Goading growers towards mating disruption: the South African experience with Grapholita molesta  and Cydia pomonella  (Lepidoptera, Tortricidae)

Brian N. Barnes and Tom L. Blomefield

• Introduction
• Materials and methods
  • Oriental fruit moth
  • codling moth
• Results
  • Oriental fruit moth
  • codling moth
• Discussion
  • Oriental fruit moth
  • codling moth
  • Factors limiting the increased use of mating disruption

Abstract - Grapholita molesta Busck. and Cydia  pomonella  L. are key pests in South African stone and pome fruit orchards, respectively. G. molesta  was accidentally introduced into the Western Cape in the late 1980's, where it has up to six generations per year. By 1991, despite comprehensive insecticide programmes, crop losses in canning peach orchards were as much as 60% and shoot damage as high as 80%. As a result, some growers were considering removing productive orchards. An area-wide mating disruption programme with Isomate-M was initiated during the 1991/92 season in 1 200 ha of peaches and nectarines in the Tulbagh Valley. The project was an outstanding success. By the end of the season only 69 G. molesta  adults had been caught in pheromone traps, shoot strikes were reduced to isolated occurrences, while not a single infested fruit was recorded from the treated area. C. pomonella  has three generations per year in South Africa and a high reproductive potential. It has recently increased in pest status due to the development of resistance to insecticides, with fruit damage in some orchards exceeding 30% after up to 13 sprays. As an exercise in resistance management, an Isomate-C and Isomate-C Plus mating disruption programme with supplementary insecticide intervention was implemented in a block of apple orchards. By the end of the third season, insecticide applications had been reduced by an average of 32%, and fruit infestation limited to an average of 0.3%. A number of factors limiting the increased use of mating disruption products in South Africa are discussed.

Key words - sex pheromone, mating disruption, Oriental fruit moth, Grapholita molesta , peaches, codling moth, Cydia pomonella , apple, pome fruit, resistance management

Introduction
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The stonefruit industry in South Africa comprises 27 000 ha of peaches, nectarines, plums and apricots. The key pest on especially canning peaches is Oriental fruit moth, Grapholita molesta  Busck. G. molesta  was accidentally introduced into South Africa in about 1987, probably on peach budwood illegally brought into the country. It was first detected in 1990, and spread rapidly throughout all stonefruit growing areas. The warmer peach-growing areas support six generations of G. molesta  per year, with four per year occurring in the cooler areas.

Initial control programmes comprised up to 13 applications of organophosphate insecticides. This heavy pesticide programme was expensive and disruptive to beneficial orchard fauna, and necessitated the application of at least two sprays against mites. In one particular area, the Tulbagh valley, shoot damage by G. molesta  to canning peaches at the end of the 1990/91 season averaged 49%, ranging from 30% to 80% (Table 2). Although fruit damage in the valley was sporadic, other nearby areas had crop losses to G. molesta  of up to 60%, despite rigorous spray programmes (W. Boonzaaier unpubl. report).

As a result of the inability at that stage to control G. molesta  chemically, some growers had started to remove productive peach orchards in favour of fruit kinds not susceptible to G. molesta. In the light of these factors the future of canning fruit industry was considered to be in jeopardy. As a consequence, the South African Preserving Company (SAPCO, a canning company in Tulbagh), and the Canning Fruit Board approached the authors for assistance, particularly with respect to the situation in the Tulbagh valley. We suggested the possibility of mating disruption using Isomate-M, the only G. molesta  mating disruption product on the market at that time. Out of this was born the Tulbagh valley mating disruption project, which we undertook to plan and coordinate.

The South African pome fruit industry comprises approximately 20 000 ha of apples and 12 000 ha of pears. The key pest on pome fruit is codling moth, Cydia pomonella  L. In South Africa's warm climate it has a high reproductive potential, and two and a half to three generations per year. Azinphos-methyl has been used almost exclusively and very effectively for its control for about 35 years. However, laboratory bioassays have shown that codling moth has recently developed a high degree of resistance to azinphos-methyl and synthetic pyrethroids in some orchards (M. Addison pers. comm.). Up to 12 sprays per season are common, and despite this some orchards suffer 30% or more codling moth damage. Resistance to these insecticides, the possibility of cross-resistance to others, and a limited number of products registered against codling moth was placing a severe strain on the pome fruit industry. A project was therefore initiated to evaluate mating disruption as a means of controlling codling moth with reduced insecticide usage, which could thereby contribute to slowing the development of resistance.

In this article we describe the two projects, and discuss factors influencing the use of mating disruption in South Africa.

Materials and methods
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Oriental fruit moth

The area-wide G. molesta  mating disruption project was initiated in the Tulbagh/Wolseley valley in August 1991. The valley comprised 2 800 ha of mixed fruit; 1 200 ha of peaches (mainly canning cultivars) and nectarines, and 1 600 ha of plums, prunes, apricots, almonds, pears and grapes. It is a discrete fruit production area and was therefore very suitable for an undertaking of this nature. The valley is about 30 km long and 10 to 15 km wide, mostly surrounded by mountains, and comprises two distinct fruit-growing areas some 10 km apart. Sixty growers were involved.

Isomate-M (Shin-Etsu Chemical Co., Tokyo), containing 93% (Z)-8-dodecen-1-yl acetate (Z8-12Ac), 6% (E)-8-dodecen-1-yl acetate (E8-12Ac), and 1% (Z)-8-dodecen-1-ol (Z8-12OH), at 75 mg/dispenser was applied to all 1 200 ha of peach and nectarine orchards in the valley. It was also applied as five-row borders to all plum, prune, apricot, almond and pear orchards. All dispensers were applied by 15 August, at the recommended density of 1 000 dispensers/ha, and in the top third of the tree canopy. The second batch of dispensers was applied 90 days after the first application. SAPCO appointed two of its staff full-time to the project, who ensured that the application of dispensers was carried out strictly according to recommendations, and took responsibility for monitoring actions.

One supplementary spray of azinphos-methyl against the first moth flight had been planned for all peach and nectarine orchards. However, this was not carried out.

The status of G. molesta  populations in the treated area was monitored with Pherocon 1C sex pheromone wing traps baited with Trécé 1-mg lures, and with terpinyl acetate bait traps. Both were deployed at one trap/10 ha throughout the treated area, i.e. 120 traps of each. They were installed over a period of a week starting on 15 August. All traps were monitored every 4 to 8 days; the pheromone traps until March 1992, and the bait traps for a shorter period as little success was obtained with them. Three pheromone traps were also placed in the canning factory bulk bin stack in mid-September. All traps were serviced and maintained according to standard recommendations. Due to the size of the project and limited human resources, no attempt was made to determine the mating status of females caught in the bait traps.

Routine visual inspections were made of shoot tips and fruit. Due to the size of the area and the limited number of monitors, inspections were not intensive, but were made approximately every month until harvest, covering all areas under Isomate-M. Fruit was inspected while still on the tree. All consignments of fruit sent to the canning factory at harvest were subjected to normal quality control during which fruit damaged by insects was set aside and inspected to determine the cause.

Another peach production area some 15 km away, the Breeriver valley, was used as a comparison. All orchards in this area were conventionally treated with insecticides, starting on 15 August 1991. Typically, two applications of acephate 750 g/kg SP (1 kg/ha) were followed by five to 10 applications of azinphos-methyl 350 g/kg WP (1.75 kg/ha), depending on the harvest date of the cultivar. All sprays were applied 14 days apart and at high volume (8X). Nine Pherocon 1C 1-mg pheromone traps were deployed in different orchards in the valley on 20 August, and were accurately monitored every 4 to 8 days until early November. Accurate trap counts ceased on this date due to excessive G. molesta  catches.

All statistics given for G. molesta  were taken from an unpublished, informal report by W. Boonzaaier (SAPCO).

Codling moth
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Mating disruption of codling moth was started in 1993 on the farm Oak Valley Estates in the Elgin area.

First season (1993/94). Three adjoining orchards forming a block of 11.7 ha, and another nearby isolated orchard of 3 ha, were selected. The cultivars comprised Granny Smith, Golden Delicious and Starking. They were mature trees of 3 to 5 m in height. The mating disruption product used was Isomate-C (Shin-Etsu Chemical Co., Tokyo), containing 51.8% (E,E)-8,10-dodecadien-1-ol (E8,E10-12OH); 29.1% dodecanol (12OH); 6.0% tetradecanol (14OH) at 165 mg/dispenser. The dispensers were applied according to recommendations at 1 000 dispensers/ha, the first batch before the emergence of the first spring moths and the second batch after 85 days. Pherocon 1C wing traps baited with Trécé 10-mg lures (10 mg E8,E10-12OH on rubber septum) were deployed in the top metre of the trees at a density of one trap/ha. They were inspected weekly and the septa changed monthly.

The full compliment of 4 sprays of azinphos-methyl 350 g/kg WP (1.25 kg/ ha) was applied against the first generation of codling moth. Thereafter, azinphos-methyl was only applied to those orchards where trap catches exceeded 1 moth/trap/ week for more than three consecutive weeks. All sprays were applied at low volume (4X).

At the end of the first two generations, and again a week before harvest (early March), fruit were visually inspected for codling moth damage on 100 trees per each of five 3- to 5-ha sample blocks, 64 trees in the border area and 36 trees in the inner area. Twenty fruit were randomly inspected from the top and 20 from the bottom of each tree without removing them from the tree, giving 4 000 fruit/sample block.

Second season (1994/95). The same four orchards used during the previous season were used again. However, 10.5 ha was added to the 11.7 ha orchard, making a total of 25.2 ha, of which 22.2 ha were contiguous. The mating disruption product used was Isomate-C Plus (52.9% E8,E10-12OH, 29.7% 12OH, 6.0% 14OH). Dispensers were applied as described for the first season, and a second batch was applied after 121 days. Monitoring traps were deployed and inspected as in the first year, and the septa replaced fortnightly instead of monthly. Insecticide application and the method of fruit sampling was as during the first season, except that seven 3- to 5-ha sample blocks were used.

Third season (1995/96). The orchards used were the same as those in the second season, with the exception that the 3-ha orchard was excluded from the trial. Isomate-C Plus was again used, the second batch being applied after 100 days. Monitoring was carried out as during the second season. Sprays were applied only to those orchards where traps recorded more than 2 moths/trap/week, or 1 or more moths/trap/week for two consecutive weeks, or where fruit damage remained above 0.1% at the end of any generation. Flufenoxuron 100 g/L DC (0.75 L/ha) was applied against the first generation, chlorpyrifos 250 g/kg WP (3.75 kg/ha) and fenoxycarb 250 g/kg WP (1.4 kg/ha) against the second generation and azinphos-methyl (as in the first season) against the third generation. Fifty trees/block in eight 3- to 5-ha blocks were sampled, 32 trees from the border area and 16 from the inner area, inspecting at random 20 fruit from the top and 20 from the bottom of each tree (2 000 fruit/sample block). The sampling times were as during the first and second seasons.

In all three seasons, data from a conventionally treated orchard in the vicinity, was used for comparison. This orchard was sprayed only with azinphos-methyl during the first and second seasons, and with the four insecticides mentioned above during the third season. Monitoring was with Pherocon 1C wing traps baited with Trécé 1-mg lures (1 mg E8,E10-12OH) which were replaced every 4 to 6 weeks.

Results
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Oriental fruit moth

Pheromone traps. Pheromone trap catches in the Isomate-M treated area and the conventionally treated Breeriver valley are given in Table 1. Between 23 August and 30 October 1991, 49 G. molesta  adults were trapped in the 120 traps covering the 1 200 ha of orchards treated with Isomate-M. After this date, no further moths were trapped anywhere in the treated area for the rest of the season. The three traps in the bulk bin stack caught 20 G. molesta  adults up to 7 November 1991, and no further moths were trapped after that date.

In the Breeriver valley, where insecticide sprays were applied every 14 days until harvest, very high catches of G. molesta  were recorded; e.g. 952 were recorded from nine traps on 7 November at which point accurate records ceased.

Bait traps. By 30 October, only five G. molesta  adults had been recorded from the 120 bait traps in the treated area. The use of these traps was therefore stopped.

Shoot damage. The percentage shoot damage at the end of the season preceding Isomate-M treatment (March 1991), and at the end of the Isomate-M season (March 1992), is shown in Table 2. In March 1991 shoot damage ranged from 30% to 80%. At the end of the 1992 season, shoot damage was virtually non-existent. In one or-chard where Isomate-M was applied late, 6% shoot damage was recorded. The only other reported shoot damage was three shoots from a 4-ha orchard of Neethling peaches and "a few shoots" (W. Boonzaaier unpubl. report) from an orchard of Black peaches.

In contrast, between 15% and 40% shoot damage was reported from the Breeriver valley at the end of the 1991/92 season.

In the Breeriver valley, fruit damage of between 12% and 15% was recorded at the end of the 1991/92 season, despite rigorous spray programmes.

Codling moth
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Pheromone trap catches from traps in the conventionally treated orchard and in the mating disruption block during the second (1994/95) and third (1995/96) seasons are given in Figures 1 and 2. Details of the extent of codling moth damage and the number of sprays in the mating disruption and conventionally treated blocks, are given in Table 3.

Pheromone traps. Trap catches in the conventional orchard indicated high codling moth populations outside the mating disruption block (Figure 1). The 10-mg traps inside the mating disruption block showed a pattern typical in these circumstances: a short period of low-level attraction immediately after lure replacement, followed by a sharp drop in catches as the pheromone concentration of the lure dropped below the high background concentration of mating disruption pheromone (Figure 2).

During the second season, trap catches indicated a build-up in codling moth populaions in the mating disruption block towards the end of the season, suggesting that chemical intervention may have been inadequate. In contrast, at the end of the third season trap catches suggested that populations were substantially lower, perhaps pointing to better overall control.

Fruit damage. The percentage codling moth damage in the conventionally treated orchard varied from 2.7% to 34% over the three seasons (Table 3). In the mating disruption block codling moth damage decreased over the three seasons from an average of 1.1% in the first season to 0.3% at the end of the third season. The range of damage in different mating disruption orchards also showed a downward trend in successive seasons, to the point where no codling moth damage was recorded from some orchards at the end of the third season.

Number of sprays. In all three seasons, 11 sprays were applied against codling moth in the conventionally treated orchard (Table 3). The average number of codling moth sprays applied in the mating disruption block remained relatively constant during the three seasons, between 7.5 and 7.8. This represents an average reduction of up to 32% in the number of sprays. The range in number of sprays hardly varied between seasons, with a lower limit of four (second season) and an upper limit of 11 (all three seasons).

Discussion
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Oriental fruit moth

That very large populations of G. molesta  existed in the Tulbagh valley at the end of the season preceding the application of mating disruption, cannot be doubted. Despite this, the area-wide use of Isomate-M to control G. molesta  in the Tulbagh valley was a great success, as testified by the total absence of G. molesta  fruit damage from 1 200 ha of fruit. As a result of this, and the extremely low incidence of shoot damage at the end of the 1991/92 season, it was suggested that the use of Isomate-M dispensers should be reduced to only one application at the beginning of the following season (P. Kirsch pers. comm.).

Unfortunately, mainly due to cost considerations, few growers took advantage of the huge benefit afforded by the mating disruption project and most of them returned to chemical control. Nevertheless, four years after the Tulbagh valley project, G. molesta  is better under control than it was immediately before the project. About 30% of growers in the valley still use mating disruption, although compliance with recommendations is poor (F. Theron pers. comm.). This is largely due to very poor technical support on mating disruption from the stonefruit industry and from distributors of mating disruption products.

We consider the following factors to have been crucial to the success of the Tulbagh valley project:

Fruit industry involvement. Four organizations worked closely together before and during the project. SAPCO and the Canning Fruit Board among them organized grower meetings in Tulbagh at which the area-wide mating disruption project was proposed and planned; financed a fact-finding mission to California by their representatives and an entomologist to gather information on G. molesta  and mating disruption; identified key growers who would support and promote the project amongst some reluctant growers; provided bridging finance to Tulbagh valley growers to assist them in purchasing mating disruption dispensers; and supervised the application of dispensers and subsequent monitoring actions.

Research and extension backup. The role played by the Stellenbosch Institute for Fruit Technology, in the persons of the two authors, was in basic planning and coordination of the project throughout its duration; providing information at grower meetings on the pros and cons of mating disruption and on application recommendations; promoting the project in the media; acting in an advisory capacity; and conducting separate 10-ha trials with Isomate-M dispensers to register the product for commercial use against G. molesta .

Support by product distributor. Pacific Biocontrol, in the person of Philipp Kirsch, made a number of visits to the area to provide information at grower meetings on mating disruption, Isomate-M dispensers and their application, and to refine aspects of the project methodology. He also provided real-time back-up on G. molesta  behaviour and the use of Isomate-M during the course of the project.

Participation by all growers. It was considered imperative to avoid pockets of stonefruit orchards within the Tulbagh valley that were not treated with pheromone, as the large populations emanating from these orchards would have placed pressure on the pheromone-treated area. The eventual participation of every one of the 60 growers was cardinal to the success of the project.

In situ supervision by SAPCO. The vested interest by SAPCO in the success of the project guaranteed their commitment. They ensured strict compliance with application recommendations, and facilitated regular monitoring over the whole area.

Codling moth
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The results of the codling moth mating disruption project should be seen in the context of the high reproductive potential of codling moth in South Africa, and its resistance to insecticides. Furthermore, spray management on the farm on which the trial took place was sub-optimal. Trees on this farm were generally large and dense, and the farm had too few spray machines for the area under fruit. The result was inadequate spray coverage.

The level of codling moth damage in the mating disruption block compared very favourably with the conventionally treated orchard, especially during the second and third seasons when poor control was achieved with a straight insecticide programme.

The average percentage codling moth damage in the mating disruption block remained at 0.3% for both the 1994/95 and 1995/96 seasons. Nevertheless, the reduced second and third generation trap catches during 1995/96 (Figure 2) suggests that populations were much lower during this third season. The total absence of codling moth damage in some orchards at the end of the third season, and the fact that it was possible in some of the orchards to omit sprays against the second and third generations, supports this contention.

The high codling moth populations in the conventional orchards surrounding the mating disruption block placed pressure on the mating disruption treatment, which partly accounted for the fact that the average number of sprays in the mating disruption orchards could not be further reduced during 1995/96. Although there were some orchards in which only four sprays were needed during the season, some orchards required the full compliment of eleven sprays. This is ascribed partly to the poor spray management on the farm -- large, dense trees and too few spray machines -- and partly to the high codling moth populations outside the mating disruption block.

Despite this, the project is considered a success by virtue of the progressive improvement in codling moth control by mating disruption in successive seasons, under a reduced insecticide programme. Greater success can be expected in orchards with less dense trees and which are under better spray management. The project has now entered its fourth successive season, in which it is hoped to reduce the number of sprays even further.

Factors limiting the increased use of mating disruption
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In our opinion the use of mating disruption in South African stone and pome fruit orchards is limited by the following factors:

Biotic factors. A hot climate resulting in more-than-usual Oriental fruit moth and codling moth generations per year, and in accelerated dispenser depletion; high codling moth reproductive potential; many orchards with large, dense trees; the probable necessity for simultaneous control of secondary pests.

Management factors. Large farms (on the average 50 ha); clashes in other orchard activities when mating disruption needs attention; increased levels of monitoring and sampling; the necessity for post-harvest control of Oriental fruit moth and codling moth (sprays for Oriental fruit moth, orchard sanitation for codling moth).

Human factors. Inadequate knowledge of mating disruption by growers and technical representatives; inadequate technical support by distributors of mating disruption products; perceived unrealistic profit-taking by mating disruption distributors; lack of sophistication of some growers; resistance by growers to change and increased effort.

Cost factors. This is probably the greatest limitation in mating disruption usage. Mating disruption of codling moth currently costs between 590 and 780 US$/ha/season, depending on the extent of chemical intervention. Mating disruption of Oriental fruit moth costs slightly less. These high costs are due to all mating disruption products having to be imported into the country. By contrast, a full spray programme costs between 190 and 275 US$/ha/season, depending on the insecticides used. However, the latter costs do not take into account the cost of the development of resistance and associated crop losses.

Many of the management and human factors can and must be addressed and improvements made where possible. However, most of the biotic factors cannot be changed. It is unlikely that the cost of mating disruption will become more affordable in South Africa, unless the products are manufactured in the country. The likelihood of this is uncertain.

Acknowledgements
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We thank the following people and organizations for their many and varied inputs into either or both projects: SAPCO, and especially Wian Boonzaaier; the Canning Fruit Board, and especially Wiehahn Victor; Pacific Biocontrol Inc., and especially Don Thomson and Philipp Kirsch (latter now of IPM Technologies, Oregon State); Oak Valley Estates, and especially Henry Brink; Unifruco Research Services; Holpro Fine Chemicals; Chris Dane; John Levings; Philip Kleinhans; John Michau; Muriel Knipe; Irene Flight and Niel du Plessis.
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