Performance and host-plant preference of two insect biological control agents on moisture stressed broom, Cytisus scoparius.

Glasshouse experiments were conducted to test the hypothesis that two broom (Cytisus scoparius) biological control agents perform better on, and prefer, broom that is growing vigorously. Insect performance and preference were assessed on host-plants subject to four moisture treatments for three months prior, and for the 12 month duration of the experiment. All measures of plant growth showed that broom grew most vigorously under moderate soil moisture treatments. Arytainilla spartiophila females avoided ovipositing on plants grown under saturated conditions, but showed no preference between other treatments. Performance (e.g. oviposition rate) was significantly higher on broom growing under moderate moisture than in saturated soil or at the lowest soil moisture content. For Leucoptera spartifoliella, preference (which treatment was selected when given a choice) was related to survival and pupal abundance, but not to oviposition performance. Results offer only partial support for the Plant Vigour Hypothesis that insects closely associated with their host-plant perform best on vigorously growing hosts, but tend to support the hypothesis that host-plant preference is related to performance of offspring for monophagous insects with immobile juvenile stages. With either agent present, several plant-growth parameters were reduced. This, combined with the ability of female insects to avoid unsuitable (less vigorous) host plants, confirms that the two insects are potentially effective control agents.