DEVELOPMENT AND APPLICATION OF POPULATION SAMPLING METHODS FOR THE STAGES OF NORTHERN CORN ROOTWORM, DIABROTICA LONGICORNIS (SAY) (COLEOPTERA: CHRYSOMELIDAE) IN QUEBEC CORN FIELD

Horizontal distribution of Diabrotica longicornis (Say) eggs around plants was random in three Quebec cornfields. Vertically, 72% and 24% eggs occurred in the surface 10 cm soil before and after fall plowing, respectively. A 5 x 10 cm x 15 cm (deep) sampling unit was appropriate for estimating egg populations before plowing. Thereafter, 20 cm depth was needed. Larvae and pupae were concentrated around the root system and a soil quadrat 20 x 20 cm x 10 cm (deep) was an easily reproducible sampling unit. Direct counting on corn plants in early morning was the most efficient method for adult sampling. Sample sizes for all stages were calculated for different precision levels. Spatial distribution of eggs and larval field populations were contagious, and a stratified random sampling is appropriate for density estimation. Spatial distribution of adults changed with corn phenology. Average mortality of overwintering eggs was 66% and of spring larvae 89%. The field population density of beetles did not change significantly during 1979-82.