MODELING WINTER OILSEED RAPE PLANT INFESTATION BY BARIS-COERULESCENS (COLEOPTERA, CURCULIONIDAE)

Baris coerulescens Scopoli is a pest attacking the tap root of winter oilseed rape. Females lay eggs in the root crown for >3 mo. The slow a ccumulation of eggs in plants poses problems in defining cohorts for m odeling larval development under fluctuating temperatures. The total n umber of immature individuals per plant was assessed throughout the wh ole reproduction period during a 4-yr study. Two supplementary data se ts were collected in 1994 and 1996. Total insect population density pl otted against degree-days (in base 10) differed among years. The speed at which the population maximum was reached was steeper during wet ye ars than during dry years. The hypothesis was that the differences in slopes were caused by drought stresses, which were known to influence egg laying from experiments in controlled conditions. A model based on the Weibull distribution and describing the growth of insect populati on density per plant from the beginning of egg laying to maximum densi ty is proposed. The insect population density is expressed as the cumu lative proportion of individuals found in plants, varying in the inter val (0,1). The initial degree-day scale is corrected by a coefficient linked to the upper soil water content. This correction ensured a good concordance among years and places. The model predicted correctly the dynamics of plant infestation under variable climatic conditions.