by Don Thomson and Glenn Thayer, April 1, 2021
Codling moth mating disruption is now widely deployed in Washington State to help control codling moth (Goldberger, J., Lehrer, N and Brunner, J. 2013). There are 2 main types of dispensing systems for mating disruption control of codling moth. Hand applied (passive) systems were the first to be registered in 1991 and remain the most widely used today. Aerosol dispensing (active) systems have become popular in the last 10 years as growers try to reduce the cost of application. Passive or hand applied dispensers are generally applied at 200 to 400 dispensers per acre. Aerosol dispensers are generally applied in apples and pears at 1 per acre. In walnuts, aerosol dispensers are applied at 1 unit per 1.75 acres due to the large canopies. The difference in application rate is large so let’s explore the release rate and the mode of action of each type of dispenser.
First, let’s talk about how the plumes of pheromone emanating from these dispensers are generated and what they look like downwind. First, hand applied dispensers contain the pheromone active ingredient and stabilizers as the pheromone is very susceptible to breakdown by photo-oxidation. The release rate of passive dispensers are a function of temperature and wind. The higher the temperature or wind speed, the greater the release rate. After diffusing through the wall to the surface of dispensers, the pheromone evaporates creating plumes that meander downwind.
Aerosol dispensers act very differently. First, the formulation is a mixture of the pheromone active ingredient, a diluent and a propellant gas. The release rate of aerosol dispensers is mechanically controlled by a program in the emitter. Every 15 minutes or so, the valve is depressed and the pheromone formulation is metered out. Upon air contact, the propellant interacts with the air resulting in a mini-explosion that breaks up the pheromone into very small droplets ranging in size from 20 to 120 microns in size. The small pheromone droplets are carried long distances downwind from the aerosol emitters. These plumes are very different from the plumes emanating from the surface of hand applied or passive dispensers. The plume extends farther both downwind and laterally. Because of the reach of the aerosol plumes, less aerosol units are needed per acre to ensure coverage. So how do codling moth males respond to the pheromone plume coming from these very different dispensing systems?
Mode of Action
Codling moth males detect the pheromone downwind of the dispensers and move upwind in search of the source. In short, dispensers stimulate male codling moth searching behavior (Witzgall et al. 1998). Instead of finding female codling moth at the end of the pheromone plume, male codling moth find a dispenser. Therefore, pheromone dispensers are competing with female codling moth for the attention of male codling moth. The mode of action is sometimes referred to as false trail following or more recently as competitive attraction (Miller and Gut, 2015). Aerosol dispensing systems work in a similar way even though the plumes are much larger and more concentrated. Research conducted at Michigan State University showed that male codling moth move along the edges of these large plumes toward the emitters and congregate behind (McGhee et al. 2016).
Importance of Point Sources
Because the mode of action is false trail following or competitive attraction, it stands to reason that the higher the number of codling moth in the orchard, the more dispensers are needed to disrupt mating. This is true for both passive and aerosol dispensers. Although the exact relationship between the number of dispensers needed relative to the number of codling moth resident in the orchard is not known, it stands to reason that the higher the deployment rate of codling moth dispensers, the greater the efficacy (Epstein, D. et al. 2006).
The number of hand applied dispensers needed per acre cannot be reduced by increasing the release rate of the dispenser. That is because, the plume only extends so far regardless of the concentration of pheromone in the plume. In short, more dispensers per acre provides better coverage of the orchard. Gary Judd of Agriculture Canada in Summerland, British Columbia showed that there was a minimum threshold of point sources required regardless of release rate in order to ensure uniform distribution of pheromone in the orchard canopy. Once the orchard is covered with pheromone, then increasing the number of point sources yields diminishing returns. Remember this: more point sources are better but once the orchard is treated with the maximum label of rate of dispensers (typically 400 dispensers per acre), adding more increases efficacy but not in proportion with the cost of adding more dispensers.
Given that the mode of action is competitive attraction or false trail following more dispensers improve efficacy but in high population orchards supplemental controls are essential to reduce populations to get maximum benefit from mating disruption. Insects such as oriental fruit moth (OFM) can be controlled by mating disruption regardless of population size but not codling moth. Why is that? It seems to be connected to the physiological responses of the 2 insects to their pheromone. OFM when exposed to high concentrations of pheromone shuts down flight. Codling moth on the other hand is stimulated to fly in search of the pheromone source (Judd et al. 2005). If the population of codling moth females is too high, it is understandable that male codling moth are more likely to bump into a codling moth female even in the presence of 400 dispensers per acre. Because OFM males do not fly, the likelihood of bumping into a female is much less. For the last 35 years, research scientists have tried to improve the efficacy of codling moth mating disruption to no avail. So it is important to understand how dispensers release pheromone and how males respond. In summary, you want to uniformly distribute roughly 400 dispensers per acre and manage populations to low levels to get the maximum results.
Potential for Resistance
For 30 years, I have been asked about the potential for codling moth to develop resistance to its pheromone. There are a number of lab studies showing that it is possible for insects to alter their blend of pheromone components both quantitatively and qualitatively. However, there is no concrete evidence of this happening in the field. Also, codling moth uses just a single component so there is no blend of components to alter. It would have evolve a new pheromone molecule that impacts both male and female and it would have to happen simultaneously. However, to be sure we collected codling moth overwintering larvae from approximately 10 orchards in Washington State. The diapausing larvae were sent to Dr. Gerhard Gries at Simon Fraser University in British Columbia. Dr. Gries and Regine Gries are well known and respected chemical ecologists. They have examined the pheromone of codling moth before. Over 150 female and males codling moths from 10 orchards were used in the study. The first step was to break the diapause of the overwintering insects. Then the codling moth were allowed to pupate and emerge as adults. The sex pheromone of the female gland was extracted and analyzed with a gas chromatograph. There was no evidence of any pheromone component other than codlemone present in the sex pheromone gland. Male codling moth were exposed to female extracts and there was the expected response to codlemone. Dr. Gries concluded that there was no evidence of any change in the sex pheromone of codling moth.
Pheromone-based IPM is the Foundation
Codling moth mating disruption is the foundation for integrated pest management in apple and pears. It is important to remember that the mode of action is false trail following or competitive attraction. Therefore, it is important to apply dispensers at the maximum label rate as dispensers are competing for the attention of male codling moth. Remember also that codling moth mating disruption cannot control high populations so it is essential that it is integrated into a pest management program that involves the effective use of insecticides and good sanitation. By managing codling moth populations as close to zero as possible provides long term control of codling moth and helps to maintain the effectiveness of the currently available insecticides. In summary, management of codling moth requires the effective integration of mating disruption, insecticides and sanitation.
- Golderberger et al. 2013. Adoption of Organophosphate Alternatives in Washington Apple IPM Programs: Survey Evidence From Pest Management Consultants and Growers. Journal of Integrated. Pest M 4(3): 2013
- Witzgall, P. et al. 1999. Behavioral observations of codling moth, Cydia pomonella, in orchards permeated with synthetic pheromone. BioControl 44: 211–237, 1999.
- Miller, J. And Gut, L. 2015. Mating Disruption for the 21st Century: Matching Technology With Mechanism. Entomol. 1–27 (2015).
- McGhee, P. et al. 2016. Optimizing Aerosol Dispensers for Mating Disruption of Codling Moth, Cydia pomonella J Chem Ecol (2016) 42:612–616.
- Epstein, D. el al. 2006. Higher Densities of Distributed Pheromone Sources Provide Disruption of Codling Moth (Lepidoptera: Tortricidae) Superior to That of Lower Densities of Clumped Sources. J. Econ. Entomol. 99(4): 1327-1333 (2006).
- Judd, G. et al. 2005. Reduced antennal sensitivity, behavioural response, and attraction of male codling moths, Cydia pomonella, to their pheromone (E,E)-8,10-dodecadien-1-ol following various pre-exposure regimes. Entomologia Experimentalis et Applicata 114: 65– 78, 2005.