Spatial patterns of western flower thrips (Thysanoptera: Thripidae) in apple orchards and associated fruit damage

Abstract

Western flower thrips, Frankliniella occidentalis (Pergande), is an economic pest of apples in orchards of North America. Western flower thrips causes damage (“pansy spot”) to apples by its egglaying activities during the bloom and immediate post—bloom periods. Difficulties in monitoring this pest and incomplete understanding of its biology during the bloom period have complicated control efforts in apple orchards. Densities of western flower thrips were monitored in seven (2003) or eight (2004) apple orchards at each of four bloom stages; in each orchard, thrips counts in blossom clusters were estimated at four to six distances into the orchard from an orchard edge that abutted native sagebrush-steppe habitat. We hypothesized that numbers of thrips in blossoms would decline with increasing distance along transects into orchards if the native habitat acted as a source of thrips. Thrips numbers in blossom clusters peaked at full bloom and petal fall. Densities showed a linear drop with increasing distance into the orchard, which we interpreted as evidence that the native habitat adjacent to each orchard did indeed act as a source of thrips moving into the orchards. Pansy spot incidence declined with increasing distance into the orchard. The major drop in damage occurred between the border row trees and samples taken at the adjacent distance (nine m away), suggesting that border rows adjacent to native habitats should be monitored with particular care. Regression analyses showed that damage and thrips density were positively correlated, albeit with substantial levels of unexplained variation in levels of damage.