Parkinsonia biocontrol: release and field evaluation of two new agents

Summary

Parkinsonia aculeata (parkinsonia) is a neotropical shrub/tree species that was introduced in the Australia as an ornamental species and for its potential value as a hedging and fodder plant. It has since spread to occupy over 8000km2 of the rangelands of northern Australia, and forms dense thickets in floodplains and grasslands, and along water courses and bore drains. It has negative impacts on the pastoral industry and rangeland production systems through limiting pasture growth, restricting stock access to water and impeding mustering. It also has impacts on the environment through providing refuges for feral animals like pigs, increasing evapotranspiration, contributing to soil erosion, and impacting wildlife habitat. At present widespread prickle bushes like parkinsonia can have control costs between $2-$300/ha/y depending on the density of infestations. Mitigating some of these control costs and improving pasture productivity can therefore assist in improving the profitability of rangeland production systems.

Mechanical and chemical control tactics for parkinsonia already exist and are already being effectively used by land managers wherever possible. But these management tactics require repeat application and are not always possible in all parkinsonia infestations (e.g. in difficult terrain or in sensitive riparian environments). Having a landscape-scale self-perpetuating form of control like biological control in these systems may therefore aid in the integrated management of parkinsonia. This was the basis for past projects funded by Meat & Livestock Australia (B.NBP.0366; B.NBP.0620) to identify candidate biological control agents, and the current project that focussed on mass rearing and release of the two most recently approved biological control agents approved for release against parkinsonia in Australia.

Based on detailed tests to demonstrate their safety, CSIRO received approval from the Commonwealth of Australia in 2012 and 2014, to release two closely related leaf-feeding moths, Eueupithecia cisplatensis and Eueupithecia vollonoides (nicknamed UU1 and UU2 respectively). In this project, we (1) determined an optimal release strategy to ensure widespread establishment of these agents across parkinsonia infestations in the rangelands of northern Australia; (2) mass-reared and released them following this strategy by forming strong partnerships between government and non-governmental agencies and regional landholders; (3) documented the establishment and potential impacts of these agents; (4) undertook a preliminary cost-benefit analysis on the potential benefits of investment in the parkinsonia biological control program, and (5) identified the role for biological control within an integrated management approach for parkinsonia.

From this project the following are the key messages from this project for producers, stakeholders and land managers.Management of parkinsonia is best done as part of an integrated management approach (Deveze 2004), and biological control is rarely a "silver bullet" management tactic on its own. Therefore, the following may be a prudent approach to parkinsonia management:

• Manage small parkinsonia infestations early by killing adult/reproductive plants using established chemical or mechanical methods.
• Use biological control in large infestations to allow them to be a chronic stressor on plant health and reproduction, while resources/time can be garnered to manage parts of parkinsonia infestations where the agents fail to establish due to some reason or are easy to access by other control tactics.
• Biological control represents a good investment despite the high initial development costs of identifying candidate agents and assessing their safety.
• Biological control does not provide instant/ near-term kill, but suppresses weed population growth and spread through impacts on plant health and/or reproduction. The impacts of biological control can take up to a decade to become apparent.
• Once agents have established permanent populations, biological control is a management tactic that is self-perpetuating requiring little additional ongoing investment other than redistributions of agents to sites that they cannot get to through natural dispersal.