|DAANE, KENT - University Of California|
Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/5/2015
Publication Date: 2/1/2016
Citation: Hogg, B.N., Smith, L., Daane, K.M. 2016. Impacts of the adventive psyllid Arytainilla spartiophila on growth of the invasive weed Cytisus scoparius under controlled and field conditions in California. Environmental Entomology. 45:109-116.
Interpretive Summary: Scotch broom (Cytisus scoparius) is a leguminous shrub from western Europe that has invaded rangeland, woodlands and rights-of-way in the Pacific western USA. The plant produces thickets that displace native species and increase the risk and intensity of wildfire. Large quantities of seeds are produced which remain dormant until the soil is disturbed, which makes restoration extremely difficult. The plant contains quinolizidine alkaloids which are toxic to many animals including livestock. Scotch broom is a target of biological control in the USA, Australia and New Zealand. The psyllid, Arytainilla spartiophila, has been introduced to New Zealand, where it is reported to significantly damage the plant. This psyllid has been accidentally introduced to the western USA, but its impact on the plant is unknown. We conducted laboratory and field experiments exposing small plants to various numbers of psyllids to determine how much they affect the growth and survival of plant. High densities of psyllids reduced plant height by 45% in the greenhouse and 22% in the field during an 8-week period. The results indicate that the psyllids can significantly reduce growth of Scotch broom plants. However, effects on Scotch broom growth were weaker in the field, and impacts on survival and reproduction of the plant may be insufficient to control the plant population. Further studies to compare the plants, insects and environmental conditions in the USA and New Zealand should be conducted to understand what is necessary to control the plant.
Technical Abstract: The post-release impact of weed biological control agents on their target weeds is rarely assessed. This study focuses on the impacts of the univoltine broom psyllid Arytainilla spartiophila Forster on the growth of its target weed, the invasive shrub Scotch broom Cytisus scoparius (L.) Link (Fabaceae), in California. In a greenhouse experiment, plant growth as measured by both height and total stem length (height + length of all branches) was reduced on plants that received psyllids, and psyllid densities were negatively correlated to proportional change in plant height. There were no effects on number of leaves, however. Furthermore, the psyllid had no statistically detectable impacts on plant growth parameters in a field experiment, underscoring how results obtained under controlled conditions may not fully predict the performance of biological control agents in the field. The high psyllid mortality that occurred in both the greenhouse and the field suggests that the impacts of the broom psyllid may be moderated by intraspecific competition.