Resilience of collembolan populations following exposure to an insecticide and their potential as bioindicators

Abstract

The effects of two organophosphate insecticides, dichlorvos and chlorpyrifos, on collembolan populations in a white clover field and mixed pasture respectively, were assessed. The rate at which the susceptible collembolan species' populations recovered in the treated area was measured as no significant difference between treatment and control populations.

Three 0.48 ha plots in a white clover field were sprayed with dichlorvos (Dichlorvos® 100E) in early summer (December 2, 1994). Samples were taken with a 'Vortis' suction machine over an eight-week period, between December 1994 and January 1995. Nine collembolan species were found in the field using this method of collection. The abundance of only two epedaphic species, Entomobrya multifasciata Tullberg and E. nivalis ssp. immaculata Womersley, were significantly reduced following the insecticide application. These two species were divided into two size classes, ≥1 mm and <1 mm. The rate of recovery into the treated plots was measured for these two species and their size classes. Recovery to control population levels 16 m into the treated area, of the larger size class of E. nivalis ssp. immaculata was within 28 days of spraying. The small size class of both species had recovered up to 4 m into the treated plots by the final sample day (28 days after spraying). The Staphylinidae community was also initially investigated for its potential as indicators of the effects of dichlorvos. The data suggests they were unsuitable as they were not abundant, were too mobile and reached peak abundance 3-4 weeks after spraying.

Two one-hectare plots in a mixed pasture were sprayed with chlorpyrifos (Lorsban® EC) in late summer (February 16, 1995). The recovery of E. nivalis ssp. immaculata and E. multifasciata of two size classes (≥1 mm, <1 mm), was measured. Samples were taken with a 'Vortis' suction machine at set distances into the treated plots to measure the rate of recovery. Five polythene walled plots each 25 m², were erected in the centre of one of the treated plots to reduce horizontal movement into the treated barrier plots from untreated areas. Recovery of collembolan populations in the centre of the one hectare plots took between 75 and 95 days. The largest size class took up to 125 days to recover into the barrier plots.

The movement of the two Entomobrya species of two size classes (≥1 mm, <1 mm) into an area treated with the insecticide, chlorpyrifos (Lorsban®, EC) was measured by setting out water traps at four different heights, 0, 0.05, 0.15 and 0.5 m. The water traps were positioned along two transects from the unsprayed area into the centre of the sprayed area. The greatest number (≥80%) were caught in the 0 m and 0.05 m traps. The majority (≥95%) belonged to the ≥1 mm size class. The main mode of horizontal dispersal was through active crawling and jumping by adults.