Using statistical methods to determine spatio-temporal distribution of aphids and aphidophagous ladybirds in alfalfa fields

چکیده مقاله

Abstract. This study was conducted to determine distribution pattern and related parameters
of spotted alfalfa aphid, Therioaphis maculata (Buckten), pea aphid, Acyrthosiphon pisum
(Harris) and two coccinellid species (Coccinella septempunctata L. and Hippodamia variegata
Goeze) in six alfalfa fields during two growing seasons, 2016 and 2017. Weekly sampling was
started when alfalfa plants reached 10 cm in height and continued until harvest. Each field was
divided into 20m×20m plots. Two samples per plot per sampling time were randomly taken
using a 1 × 1 m quadrat, to count adult coccinellids. Twenty alfalfa stems (longer than 10cm)
per plot were randomly cut and shaken eight times on a white pan to record the aphids' density.
The aphids that fell into the pan were counted and recorded. The ratio of variance to mean, as
well as Taylor’s power law (TPL) and Iwao’s patchiness index (IPI), were used to determine the
spatial distribution patterns of the insects. The variance exceeded the mean on all sampling
dates, which may imply an aggregative distribution. Aggregation was also confirmed by using
regression coefficient of both TPL (parameter b) and coefficient of IPI (β), which both were
significantly above unity. Correlation between aphid counts and those of the coccinellids
indicated that there was a positive but weak relationship between predator and prey densities in
the fields, although a weak correlation may imply that factors other than prey accessibility
contribute to the spatial distribution of the predators. Using different statistical methods
provides useful information about the spatial distribution and dispersal pattern of aphids and
natural enemies in alfalfa fields, and this information can be used in the biological control of pests.