Semiochemicals as tools for guava moth control
Andrew Twidle is a chemist at Plant & Food Research and a PHD student at the School of Chemical Sciences at the University of Auckland. Andrew is studying the semiochemicals of Carposinidae moths with the hopes of finding environmentally friendly tools to help to control them. One of the moths he is working on is guava moth, which he spoke about at the recent New Zealand Feijoa Growers conference in Hamilton.
Whenever I am asked what I do, it generally leads to another immediate question: What are semiochemicals?
Well, semiochemicals are chemical signals that organisms use to communicate with one another in their environment.the most well recognised example of these are pheromones, where members of the same species communicate with each other using specific odours. However, there are many other types of semiochemicals, relaying many different types of messages. From the smell of freshly cut grass to that of ripe fruit on the tree, odours tell a story and the story changes depending upon who is receiving the odour message. In our study of guava moth we are using a dual approach, trying to find semiochemicals that affect both male and female moths. For the males we are looking at pheromone analogues to disrupt their ability to locate female moths, and for the females we are trying to develop an attractant based on host plant volatiles to trap and kill them. So how do we go about identifying these chemical signals and using them to create pest management tools?
The first step in this process is isolating the system and collecting the volatile compounds being produced. To find a female attractant for guava moth, we started by investigating the volatiles produced by their host plants, specifically their native host, magenta lilly pilly, and a nonnative host, commercial feijoa, since it is these plants that the females search for to lay their eggs on.we collected the real-time volatiles produced by the fruit and leaves of these trees and then analysed them using a gas chromatograph coupled to a mass spectrometer (GC-MS). The GC separates the complex mixture into individual compounds which then pass into the MS, providing
structural information about the compounds, allowing us to tentatively identify them. While it is good to know what compounds the plants are producing, we are really interested in what compounds the moths are detecting.
To discover which compounds produced by the plant are biologically active with the female guava moth we use another technique, electrophysiology. Here we use a gas chromatograph coupled to an electroantennogram detection unit (GC-EAD) to identify the biologically active compounds. First the volatiles are separated out using the GC, and then the compounds are individually passed over the moth’s antenna as they come out of the Gc.when a compound the female guava moth can perceive passes over the antenna, it causes an electrical depolarisation in the olfactory receptor neurons of the antenna, which we see as an electrical impulse on the EAD. Of the many volatile compounds in the sample, we are able to narrow down the complex mixture to only those compounds that the female guava moth can detect. These bioactive compounds are tentatively identified from their mass spectra. Candidate compounds are then synthesised to confirm the structure, whereby the new semiochemicals may be evaluated as potential pest management tools. From the guava moth host plant testing we have found three antennally active compounds that are in both host plants. We have successfully identified these compounds and now plan to test them in trapping trials of the female moths over the coming season.
The procedure for developing a pheromone analogue for male guava moth disruption is a little different, since we already know the active odours of the pheromone.the question we asked instead was: Can we make something more simple and cost-effective than the current four-component pheromone to disrupt male behaviour? To understand how this could work, let’s first look at how moths perceive pheromones. When moths detect a pheromone they begin a zig-zagging flight behaviour as shown in the diagram on page 48. While they are in the plume, the antenna is firing; when they are out of the pheromone plume, the antenna stops firing. Once the antenna stops firing the moth turns back into the plume, continuing this behaviour until they reach the female. This system relies on enzymes in the male moth antenna being able to remove the pheromone molecule very quickly from its antennal receptor cells so it can follow the plume.what we propose to do with the pheromone analogue is to construct a very similar molecule which fits inside the male moth’s antennal receptor cells but which is sufficiently different so that the antennal enzymes are unable to remove it quickly. When unable to remove the pheromone analogue instantly from the antennal receptor cell, the male moth won’t know when it exits the odour plume and hence it will be unable to follow the pheromone plume, as shown in the example here on the right. So far we have synthesised six different analogues, but how do we know if they are effective?
Like the female attractant work above, we use electrophysiology. In the first instance we use the male antenna connected up to the electroantennogram detection unit. We then puff the pheromone analogues over the antenna in parts per million concentrations to see if the male moths respond to them. Once we have a compound that the male moth antenna is responding to, we then need to check that it is the pheromone receptors that are firing.to do that we use another electrophysiology technique, single sensillum recording (SSR). Here the response is recorded from individual sensory sensilla, in our case the pheromone receptor of the guava moth. Examination of guava moth antennae under an electron microscope shows that the male antenna is covered in long hairs, which are absent from the female antenna. It is these long hairs that are the likely pheromone receptors, and hence their responses with the antennally active analogues we recorded.two of the analogues have produced responses in SSR tests, and they will now be tested with male guava moths in the field for behavioural activity during the 2020 season. This research project is supported by the Sustainable Farming Fund (Project 405237 Sustainable Management of Guava Moth). Industry co-funding comes from NZ Feijoa Growers Inc. and NZ Macadamia Society Inc. Etec Crop Solutions, Northland Regional Council, Hawke's Bay Regional Council and the Gisborne District Council have also contributed to this research with funds or in-kind contributions.this project is also supported by Plant & Food Research who are funding Andrew Twidle’s PHD.