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Sustainable pest control – now and in a changing climate

Sustainable pest control – now and in a changing climate

With warmer temperatures potentially promoting changes in vineyard pest impacts, pest control becomes more important than ever.

Summary

Increased temperature associated with climate change may lead to changes in vineyard pests, including in their distribution, generation and emergence times. While we have tools at our disposal to assist in predictions, information from growers remains crucial to understanding these changes and their implications for the industry. There are lots of natural enemies in every vineyard and there is no doubt that these make a major contribution to pest control: how large is in the hands of growers. Natural enemy contribution to pest control under climate change can be enhanced by continued commitment from growers for their support with sensitive chemical use and provision of resources.

Insect pests:

affect production costs and crop loss - the profile of mite pests is lower now than in the past
effects often go unrecognised and indirect effects are underestimated - especially effects attributed to viruses transmitted by pest vectors
industry has invested significantly in sustainable control of pests by natural enemies
there are lots of natural enemies in every vineyard - there is no doubt that natural enemies DO make a major contribution to pest control, depending on management
pest and natural enemy distributions and life histories may change with climate change
natural enemies can offer protection when pest pressures are unpredictable, including during exotic incursions.

Sustainable pest control - now

Pests: Analysis of economic data on the cost of pest and disease control and production loss confirms what growers know - diseases are the biggest problem (and cost) in growing grapes, but pests also make a substantial direct contribution. Lightbrown apple moth is the most important insect pest, followed by weevils, trunk insects, mealybugs and scale all of which can have a severe impact in some regions or in particular seasons (see Table 1). Viruses vectored by insects, especially leafroll viruses, increase the potential impact of insect pests. For example, the most widespread virus in Australian and New Zealand grapevines are the group of viruses called grapevine leafroll viruses (GLRaVs): several GLRaVs are spread between vines by mealybugs and scale, increasing the cost of these insects to the industry and highlighting the importance of effective control. A single mealybug may transmit leafroll virus to a healthy plant. Leafroll is one of the most important virus diseases of grapevines, with infected vines less vigorous than healthy vines, significant yield loss (up to 30-50%) and effects on fruit including delayed ripening. Production may be influenced without grower awareness as infected vines do not always show symptoms (Fuchs 2007).

Natural enemies

A diverse range of natural enemies contributes to control of vineyard pests (Figure 1) with hundreds of natural enemies of insect pests present in well managed vineyards. Predators are perhaps the most visible reminder of pest control activity: these include spiders stalking prey on the ground and in the canopy, stretching huge webs between rows to tiny webs within developing bunches, ground beetles (carabids), large rove beetles (staphylinids) on the ground and tiny mite-eating rove beetles in the canopy, colourful ladybird beetles (coccinellids), a range of predatory flies including the fascinating hoverflies (Syrphidae) appearing to fly yet stationary as they 'hover', while predatory midges (Cecidomyiidae), swarms of brown and green lacewings are seen around lights on a summer evening, along with predatory bugs, and even some predatory thrips. Predatory mites are tiny, like their prey ,but are essential to the control of bunch, bud, blister, rust and two-spotted mites. Most parasitoids are really small and hence less obvious pest control agents in the vineyard. For example, Trichogramma, the egg parasitoid of light brown apple moth, is one of the world's smallest insects yet can achieve high levels of parasitism of eggs laid. There many other wasp parasitoids attacking eggs, larvae and pupae of vineyard pests, and also fly parasitoids like the tachinid fly attacking lightbrown apple moth caterpillars. The predatory potential of some common vineyard residents can be unrecognised: recent video analysis from New Zealand confirmed European earwigs as important predators of lightbrown apple moth larvae in the canopy (Frank et al. 2007) and ants are predators of lightbrown apple moth eggs.
A couple of examples demonstrate the high diversity of natural enemies that can attack a single pest in a vineyard. Lightbrown apple moth has 26 different parasitoid species attacking eggs, caterpillars and pupae in addition to a long list of predators: spiders, earwigs, ladybird beetles, predatory bugs, carabid beetles, staphylinid beetles, brown and green lacewings and ants. A recent detailed survey of scale in vineyards revealed a similarly wide range of natural enemies: green lacewings, ladybird beetles, larvae of other beetles such as the Carabidae, wasp egg parasitoids and predators and another surprising predator, the scale-eating caterpillar of the moth Mataeomera dubia (Rakimov 2010).
Predators and parasitoids not only have potential to decrease the need for pesticide applications, but are especially important in control of the pests that are difficult to access. Light brown apple moth are protected in their webbed leaf rolls and in developing bunches; adult scale and mealybugs not only hide under bark or in fence posts, but also have protective outer coverings which are difficult for pesticides to penetrate; cane boring larvae of weevils and other trunk borers are protected within canes. For much of their lifecycles, mites are protected under leaves or in leaf buds.

Increasing abundance and diversity of natural enemies in the vineyard

There are two essentials to increasing the role natural enemies can play in pest control in a well-managed vineyard: limiting chemical stress and providing resources. Pest control is provided by chemicals and natural enemies HOWEVER … 'Chemical control of the pests and diseases can be potentially hazardous for predators' (Nicholas et al. 2007) and there are many observations linking pest problems with pesticide use, such as 'Outbreaks of scale and mealybug are encouraged by overuse of insecticides which reduces predatory insect populations "(beneficials)" (GWR 08/04). Decreased predation of lightbrown apple moth eggs is seen with increased chemical stress (Figure 2a, see page 52). Though the use of chemicals may be necessary, chemicals differ in their impacts on beneficials, including natural enemies and selection of chemicals known to have less impact, while still providing pest control, will support increased natural enemy populations. There are many sources of information on chemical effects on beneficials and we have provided a tool for grower use to provide easy access to this diverse information on a website of chemical data (IMPACT for viticulture: Invertebrate Management of Potential Agro-Chemical Toxicity: maximising your beneficial bugs, at (http://cesar.org.au/index.php?option=com_collateral_manage) together with a recently published Innovators Network Factsheet† 'Pesticide impacts of beneficial species' and published information (Thomson and Hoffmann 2006a; 2007).

Resources

By providing resources such as shelter, overwintering sites, alternative hosts and food sources from pollen and nectar, vegetation can influence invertebrates present not only in the vegetation itself, but also in the vineyard. Opportunities for provision of resources include vegetation in the vineyard midrow, woody vegetation planted adjacent to vines as 'insectaries', shelterbelts or remnants or even at the wider landscape scale. As invertebrates affected by vegetation include natural enemies controlling vineyard pests, vegetation has the potential to lead to increased numbers of natural enemies within the vines and improved pest control (Thomson and Hoffmann 2006b, 2008, 2010a) (Figure 1b). There are many opportunities to increase vegetation within and around a vineyard, and analysis shows that the increase in abundance of natural enemies within the vines can potentially cover the cost of establishing vegetation, with benefits resulting from 100m of vegetation being as high as $8000 (Thomson and Hoffmann 2010b). However, care needs to be taken to ensure that vegetation does not harbour pest invertebrates and birds. Midrow and undervine planting can also provide resources for invertebrates, increasing the abundance of natural enemies and control of pests such as lightbrown apple moth (Fig. 1c) (Thomson et al. 2009; Innovators Network Factsheet † Native cover crops in viticulture).

Sustainable pest control in a changing climate

Temperature is a principal determinant of where an organism can live, hence, it is anticipated that climate change will bring direct changes in pest distributions and changes in natural enemy distributions as insects respond to temperature changes (Thomson and Hoffmann 2010c). There may also be changes in pest and natural enemy life cycles (generation times, times of emergence) in response to temperature, and changes in abundance of invertebrates in response to shifts in vine management including the potential adoption of new varieties, clones or rootstocks, changes in water availability and management potentially leading to changes in vine 'stress' and implications. Management may also be affected by increasing industry and external demands for 'sustainability' and reduced 'environmental footprint' increasing pressure on the industry for changes including reducing chemical input and increasing non-crop vegetation.

Current pests as future pests

To consider the potential impact of grape pests under a changed climate, there are three questions we might ask:
will there be changes in distribution?
Will there be changes in vine and pest phenology such that more damaging interactions will potentially occur?
Are there potential changes in vines that make them more vulnerable to attack?

Distribution

Predictions of changes in distribution of both pests and natural enemies are possible using current distribution data and models. The models can help predict where pests will occur in future and model predictions are improved by direct measures of the temperature limits of the organisms. As future distributions are predicted from current distributions, there is a need for accurate current distribution records. Two examples illustrate the impact of good records on outcomes for the industry, lightbrown apple moth and mealybugs. Due to its economic importance in grapes and other crops and availability of extensive records, lightbrown apple moth was one of the early pests selected for modelling (Sutherst 2000). This early exercise and subsequent work by us indicates that its pest status may decrease in some areas (Thomson and Hoffmann 2011).
In contrast, mealybug records are sparse. The three species commonly occurring in vineyards are often recorded simply as 'mealybugs', hence, accurate mapping of the distribution of each of the species is difficult. Our field surveys (2011-2012) observed mixed populations at all sites visited, with a minimum of two species at most sites. Mealybugs are an economic problem in grape vines not only because of direct damage to the crop and costs for their control, but also their role in transmitting grapevine leafroll viruses. Overseas, 'mealybugs have become increasingly important vineyard pests - a result of their direct damage to the vine, their role in transmitting grapevine leafroll viruses and the cost for their control ...' (Kent Daane, IPM Davis California). Input from growers has the potential to greatly improve knowledge of occurrence of each species, contributing to accurate current distributions and hence, accuracy of future predictions.Development time, emergence time and number of generations per year
Distribution changes may not be the only or even the most important affect of climate change on vineyard pests. Climate (especially temperature) not only influences where a species can live, but also phenologies and entire life cycles. Insect development time is determined by temperature leading to changes in life cycles with implications for pest control, especially the number of generations pest breeds in a year and their emergence time (Table 2, page 55). Some pests like fig longicorn have one generation per year. There may be a narrow window for control of these types of pests depending on the presence of the most susceptible stage. Other pests have multiple generations per year and this can mean variation in appearance of the vulnerable stage for control, as well as more generations for pest build up in the future. Longtailed mealybug currently has 3-4 generations per year, depending on temperature, so that crawlers will be emerging at different times. For some pests, different species may have different life cycles: the most commonly occurring scale (71% of vineyards recently surveyed Rakimov 2010), grapevine scale (Parthenolecanium persicae), has a single generation per year, with nymphs emerging in different regions from October to November, whereas the soft brown scale (Coccus hesperidum) has several generations per year, 4-5 in northern Australia but only 2-4 in southern Australia. Soft brown scale occurs in 9% of the vineyards surveyed by Rakimov (2010): what would be the consequences of a wider distribution of this multivoltine scale? Lightbrown apple moth can have two to five generations per year, depending on region. This is reflected in costs of control: currently growers in Tasmania report cost of control $23/ha/year compared with $80/ha/year in the warmer Riverland (GWR 08/04). How much will control costs rise if the number of generations in Tasmania is increased?
Timing of chemical applications to the most vulnerable or exposed stage of a pest is important for successful treatment. For mealybugs, mites and scale, the newly hatched first instars or 'crawlers' can be accessed while dispersing. Fig longicorn and vine borer larvae are exposed before tunnelling into vine canes, and spraying targets black vine weevil adults in the window following emergence and before egglaying. Control will be enhanced by accurate knowledge of life cycles at different locations. For some pests with a single generation each year (Table 2) the adult stage is the most vulnerable stage, while for others, emerged larvae or nymphs can be the most vulnerable. If access for control is limited or control is improved by targeting vulnerable stages, knowledge of local emergence times may be required.
A suite of other effects may result from changes in generation time. Overall abundance of pests may increase if there are more generations. Changes in both generation time and emergence time may result in pests becoming associated with a different stage of crop development, especially if changes in crop phenology do not synchronise with those of the pest. Accurate monitoring and pest-keeping records will become more important in detecting these changes and keeping the industry informed of potential problems.

Vine stress

There is clearly potential for climate change to induce changes in vines that will increase their vulnerability to pests. Stressed vines may be more susceptible to trunk boring insects. This suggests that research into optimising irrigation benefits on vine growth may have added benefits of protecting vines from trunk insects. Our preliminary modelling, using available pest records, indicates that rainfall may have a strong influence on potential distribution of trunk boring pests, including fig longicorn, elephant weevil, vine weevil and common auger beetle. The economic impact of trunk boring insects is greatest in 'warm dry' regions. Does this suggest the impact may become greater under the hotter and drier conditions predicted with climate change in many regions?

Exotic pests

Climate change is predicted to lead to an increase in number of successful invasions (beyond what might be expected due to increased travel). Four insect pests are recognised as high priority security pests of viticulture, two mealybugs (vine and grape) and the glassy-winged sharpshooter (GWSS). The impact of the GWSS is due to its role as a vector of the devastating Pierce's Disease.
Exotic threats to vineyards can occur because of the arrival of a new pest carrying a new disease, or due to replacement of one pest by another within a complex. Comparison of an Australian native mealybug, longtailed mealybug, to one of the potentially invasive exotics, the vine mealybug Planococcus ficus illustrates this latter point. Vine mealybug is a highly successful invader originating in the Crimea but now a major pest throughout Europe and the Americas. Longtailed mealybug has been resident in California since at least 1933; it remains limited to central coast vineyards, where it has three generations a year. In contrast, the vine mealybug was first identified in California in the Coachella Valley in the early 1990s. It has since spread into California's San Joaquin Valley and central coast regions, with new infestations reported each year. Vine mealybug's 4-7 generations per year in much of California's grapegrowing regions are resulting in rapid population growth, making it particularly damaging and difficult to control. It is also potentially a more efficient vector of leafroll viruses, with viruses now of more concern in regions where vine mealybug occurs (Daane et al. 2008).

Final remarks

Progress towards an accurate predictive framework for future changes in economically important vineyard pests under climate change and other drivers will require accurate information on current pest distributions and current outbreaks. This means accurate pest monitoring and record keeping along with support services for pest identification. Accurate records may allow potential changes in pest life cycles to be detected with implications for shifting control recommendations. Awareness of current pests provides some protection from exotic pest incursions.
Information from growers is crucial to understanding pest changes. Addition to distribution data will greatly improve the accuracy of prediction of distribution changes and monitoring will improve our understanding of changes in emergence time and generation time. At the same time, natural enemy contribution to pest control under climate change, as currently, can be enhanced by continued commitment from growers for their support with sensitive chemical use and provision of resources.

Acknowledgements

This research was funded by the Grape and Wine Research and Development Corporation with support from Australia's grapegrowers and winemakers through their investment body.
†Innovators Network Factsheets can be accessed at the GWRDC website http://www.gwr dc.com.au/site/page.cfm?u=129

Authors

Linda Thomson (primary author, email lthom@unimelb.edu.au), Michael Nash, Angela Corrie, Ian Smith and Ary Hoffmann, Bio21 Institute, Zoology Department, University of Melbourne, Parkville VIC, 3010

References

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Frank, S.D., Wratten, S.D., Sandhu, H.S. And Shrewsbury, P.M. (2007) Video analysis to determine how habitat strata affects predator diversity and predation of Epiphyas postvittana. Biological Control 41, 230-236.
Fuchs, M.F. (2007). Grape leafroll disease. Cornell University Integrated Pest Management. Www.nysipm.cornell.edu/factsheets/grapes/diseases/grape_leafroll.pdf
GWR 08/04 (2010) Final Report Assessment of Economic Cost of Endemic Pest & Diseases on Australian Grape & Wine Industry.
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