I started my PhD in October 2017 at the University of Auckland.
My supervisor is Associate Professor Greg Holwell. Greg is interested in evolution—particularly sexual selection—and uses arthropods to work on some of the biggest questions in this area. My co-supervisor is Dr Gonzalo Avila, leader of the biocontrol team at Plant & Food Research, Auckland. Gonzalo is interested in evaluating parasitoids for their uses in classical biocontrol programmes.
This post is a brief overview of the rationale and methods of my project.
The overarching goal of my project is to improve risk assessment of classical biocontrol agents before they’re released. To achieve this I’m evaluating how useful it is to conduct behavioural and chemical ecology studies alongside traditional host testing methods.
Classical biocontrol is the deliberate introduction of an exotic natural enemy so it will permanently establish and provide long-term control of a pest. Before starting a biocontrol programme, natural enemies need to be screened for potential non-target risks—the chance they will attack species other than the target—before they can be approved for release in many countries. This is also the case in New Zealand, where the EPA facilitates a process of engagement with stakeholders and the public for any proposed new releases.
One of the most common and successful types of natural enemies used as biocontrol agents are the parasitoid wasps. These wasps are often smaller than social wasps, don’t sting people, and don’t live together in colonies. They make their living by laying eggs inside or on the surface of a host, typically the larvae or eggs of another insect. The wasp larva then feeds on the host, eventually killing it, before emerging as an adult. We have many native parasitoid wasps in New Zealand, but when you’re dealing with introduced pests you normally have to consider using an exotic natural enemy for their control – usually from the homeland of a pest of foreign origin.
The standard approach for testing parasitoids for their non-target risks are no-choice oviposition tests. A host is offered to a parasitoid inside a sealed plastic tube in a containment lab and the observer records attack behaviour, and eventually emergence rates of the parasitoid offspring. This process is replicated a number of times. If no attacks are observed and no offspring emerge it is very unlikely this particular non-target species is a physiological host (a host which the parasitoid is capable of developing in).
However, when parasitoids search for a host out in the field they use lots of different chemical cues or odours associated with hosts or the plants their hosts feed on. This is a very different environment to a plastic tube in a lab where the parasitoid is confined in close quarters with a host.
Physiological vs ecological host range
No-choice tests are great for identifying physiological hosts, but just because a parasitoid is capable of developing on a particular host species, it doesn’t mean the parasitoid will seek out or attack this species in the field. This leads to some important questions:
- What if the parasitoid attacks the host in the lab because it feels it may have no other choice, or it has an urge to dump its eggs?
- What if the parasitoid would never even notice or be attracted to the chemical cues associated with this species of host in the field?
These kinds of questions illustrate the need for information on ecological (i.e. realised) host range, not just physiological (i.e. fundamental) host range. An ecological host range is a smaller subset of the physiological range, made narrower by environmental cues which act as a filter to exclude certain physiological hosts which, for whatever reason, are not attractive or do not elicit attacks under field conditions. In order to understand ecological host range, we need to bring in a variety of other tests to examine the underlying processes of host-finding behaviour in natural enemies.
The wasp and the stink bug
I’m aiming to combine several different methods to better understand the ecological host range of the samurai wasp, Trissolcus japonicus, if it is introduced into New Zealand against brown marmorated stink bug (BMSB), Halyomorpha halys.
BMSB is a serious plant pest. It feeds on hundreds of different plant species and damages valuable crops. Native to East Asia, BMSB has recently spread through North America and Europe, where it is causing high levels of damage to crops and costing growers millions of dollars every year. While it has not yet established in New Zealand, hundreds of live stink bugs are intercepted at our border every year. I’ve written a short primer with images about how to separate BMSB from NZ stink bugs here.
The samurai wasp is a tiny egg parasitoid, meaning it lays its eggs into the eggs of a host species. The samurai wasp is restricted to hosts from two families of shield bugs, and we only have one of these in New Zealand, the stink bugs (Pentatomidae). Testing overseas has shown that this wasp is likely to be the most effective natural enemy of BMSB. An application for conditional release of the samurai wasp has been approved (with controls) by the Environmental Protection Agency (EPA). This means once the stink bug has established in New Zealand, the samurai wasp can be released under these conditions.
My work explores what might happen to New Zealand’s stink bug species if the samurai wasp were to be released in New Zealand. I’m trying to better understand its ecological host range by incorporating different types of tests, to move beyond no-choice oviposition tests. One of my key aims is to incorporate chemical ecology techniques such as GC-EAD, to identify individual compounds associated with hosts which elicit attractive behavioural responses in the wasps.
My current plan
- No-choice oviposition tests against an endemic New Zealand stink bug. My co-supervisor led a team of researchers who managed to conduct no-choice oviposition tests with the wasp against every species of stink bug known to be present in New Zealand except one: The alpine shield bug, Hypsithocus hudsonae, which couldn’t be found in time for their testing. We’ve now found this species and I’ve concluded my testing.
- Electrophysiological tests. The next step is to use EAG, GC-EAD, and other chemical ecology techniques to try to understand how the samurai wasp might respond to volatile organic compounds associated with non-target stink bugs. I’m working with Dr Kye Chung Park and Lee-Anne Manning from Plant & Food Research Lincoln to examine the volatile profile of different species, and to work out which compounds elicit a nervous response in the wasps. I’ve written more about this work here.
- Olfactory bioassays. We can use y-shaped glass tubes to test whether or not particular compounds or particular stink bug species are attractive to the wasp. We can use a blank chamber as a control, and then place a female stink bug in the other chamber. Air is then pumped down each arm through the chambers and into the main corridor. The wasp is released at the base of the main corridor and, ideally, makes a decision about which arm to venture down or hang around in based on what it is smelling. I’m also investigating open arena arrestment experiments. These involve coating a piece of filter paper with stink bug volatiles and measuring the retention time of the wasp, compared to a control where only the solvent is used on the filter paper.
- Non-target rearing program. What happens to the host preferences of an introduced parasitoid reared on a non-target host? I want to look into this question further by rearing samurai wasp on a non-target species of stink bug. If we can detect a shift in host preference after only 10 generations, then this would be an important factor to consider when designing biocontrol programs with parasitoids in the future.
- Competition experiments with an existing natural enemy of stink bugs. Trissolcus basalis (Wollaston) is closely related to the samurai wasp. It was introduced to New Zealand in the 1940s, so it’s been here for 70+ years. We know it attacks non-target species to varying degrees, but how will it respond to the samurai wasp? Competition between biocontrol agents over non-target species has been given little attention, but it could be an important consideration when weighing up non-target issues overall. I want to see what happens when these parasitoids oviposit into non-target species one at a time in sequence, and what happens when both are present on the egg mass at the same time.
Classical biocontrol agents can be a very useful tool for managing pests in the long term in a self-sustaining, environmentally friendly, and cost-effective way, and they can also help to reduce our dependence on pesticides. However, to do biocontrol the right way we need to assess potential risks the candidate biocontrol agent may pose to non-target species when released into a new environment.
In New Zealand the EPA decides which species are allowed to be released after consulting with science, industry, and the public. It is a legal requirement for an applicant to carry out a risk assessment before submitting their application to release a new species.
My project aims to try out different methods for assessing non-target risks and see what they tell us about the risk of non-target attack. How difficult is it to use these techniques? Do they complement each other well? Is it worth including the lesser-used techniques like non-target rearing or competition experiments? How much extra certainty can we provide with chemical ecology methods? Ultimately, the goal is to help decision makers to better understand non-target risks associated with proposed biocontrol agents.
I’m happy with my progress to date but I still have a long way to go. I’ve scrapped plans for some chapters, modified others, and introduced new ideas into the project. Of course challenges crop up, colonies die, and insects don’t behave how you expect them to. But I’m learning from a group of very intelligent and experienced colleagues, and I’m optimistic about the results I’ll collect.