Increasing consumer concerns over food safety, combined with the desire for more sustainable production practices has led to the search for alternatives to calendar applications of organophosphates, which have provided the basis for pipfruit pest control for over 30 years. Leafrollers are without doubt recurring, widespread, and potentially very costly pests of our export horticulture industries, and they do require control. Mating disruption, using synthetic sex pheromones applied in orchards at 1000 plastic twist-tie dispensers per ha is one system giving promising results against several species, and has counteracted the threat of insecticide resistance in leafrollers and codling moth. In this article, we look at new technology, including a bio-sensor device using a moth antenna and portable electronics to detect pheromone in treated areas, which is improving prospects for making disruption more viable on a wider scale.

A Solution for Insecticide Resistance

Despite regular insecticide applications, exports to certain markets have from time to time been jeopardised, where the occurrence of leafroller damaged fruit has exceeded the nil tolerance level. Fortunately, cases of insecticide resistance in two species of leafroller have remained localised, in contrast to some overseas leafrollers, such as the tufted apple budmoth, which has widespread resistance in the eastern USA. The recent development of mating disruption against lightbrown apple moth and other NZ leafrollers has significantly reduced the threat from insecticide resistance, by providing a method which controls resistant insects, and which does not leave any offspring in the treated area. This approach, first used against lightbrown apple moth in Nelson, is now showing good results in trials by Dr. Howard Wearing against resistant P. octo in Central Otago.

Other trials with mating disruption in parts of Nelson have generally provided similar levels of insect control to the standard insecticide program, despite the omission of two insecticide applications (with low residue fruit resulting from the new program). In recent (pre-commercial) trials in apple orchards, disruption costs about the same as three-four insecticide applications, but only offers the saving of two sprays to offset the increased costs. Trials with double rate pheromone (2000/ha) gave better control than the standard rate, which was itself better than the reduced insecticide program alone.

Mating disruption of codling moth is also progressing well in NZ, and the technique is being used on a large scale in the Pacific Northwest (USA), where resistant codling moths now occur. Trials funded by the NZAPMB, growers and the Crown, and conducted by John Clearwater in the Auckland region have shown that repeated use of disruption over several seasons can greatly reduce even large populations of codling moth - and this season organic apples were exported to Europe from these North Island trials, as well as from our trials at Winchmore Research Orchard ("BioGro" certified) in South Canterbury.

Future Development of Mating Disruption

For conventional growers, mating disruption offers benefits of a) resistance management, b) improved control - if overlaid onto the standard insecticide program, or c) reduced residues at harvest if used with a reduced pre-harvest program, as in the Nelson trials. The increase in quarantine security against leafrollers from deploying pheromone dispensers over an existing full insecticide programme may well be the most significant benefit for many growers. This increase in quarantine security may be important enough to warrant pheromone use this season. However, for production of low residue fruit, pheromone currently costs more than the saving of two insecticide applications (and has slightly greater uncertainty if used with reduced insecticides, until shown otherwise). How then can we proceed from here? Can the pheromone-based control program be integrated with the existing control measures used by growers, without increasing costs or risk of damage?

Other trials funded by the NZAPMB have shown that improved leafroller control can be achieved from higher rates of pheromone, which offers the basis for optimism that the technique can be made to deliver even better levels of control, provided that greater understanding of the requirements for success can be translated into an economically viable system. In some locations, it will be necessary to integrate the system with an alternative pre-harvest insect control tactic, to reduce the risk of live larvae being present at harvest. Attempts to develop such residue-free alternatives, such as Bt (used against leafrollers on kiwifruit and berryfruit), or natural pyrethrum, have not yet been effective enough. However, research is continuing on low residue pest control options.

Understanding The New Technology

Fig. 1. Replacing the antenna in the antenna holder, which is used to contain the moth antenna used to detect pheromone in the air of orchards treated with mating disruption dispensers. (Photo: D.M. Suckling)

Some questions still remain on how we can improve the efficacy of the mating disruption system. For example, the optimal placement of pheromone dispensers is not known. One exciting development which should help with these questions, is the use of a portable electroantennogram, as a bio-sensor for pheromone in the air of treated orchards. This equipment relies on the most sensitive detector available - the moths own antenna! The fresh antenna is mounted between wells containing a weak salt solution and electrodes, and is hooked up to electronics and a computer, that records a change in electrical potential when the antenna detects pheromone filaments in the air. A glass housing and charcoal filter over the incoming airstream mean that we can determine the amount of pheromone reaching the antenna, and hence calibrate the system. Using this system, we have learned that the apple leaves are a significant factor in the amount of pheromone in the air - since the leaves pick up and release pheromone from the air. Leaves were shown to pick up pheromone quickly (over seconds), and release it slowly, over several days. Therefore, disruption will be expected to work better where there is a full canopy. This result is in line with results from disruption of European grape moth in German vineyards, where disruption is only registered against the second generation of the pest, as too little foliage is present in the spring.

We will need to have a cost effective blend against several species, and our research is targeting this type of "cure-all" blend for native leafrollers. Mating disruption can operate by different mechanisms, and a knowledge of these mechanisms can help us to get better insect control. The main mechanisms are thought to be: a) "false trails", where mating is reduced through competition between female moths and dispensers, b) habituation, where the sensory system of male moths is overloaded by the constant stimulation of pheromone puffs at high doses, or c) trail masking, where the attractant blend being released by a female moth is hidden by a cloud of similar or repellent chemicals. For native leafrollers, such as the green-headed leafroller (Planotortrix octo), a pest in Waikato, Hawkes Bay, Canterbury and Central Otago orchards, we have shown that disruption is a promising option, using a non-attractive blend of pheromone and inhibitor. This blend is therefore only capable of disrupting P. octo by habituation or trail masking mechanisms. However, since the same blend is the true pheromone for another related pest leafroller, we can see that the false trail mechanism may also apply - against another species.

It is about 30 years since the idea of mating disruption was first suggested, although the first commercial system for tree fruits worldwide is less than 10 years old. The very first mating disruption trial against New Zealand leafrollers was conducted in 1984, so we have made reasonably good progress in the last 10 years, in learning about the problems and prospects for this new technology. Our team is optimistic that mating disruption will be a part of orchard pest management systems of the future, but certain aspects need to be better understood before the technology will offer something to all growers.