Mating disruption for caterpillar control in organic pipfruit orchards in New Zealand
Codling moth and five leafrollers are key insect pests of organic pipfruit orchards and can cause significant crop loss, if not controlled. The economic thresholds for these pests are likely to be determined by the end use of the fruit, with less stringent standards generally being accepted by the local market. In contrast, quarantine restrictions demand a nil tolerance for all export fruit. Mating disruption, using synthetic sex pheromone, is a promising control tactic for these species. It involves the placement of pheromone dispensers into the crop that continuously release pheromone for several months. The resulting pheromone ‘cloud’ disrupts mate location by males, reduces mating and consequently reduces damage by caterpillars in the next generation.
However, mating disruption has several important limitations that relate to organic production. Each pest species has a unique pheromone and may require a separate dispenser (with associated higher costs), unless a common pheromone blend can be developed for related species. Mating disruption is most effective at low population densities. The size of the area to be treated is also important because mated female moths may immigrate and lay eggs. Areas greater than 3 ha are needed to achieve the best results. A cost-effective combination of pheromone and larval control, such as with Bacillus thuringiensis, codling moth granulosis virus, or other larval insecticides, will be needed to achieve quarantine standards for export.
Pheromones are chemicals used for communication amongst insects of the same species, and are frequently blends of several compounds. Sex pheromones were first identified and used against insect pests in the 1940s when crude extracts from virgin females were used to lure male gypsy moths. Pheromone-baited traps continue to play an important role in many cropping situations, where they are used to indicate pest presence and relative abundance (Suckling 1993).
Sex pheromones are also used for mating disruption (disrupting sexual communication and preventing mating). This technique involves permeating the entire crop environment with synthetic sex pheromone. In New Zealand, this has been achieved using twist-tie dispensers marketed by Shin-Etsu Chemical Co. (Tokyo) (Suckling 1993), although other types of dispensers are being tested or marketed elsewhere. Up to 1000 Shin-Etsu dispensers per ha are placed in the orchard, where they release pheromone for several months. This has the effect of confusing the males, by preventing them from detecting the pheromone of virgin females. This approach has been successfully used in a number of overseas pest/crop situations (Suckling 1993).
Scientific evidence shows that mating disruption works in many situations, but failures have also been reported. The mobility of insects, and their ability to re-colonise the treated area have been suggested as the main reasons why these attempts have failed. This paper reviews the main issues that need to be addressed to optimize mating disruption as a control technique against lepidopteran insect pests in New Zealand pipfruit orchards.
New Zealand has a long and narrow landform, with many hills and mountains rising in opposition to the predominantly westerly airflows. These factors, and the cycle of cyclones and anticyclones, make for a vastly different climate and weather to that experienced in continental areas, such as Europe and the United States. Orchards also experience a wide range of conditions, ranging from sub-tropical temperatures and high humidities in the far north, through to extremes of hot dry summers and long cold winters in Central Otago. The main species of lepidoptera that attack pipfruit crops include codling moth (Cydia pomonella), the lightbrown apple moth (Epiphyas postvittana), and four endemic leafroller species. After analysis of their pheromones (Galbreath et al. 1985; Clearwater et al. 1991), these four species were identified as two greenheaded leafrollers, (Planotortrix octo, and P. excessana), and two brownheaded leafrollers, (Ctenopseustis herana, and C. obliquana) (Dugdale 1988). Both C. pomonella and E. postvittana are found throughout the fruit growing areas of New Zealand. However, the species composition and abundance of the endemic leafrollers varies between regions. This complexity makes the implementation of a universal mating disruption programme very difficult, as pheromones are specific to each species. Research on disruption in orchards initially concentrated on Nelson, where one species (the lightbrown apple moth) is dominant, making it is easier to understand all the variables that go into making disruption successful (below). Research has been aimed subsequently at developing a multi-species pheromone blend in order to disrupt all of the endemic species that are abundant in this and other districts. This should be possible because the individual pheromones of the native species share similar components. At present, three dispenser formulations have been evaluated for use against codling moth (Isomate-C), the lightbrown apple moth (Isomate-LBAM) and the native leafrollers (Isomate-Z5/Z8). The development of a common dispenser against codling moth and LBAM is underway for use in Australian apple orchards, where no other leafroller species require control. Similar work is needed in New Zealand, incorporating the blends of all pest species.
Secondary pests such as mealybugs, scale insects, woolly apple aphid and Frogatt’s apple leafhopper, also require management in orchards, and these pests will be dealt with elsewhere (Wearing et al. this volume).
The adoption of mating disruption as a technically and economically viable control tactic by conventional New Zealand pipfruit growers has been slow, in part because of the low cost and generally high effectiveness of broad spectrum insecticides. The majority of our fruit is grown for export to diverse markets such as Europe, the United States and Asia. It is a high quality product aimed at the ‘top end’ of these markets. To ensure that the fruit is blemish free and of the very highest quality, synthetic insecticides such as azinphos-methyl and chlorpyrifos are used. These insecticides ensure that the strict phytosanitary restrictions placed on our fruit can be met. Development of integrated pest management, with lower reliance on broad spectrum insecticides, has been underway for many years in New Zealand pipfruit (Wearing et al. 1993).
For the organic grower, quarantine restrictions are the main barrier preventing entry of fruit to export markets. There are no insecticides currently on the market which are able to lower leafroller and codling moth numbers to quarantine-acceptable levels at a reasonable cost and still fit the criteria for organic certification. Some work has been done on Bacillus thuringiensis, but the high levels of ultra-violet radiation in New Zealand quickly deactivate current formulations. Codling moth granulosis virus has shown some promise (Wearing 1990) but this will only control one species, and doesn’t affect any of the leafrollers. Mating disruption is currently one of the only viable methods of controlling the main tortricid insect pests, while meeting the certification criteria of the Biological Producers and Consumers Council. However, at $250-$300 (pre-commercial price) per hectare, for an application of each type of dispenser, this system is expensive. A multi-species dispenser will be needed for organic orchards to reduce the cost to growers.
There are no rules that will absolutely guar–antee successful pest control at any site, since the tactic of disruption is targetted at the adults of the generation before the occurrence of the damaging larval stages. However, careful planning and attention to the criteria described below can greatly improve the chances of success. Physical site characteristics are critical to the success of disruption, and experience has shown that certain "types" of orchards will be more amenable to this approach than others.
Size, shape, and foliage density
Immigration of mated females is a major problem, which is influenced by the factors of block size and shape. The size of treated areas should exceed 3 ha and blocks with reduced edge effects (i.e. square compared to rectangular) are preferred. Foliage is an important buffer of atmospheric pheromone concentrations, acting to reduce wind speed, as well as to absorb and release pheromone (Karg et al. 1994; Suckling et al. 1996). Hence disruption is improved when the maximum amount of foliage is present (e.g. more uniform plantings of larger trees, more buffering of pheromone concentrations in summer compared to spring). Blocks with missing trees are more at risk of failure of disruption on the edges of these gaps.
Topography and shelter
Pheromone concentrations are likely to be lower in windy or exposed situations, or where air drainage permits loss of pheromone. This has been shown, by using insect pheromone traps to monitor the success of disruption (see below). Moth catch is rare within pheromone-treated blocks, but catches sometimes occur at hilltop sites and where wind is able to remove the pheromone more rapidly than it can be replenished. Being heavier than air, pheromone will also drain (with air movement) down slopes, giving higher concentrations on the lower slopes. In general, pheromone concentrations are higher where shelter is present to reduce wind speed.
Population size
The greatest success of mating disruption has been obtained in orchards that have a low population of the target species. This is because disruption works best with lower pest densities (Suckling and Shaw 1992). From codling moth mating disruption trials in Vancouver, the initial population density required for excellent control has been shown to be capable of causing less than 0.5% damage to fruit from this insect in the previous season. However, dramatic reductions from a high initial population were recorded at an isolated orchard in the Coromandel district of New Zealand (Clearwater and Muhlbacher 1993). Based on our New Zealand trials, the least risk option available to organic growers is to reduce population size using other tactics first, such as codling moth granulosis virus (CMGV), and/or by the removal of windfall fruit, before using mating disruption. As an initial guideline, >10% fruit damage in the previous season requires the use of mating disruption with CMGV; if there is <10% damage, mating disruption can be used alone. For a population causing >10% damage, it may take up to three years to bring the population and damage down to economically acceptable levels. In the absence of knowledge of codling moth density, it would be wise to apply CMGV in the first year, particularly in the North Island. The CMGV may also be necessary to successfully manage codling moth infested trees which are planted on sloping ground.
The use of Bacillus thuringiensis formulations currently available will provide limited control of leafrollers (Suckling et al. 1993; Walker unpublished data), and should be supplemented by management of host weeds through grazing or other means.
Isolation
External sources of codling moth or leafrollers can provide significant immigration of mated females. Dispersal distances of mated leafroller females in New Zealand can exceed 100 m, although distances under 50 m are more common (Suckling et al. 1994a). Problem sources include home gardens, unsprayed fruit or nut trees, and surrounding weeds. In the case of codling moth, which has a limited host range (pipfruit, walnuts), it may be possible to remove the sources of immigrant females. For leafrollers, which are polyphagous, removal of important alternative host plants such as scrub weeds (e.g. gorse and broom) may also help to reduce the risk of immigration during the season. Immigration of mated females represents the most serious defect of this control tactic, and large block size is therefore critical to reduce the impact of immigration, by diminishing the edge effect.
Application of Dispensers
Dispensers should be applied at the rate of 1000 dispensers per ha, and placed as high as possible in the tree canopy, to minimise the risk of mating in the tops of trees. This is likely to be more important in blocks affected by wind, which will cause more of a vertical gradient in atmospheric concentration due to the increasing wind speed with height. Approximately four hours of labour are required to place 1000 dispensers. Dispensers should be applied loosely around lateral branches, preferably under foliage on the South (shady) side of the tree. Application of extra dispensers in place of missing trees (on trees around the gap), or in exposed situations is advised. For codling moth, the recommended dates for dispenser application are: Auckland - last week of September/1st week of October; Hawkes Bay - 1st week of October; Nelson, Canterbury, Otago - 15 October. For Otago, Canterbury and Nelson one application is likely to be sufficient. However, in Hawkes Bay and Auckland, two applications are likely to be needed, with the second application (at half rate, 500 per ha) in mid-January (Bradley et al. 1995a and b). To date, most researchers overseas have reported the best results using the Isomate-C formulation, although other dispensers are being developed.
Similar recommendations apply for leafrollers, although it appears that Central Otago (Wearing 1995) and Canterbury (Suckling unpublished data) orchards will need a second application (500 per ha). It is possible to predict pheromone release rates from the dispensers applied against lightbrown apple moth or native leafrollers (Bradley et al. 1995a and b), and higher rates have been shown to be more effective (Suckling and Shaw 1995). However, more information is needed on the atmospheric concentrations of pheromone required for success.
Monitoring
Pheromone traps are the most cost effective means of monitoring disruption efficacy, although they only indicate ambient male moth activity, and may not accurately predict immigration of females or population size. To assess codling moth and leafroller populations in the first season of disruption, it is therefore advisable to monitor the pest pressure from both inside and outside the orchard, by operating traps externally and internally. Larger catches in traps outside the block will indicate the potential for pest immigration from outside the orchard, and be a guide to the need for the removal of nearby host plants.
Standard pheromone traps placed inside orchards treated with dispensers are generally "shut-down" once dispensers have been applied. A catch of moths in these standard traps can indicate failure of the disruption system. In some cases this has resulted from poor application technique (Suckling and Shaw 1995), or because a second application was needed when the release rate had become too low. For both codling moth and leafrollers, higher strength lures for traps are being evaluated, and these show considerable promise for indicating high flight activity. Peak activity may indicate the need for further pest control action, in addition to disruption. Lures intended for use inside treated areas may need be 10 times stronger than standard lures, to measure these effects under the disruption treatment. Peak catch in external traps or in traps baited with high strength lures inside the pheromone-treated area usually reflect peak flight, and therefore mating in the treated area (Suckling and Shaw 1992; Suckling et al. 1994b; Suckling and Burnip 1995). It may then be possible to time CMGV, Bt or other organic sprays to coincide with hatching codling moth or leafroller eggs, using information from these traps. However, no action thresholds have yet been developed.
Inspections of fruit for the presence of codling moth larval injury is best performed 10-20 days after the flight peak detected in external or high dose pheromone traps. This will provide a further check to the efficacy of the control measures used. Banding of trees prior to the prepupal stage of codling moth can give an indication of larval populations present in individual trees, but this is time consuming, and is not effective as a control measure because the bands simply provide additional overwintering sites for the larvae which would not normally be available. Leafroller larvae are much more abundant on foliage than on fruit earlier in the season. Closer to harvest, a crude estimate of population size can be made from examination of fruit clusters. A more rigorous protocol for estimating leafroller larval population size is under development. Sampling for secondary pests (Wearing et al. this volume) would also be necessary in many situations.
Mating disruption, using synthetic sex pheromone is likely to be a useful part of a pest management programme for the caterpillar species attacking organic pipfruit orchards in the future, but will be most valuable as part of an integrated package of measures, rather than used in isolation. The cost of current pheromone dispenser formulations remains a barrier to adoption, especially where more than one caterpillar pest species is present. Increased returns from fruit produced without chemical insecticides could therefore greatly assist growers with the adoption of mating disruption, as would the development of a less expensive dispenser.
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