Release the wasps! How a parasitoid wasp might aid in the management of a fruit fly pest issue

In 2018, Agriculture and Agri-Food Canada research scientist Dr. Paul Abram was embarking on a new project to tackle the spotted wing drosophila (SWD). The original plan was to introduce its natural enemy, a parasitoid wasp, here in British Columbia (B.C.) to control populations. That is, until scientists discovered that the wasp had beat them to the punch and was already present in B.C.'s Fraser Valley. Now, Dr. Abram has the unique opportunity to study the efficacy of this method of management in real life and inform other scientists considering intentional releases across Canada.

A taste for fresh fruit

While other flies in the Drosophila genus tend to feed on rotting and fallen fruit, the SWD stands out for its taste for fresh and ripening fruit. This is made possible due to its sharp saw-like ovipositor, an appendage which allows it to cut through the harder skin of fresh fruit and deposit its eggs within, eventually causing the fruit to decompose. For B.C.'s fruit growers, the economic impact of this fly has been felt through the losses of soft- and thin-skinned fruits such as raspberries, blueberries and cherries, all of which had a combined export value of $623 million in 2020. Insecticide applications have been the main course of action to limit the impact of the SWD, but this method can be costly and harmful to the environment.

To address these issues, Dr. Abram and his team at the Agassiz Research and Development Centre (RDC) turned to the pest's natural enemy for management – a method commonly referred to as biological control. This is a common approach to help manage invasive insect pests that has resulted in many effective and safe pest control programs around the world. In its native environment, the SWD's natural enemies are two tiny parasitic wasps – the samba wasp and the ronin wasp – both of which attack the pest developing inside fruits. Biological control can pose as an attractive option for pest management due to its self sustainability (that is, the wasps reproduce on their own) and its ability to precisely target just the pest, saving farmers on costs and time.

Benefits outweigh the risks

To Dr. Abram's surprise, both wasps were commonly found across most of the southern coast of B.C. by 2020. This discovery has given scientists a unique opportunity to study two things: how effective the wasps are in controlling SWD populations and the ability to monitor any impact they have on agriculture and surrounding ecosystems. So far, the results have been promising.

In 2020, both wasps were shown to kill an average of 13%, and in some cases up to 53%, of SWD larvae in natural habitats on 13 species of crop and non-crop fruiting plants. Of the two wasp species, the samba wasp has been the most promising. While the ronin wasp attacks several species of Drosophila, the samba wasp is highly specific in targeting and attacking SWD and has therefore been deemed the better candidate for deliberate introductions. Additionally, there have been no perceived negative impacts on the wider ecosystem. In fact, this method may even prove beneficial in protecting native biodiversity by mitigating the negative impact that SWD has on animals that feed on fruit, such as birds.

While Canadian scientists continue to monitor other fruit-growing regions to see if these parasitic wasps arrive on their own, the results observed in the Fraser Valley will ultimately lay the groundwork to consider the intentional redistribution of the samba wasp across Canada.

Researchers at Agassiz RDC have already begun to use this experience to help other scientists studying the same pest. They have partnered with their American colleagues to publish a series of recommendations for sampling and identifying the wasps. The recommendations include details on how to separate the wasp larvae from host fruit material to accurately estimate their numbers and establish their interaction with the host. They have also developed instructions to identify emerging parasitoids along with a key to the common families of parasitoids that attack SWD.

Establishing regional guidelines going forward

In the future, these guidelines will help form the basis for a large, multi-research team sampling effort to study and define how natural enemies kill the pest. It will also help scientists examine how plants and other living organisms in the surrounding area are impacted by both accidentally and intentionally introduced parasitic wasps that are helping control SWD in several regions around the world. Over the long term, the self-sustaining biological control effects of these parasitoids could lead to a reduction in pesticide uses in some cases – a bonus to fruit growers as well as the environment!

Currently, the introduction of a biological control agent into Canada is a highly regulated operation, but the same cannot be said for introductions within Canada. Canada is home to several different ecozones, and what works in one area may not be as successful in another. The process for redistributing a biological control agent within Canada is vague, but Dr. Paul Abram hopes to change that. One of his long-term goals is to develop a formal consultation process for redistribution within Canada that ensures due diligence.

Key Discoveries/Benefits:

• The surprise discovery of the spotted wing drosophila's (SWD) natural enemies, the ronin wasp and the samba wasp, in British Columbia has given Dr. Abram the unique opportunity to study the efficacy of this method of control.
• The samba wasp appears to be highly specific in targeting and attacking SWD and has thus been deemed the better candidate for deliberate introductions. Additionally, there have been no perceived negative impacts on the wider ecosystem.
• Now, Dr. Abram's team has the opportunity to lay the groundwork for scientists considering deliberate releases of the samba wasp in other regions of Canada.
• In collaboration with American colleagues, Dr. Abram's team has contributed to publishing a series of recommendations for sampling and identifying the wasp and have developed instructions to identify emerging parasitoids along with a key to the common families of parasitoids that attack SWD.
• One of Dr. Abram's long-term goals is to develop a formal consultation and documentation process to inform the introduction of biological control agents within Canada.

Photo gallery

Related information

• AAFC Profile – Dr. Paul K. Abram