Year of Award


Document Type


Degree Type

Master of Science (MS)

Degree Name

Environmental Studies

Department or School/College

Environmental Studies Program

Committee Chair

Neva Hassanein

Commitee Members

Helen Atthowe


arthropod, Brussels sprouts, Bt, no-till, Pieris rapae, predator, pyrethrin


University of Montana


Agricultural systems are being re-engineered with hedgerows, living mulches, or minimum tillage activities in hopes of retaining populations of beneficial, predatory insects that may decrease the need for pesticide use. The purpose of this research was twofold. First, this on-farm research assessed the population and activity of six beneficial arthropod predators - the carabid beetle (Carabidae: Coleoptera), minute pirate bug (Orius spp.: Hemiptera), nabid bug (Nabis spp.: Hemiptera), lady beetle larvae (family Coccinellidae: Coleoptera), syrphid fly larvae (Syrphidae: Diptera) and spiders (Araneae) - and one crop pest - the imported cabbage worm (Pieris rapae: Lepidoptera) - within a no-till Brussels sprouts (Brassica oleracea, gemmifera group) and red clover (Trifolium pratense) living-mulch system. Second, using two common organic insecticides - Bacillus thuringiensis var. kurstaki (Bt) and a pyrethrin/rotenone blend – this research assessed the capacity of these aforementioned predatory or parasitic arthropods to control the P. rapae population and crop damage through biological (as compared to chemical) means. Field investigations for predators/pests involved weekly sweep-net sampling, pitfall trap installation, and direct plant examination. Insecticides were applied as a bi-weekly “calendar” application (pyrethrin/rotenone) or as a pest-density “threshold” dependant application (Bt). Generally, pest control and damage prevention were more successful in Bt treatments than in pyrethrin/rotenone treatments. Bt pesticides had no significant effect on any arthropods sampled, while the pyrethrin/rotenone insecticide appeared to significantly reduce the activity or population levels of all arthropods sampled. P. rapae activity and crop damage was lowest in Bt treated plots, moderate in control plots (no pesticides were applied, yet natural levels of arthropods were present) , and highest in plots treated with pyrethrin/rotenone sprays. Preliminary results indicate that Bt treatments worked as an additive control measure, which then augmented natural predator populations. The increased pest activity and damage in pyrethrin/rotenone treated plots - which coincided with reduced “beneficial” insect numbers as compared to the other treatments - may indicate a disruption of the multiple-arthropod predator assemblage that kept pest impacts lower in the “control” plots.



© Copyright 2006 Ethan A. Smith