Modeling Winter Oilseed Rape Plant Infestation by Baris coerulescens (Coleoptera: Curculionidae)

Abstract

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 accumulation of eggs in plants poses problems in defining cohorts for modeling larval development under fluctuating temperatures. The total number of immature individuals per plant was assessed throughout the whole reproduction period during a 4-yr study. Two supplementary data sets were collected in 1994 and 1996. Total insect population density plotted against degree-days (in base 10) differed among years. The speed at which the population maximum was reached was steeper during wet years 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 population density per plant from the beginning of egg laying to maximum density is proposed. The insect population density is expressed as the cumulative proportion of individuals found in plants, varying in the interval (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.