Broad-leaved pepper tree

Broad-leaved pepper tree (Schinus terebinthifolius) invades coastal dune areas, wetlands and streams in NSW. It displaces native vegetation and forms thickets that shade and outcompete smaller plants. This sub-project’s goal was to reduce the threat of broad-leaved pepper tree to natural areas of NSW by developing a self-sustaining biocontrol solution.

This sub-project focused on the leaf-feeding thrips (Pseudophilothrips ichini), which originates from Brazil and was approved for release as a biocontrol agent in Florida, USA in 2019. Feeding damage by the thrips not only kills meristems and causes flower abortion, but also suppresses the growth of young plants and curtails seed production in mature trees. The thrips has been found to be highly host-specific during testing performed as part of the USA biocontrol programme. Additional host-specificity testing, comprising non-target species relevant to the Australian context, was conducted to complement previous results. In both multi-generational no-choice and choice tests performed, P. ichini completed its lifecycle on the native plant species Rhus taitensis, albeit in lower numbers that on the target weed. Based on these results, it was not possible to proceed with the submission of a release application for P. ichini because it would pose a considerable threat to this Australian native plant species and thus approval to release would not be granted.

Michelle Rafter of CSIRO was leading this sub-project.

Background

What is the weed problem?

While typically planted as an ornamental garden tree, broad-leaved pepper tree has no beneficial aspects in Australia. Invasions are more prevalent in disturbed, successional plant communities, such as abandoned farmland. Stands of broad-leaved pepper tree provide poor shelter for wildlife and shades out small native plants thus reducing both animal and plant biodiversity. Broad-leaved pepper is toxic to humans and animals, containing resins similar to those present in poison ivy (Toxicodendron radicans), another plant from the Anacardiaceae Family. These resins cause skin irritations such as itching, lesions, rashes, swelling and ulcers.

How is the weed currently managed?

As a woody weed with seasonal growth phases, broad-leaved pepper tree can be difficult to control. In summer, the vegetative growth phase causes plants to quickly regrow from above-ground stems, the trunk base, root crowns or below-ground root suckers. Manual control includes hand-pulling or chipping out seedlings. Cutting down broad-leaved pepper tree at the stump requires follow-up treatment for 6 to 18 months. Alternatively cutting trees 4 cm below the soil, chipping away the bark and nailing a tin plate over the cut stump can extend the duration between follow up treatments. Mulching of plant parts can reduce likelihood of regrowth. A number of herbicides can be used to control broad-leaved pepper tree.

Profuse regrowth can occur after the use of control methods such as; cutting at the stump, bark girdling, fire, and herbicide application. Control methods should therefore be considered alongside revegetation with native species, control of other weedy species, and follow up maintenance and treatment after the initial control effort.

What can biocontrol offer to the weed’s management?

Biological control offers promise as a control method because it is non-destructive, cost effective and sustainable, with little chance of off-target damage. This weed management tool is particularly suitable in culturally and ecologically sensitive areas such as riparian zones where broad-leaved pepper tree occurs. A successful classical biological control program for broad-leaved pepper tree would reduce the growth and seed production of populations which in turn should reduce the local abundance and dispersal of this weed. Biological control has proven to be an effective management tool for other invasive woody weeds such as melaleuca in the USA.

Previous research

Exploratory surveys for natural enemies of broad-leaved pepper tree in South America, its native range, were first conducted by US researchers in the 1950s. Three insects were identified as potential biocontrol agents and were subsequently released in Hawaii. Of the three agents two established, but caused limited damage to the weed. In 1980, efforts were renewed to find effective biocontrol agents for broad-leaved pepper tree for mainland USA and Hawaii. Multiple candidate agents were identified in Brazil and tested in USA quarantine facilities. Two candidate agents were found to be highly host specific – the leaf galling psyllid Calophya latiforceps (Hemiptera: Calophyidae) and leaf-feeding thrips Pseudophilothrips ichini (Thysanoptera: Phlaeothripidae). The thrips was approved for field release in 2019, while the psyllid is awaiting final approval for release in the USA.

Of the two species, the thrips appeared to be the most promising agent based on laboratory testing and climate modelling. In laboratory tests, plants subjected to thrips herbivory were shorter and had fewer leaves and stems than control plants (no herbivory), with plants dying within 2.5 months. The thrips also attack the growing tips of broad-leaved pepper tree causing tip death. The damaged tips are unlikely to produce flowers or fruit, thereby reducing sexual reproduction and spread by seed. The thrips has a short generation time of just 20 days and reproduces all year round in suitable climatic conditions, completing at least 12 generations per year.

Manrique VDR, Erazo L, Reddi N, Wheeler GS, Williams D, Overholt WA (2014) Comparison of two populations of Pseudophilothrips ichini (Thysanoptera: Phlaeothripidae) as candidates for biological control of the invasive weed Schinus terebinthifolia (Sapindales: Anacardiaceae). Biocontrol Science and Technology. 24: 518-535.

Wheeler GS, Kay FM, Vitorino MD, Manrique V, Diaz R, Overholt WA (2016) Biological control of the invasive weed Schinus terebinthifolia (Brazilian Peppertree): A review of the project with an update on the proposed agents. Southeastern Naturalist. 15: 15-34.

Wheeler GS, Manrique V, Overholt WA, McKay F, Dyer K (2017) Quarantine host range testing of Pseudophilothrips ichini, a potential biological control agent of Brazilian peppertree, Schinus terebinthifolia, in North America and Hawaii. Entomologia Experimentalis et Applicata. 162: 204-217.

Trust-funded sub-project

The leaf-feeding thrips Pseudophilothrips ichini was selected as the most promising candidate agent of broad-leaved pepper tree to pursue for the sub-project supported by the NSW Environmental Trust because it:

• has been well studied, rearing protocols exist and a starter culture was readily accessible from colleagues in the USA,
• was tested on an extensive number of non-target species as part of the USA program, and thus limited additional testing was required (only seven genera are found to occur in the Family Anacardiaceae in Australia),
• has a short generation time, broad environmental tolerance and can cause damage that is likely to contribute to the management of the weed.

The key activities of the sub-project were to:

1. Prepare and submit to the national Environment and Invasive Committee an information dossier to support the nomination of broad-leaved pepper tree as a target for biocontrol in Australia.
2. Undertake host-specificity testing of P. ichini on closely-related plant species to broad-leaved pepper tree that occurs in Australia and, pending results indicate that the insect does not pose a threat to non-target species.
3. Prepare and submit to the relevant authorities an application for its release in Australia for the biocontrol of broad-leaved pepper tree, pending results indicate that the insect does not pose a threat to non-target species.

Summary of achievements

October 2021

The sub-project prepared an information dossier to support the nomination of broadleaved pepper tree as a weed target for biocontrol, which was submitted in April 2018 to the national Environment and Invasives Committee (EIC) and endorsed in May 2018.

A colony of the thrips was established in the high-level quarantine facility (BC3 micro) in Brisbane. A Standard Operating Protocol was developed to demonstrate to the authorities, the Federal Department of Agriculture, Water and the environment (DAWE), that the thrips could be effectively contained in cages. Based on this, permission was obtained in July 2020 to undertake research on the thrips at the lower level of quarantine (BC3 macro) in the facility. The thrips was re-imported in August 2020.

The proposed plant list for testing the thrips P. ichini was developed based on currently accepted phylogenetic information available in the literature. Test plant species were selected based on their phylogenetic relationship to the target weed, according to the centrifugal phylogenetic method. Representatives from all genera of the Anacardiaceae family present in Australia were included. Plants or propagation material of different accessions of 20 of the 23 non-target species on the proposed test list were acquired. Many native plant species were obtained from bush regeneration and Landcare nurseries in Northern Queensland. The focus once species were obtained was to keep accessions in a pest free state and with flushing foliage ready for inclusion in host-specificity testing.

Initial no-choice tests conducted by collaborators at the University of Florida, followed by additional no-choice tests in the quarantine facility in Australia, indicated that P. ichini can oviposit and complete development on the native species Rhus taitensis, Rhodosphaera rhodanthema and Dodonea viscosa. In no-choice tests involving R. taitensis and R. rhodanthema the numbers of larvae and adults produced were comparable to that on broad-leaved pepper tree, under these testing conditions. Considering these results, the decision was made to conduct multi-generational no-choice tests with R. taitensis and R. rhodanthema as a priority. Dodonaea viscosa had previously been included in multi-generational no-choice tests conducted as part of the US biocontrol program for broad-leaved pepper tree and found not to be suitable to sustain populations of the thrips. The multi-generational no-choice tests performed in the sub-project were at the expense of conducting simpler no-choice tests with other plant species of the test list, even though they had been sourced and propagated. To further assess if R. taitensis or R. rhodanthema were suitable hosts for P. ichini, collaborators at the University of Florida were subcontracted to concurrently conduct choice tests with these species as well as a couple of other species that did not support development of the thrips in no-choice tests.

Pseudophilothrips ichini completed its lifecycle on R. taitensis in both multi-generational no-choice and choice tests, albeit in lower numbers that on the target weed. This candidate biocontrol agent thus represents an unacceptable risk to this native species, which geographically overlaps with the target weed. While no-choice testing revealed that R. rhodanthema is within the fundamental host range of P. ichini, subsequent multi-generational no-choice tests indicated that it is less at risk than R. taitensis as most replicates failed to sustain P. ichini over multiple generations. Very few larvae developed on R. rhodanthema in the choice tests performed.

Based on results obtained during the sub-project, it was not possible to proceed with the submission of a release application for P. ichini because it would pose a considerable threat to the Australian native plant species R. taitensis and thus approval to release would not be granted.

Acknowledgement

The assistance of Tracey Steinrucken, Kerri Moore and Tim Vance of CSIRO is greatly acknowledged.

Overseas collaborators: Gregory Wheeler (USDA IPRL) and Carey Minteer (UFL).