The impact of host-plant stress on the performance of two insect biological control agents of Broom (Cytisus scoparius)

Weeds often grow over a wide range of environments, experiencing a number of environmental stresses with varying intensities. As a consequence these weeds will often exhibit differences in morphology and physiology. It has been suggested that these differences in host-plant morphology and physiology will affect insect herbivores. Determining how insect herbivores will be affected is important for weed management programs, so that environments where greatest establishment, population growth and impact on host-plants by weed biological control agents can be identified. The Plant Stress Hypothesis (PSH), Plant Vigour Hypothesis (PVH), and Insect Performance Hypothesis (IPH) have been used to explain changes in insect herbivore performance, when host-plants are experiencing environmental stress. A review of more than 200 studies on insect performance across five insect feeding-guilds and 12 stress types was conducted. It was found that both insect feeding-guild and stress type were important determinants of predicting insect performance, which were not considered by these three hypotheses. The performance of Leucoptera spartifoliella and Arytainilla spartiophila, two biological agents of Scotch broom (Cytisus scoparius) were assessed over a range of host-plant soil moistures and shading levels. It was found that overall performance of L. spartifoliella and A. spartiophila were greatest on broom growing in drier soil conditions. Populations of L. spartifoliella were greatest in sunnier environments under laboratory conditions or in the field when populations were low. However, when populations in the field were medium or high, there were no differences between shaded and non-shaded environments. The greatest impact to broom might occur in shaded environments, where broom is not growing optimally and is combined with the added pressure of L. spartifoliella attack. As the laboratory experiments did not allow ovipositing females to select between hosts of varying quality, further laboratory experiments were conducted, giving ovipositing females the choice of host-plants. Relationships between oviposition preference and oviposition performance were similar in most cases for L. spartifoliella and A. spartiophila across a range of host-plant soil moisture and shade gradients. The results give some support to the hypothesis that preference and performance are linked in monophagous insects that have immobile juvenile stages. These positive preference-performance results also increase the strength of predictions made in the earlier laboratory experiments. In a survey on L. spartifoliella pupal populations in New Zealand and in Australia, pupae were found on all ages of broom surveyed, from plants as young as two to 23 years old. No evidence was found to suggest that L. spartifoliella pupal populations were influenced by the age of host-plants. However, populations increased as the amount of available feeding resource (one-year-old shoots) increased. The continual feeding damage inflicted by L. spartifoliella on plants of a young age may hasten plant senescence and death. This study showed that environmental stress significantly affects insect herbivore performance, and that both insect feeding-guild and stress type need to be considered when predicting insect performance. Broom management strategies can be enhanced as environments where insect establishment, population levels and impact may be greatest have been identified. Control of broom by these two biological agents will be greatest where broom is growing in dry and shaded environments.