This post is a summary of Review: Exploring correctness, usefulness, and feasibility of potential physiological operational welfare indicators for farmed insects to establish research priorities, an open-access article in the peer-reviewed journal animal. The original paper was written by Meredith Johnson and Meghan Barrett; this review was funded by Rethink Priorities as part of our research agenda on understanding the welfare of insects on farms.
This post was written by Elisa Autric and reviewed for accuracy by Meghan Barrett. All information is derived from the Johnson and Barrett (2025) publication unless otherwise cited, and some text from the original publication is directly adapted for this summary.
Introduction
Over 2.5 trillion insects are reared annually for food and feed (Shah, 2024), and projections suggest the industry will continue this upward trend in the coming years (van Huis and Gasco, 2023). However, little research is currently available on the welfare of farmed insects compared to farmed vertebrates.
The paper summarized here aims to guide efforts to fill this knowledge gap by evaluating physiological indicators of farmed insect welfare. It examines how correct, useful, and feasible[1] various indicators are, so that the most promising ones can be prioritized for validation and use in future research. These indicators are operational, meaning they can be used to putatively assess the welfare of animals in the absence of true indicators of affective states. Examples of operational welfare indicators for vertebrates include body condition, heart rate, body temperature, health status, stress hormone levels, and others. While each individual indicator may have weaknesses, using them together can offer more convincing evidence of the animal’s true affective state.
The authors draw on vertebrate operational welfare indicators to categorize possible indicators in insects as either:
- ‘Short-term’ priority: indicators that are likely to be correct and feasible on farms;
- ‘Long-term’ priority: indicators that are likely to be correct but not feasible on farms;
- ‘Low’ priority: indicators that are least likely to be correct or feasible on farms.
The authors review these potential welfare indicators in detail, organized by physiological systems, beginning with whole-body indicators, then covering immune, neurobiological, and respiratory/cardiac indicators.
Whole-Body Indicators
Whole-body indicators are often low-cost, both technically and in terms of impact on the animals, and can be feasible in farmed conditions. Thus, the authors recommend that several of these indicators be further refined for on-farm application.
As with vertebrates, unusual body sizes and mass, or scaling relationships[2] in insects could indicate deteriorated physical health and negative affective states. While some methodologies employed to assess these parameters (namely, visual inspection and scoring) in vertebrates do not straightforwardly apply to most insects due to their fixed exoskeleton, the low cost and high feasibility of this type of welfare assessment in farm conditions make it a short-term priority tool to be further investigated. Additionally, some insects–particular adult black soldier flies–have clear body parts that allow the visualization of their internal organs and fat bodies, potentially allowing for traditional vertebrate-like scoring.
Another whole-body indicator commonly used to assess vertebrates’ welfare is physical injuries and/or deformities. Not all injuries or deformities will cause low welfare in insects; however, some may be in body regions innervated with nociceptors (Barrett and Fischer, 2024) or otherwise crucial to behavioral expression. Counting present and past injuries (through melanization spots on an insect’s cuticle) could be a feasible method for informing at least a partial assessment of welfare in some insects under farm conditions and is therefore a short-term research priority.
Finally, volatile organic compounds (VOCs) and nonvolatile chemical production could reflect stress or health issues. In vertebrates, VOCs can sometimes be used to assess the presence of disease (Ellis et al., 2014), and odors produced by rats can tell conspecifics if they are hungry (Schneeberger et al., 2020). Insects also use odors for conspecific communication, which makes VOCs a potential target for understanding internal states. Further, VOCs produced by black soldier fly larvae (which are photophobic) that were fasted in light (e.g., high stress) were different from those produced by larvae fasted in the dark (e.g., lower stress; Cattaneo et al., 2025). This suggests there may be chemical signatures of stress that could be used as indicators of farmed insect welfare and should be short-term research priorities.
Immunological Indicators
Many immunological indicators are of low priority for further research due to their low likelihood of correctness and/or low feasibility.
Cuticular melanization can increase due to immune stress in insects (Nakhleh et al., 2017) and is feasible to measure on farms. However, it can also vary with age or genetic heritability (Song et al., 2022). The variability of factors that lead to melanization reduce its informativeness and correctness of this indicator.
As with poikilotherms (i.e., an “organism that cannot regulate its body temperature except by behavioral means”[3]), insects are particularly sensitive to temperature changes in their environment, such that their body temperature may vary significantly without suggesting any particular welfare challenges. Additionally, while fever can sometimes signal a heightened immune response to pathogens, it is not systematically involved (Stahlschmidt and Adamo, 2013; Adamo, 1998). This indicator is therefore not sufficiently accurate at the individual level, but could potentially be explored as a measure of population health for insects that produce a substantial amount of metabolic heat in groups.
Finally, molecular markers of immune function could be used as indicators of insect welfare as they have been shown to correlate with various stressors such as forced flight (hemocyte levels, see Adamo, 2010), heat, infection (cytokine levels, see Matsumura et al., 2018 & Ishii et al., 2010) or predation (antimicrobial peptide levels, see Adamo et al., 2017). However, some measures of correctness are unlikely to be found with these markers, and they may be difficult to measure on farms, often requiring the sacrifice of animals to be assessed. Thus, given the low feasibility and uncertainty around correctness, these indicators are deemed low-priority for further research by the authors.
Potential Neurobiological Indicators
Neurobiological indicators are considered by the authors to be promising candidates for welfare assessment. While they are of relatively low feasibility in farmed conditions, these indicators are likely to be correct and are thus recommended as long-term research priorities for use mainly in lab validation of other, more feasible indicators.
In vertebrates, the volume and complexity of certain brain regions have been associated with some affective state changes, like depression (Frodl et al., 2006). Similarly in insects, the brain has been shown to play an analogous role, for instance, in modulating depression-like states; brain size is well known to change in response to variations in the external environment (see Wang et al., 2022 and Maleszka et al., 2009, for honey bees; Technau & Technau, 2007, for fruit flies; Cayre et al., 2007, for crickets). However, obtaining data for this indicator is costly, technologically demanding, and generally requires sacrificing an individual. The authors therefore categorize it as a long-term priority indicator to explore further.
Furthermore, the close relationship between affective states and biogenic amine levels in vertebrates (Beaulieu, 2023), and the well-conserved neuromodulators in at least some insects, means that biogenic amine levels in the brain (e.g., dopamine, serotonin) may be likely to serve as a correct and useful welfare indicator. However, as with brain region volumes and complexity, such measures are technically difficult to obtain and would most likely require sacrificing insects. This leads the authors to recommend it as a long-term rather than a short-term priority to explore.
Respiratory and Cardiac Indicators
Respiratory and cardiac indicators are of varying promise for on-farm use. While some are likely to be infeasible and potentially incorrect or challenging to use due to insects’ poikilothermic nature (respiratory patterns), others are either highly likely to be correct (measures of oxidative stress) or potentially feasible (measures of heart rate and heart rate variability), thus being recommended respectively as long-term and short-term research priorities.
As a marker of the fight or flight response (McCarty, 2016), vertebrates’ respiratory patterns can be used to assess their welfare. In insects, respiratory patterns may also change in response to environmental conditions or their behavior; however, the range of these patterns in the insects even at rest is markedly more variable across and within species (depending on life stage and phenotype/genotype) than in farmed mammals. These factors make it more difficult to identify accurate and useful potential indicators related to this phenomenon. Moreover, disentangling changes in respiratory patterns resulting from activity or temperature changes from those due to stress can be difficult. Finally, methods for measuring respiration in most insects are expensive and time-consuming, making it likely to be infeasible on farms. The authors conclude that using respiratory pattern indicators is infeasible for deploying on farms in the near future and categorize it as low-priority.
A more promising respiratory/cardiac indicator is oxidative stress, which is likely not only a correct operational welfare indicator but also a true marker of affective states in insects. Oxidative stress is defined as a high ratio of reactive oxygen species (ROS) to antioxidant defenses. It is extremely dangerous in both vertebrates and invertebrates (Beaulieu, 2024b). Markers of oxidative stress have been shown to correlate with a negative affective state such as depression in vertebrates (Bhatt et al., 2020), and in some insects like fruit flies (Jiang et al., 2017). The measurements involved in testing for markers of oxidative stress damage are likely to be lethal in the vast majority of cases and are unlikely to be easy to use on farms, but the potential for correctness of this indicator warrants further investigation (possibly to use the indicator as a resource in validating other, more feasible on-farm measures).
Finally, heart[4] rate and heart rate variability are potentially feasible on-farm indicators of welfare, using non-invasive and/or visual measurement methods. This could be done by observing the pumping through clear body segments (adult black soldier flies) or potentially through the use of infrared sensors, though measures of pumping direction are deemed infeasible on farms by the authors. When used in conjunction with baseline measures of heart rate at a specific temperature and life stage, the level of heart rate elevation or depression compared to baseline could be explored as an indicator of stress response in insects (Davis et al., 2021). Because of its relationship with temperature, insects’ heart rate alone is likely a poor indicator of welfare (similar to respiratory patterns or body temperature). Thus, heart rate variability may better reflect a putative welfare state. Regardless, the high feasibility of this indicator on farms leads the authors to categorize it as a short-term research priority.
Conclusion
In this review of potential physiological indicators of farmed insect welfare, the authors identified seven (heart rate, heart rate variability, fat mass, visual body condition scoring, body mass and scaling data, physical injury, and volatile organic compounds) potential operational indicators as short-term priorities for further research. These are potentially correct and feasible on-farm indicators of welfare. Five (biogenic amines, neurohormones, markers of oxidative stress, brain region volumes, and cuticular hydrocarbon profiles) are classified as long-term priorities. Changes in total cuticular melanization, fever/body temperature, molecular markers of immune function, respiratory patterns, heart rate pumping, and growth rate were categorized as low priority for further research due to their unlikely correctness and/or lack of feasibility on farms.
Further research is warranted into other operational welfare indicators, like behavioral indicators or telomere attrition (a potential genetic marker of chronically stressful conditions), as well as into valid indicators of affective states (as opposed to operational indicators of welfare as reviewed here). Throughout the review, the authors suggest biogenic amines, markers of oxidative stress, and possibly shifts in brain region volumes are the most likely to be valid markers of affective state in insects.
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- These assessment criteria categories are derived from Browning, 2022. Correctness criteria include indicator validity, accuracy, completeness and reliability; usefulness criteria include range of applicability, scale type and informativeness; feasibility criteria include ease of use and current data availability. ↑
- Between one measure of body size or mass and another. ↑
- Oxford Online Dictionary. ↑
- Note that insects do not technically have a ‘heart’ but a dorsal vessel, accessory pulsatile organs, and diaphragms that divide the body cavity and make up their circulatory system. We use the term heart as a functional signifier for these various body parts, as do many entomologists. ↑