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Alternatives to Strychnine

James A Tansey, PAg, Provincial Specialist Insect/Pest Management, Richard Wilkins, PAg, Provincial Specialist Pesticide Regulatory, Raul Avilla, PAg, Provincial Specialist Pest Regulatory, and Brooke Fiala, Crop and Lab Field Technician

June 2022

The best rodenticide for Richardson’s ground squirrel (RGS) control in prairie agricultural settings was thought to be strychnine. In large part because of environmental concerns, the Pest Management Regulatory Agency of Health Canada has issued a final decision to de-register strychnine for RGS control. This, coupled with limited supplies and burgeoning RGS populations, means that an examination of alternatives is needed. Given the great damage this pest can cause, its status as a regulated pest in Saskatchewan and pending loss of an effective control product, solutions for and recommendations to the agriculture community are needed.

Several products are registered to control of this pest (Table 1). The Saskatchewan Ministry of Agriculture, Crops and Irrigation and Regional Services Branches, in collaboration with Saskatchewan Association of Rural Municipalities Plant Health Officers and Alberta Agriculture, Forestry and Rural Economic Development, completed a comparative study to evaluate the relative efficacies of strychnine and several registered alternatives.

Product Registration number Active ingredient Toxicant effect Formulation Notable considerations
2% Liquid Strychnine Concentrate 30433 Strychnine Respiratory failure Liquid concentrate. Requires mixing with grain bait to achieve 0.4% application concentration Deregistration decision March 2021. Last permitted use March 2023
Burrow Oat Bait 24795 Zinc phosphide Central nervous system depressant Coated oats Pre-baiting for best results. Not required
ZP Rodent Oat Bait AG 29030 Zinc phosphide Central nervous system depressant Coated oats Pre-baiting for best results. Not required
Rozol RTU Field Rodent Bait 29545 Chlorophacinone Anticoagulant Pellet Re-baiting required after 48 hours. Third baiting may be required
Ramik Green 11669 Diphacinone Anticoagulant Pellet Re-baiting required after 48 hours.

Table 1. Products registered for use to control Richardson’s ground squirrel populations in Canada and tested for efficacy and non-target effects as part of this study.

Sites characterized by high RGS populations in grassland and pasture systems were chosen in Saskatchewan (Rural Municipality of Maple Creek) and Alberta (MD of Willow Creek). Sites were scouted before establishment to ensure thriving RGS populations. Sites were established in April 2022 by staking 0.16 ha (40 m x 40 m) plots. A central stake was equipped with two low-latency game cameras, capable of night-vision. The combined fields of view of both cameras covered the entire plot.

Live RGS's were counted daily pre-application to determine baseline numbers and post-application for one week to evaluate reductions in populations. In addition, the entirety of all plots was walked daily to detect any dead RGS’s or non-target species. An additional perimeter of 10 m around each plot was also walked daily. All anticoagulant treatment (Rozol RTU and Ramik Green) sites received second applications.

No non-target mortality was observed for any treatment.

Significant reductions in on-site RGS populations were found for all toxicant treatments. On the Saskatchewan sites, comparable control was achieved with strychnine bait and with the zinc phosphide products. On Alberta sites, no differences in control were detected among toxicants.

Costs of each product per acre, applied at maximum label rate, were calculated based on observed densities of 25 holes per 1600 square metre plot size. Zinc phosphide products were the least, and anticoagulants were the most expensive. Rozol RTU, can require three applications for efficacy, meaning that costs for this product could be $44.19 per acre.

Product $ per acre
2% Liquid Strychnine Concentrate 12.97
Burrow Oat Bait 4.54
ZP Rodent Oat Bait AG 8.80
Rozol RTU Field Rodent Bait 14.73
Ramik Green 10.60

Table 2. Application costs comparison calculation for the registered Richardson’s ground squirrel control products tested. Rozol and Ramik can require multiple applications. Costs are per application.

An important consideration with all baits is timing. Best results are achieved if baits are applied as RGS are emerging from overwintering, before spring green-up. This can be as early as late February in some regions. Once there is a selection of green growth, bait acceptance can be reduced. In the case of anti-coagulants, fresh plant material, particularly legumes, provide RGS with vitamin K. This is the antidote for anticoagulants. Strychnine and zinc phosphide have no antidotes.

Concerns over the de-registration of strychnine for RGS control have been expressed by many growers in a year that is becoming increasingly characterized by high RGS populations and pressures. However, alternative products are available and effective.

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