Executive Summary

What the report is about
Pest and virus damage has cost the Northern Adelaide Plains (NAP) horticultural industry up to $20 million a year in lost crops and overheads. This study was conducted to determine whether indigenous perennial native species are less suitable hosts of invertebrate pests, particularly of Western Flower Thrips (WFT), than common weeds, when grown near horticulture on the NAP.

This report evaluates invertebrate populations on different native plants and grasses for revegetation to replace weeds and suppress agricultural pest populations and disease on the NAP. Emphasis was placed on plant species that benefit Natural Resources Management (NRM), maintain populations of beneficial insects and have the potential to generate profit.

Background
A key challenge for natural resource management in horticulture is the integration of native vegetation into commercial practices. Revegetation programs have not been well received in horticultural producing areas because of perceptions that native plants host crop diseases and harbour pests. On the NAP, weeds are recognised as a major issue because they harbour WFT, Frankliniella occidentalis, which are efficient vectors of Tomato Spotted Wilt Virus (TSWV). Current strategies to control WFT, including ‘bare earth’ buffers around crops and high chemical use, are expensive and not sustainable.

An alternative would be to replace weeds with native plants that can be shown not to host TSWV or harbour WFT. In addition, perennial native plants may support integrated pest management (IPM) strategies by providing a stable refuge for natural enemies near ephemeral horticultural crops.

However, it is well recognised that for revegetation to occur, there needs to be an incentive for landholders. Perennial native plants that generate profit are most likely to present a reasonable prospect to engage growers to revegetate threatened land that is under commercial production.

For commercial growers to be confident in their revegetation choices, the relative risks and benefits that weeds and/or native plants could pose to their crops needs to be understood and addressed.

Objectives
This project aimed to provide a framework of information for those involved with the horticulture industry and natural resources management to make informed decisions about selecting native vegetation to reduce the pest and disease pressure on crops on the NAP. In this study, the potential for indigenous native plants and grasses to either host pests or beneficial insects was examined.

Methods and Results
Initially, information on the abundance and diversity of pest and beneficial insects was collected through surveys of weeds and remnant native vegetation. Following surveys, selected indigenous plants (Maireana brevifolia, Atriplex semibaccata and Rhagodia parabolica) and grass (Themeda triandra, Chloris truncata, Enneapogon nigricans and Danthonia linkii) plots were established near commercial glasshouses. The invertebrate fauna from these plots were regularly collected and categorised into functional guilds. The prevalent pest and beneficial taxa on the native plants and grasses were compared with each other, and with nearby brassica weed plots.

Survey results indicate that the abundance of WFT on brassica weeds was up to 300 times higher than on native plants, and up to 160 times higher than the native grasses. The native plants and grasses also recorded far fewer tomato and onion thrips, which can also vector TSWV. A reduction in TSWVvectoring thrips would be advantageous, but it is unclear whether some other invertebrates may in time emerge as new pest problems. The most abundant invertebrates likely to become pests on native plants and grasses were leafhoppers and aphids, respectively. However, the common weeds in the area would also harbour significant populations of these potential pests. Regardless, the saltbushes, and, in particular, A. semibaccata, require taxonomic evaluation to determine the disease vectoring capacity of leafhoppers present. Likewise, host preference testing of important pest aphids, such as lettuce aphid, on native plants and grasses should be considered.

Native plants and grasses harbour more diverse beneficial invertebrates and fewer WFT, than weeds.

Native plants and grasses have the potential to support a range of natural enemies, and can provide a stable refuge area near cropping activities. In addition, there is a substantial group of invertebrates that represent species of unknown function. Many of the species will not impact on nearby horticultural crops, but some beneficial or pest species may emerge with larger plantings and in subsequent seasons.

Implications and recommendations
Overall, the results suggest that using selected native plants, a reduction in the key thrips species that host TSWV is possible. However, it would also appear that the native species chosen in these trials have a range of agronomic and host specificity characteristics that can influence IPM outcomes; in both beneficial and potentially undesirable ways. This confirms the concept of informed and deliberate plant choice, which we collectively refer to as ‘revegetation by design’.

Considerable knowledge and insights have been gained, but to make improved and reliable recommendations further investigations are required. Areas of particular interest include the beneficial suite of invertebrates, in particular the parasitic wasps associated with control of thrips, predatory Erythraed mites found on native grasses, and brown lacewings warrant investigation for potential in IPM strategies to control lettuce aphid. Areas for further study include, evaluation of the taxonomy and host preference of whitefly found on Rhagodia parabolica, the diversity of coccinellid species on the native grasses and Rhagodia parabolica and Chirothrips manicatus as alternative hosts for parasitic wasps associated with the control of pest thrips.

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