Abstract:
To estimate population density of the cabbage aphid, Brevicoryne brassicae (L.), a
stratified random sampling was conducted in two unsprayed canola fields in Isfahan
(central Iran) during 2011-2012. Population density was determined for apterous, and
alates, as well as the total population, at two plant growth phases (PGP1: From plant
emergence to the end of rosette, and PGP2: From the beginning of stem elongation to
ripening) on the whole plant as the sampling unit. Sources of variation in the sampling
procedure were analyzed with a nested analysis of variance (NANOVA). In PGP2, aphid
density in upper (10-15 cm upper part of stem) and lower (the rest of stem) parts were
compared using Student’s t-test. Temporal changes in spatial pattern during the growth
season were evaluated using 1/k (aggregation index) and Lloyd’s Patchiness Index.
Results showed that differences among fields accounted for the majority of total variation
observed in aphid densities and the aphids significantly preferred upper parts of canola
plants. Among different indices used for analyzing spatial distribution of the aphid,
Taylor’s Power Law (TPL) described well the relationship between variance and mean of
the population. In both PGPs spatial patterns of apterous, alates, and total population
were aggregated, random, and aggregated, respectively. Estimates of 1/k and changes in
the value of patchiness index revealed that the aphid population was aggregated at the
beginning of rosette stage and became more dispersed with time.
Keywords: Brassica napus, Between-plant distribution, Isfahan, Population density, Withinplant
distribution.
