This post is a short summary of a peer-reviewed, open access publication on yellow mealworm welfare in the Journal of Insects as Food and Feed. The paper and supplemental information can be accessed here. The original paper was written by Meghan Barrett, Rebekah Keating Godfrey, Alexandra Schnell, and Bob Fischer; the research conducted in the paper was funded by Rethink Priorities. 

This post was written by Abraham Rowe and reviewed by Meghan Barrett. Unless cited otherwise, all information is derived from the Barrett et al. 2023 publication.

Summary

• As of 2020, around 300 billion yellow mealworms (Tenebrio molitor) are farmed annually (though recent estimates now put this figure at over 3 trillion individuals (Pells, 2023)).
• Barrett et al. 2023 is the first publication to consider species-specific welfare concerns for farmed mealworms.
• The authors identify 15 current and future welfare concerns, including more pressing current concerns such as:
  • Disease — Bacterial, fungal, protist, and viral pathogens can cause sluggishness, tissue damage, slowed growth, increased susceptibility to other diseases, and even mass-mortality events. 
  • High larval rearing densities — Density can cause a range of negative effects, including increased cannibalism and disease, higher chances of heat-related death, competition over food leading to malnutrition, and behavioral restriction near pupation.
  • Inadequate larval nutrition — This may result from not providing enough protein in the animals’ largely grains-based diet. 
  • Light use during handling — Photophobic adults and larvae may experience significant stress due to light use during handling.
  • Slaughter methods — While we have high empirical uncertainty about the relative harms of slaughter methods, it is clear that some approaches to slaughter and depopulation on farms are more harmful than others.
• Future concerns that haven’t yet been realized on farms include:
  • Novel, potentially toxic, or inadequate feed substrates — Polymers (like plastics) and mycotoxin-contaminated grains may be more likely to be used in the future.
  • Selective breeding and genetic modification — In vertebrate animals, selective breeding has caused a large number of welfare issues. The same might be expected to become true for mealworms.

Current rearing and slaughter practices

Yellow mealworms are the larval instars of a species of darkling beetle, Tenebrio molitor. Larvae go through a number of molts prior to pupation, which can take between a few months to two years depending on nutrition and abiotic conditions. Mealworms take up to 20 days to pupate. After pupating, the emerged adult beetles will mate within 3-5 days. Mealworms are a popular insect to farm for food due to their rapid growth, high nutrient content, and ease of handling. Adults are typically only used for breeding, while large larvae are sold as food and feed.

Mealworms typically consume decaying grains, but have been reported to eat a wide variety of other foods in certain circumstances (including dead insects, other mealworms, and decaying wood). In farmed conditions, larval mealworms are fed a diet of 70%-85% cereals and other carbohydrates, and may be provided with supplementary protein, fruit, or vegetables. Mealworms are reared in stackable crates, usually with screened bottoms to allow frass (insect excrement) to fall through and not accumulate. Mealworms may be reared in up to 24-hour darkness, as they are photophobic.

Insects bound for slaughter are collected at around 100 mg. Prior to slaughter, insects are sieved out of the substrate, washed (to remove frass and other waste from the exterior surface of their bodies), and prevented from eating for up to two days (called ‘fasting’) to clear their digestive systems. Slaughter is typically done by microwaving, baking, air freezing, shredding, or boiling. The method might vary depending on the producers’ need, as it directly impacts final insect meal quality.

Larvae and adults to be used as breeding stock are typically kept separate from insects raised to be food or feed. The final instar of larval mealworms can last an extended period, so cannibalism of pupae by larger larvae is somewhat common. Producers often rear adult breeding stock in similarly aged cohorts, so that culling of adults after fertility declines can happen simultaneously. This is often done via freezing in air.

Welfare concerns

The authors outline eleven current concerns and four future concerns for mealworm welfare, and make recommendations on addressing these concerns.
 

1. Concern: Inadequate nutrition for larvae and adults

Larval and adult diets are very homogenous. The prominence of grain-only diets results in higher pre-slaughter mortality and extended developmental time.

Recommendations: Supplement mealworm diets with eggs, soy, or yeast protein (at 20% for larvae and 40% for adults). Provide larvae the opportunity to self-select foods from a heterogeneous mixture.
 

2. Concern: Inadequate hydration for larvae

Since water increases the likelihood of fungal growth, larvae are often raised with minimal access to water. However, the humidity on farms also can be relatively low. This leads to increased mortality and reduced growth of larvae.

Recommendations: Provide larvae with 70%-75% relative humidity, and provide continuous access to water sources that are changed and cleaned regularly. Larvae should have access to water within 5 cm at all times.
 

3. Concern: Handling-induced stress during, especially during washing, and fasting, and slaughter

While some handling is necessary to farm mealworms, handling can cause significant stress to insects, especially since it often needs to be done under bright lights so workers can see what they are doing.

Recommendations: Use red lights when handling mealworms, and reduce the severity and duration of handling during rearing. Eliminate or reduce pre-slaughter washing and fasting, given the low-level of evidence on their efficacy (with currently evaluated substrates). If using fasting, provide inedible burying substrates for larvae during the fasting period.
 

4. Concern: Stocking densities leading to injuries, cannibalism, restricted behavior, and other stressors

While insects are often found at high densities in nature, high stocking density can lead to a variety of issues, like injuries, cannibalism, and restrictions in behavior on farms. Additionally, higher densities can increase disease burden on farms.

Recommendations: Stock mealworms at or below 0.5 larvae/cm², separate larvae and pupae, age-match larvae during rearing, reduce stocking density when nearing pupation, and monitor temperature in crates during high-density periods to ensure it stays below 37 °C.
 

5. Concern: Diseases and parasites

Diseases, especially those caused by fungi and protists, are common on farms. Parasites, such as mites, also often live on insect bodies. These issues can cause significant welfare issues and increase the rate of pre-slaughter mortality.

Recommendations: Monitor populations for indications of disease, avoid feeding waste to larvae, heat-treat feed prior to introducing it to a farm, follow hygiene recommendations, and explore methods like immune-priming and probiotic use to reduce rates of disease and improve physical health.
 

6. Concern: Inhumane slaughter and depopulation

No regulations exist on insect slaughter, and often depopulation (e.g., killing a population due to disease outbreaks) is done by removing insects from farms and letting them die.

Recommendations: Use slaughter and depopulation methods that are more likely to be instantaneous, avoid freezing insects in air (especially as a method of stunning prior to any heat-based slaughter method), and develop and use stunning and anesthesia protocols.
 

7. Concern: Poor ventilation during rearing

Due to rearing taking place in relatively confined quarters or sealed crates, poor ventilation can lead to hypoxia, especially in dark conditions when mealworms produce more gasses, as activity is increased.

Recommendations: Never keep mealworms in sealed crates, and ensure good airflow through crates, especially during periods with low lighting. Provide appropriate light cycling for mealworms to reduce gas outputs.
 

8. Concern: Rearing adults on mesh without substrate

Rearing of adults is often done on steel mesh above substrates, to prevent egg cannibalism. This means that adults cannot do natural behaviors, such as burrowing, which could lead to welfare issues, especially when lights are bright.

Recommendations: Give adults access to oviposition substrates that they can bury themselves completely in.
 

9. Concern: Inbreeding leading to poor physical health

Due to breeding stock being effectively limited over many generations, inbreeding can be common on mealworm farms. This can lead to increased morphological defects, decreased longevity, and reduced stress and disease resistance.

Recommendations: Monitor genetic diversity and introduce new insects to increase genetic diversity of breeding stock.
 

10. Concern: Morphological defects from poor developmental conditions

Up to 4% of adult mealworms on farms have morphological defects from poor developmental conditions, restricting movement, mating, and potentially digestion, and/or leading to early mortality.

Recommendations: Do not store larvae or pupae below 10 °C, and monitor and humanely slaughter severely deformed adults.
 

11. Concern: Inadequate environmental conditions during shipping, rearing, and storage for live-sold mealworms

Mealworms are shipped live to pet stores and consumers, which can cause significant stress.

Recommendations: Do not ship live insects in poor climate conditions, reduce stress by marking packages ‘fragile’ and shipping in vibration reducing material, do not refrigerate pupae or older larvae below 10 °C, provide appropriate nutrition during transport, and provide care instructions to pet store owners and consumers.
 

1. Future Concern: Feeding of plastics or polymers to mealworms

Mealworm larvae are able to be fed some plastics and polymers. However, because these materials are not nutritious, this can lead to higher rates of malnourishment, resulting in mortality and cannibalism.

Recommendations: Avoid feeding mealworm larvae polymers and plastics.
 

2. Future Concern: Feeding of mycotoxin contaminated grains to mealworms

Insect producers are interested in exploring the use of mycotoxin contaminated grain as a low-cost feed for mealworms (since this grain is typically destroyed). Feeding certain mycotoxins to mealworms can lead to increased rates of mortality, decreased growth rates, or behavioral avoidance of feed, especially at early instars.

Recommendations: Avoid feeding mycotoxin contaminated grains to mealworms unless the mycotoxins have been identified to be safe (or to be present at levels known to be safe) for mealworm larvae. Avoid feeding early-instar mealworms any mycotoxin contaminated grains.
 

3. Future Concern: Genetic modification and selective breeding harming health and welfare

Genetic modifications and selective breeding are being explored for mealworms, to speed up growth rates and increase body size to improve the efficiency of farming. 

Recommendations: Novel strains of mealworms should be studied for their lifecycle welfare prior to their adoption by industry, and strains with worse welfare outcomes should be avoided. Genetic tools could be further explored to enhance mealworm welfare.
 

4. Future Concern: The use of diatomaceous earth for rearing adults harming their physical health.

Diatomaceous earth has been discussed as a potential insect-rearing substrate to reduce egg cannibalism by adult beetles. It can cause abrasions, dehydration, and mortality.

Recommendations: Avoid the use of diatomaceous earth for rearing adult beetles.

Directions for future research

There are significant evidence gaps on both mealworm sentience (see Coleoptera, in this summary of Gibbons et al. 2022) and welfare-impacting practices on farms (as described throughout Barrett et al. 2023). 

Future research should focus on addressing the sentience question, and learning more about disease, cannibalism, welfare impacts of larval stocking density, the effects of hormone-induced overgrowth of mealworm physiological health and behavior, and the relationship between feed efficiency and welfare on farms. Larval nutrition is also understudied, and empirical work to understand the best slaughter and depopulation methods is lacking.
 

Acknowledgements

This research is a project of Rethink Priorities. It was written by Abraham Rowe, summarizing work by Meghan Barrett, Rebekah Keating Godfrey, Alexandra Schnell, and Bob Fischer. Thanks to Meghan Barrett for helpful feedback, and Adam Papineau for copy editing. If you like our work, please consider subscribing to our newsletter. You can explore our completed public work here.

References

Barrett, M., Godfrey, R.K., Schnell, A., & Fischer, B. (2023). Farmed yellow mealworm (Tenebrio molitor; Coleoptera: Tenebrionidae) welfare: species-specific recommendations for a global industry. Journal of Insects as Food and Feed (published online ahead of print 2023). https://doi.org/10.1163/23524588-20230104

Pells, R. (2023). How France became the unlikely home of the insect-farming industry. Nature (Spotlight). https://doi.org/10.1038/d41586-023-00290-z