiques. De plus le colza, ne semble pas être la plante-hôte la plus favorable à la multiplication des baris, en raison de la précocité de sa maturation.
Résumé / Abstract : Baris coerulescens is a new pest on oilseep rape in the western part of France. A previous outbreak occured in other regions (Alsace, Rhône valley) in the seventies but never reached such a number of larvae per root. Growers were concerned that the damage caused to the roots could be detrimental to yield. As literature on the species (or even the genus) was scant, a whole investigation was started to find and understand basic elements of the biology of the pest. Experiments were conducted in open field or laboratory conditions. Baris coerulescens starts invading fields at the end of winter when maximum air temperatures are in excess of 15°C, with no strong wind and no rain. These conditions can happen any time between february and april in the region. Field sampling showed that new adults reach the fields till may, then th enumber starts to decrease. The spatial distribution of adults and eggs is agregative, but all plants are infested. Eggs are inserted under the epidermis of the lower part of the stern (2%), the root crown (88%), or the upper part of the root (10%). To avoid rejection of the egg by the plant, the laying female bore a semi-circular trench around the egg. These are four larval instars before nymphosis, which were characterized by the width of head capsules. New adults leave the decaying roots at the end of september. Some of them reach new sown rape fields where they feed for some time before overwintering in the soil, but most "disappear" in other overwintering places. In semi-artificial conditions, females can be active (to feed) on warm sunny winter days but do not mature sexually. There is an obligatory reproductive diapausis. After the overwintering period, vitellogenesis is under the influence of temperature and growth stage of the plant. In natural conditions, because of the length of the infestation flight, a field population is totally mature only after at least 47 days after the first captures. In laboratory conditions, after 7 to 8 weeks of overwintering, females reach full sexual maturity in 2 to 3 weeks at 20° with plants at growth stage D and a photophase of 16th. The potential fecundity of females was studied in laboratory conditions at 20°C. Laying lasted 4 months and females laid a mean number of 315 eggs. The mean daily fecundity was 3,5 eggs per female. It was subject to variations according to plant quality. Female given drought stressed plants during 8 days laid twice fewer eggs. When given well watered plants these females resumed a normal ovarian production. Egg laying was highly dependant on temperatures : twice more eggs were laid at 20° than at 15°C. When given the choice, a female lay more eggs on roots with big diameters than on small ones ; the relationship between diameter and number of eggs is proportional. Larval development was studies at 6 constant temperatures : growth was almost inhibited at 10°C (it took an average of 78 day to obtain 1st instar larvae) ; the optimum temperature for rapid growth without too much mortality was around 30°C. These high thermal requirement explain why, in natural conditions, the pest never completes its whole development (egg to nymph) before harvest, at the beginning of july. Final success of the most developped larvae will depend on how long the root will retain a sufficient quality to sustain development. For example, harrowing the stubble left in place after harvest to hasten dessication of the roots leads to a very high mortality : emerging adults are twice more numerous on the part not harrowed. The development of many larvae (5 to 10) in a root can cause yield losses as high as 19% as demonstrated in a cage experiment. Yield losses depend on the earliness of attack and are mainly caused by a reduction in 1000-seeds weight. Losses can be even more drastic as fungal diseases are more frequent on early attacked plants leading to premature drying of the stem. In conclusion, rape do not seem to be the best host plant for this pest, usually found on cabbage, because it is harvested too soon, and outbreaks of the pest will depend on very favorable weather conditions which do not seem to be frequent.