Sentience Of Adult Versus Juvenile Insects

Every year, over a trillion insects are farmed for food, clothing, and medicine. While insect farming may be less demanding on the environment than traditional animal farming, many animal advocates believe it is still unethical.

This paper estimates the relative difference between adult and juvenile insects in sentience and the capacity for welfare. An animal’s level of sentience is its ability to experience emotions — or realize its own welfare — while its capacity for welfare is its welfare range (how well or badly things can go) multiplied by its lifespan. In other words, if adult insects are more sentient and can suffer more than immature insects, then we should prioritize improvements to the lives of insects in their adult forms. The paper looks at several commercially important insect species: yellow mealworms, black soldier flies, western honeybees, house crickets, and mealworms. 

Yellow mealworms, black soldier flies, and silkworms used commercially as juveniles, whereas honeybees and house crickets are largely farmed as adults. Insects differ in how they develop from juvenile to adult insects. With complete metamorphosis, the immature stage has fewer structures and functions than the adult (e.g., caterpillar and butterfly). Conversely, with incomplete metamorphosis, the immature stage is simply a smaller version of the adult. Of the species of interest, only house crickets undergo incomplete metamorphosis. In general, we expect cognition to be more different between juvenile and adult insects who undergo complete metamorphosis, compared to those who undergo incomplete metamorphosis.

The researchers developed an Invertebrate Sentience Table that includes a variety of traits that affect the likelihood of sentience (e.g., brain size, defensive behavior). Similarly, the researchers’ Welfare Range Table is used to examine differences in capacity for welfare (e.g., personality, communication). The literature review revealed that many of the traits in both tables have not been studied in the five species. In fact, direct evidence was found for only 19 of the 150 traits for any species.

The researchers created a table with the 19 traits relevant to both welfare ranges and probability of sentience. They used a simple scoring metric to crudely estimate the differences between life stages and species. According to their estimate, adults of all species have a higher welfare range than juveniles of that species. Further, adults of all species other than the silkworm are more likely to be sentient than juveniles of that species. 

Despite the findings, the researchers’ high level of uncertainty prevents strong conclusions. Gaps in the literature made it challenging to collect data. Because adult insects are more commonly studied than juveniles, the investigation was biased against juvenile insects. Therefore, the conclusions are tentative and could be revised with more research. 

That being said, it seems probable that there are some differences in sentience and welfare ranges across insect life stages. The data is suggestive, and scientists may have avoided studying certain traits in juvenile insects because they knew there would be a null result. However, these differences are likely small. We have far too little evidence to reliably conclude that there’s a large difference. For example, the neurological evidence often points to juvenile insects having the same capabilities as adults.  

The sentence and welfare ranges of an insect are based on the life requirements at each stage, which vary across species. For example, immature insects vary in their independence and interaction with the environment. For some species, food, shelter, and security are provided for by adults. Since these immature insects don’t need to search for resources or engage in defense behaviors, they may not require the same capabilities as adults. In these cases, immature insects may be less likely to be sentient than their adult forms.

Similarly, changes in the rearing environment can affect brain development. When honeybee colonies are transported for pollination, the hive is sealed. Young bees confined to the nest miss stimuli at critical points in their development, which can stunt traits related to coordination, learning, and memory. In that case, farmed bees may have a lower capacity for welfare than wild bees. 

More research is needed in this area, but one thing is clear — advocates should avoid assuming that juvenile insects matter less than adult insects, since we don’t have the information to draw this sort of conclusion.