Pest control for organic production in New Zealand
Organic apple production in New Zealand is affected by a pest complex which includes codling moth, leafrollers, scale insects, mealybugs, woolly apple aphid, and mites. While some damage from these pests can be tolerated on fruit before grading for the local market, the overseas quarantine requirements demand essentially pest- and damage-free export apples. This includes additional "hitch-hiker" species which move onto the export crop after harvest. Of all these pests, only mites can be controlled to export standards in all regions by reliance on their existing natural enemies, such as the predatory phytoseiid Typhlodromus pyri. Other pests require intervention with selective control methods which do not disrupt the ecosystem and natural enemies.
Sources of immigrant codling moth, such as neglected apple trees close to the orchard, must be eliminated. The resident population can then be managed effectively using mating disruption, especially on large orchards, and/or granulosis virus sprays. Despite the presence of a complex of natural enemies, the five pest species of leafrollers cannot yet be controlled to export standards with the bacterium Bacillus thuringiensis, pyrethrum, and/or mating disruption. Research is continuing with these techniques and with non-chemical methods of postharvest disinfestation. Mineral and plant oils can assist natural enemies in the control of secondary pests, such as scale insects; resistance of rootstocks to woolly aphid plays a similar role with this pest. Further introductions of parasites and predators are in progress for control of scales, mealybugs and leafhoppers.
Progress with organic apple production for export is severely limited by quarantine requirements which especially require new methods for effective control of leafrollers acceptable to organic certification.
Insect pests are one of the major constraints to expansion of organic apple production in New Zealand. This is not because New Zealand is afflicted with especially severe or unique pests but because the economic success of the apple industry is based on exporting. Our export markets welcome the reduction and elimination of pesticide residues but this is accompanied by quarantine regulations and quality standards which require fruit to be free of insect pests and their damage. This nil tolerance of insects is very difficult to achieve in apples even with conventional chemicals and it is a much bigger problem using biological control methods, such as the use of natural enemies. This approach often relies on the presence of low densities of the pest insects in the orchard to maintain populations of the biological control agents. International trade in organic apples between major trading blocks is minimal, and is severely restricted by quarantine requirements. Exceptions to this occur only within "common markets", from temperate to tropical areas, or between trading partners who have agreed that they share the same complex of pest species. New Zealand requires major technical innovation in pest control for organic apple production to overcome these constraints to export.
The local and processing markets for organic apples in New Zealand are much less affected by pest problems than the export market. The key apple pest, codling moth, Cydia pomonella L., which attacks the fruit directly, must be controlled very effectively on local market and process fruit, but a low level of damage (ca 1%) by this and other insect pests can be tolerated. There are excellent prospects for satisfactory organic control of the pests of apples on fruit destined for the domestic and process markets in New Zealand.
The very high efficacy of pesticides has enabled conventional orchardists to grow apples under a wide range of climatic conditions, regardless of the effects of climate on the incidence of pests and diseases. Organic production requires that careful consideration be given to location before deciding where to grow a given crop. This is important for organic apple production in New Zealand. The cooler and drier areas of the country assist by limiting disease infection risk in summer, such as for black spot, avoiding some pests, such as mealybugs (Pseudococcus spp.), and reducing the numbers of pest generations completed each year, such as for leafrollers. On the other hand, a wet winter can assist the decay of fallen leaves and reduce the spring inoculum of diseases such as black spot.
This paper describes the latest technological developments for the control of apple pests in New Zealand using biological methods, relates their efficacy to market requirements, and identifies shortcomings which are constraining the expansion of organic production.
Codling moth
Codling moth is the primary pest of apples in New Zealand. Mean fecundity (eggs laid per female) varies from 18 to 136 (Wearing and Ferguson 1971; Wearing 1979) and eggs are laid singly on or close to the fruit. Natural larval mortality between hatching and entry to the fruit is 8-14% (Wearing 1979). As a result, a population density of only one pair of moths per tree can cause economic damage and a high level of control is required for export, local, or process markets. Natural enemies, such as the silvereye, Zosterops lateralis (Latham) and the parasitoid Ascogaster quadridentata Wesmael, are inadequate to provide this (Wearing 1979). However, with only a restricted range of host plants, the destruction of hosts of codling moth near to orchards can make a major contribution to control by reducing the immigration of adult moths. This procedure is an essential component of organic production and must be combined with control practices within the orchard.
Three options have been thoroughly investigated for organic control of codling moth within New Zealand apple orchards - ryania, mating disruption, and granulosis virus. Ryania is the ground powdered stems of Ryania speciosa Vahl. and this botanical insecticide is selectively toxic to codling moth, allowing survival of many natural enemies (Wearing 1990). However, the alkaloid active ingredients of ryania show toxicity to both vertebrates and invertebrates and its selectivity is partly due to its formulation and the need for ingestion. Research in the 1960s and 1970s in Nelson indicated that control of codling moth could be achieved with six sprays of ryania per season (Wearing 1979); at current exchange rates, this would cost more than $1000/ ha for materials (Wearing 1990). The EPA has recently required re-registration of ryania in the U.S.A. as part of a wide-ranging programme for re-registration of pesticides and until this is completed, ryania is unlikely to be registered for use in New Zealand. Even when registered, its use on export crops could be limited by the impact of heavy spray deposits on fruit colour and appearance.
Mating disruption of codling moth has been investigated in Auckland, Waikato, Hawkes Bay, Horowhenua, Nelson, Canterbury and Central Otago. This technology is based on the release of high concentrations of female pheromone components into the orchard, which prevents males from locating and mating with adult female moths. It is highly selective due to the specificity of the pheromone for each insect species. Pheromone dispensers (Shin-Etsu Chemical Co., Japan) are placed in the orchard in early spring and they release pheromone through the summer. One application of dispensers is required in the south, with a second half-rate application in the north of New Zealand, depending on the emergence period and number of generations of codling moth. The efficacy of disruption improves as population density falls and excellent results have been obtained, particularly where large apple blocks have been treated. This technology is discussed in detail at this Conference by Suckling et al. (1994a). The cost of materials for mating disruption is $233/ha for 1000 dispensers/ha and $117/ha for 500 dispensers where a second application is needed.
The granulosis virus of codling moth (CMGV) is available as an imported proprietory product (e.g. Madex-3, Key Industries Ltd, Auckland). The virus is highly specific to codling moth, does not spread easily by natural means, and must be applied as a spray by conventional machinery. The target of spraying is the 1st instar larva as it hatches from the egg and before it enters the fruit. The virus must be ingested and, while some larvae die rapidly, others cause superficial fruit damage before dying and a few survive even longer. Larval mortalities of 98-99% have been achieved in spray trials with Madex-3 (Wearing 1993) and, due to the slow action of the virus, it is in the following season that the major impact of this mortality on the population is observed in reduced damage. Madex-3 is available under an EUP-LS (Experimental Use Permit, Limited Sale) and the cost of one application was estimated by Wearing (1993) at $130/ha for materials at the recommended rate (100 mls/ha in 600 litres of water per hectare). CMGV is susceptible to UV, which reaches high levels in New Zealand, and milk powder (at 250g/100 litres) or Nufilm-17 (at 50g/100 litres) can be added to the virus sprays to reduce UV degradation (Wearing 1993). Even with this protection, 1-2 weekly spraying is necessary and six sprays at the recommended rate would cost $780/ha. However, more frequent lower-rate sprays are being investigated to improve efficacy and reduce costs.
The combination of mating disruption and CMGV provides a powerful means of rapidly reducing high densities of codling moth in organic orchards and this has been used successfully in a number of HortResearch trials. Mating disruption facilitated the first exports of New Zealand organic apples in 1992 (Wheeler 1992) and nashi in 1993 and this provides a clear indication of its high level of efficacy. Application of mating disruption and CMGV should enable expansion of production of organic apples for the local and process markets.
Leafrollers
The leafroller complex in New Zealand apple orchards comprises one introduced and four native species: Lightbrown apple moth, Epiphyas postvittana, from Australia, brownheaded leafrollers, Ctenopseustis obliquana and C. herana, and greenheaded leafrollers, Planotortrix octo and P. excessana. These are all polyphagous species which damage foliage and fruits, and all are bi- to multi-voltine depending on latitude. Natural enemies were imported from Australia in the 1970s to improve their biological control (Thomas 1989). Despite successful establishment of parasitoids such as Xanthopimpla rhopaloceros Kreiger and Trigonospila brevifacies (Hardy), and circumstantial evidence of reduction of leafroller populations associated with these species, control intervention is still necessary to reduce damage which may otherwise reach 30% of apples in untreated orchards.
Many organic growers rely at present on Bacillus thuringiensis Berliner sprays for control but these have limited effectiveness in the field, probably principally due to their susceptibility to UV breakdown and rainfall (e.g. Nyouki and Fuxa 1994) on the exposed apple surface. Replicated trials over the past six years with a variety of formulations and strains of B. thuringiensis have failed to obtain control consistently below 3% fruit damage from leafrollers (Suckling et al. 1993, Walker and White unpublished) despite the known high toxicity of B. thuringiensis to leafrollers in laboratory bioassays (Suckling et al. 1994b). Damage levels have generally been reduced by an average 55% (range 0-90%) compared to that in the untreated control trees. While this may be acceptable for local and process markets, it is not sufficient for export quarantine requirements in most orchards.
Natural pyrethrum (Suckling et al. 1993), ryania (Wearing 1990), and other botanical insecticides such as Neem oil are also toxic to leafroller larvae and have been investigated in replicated trials (Walker and White unpublished). Among these alternatives, pyrethrum has been significantly better than B. thuringiensis in protecting apples from leafroller damage (Suckling et al. 1993).
Mating disruption of leafrollers has been used successfully for the management of resistance to insecticides in lightbrown apple moth (Suckling et al. 1990) and P. octo (Wearing and Ogle 1993) but there is limited experience in the use of disruption alone for leafroller control in apple orchards (Clearwater 1993; Suckling et al. 1994c; Wearing 1994). The wide host range and dispersal ability of leafrollers (Suckling et al. 1994d) means that there is usually considerable potential for immigration of mated females into pheromone-treated orchards and this could undermine the disruption process. It would therefore be essential to use mating disruption of leafrollers alone only on large areas, much greater than the dispersal range of mated females. However, the combination of mating disruption and insecticides used in resistance management (Suckling et al. 1990) could be matched in the organic context by combining disruption with an approved botanical insecticide. This has yet to be attempted.
Thomas (1975) provided a number of crop management practices which could assist with control of leafrollers, including regular mowing in summer and grazing of the ground cover in winter to destroy leafrollers on alternative herbaceous host plants. Recent research has confirmed the benefits of these practices (Thomas and Burnip1993).
The foregoing control methods for leafrollers provide crop protection adequate for local and process marketing but greater efficacy is still required for reliable export quality. Since good progress has been made with control of codling moth (see above), inadequate leafroller control has become the principal pest constraint to the expansion of export production.
Scale insects
There are three important introduced armoured scale pests of apples in New Zealand, San Jose scale (SJS), Quadraspidiotus perniciosus (Comstock), oystershell scale (OSS), Q. ostreaeformis (Curtis), and mussel scale (MS), Lepidosaphes ulmi (L.). They feed principally on the bark of the trees but also disperse onto foliage and fruits where they can be a quarantine problem. All three species are attacked by an array of natural enemies (Hill 1989) which can reach very high levels (e.g. Collyer and van Geldermalsen 1975). Nevertheless, research on SJS in Nelson in the 1960’s showed that this pest continued to increase to damaging levels, even killing major leaders and entire trees, despite the presence of high parasitism (Collyer and van Geldermalsen 1975; Wearing unpublished). Dense populations of SJS, OSS and MS have been observed consistently on unsprayed neglected apple trees and in mature organic orchards. High populations of SJS and OSS also occur on alternative hosts outside orchards, such as willows and poplars, from which orchard invasion can occur. The research in Nelson and all other observations indicate that our present complement of natural enemies is inadequate to prevent dense populations and economic damage from these pests.
HortResearch has recently established the predatory mite Hemisarcoptes coccophagus in New Zealand (Hill et al. 1993) to assist the control of introduced armoured scale pests of kiwifruit and this predator is being distributed to populations of SJS, OSS, and MS where its performance will be monitored (Charles unpublished). Other potential specific parasitoids are also being investigated for future introduction. In the meantime, control of these scales in organic orchards can be achieved through the use of dormant and green tip mineral oils (1-2%) applied at high volume to ensure good coverage. If this procedure is followed every season, scale insects should be controlled to export standards, although further research is needed to confirm this. Summer oil (1%) may also assist. Research is needed to determine the best spray timing for minimal impact of oils on the existing and introduced natural enemies.
Mealybugs
Three species of mealybugs are found commonly in pip-fruit orchards from Nelson northwards, obscure mealybug, Pseudococcus affinis (Maskell), citrus mealybug, P. calceolariae (Maskell), and longtailed mealybug, P. longispinus (Targioni-Tozzetti). These pests are essentially rare or absent from orchards in Canterbury (although P. affinis is present in this region) and Central Otago. Like scale insects, mealybugs overwinter in cracks and crevices or under the bark of the trees and disperse to foliage and fruits in summer. The presence of mealybugs in the calyces of apples is not acceptable for export.
Mealybug infested fruits remained at very low levels throughout a long term organic experiment in which ryania was the only insecticide used for pest control at Appleby Research Orchard from 1960 to 1975 (Collyer and van Geldermalsen 1975). This was attributed to the presence of natural enemies of the mealybugs (P. calceolariae and P. longispinus). Charles (1993) has recently surveyed the natural enemies of mealybugs in New Zealand and parasitoids and predators are widespread and common, except for those attacking P. affinis. This research and the Appleby experiment have suggested that biological control of P. calceolariae and P. longispinus may be sufficient for organic apple production, even for export. However, this is not the case for P. affinis and the parasitoid, Pseudaphycus maculipennis Mercet, is proposed for introduction to New Zealand. An application to import this parasitoid has been made and, if successful, the programme of introduction will be carried out over the next two years.
Little research has been carried out on control of mealybugs with organically-approved insecticides. van Epenhuijsen et al. (1992) reported limited toxicity of pyrethrins, insecticidal soaps, and oils to P. longispinus. Mealybugs may not be a serious constraint to organic production, even for export, except where P. affinis is the dominant species. However, further research is needed on the ecology and control of the whole complex.
Leafhopper
Froggatt’s apple leafhopper, Edwardsiana crataegi (Douglas), is a foliage-feeding pest of apple from Europe. In the absence of conventional insecticides in organic orchards, this leafhopper can reach extremely high levels and cause severe leaf damage and honeydew production. Charles (1989) reviewed the natural enemies of this leafhopper and as a result of this, and further research in collaboration with the University of Wales, has proposed the introduction of the dryinid parasitoid Aphelopus melaleucus (Dalman) from Europe in the near future. An Importation Impact Assessment for this species has been completed.
Ryania provides effective control of this pest (Collyer and van Geldermalsen 1975; Wearing 1990) and van Epenhuijsen et al. (1992) demonstrated that a range of organically approved pesticides were also toxic, especially those containing saponified fatty acid and oil, pyrethrins, and ryanodine. Froggatt’s apple leafhopper is not a quarantine pest and does not limit the export production of organic apples except through effects on tree vigour and productivity and contamination of the fruits with honeydew.
Woolly apple aphid
Woolly apple aphid, Eriosoma lanigerum (Hausmann), causes galls while feeding on the stems and roots of apple trees and it often disperses to the fruit and settles in the calyces of apples. The honeydew produced there encourages the growth of sooty mould and both the aphid and the mould is unacceptable on fruit for export. Control of woolly apple aphid in organic orchards is achieved through the combined action of rootstock resistance and natural enemies (Walker 1989), notably the parasitoid Aphelinus mali (Haldeman) and the predator Micromus tasmaniae Walker (Wearing and Thomas 1978). Rootstocks resistant to woolly apple aphid attack were developed by the East Malling Research Station, U.K., from the North American cultivar ‘Northern Spy’ during the 1920’s. These rootstocks, which include the Malling-Merton series, are used by New Zealand apple industry and are widely used elsewhere. More recently, with an international trend toward smaller tree size, less vigorous aphid-resistant rootstocks are required. Rootstocks suitable for New Zealand conditions are being developed by HortResearch from an Asiatic species, Malus sieboldii, and aphid resistance is an important factor in that programme.
Apple leafcurling midge
The apple leafcurling midge, Dasineura mali (Kieffer), has been present in New Zealand only since 1950. The larvae roll the edges of apple leaves and causes damage to shoots which can seriously stunt growth of young trees. The parasitoid Platygaster demades was successfully introduced into New Zealand in the 1920s against the pear leafcurling midge, D. pyri (Bouché) and is present in organic orchards where it attacks both midge species. However, despite high levels of percentage parasitism, apple leafcurling midge populations still increase to high levels and can cause significant damage to young trees (Todd 1959; Collyer and van Geldermalsen 1975). Mature trees are able to tolerate the presence of this insect and it is only a minor pest of mature organic orchards, where there is anecdotal evidence that parasitism, and predation by the mirid Sejanus albisignata (Knight) (Clearwater unpublished), prevent high populations from developing. Research is needed to confirm this or determine whether additional natural enemies should be considered for introduction to New Zealand.
van Epenhuijsen et al. (1992) investigated a range of organically-approved control agents against leafcurling midge but reported limited success. Pyrethrins and a vegetable oil were the most effective of the materials tested. However, spray intervention is unlikely to be required for this pest in organic orchards except on young trees and it is not at present a barrier to organic apple production, even for the export market. With recent increase in the pest status of apple leafcurling midge (Tomkins et al. 1994), this situation may change and a programme to evaluate the effectiveness of natural enemies is proposed before introduction of further biological control agents.
Noctuids
The young caterpillars of a range of noctuid moths (especially Graphania mutans (Walker)) damage the young fruitlets and foliage of apples, particularly during the flowering period, before descending to the gound cover to complete larval development. Noctuid larval feeding can typically damage 2-6% of the young fruitlets from late bloom in export orchards and this damage can increase to more than 10% when fruitlets are left unprotected for long periods in early summer. It is possible that B. thuringiensis sprays applied over flowering could address part of this problem and this is being investigated. The adults are strong flyers and can occur in very large numbers in pastures and fields surrounding orchards. Their greatest economic impact on conventional orchards is when eggs are laid on fruit during harvest which can result in the rejection of export consignments. Presence of eggs on the fruits at harvest is a potential quarantine problem for export of organic apples, although the incidence appears to be rare.
Phytophagous mites
The principal mite pests of apples in New Zealand are the European red mite (ERM), Panonychus ulmi (Koch), and the two-spotted mite (TSM), Tetranychus urticae Koch. Both feed on the foliage but can contaminate the fruit prior to harvest, when ERM is laying its winter eggs and TSM diapausing females are seeking overwintering sites. This contamination causes significant marketing problems only at high population densities which are rare in organic orchards due to the presence of a wide range of effective natural enemies. The predatory mite Typhlodromus pyri Scheuten is particularly important for the control of ERM, and it is assisted by a ladybird, Stethorus bifidus Kapur. There are other natural enemies in the families Miridae (S. albisignata), Cecidomyiidae, Anthocoridae, and Phlaeothripidae (Walker et al. 1989) and a further predator, Orius vicinus (Ribaut) (Anthocoridae), was recently discovered in Otago (Wearing and Larivière 1994). Predatory mites are also important in the control of TSM, notably Typhlodromus caudiglans Schuster, Amblyseius cucumeris (Oudemans), Phytoseiulus persimilis Athias-Henriot (from Canterbury northwards), Neoseiulus fallacis (Garman), and Galendromus occidentalis (Nesbitt) (in Otago and Canterbury)(Thomas and Walker 1989). The insect predators which feed on ERM are also predatory on TSM, and S. bifidus (and S. histrio in some orchards) can play an important role in control (Charles et al. 1985).
Apple cultivars also affect the levels of ERM in apple orchards. For example, ‘Delicious’, ‘Red Delicious’, and ‘Braeburn’ are especially susceptible whereas ERM rarely reaches damaging levels on ‘Jonathan’. However, the complex of natural enemies should maintain control of ERM in organic orchards even on susceptible cultivars, provided there is care in the use of organically approved sprays (see Integration of control methods). TSM may damage organic apple orchards in hot dry seasons, which provide conditions more suited to the mite than to its natural enemies.
If intervention is needed for mite control in organic orchards, summer oils (1%) can be of great assistance and allow the survival of predatory mites. Dormant or green tip mineral oil sprays (1-2%) can similarly assist ERM control while allowing survival of natural enemies of mites.
Integration of control methods
Organic production aims for sustainable management of the crop ecosystem and this requires that the interactions between crop management and control methods for pests and diseases are understood. It cannot be assumed that organic control agents are inherently safe to use, safe to the crop and its ecosystem, and safe to the wider environment and human health. For example, copper and sulphur fungicides can cause crop damage, and the russetting of fruit can cause significant problems in marketing of the fruit. Ryania contains alkaloids toxic to both vertebrates and invertebrates.
Natural enemies of insect and mite pests are susceptible to some of the sprays approved for use on organic apples. Pyrethrum is a broad spectrum chemical pesticide which achieves much of its selectivity through brief efficacy in the field. However, it is toxic to a range of beneficials, such as the predatory mites important for control of ERM and TSM and to some parasitoids (see e.g. Martin 1993). Insecticidal soaps and sulphur fungicides can also be harmful to predatory mites (Martin 1993). The use of sulphur in apple orchards has long been associated with mite outbreaks arising from toxicity to a range of predatory mite species. However, the severity of this effect varies with sulphur formulation and predator species. T. pyri was found to be tolerant of colloidal sulphur sprays by Collyer and van Geldermalsen (1975) whereas sulphur is highly disruptive to mite control by M. occidentalis (Hoy and Standow 1982) (see also Martin 1993). The specificity of these effects is also shown by ryania, which is safe to many beneficial species but was found to be toxic to some predatory bugs (e.g. Criocoris saliens Reuter), parasitic Hymenoptera (e.g. Macrocentrus ancylivorus Rohm which attacks oriental fruit moth, Cydia molesta (Busck)) and coccinellids (see review of Wearing 1990).
Research is needed with all pest control products and practices used in organic apple orchards to determine their impact on key natural enemies. Inherent selectivity can often be found in natural products but this is by no means assured, and formulation, placement, and timing may be very valuable in achieving this.
Local and process markets
There are excellent short-term prospects for the organic production of apples for the local and process markets. The availability of mating disruption and the granulosis virus of codling moth addresses the major pest barrier to production. These control methods are costly and methods to improve their cost effectiveness are required. Equally important is the recent availability of apple cultivars resistant to diseases, especially to black spot, Venturia inaequalis (Cooke) and to a lesser extent powdery mildew, Podosphaera leucotricha (Ellis and Everh.). These cultivars can produce fruit of good quality for the local and process markets and HortResearch is evaluating a wide range, including ‘Dayton’, ‘Freedom’, ‘Jonafree’, ‘Liberty’, ‘Prima’, ‘Priscilla’, and ‘Red Free’. Some of this research is described by McCarthy (1994) at this Conference. Many of these cultivars also have resistance to fireblight. They lack the long storage characteristics essential for successful exporting.
Organic growers who produce apples for the local and process markets can tolerate low levels of damage from the other pests described in this paper. However, the essential but costly control of codling moth requires that other costs of pest control are kept to a minimum. Present practices, in which B. thuringiensis sprays, oils and other products are applied to the crop, cannot be sustained in addition to codling moth control without substantial price differential in favour of organic apples. Improvements in biological control of scales, mealybugs, and leafhopper through existing research programmes could reduce these pests to levels where spray interventions (such as oil sprays) are not required.
Similarly, improvements in the efficacy of B. thuringiensis or more effective and cheaper methods for leafroller control are being investigated. Woolly apple aphid will remain a minor problem for organic growers but only as long as commercial rootstocks remain resistant to this pest. Recent trends for apple growers to use aphid-susceptible rootstocks (e.g. Malling 9, Malling 26 and ‘Mark’) should be strongly opposed and HortResearch has a programme to develop new rootstocks resistant to woolly apple aphid (S. Tustin, M. Malone and J.T.S. Walker, pers. comm.). This is essential for future organic production. In the interim, interstems can be used to reduce the vigour of aphid-resistant roostocks to produce smaller and more compact apple trees.
Export market
The production of organic apples for export can be achieved at present but is difficult and demanding because of fruit quality and quarantine requirements. Reliance on copper and sulphur fungicides for the control of black spot and powdery mildew on conventional cultivars inevitably results in some russetting of the fruit and lower export pack-outs. The development of disease resistant apples suitable for export is the highest priority for research and development and a programme with this objective is being led by Allan White of HortResearch. While codling moth mating disruption can achieve control of this key pest, leafroller damage and infestation of the harvested fruit is difficult to prevent with available technology, particularly at reasonable cost. Similar problems are evident with mealybugs. Short-term prospects for expansion of organic apple exports are therefore limited, but are best in the southern and drier areas of New Zealand, such as Otago and Canterbury where the disease incidence and pest complex is more readily controlled. There are also better prospects for organic production of pears for export because of their fewer pest, disease and phytotoxicity problems.
Medium to long term prospects for the organic production of export apples are good provided that the current research and development initiatives are continued, and preferably expanded. The principal needs yet to be met are disease resistant cultivars and control methods for leafrollers. It should soon be possible to produce transgenic apple cultivars which contain B. thuringiensis genes that protect the crop from attack by leafrollers. This option should be considered seriously by organic growers. These genes are the same as those present in the B. thuringiensis sprays already being applied to organic apples close to harvest. Although there is opposition to the use of transgenic crops by organic movements, there is a need to evaluate each transgenic development on its own merits. Bt-apple cultivars present a major opportunity for organic production using a novel application of a biological control agent with an established record of safety to the environment and human health. On present evidence, many of the plant-derived pesticides approved for organic production may present greater hazards to the environment and to consumers than would Bt-apples. A recent review of natural and synthetic insecticides (Coats 1994) reached three clear conclusions: the biological activity of a chemical is a function of its structure rather than its origin; the biological properties, especially safety, of a chemical depend on its structure and the way in which it is used; and perceived risks are not always consistent with actual risks. The safety of all chemical control agents require objective evaluation, whether natural or synthetic, or applied as sprays or transgenically.
Another key principle for the future success of pest control in organic apple production is the development of an integrated programme which addresses the interactions between management practices and their effects on the whole crop ecosystem. Like conventional practices, organic pest control methods will impact on other pests, their natural enemies, natural disease control agents, tree vigour, fruit quality and productivity.
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