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Animal Welfare
Animal Welfare , Invertebrate Welfare

Research summary: Grinding parameters for humane slaughter of yellow mealworm larvae

William McAuliffe
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  • Early research by Rethink Priorities has shown that insects raised for food and feed are among the most numerous groups of farmed animals, and that they are unfortunately farmed with little to no welfare standards.
  • As part of its research agenda on understanding these animals’ needs, Rethink Priorities’ Animal Welfare Department is funding a series of research papers investigating the humane slaughter methods for the insect groups most frequently farmed for food and feed, namely: black soldier fly larvae, yellow mealworm larvae, and crickets.

This post is a short summary of Fillers at the end of processing improve instantaneous death via grinding for farmed yellow mealworm larvae, a peer-reviewed, open-access publication in Journal of Insects as Food and Feed under a CC BY 4.0 license. The paper and supplemental information can be accessed here.

The original paper was written by Mariangeles Paloma Zacarias, Craig D. Perl, Rodrigo Rodriguez-Guevara, and Meghan Barrett; the research conducted in the paper was funded by Rethink Priorities.

This post was written by William McAuliffe and reviewed for accuracy by Meghan Barrett. All information is derived from the publication, some text of which is directly adapted for this summary.

Summary

  • Grinding may be a humane method for slaughtering farmed yellow mealworm larvae. However, it is unclear what standard operating procedure producers should adopt if their machinery varies from the type used in the study.
  • A single cut to any body segment rarely caused immediate death, as measured by a lack of response to mechanical stimuli, in larvae of any body size.
  • By independently manipulating larval body size, particle size, plate hole diameter, and use of a filler (for the last larvae to exit the grinder), experimenters were able to assess the likelihood of instantaneous death using an industrial meat grinder.
    • The share of individuals who died instantaneously ranged from 73.27% to 96.47% depending on body size, use of filler, and particle size plate hole diameter.
    • Using a smaller plate (2.55 mm) and a filler increased the probability of instantaneous death.
  • When machinery specifications differ from those used here, producers should follow directions in the supplementary materials to optimize the humaneness of their own protocol.

Introduction

Hundreds of billions of yellow mealworm larvae are farmed each year. Larvae may be killed via grinding or shredding in industrial meat grinders; little is known about the welfare impacts of this method. However, previous experimentation on black soldier fly larvae suggests that the details of the procedure, such as the hole diameter of the particle size plates, affect the percentage of individuals that die within a second. Species-specific procedures are needed to assess the most humane standard operating procedures for grinding different insects. Accordingly, this study assessed: 1) if single cuts to different body segments result in instant death for slaughter-weight yellow mealworms; 2) if body size, particle size plate hole diameter, or the use of a filler at the end of a batch of larvae (to push remaining larvae through the particle size plate) had an effect on the likelihood of instant death for yellow mealworm larvae. Instant death was determined to be when larvae did not move in either half of the body when poked with a sharp needle one second after a cut was received.

Major Results

  • Only 3 of 520 larvae died instantaneously from a razor blade cut in any body segment. The vast majority of larvae were still alive 90 seconds after the cut.
    • Larvae had many behavioral responses following the cut (videos can be found in the supplement), including rolling, walking or walking backwards, and twitching.
  • Getting stuck at the blade typically results in single cuts, instead of total mechanical disruption and thus instant death (Barrett et al. 2024). Larger body sizes and the use of a filler can reduce the percentage of larvae stuck at the blade to 0.08%.
  • An instantaneous death rate of 96.47 ± 1.19% could be achieved with a filler and the smallest particle size plate diameters (2.55 mm).

Discussion

2.55 mm diameter holes for the particle size plate and a filler at the end maximize the likelihood of instantaneous death; larger body sizes can also increase the likelihood of instant death for individuals that are past the particle size plate and which do not stick at the blade. However, due to the sheer scale of yellow mealworm farming, if all producers adopted this protocol, there would still be 11-21 billion individuals that do not die right away.

Limitations of the study include:

  • Injuries in the grinder prior to death could not be assessed.
  • The grinder used is too small for farms with high volumes of output. However, the supplementary materials include instructions for creating and testing the error rate of grinding standard operating procedures with larger machines.
  • The study may overestimate the proportion of larvae that are not instantly dead, as ventral-nerve-cord-mediated reflexes would still have counted as “alive” in the response categories used to determine whether larvae cut in each body segment were alive or dead. Therefore, the likelihood of death estimates are conservative, and, in fact, the error rate of these methods may be smaller than the 3.53% reported.

Future research should explore what rates are obtained when using a different substrate as filler, smaller and larger larvae, different hole diameters, and different grinders.