This article is republished from Grapegrower and Winemaker, Issue 705, October 2022, with permission from the authors and Grapegrower and Winemaker.
Vineyards usually have a groundcover made up of spontaneously growing grasses and broadleaved weeds or have bare earth, either directly under the vines or across the whole vineyard. Each of these options has problems. Bare earth increases the risk of soil erosion by wind and water run-off and can lead to poor soil structure. Conversely, a weedy groundcover uses valuable water and requires mowing to prevent excessive growth. If the weeds are too vigorous, they can block air flow and favour development of fungal diseases and frost damage. Reflecting this challenging situation, there has been growing interest both in Australia and overseas in the use of alternative types of groundcovers. That’s where cover crops come in. These can be thought of as form of groundcover that is deliberately established and often more actively managed than a spontaneous coverage of weedy plants. Cover crops can deliver a range of benefits in the vineyard. Early work in South Australia, for example, showed scope for weed control using under-vine cover crops. In 2021, Wine Australia awarded funds to a new project that aims to generate evidence-based guidelines for using a range of cover crop types to deliver multiple benefits to vineyards. Results from the first year of this project, run by Charles Sturt University, are the focus of this article.
One feature of the new project is the use of laboratory studies to assess the benefits of a range of potential cover crop plants to parasitic wasps that attack light brown apple moth, one of the key vineyard pests in many districts. The caterpillars of this pest damage bunches and leave them more susceptible to infection by botrytis fungus, leading to bunch rot. Our work has focussed on minute Trichogramma wasps that lay their eggs in the pest’s eggs, ‘hijacking’ them so they give rise to more wasps rather than developing into damaging caterpillars. Whilst Trichogramma wasps are widely distributed in Australia, they live for only a few days unless able to feed on nectar. Unfortunately, nectar plants tend to be scarce in vineyards making them inhospitable locations for effective biological control. Our laboratory research has identified a range of plant species that may remedy this problem by providing nectar that is suitable for use by Trichogramma wasps. One example is buckwheat which produces nectar that extends the lifespan of these tiny wasps and more than doubles their egg laying. Buckwheat can be grown as a mid-row cover crop in vineyards but is too tall (about 1 m) to be suitable for use directly under the vines. Here, another plant, alyssum, seems to have good scope as it grows to only a quarter of the height of buckwheat. Alternatives to these exotic plants include several native species that might be favoured in settings where the manager wished to soften their overall environmental impact and make the vineyard a setting where native invertebrates and small animals find harbour. But our screening of various options has revealed an important practical message. Some native plants, including Acacia and Kunzea species, appear to provide no benefit to Trichogramma. Fortunately, many other species do greatly boost egg laying by wasps. These include the prostrate-growing species Grevillia lanigera, Myoporum parvifolium and Leptospermum ‘Pink Cascade’. Our laboratory tests help inform which plants might be used as vineyard cover crops, as well as for selecting species suitable for ‘insectary plantings’ beside vineyards, and even help understand which native woody plants might be valuable in shelterbelts and the wider landscape.
Geoff Gurr checking early season establishment of buckwheat sown as a mid-row cover crop.
Buckwheat plants blooming mid-season and providing nectar to beneficial insects.
Light brown apple moth: one of the key vineyard pests and for which cover crop can provide protection by attracting beneficial insects such as nectar-feeding Trichogramma wasps.
So much for boffins playing with bugs in the lab; what actually happens in the field? Vineyard trials were conducted in 2021-22 in the Orange district to assess ease of establishment of various cover crop species and measure the benefits of each. An important finding from this season-long trialling was that some of the plants that were shown to be useful to Trichogramma wasps in the laboratory led to reduced levels of field damage to bunches by light brown apple moth. In the case of alyssum established as under-vine plots, for example, the numbers of damaged fruit bunches were reduced by two thirds at both the organically-managed site (See Saw Wine’s Balmoral Vineyard) and a conventionally-managed site (Angullong Vineyard). Moreover, the alyssum cover crop led to lower numbers of fruit bunches affected by botrytis and, for those affected bunches, the severity of rot was reduced. Similar effects, though slightly less pronounced, were apparent when a mix of low-growing, perennial native plant species was established under the vines. This was an exciting finding because those natives were established as small plants that took time to grow yet they were still delivering benefit in year one. By the end of the first season several, especially Dampiera and Myoporum, were aggressively covering more ground and out-competing the adjacent weeds but still retained their prostrate growth habit. This meant that they did not climb into the vine foliage and, at an average of less than 20 cm high, were considerably shorter than the weedy plants in the control plots in the trials which comprised the original groundcover. Those grassy and broadleaved weeds were typically around 50 cm high so much more likely to impede air flow through the vines and around bunches. We anticipate that the benefits of these native plants in year two will be even stronger as they cover more area and provide more nectar resources to beneficial insects.
- Several native and non-native plant species have been identified as agronomically well suited for use as vineyard cover crops both mid-row and under-vine;
- Several species (including perennials) established and achieved significant levels of ground-coverage and competed well with weeds;
- Several species exhibited a prostrate growth habit in vineyard settings to the extent that they were shorter than the weedy vegetation they replaced and thus can be assumed to improve air flow, so reducing frost risk and fungal disease severity;
- Fruit bunch attack by light brown apple moth was greatly reduced by some cover crop treatments such as alyssum;
- Both incidence and severity of botrytis bunch rot was reduced by some cover crop treatments, including alyssum, which suggests that reductions in the severity of light brown apple moth attack can have a knock-on effect in reducing the botrytis fungus infection within bunches.
The Australian grapevine moth can cause serious late season defoliation but is attacked by a range of beneficials such as parasitic wasps; this individual has several wasp eggs on the ‘neck’ area from where it is unable to groom them off.
The low-growing native shrub, Myoporum parvifolium, grew vigorously beneath vines producing large numbers of white flowers that laboratory tests showed were effective in boosting the performance of Trichogramma wasps.
The familiar garden ornamental plant, alyssum, appears well suited as an under-vine cover crop where it competes with weeds, forms a dense but prostrate groundcover that provides good air flow beneath the vine canopy. It also reduced damage by light brown apple moth and late season botrytis bunch rot.
From a practical perspective, establishing cover crops remains a challenge. At the relatively small scale of research plots in this study it was possible to use small, motorised machinery and hand tools, but this would not be economical at a commercial scale. Tractor-mounted machinery can be used to prepare a seed bed for mid-row cover crops; indeed, this is not uncommon, but establishing under-vine cover crops requires specialised machinery to work around vines, posts and irrigation lines. Currently, such equipment is not common and relatively expensive. Potentially, however, there is no need to use tillage to establish a fine seed bed if herbicides are used to reduce weed competition and allow cover crop plant seeds to be surface-sown (suitable for species such as alyssum) or if using small ‘tube stock’ native plants. The costs of planting material are also a consideration. Some cover crops – such as buckwheat, alyssum and several commercial blends – are inexpensive, widely available and grow vigorously enough to give a good ‘strike’. Alyssum offers the advantage of self-seeding to provide a perennial strip and has an extremely long flowering period which is important in order to give season-long nectar availability to beneficials. Buckwheat tends to bloom very rapidly but sowing time is important because it is very susceptible to frost so cannot be sown early in cooler districts. Whilst buckwheat has a short blooming period it is possible to extend nectar availability by slashing part of the width of mid-row strips to encourage a new flush of growth. Native species are the costliest of the treatments we evaluated because small-potted plants were used since seed availability was poor for most species. Whilst the natives are perennial and likely to provide benefit for many years, the up-front costs will be prohibitive in some circumstances.
Some native shrubs such as this Grevilia lanigera will grow too tall to be used under vines but have potential value as mid-row plants or for use in insectary plantings beside the vineyard. Like alyssum and Myoporum, laboratory studies showed that it provides nectar that boosts performance of parasitic wasps such as those that attack light brown apple moth and Australian grapevine moth.
Our year-one results appear very promising but there is still much to be learned. During spring 2022 we will be monitoring how the various cover crop species survived winter frosts and grazing by sheep. These are important aspects because the plants need to be in a vigorous state early in the season in order to prevent weed growth and provide habitat for beneficials. They also need to bloom as early as possible to attract and support parasitoids such as Trichogramma wasps so that the first generation of light brown apple moth is checked. Species such as buckwheat will require re-sowing each year. Further, it is important in such field trials to base conclusions on more than just a single season, especially since 2021-22 was so moist. If conditions become drier in the upcoming season it will, for example, provide an opportunity to assess cover crop vigour when vines are competing for water and see if they are less ‘thirsty’ than the grassy weeds they are replacing.
This work is supported by Australian grapegrowers and winemakers through their investment body, Wine Australia, with matching funds from the Australian Government. We thank See Saw Wines and Angullong Wines for hosting vineyard trials.