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

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.

Keywords

• distribution patterns
• taylor’ s power law
• iwao`s regression
• aggregation