Investigating the insect-parasitic nematode, Bradynema listronoti, as a biological control agent of the carrot weevil, Listronotus oregonensis

Project Code PRR19-010

Project Lead

Annie-Ève Gagnon, Agriculture and Agri-Food Canada

Objective

To improve carrot weevil control in carrot crops by evaluating the feasibility of establishing a parasitic nematode population in commercial fields

The carrot weevil, Listronotus oregonensis, is a major pest of carrots, and also affects celery, parsley and celeriac. This pest causes serious economic damage, which may result in up to 40 percent yield reduction and is prevalent in the largest carrot producing regions of Ontario, Quebec and Nova Scotia. Soil-dwelling weevil larvae chew holes in roots, rendering carrots unmarketable and creating entry sites for pathogens, which may lead to post-harvest infections during storage. Although noted as having only a single generation in much of the existing literature, emergence in late summer of a second generation of the weevil was recently confirmed in Ontario and Quebec, possibly due to climate change.

The most common management practice to control carrot weevil is the use of insecticides. However, despite multiple spray applications of insecticides, carrot producers have been increasingly struggling with considerable yield losses. Resistance of carrot weevil to insecticides (e.g., phosmet) has limited the efficacy of existing control tools. A growing need for alternative management options has therefore led to intensive research to develop a new and efficient biological solution.

Following the discovery in Quebec soils of a native, parasitic castrating nematode (Bradynema listronoti) capable of sterilizing female carrot weevils, this research was undertaken to further elucidate the biology of B. listronoti. This 3-year project aimed to: a) optimize nematode rearing techniques; b) improve understanding of the nematode’s mechanism of action and its infection potential, and c) assess its ability to establish in commercial fields without a previous presence of the nematode.

Results

Laboratory and field studies were conducted from 2019-2022 at the Agriculture and Agri-Food Canada’s (AAFC) Saint-Jean-Sur-Richelieu Research and Development Centre in Quebec. Mass rearing of B. listronoti requires continuous maintenance of its living host, L. oregonensis. The nematode parasitizes the weevil larva, feeding and reproducing inside the larvae, without killing the host. The infected weevil larva carrying the nematodes continues its development to the adult stage, but infection will lead to complete castration, destroying the ability of infected females to produce eggs. The parasite strongly represses genes involved in the maturation of the weevil’s reproductive system. Although sterile, the female weevil continues its egg-laying behavior by visiting many carrot plants, but only releasing nematode offspring, which can infect more weevils. Each infected female weevil is estimated to release an average of 5,500 new nematodes. All weevil stages appear susceptible to infection when exposed to B. listronoti inoculum, with larvae being the most vulnerable stage. This beneficial nematode is species specific, only parasitizing the carrot weevil, so it is not a threat to non-target indigenous species.

Optimal rearing conditions (i.e. substrate, age of nematodes, dose) for the production of large quantities of nematodes have been identified in this project. Furthermore, the biological control potential of the carrot weevil using B. listronoti was confirmed with high parasitism rates (> 50%) observed in controlled environment experiments. The establishment of nematode populations in carrot fields represents a challenge, specifically the synchronization of inoculation with the most vulnerable weevil life stage. Trials undertaken so far have not confirmed B. listronoti establishment in fields with no previous nematode history. Additional studies are necessary to validate its impact in the field under commercial farm conditions.

Conclusion

The results from this project provide an important basis for a better understanding of the weevil-nematode interaction and offer avenues for setting up an application technique for its delivery and use in the field. The project’s findings will inform future work to further investigate and develop B. listronoti as a native, Canadian biological control alternative contributing to improved carrot weevil management.

The research work on this biocontrol agent is continuing under a new project aiming to scale up field studies. In the meantime, AAFC is currently pursuing a legal process to patent this biocontrol technology and is investigating large scale commercialization opportunities of the beneficial nematode with some companies. Ultimately, the development of an integrated management strategy that includes B. listronoti as a biocontrol tool will lead to a long-term sustainable approach to carrot weevil control with reduced reliance on the use of insecticides.