Discussion
Our results indicate that biodegradable plastics are not any better than
common plastics. This is based on the adverse outcomes to fitness after
consuming either plastic oil-derived or compostable plastics for almost
all fitness variables. More specifically: compared to the control group,
consuming these products delayed the developmental duration of both
larval and pupal stages, hastened animal mortality, which resulted in
lower weight (with the lightest in the PS and CP groups), and led to
reduced survival. We will discuss each of these different effects below.
One negative effect of consuming plastics oil-derived and bioplastics is
the delay in developmental time. Although, the presence of micro- and
oil-derived nanoplastics did not influence larval growth in Bombix
mori (Muhammad et al., 2021). A similar result was reported in T.
molitor and Hermetia illucens larvae after eating bioplastics
(Heussler, et al. 2024; Kokalj et al. 2024). Our results showed that
both types of plastics delay the development time. The differences could
be explained by time, since our animals fed on both types of plastic
from very early larval stages compared to previous studies. One related
question is that of the cost of delayed development. A consequence is
that animals can face more threats, such as predators, parasites, and/or
parasitoids (Nylin and Gotthard, 1998). Increasing development times is
common in insects, largely explained by not reaching a certain body mass
threshold necessary to complete metamorphosis (Nijhout, 2003). This
aligns with our result of reduced weight in animals that consumed
plastics or compostable products: reduced weight could prolong
development. There are several non-mutually exclusive mechanistic
explanations for this. First, stressed animals may not gather enough or
do not have access to particular nutrients for their development (e.g.
Welden and Cowie, 2016). Second, the presence of toxic elements in both
petroleum-derived and compostable plastics may cause negative effects
(Wang et al., 2020; 2021; Zimmermann et al., 2021). Third, either
plastics or compostable products may obstruct the digestive system and
impede nutrient acquisition (Sigler, 2014). And fourth, plastics may
affect the animal’s microbiota (Antonelli et al., 2022).
Our results also showed that mortality increased when larvae were fed
with biodegradable and non-biodegradable plastics, where even mortality
was higher in the CP group. Similar studies showed that Drosophila
melanogaster males had a higher mortality compared to females after
ingesting PS microplastics (El Kholy and Al Nagar, 2023). In contrast,
in yellow mealworm beetles and black soldier fly larvae that were fed
bioplastics did not show increased mortality (Heussler, et al. 2024;
Kokalj et al. 2024). However, similar to other studies (Wang et al.,
2021), we found a trade-off between investment in development and
survival. Also, the number of offspring after reproduction in surviving
pairs of treatment groups was significantly lower than control. As
explained above, the trade-offs may be understood by the different
mechanisms underlying the ingestion of non-natural material. One
challenge to clarify is how the different effects plastics
non-biodegradable and bioplastics can cause on survival, reproduction,
and growth may balance each other, resulting in some traits being less
affected than others. For example, we are aware of the effects of oil
derived microplastics on antioxidative stress response, sex hormone
disruption, and disturbed transcription of steroidogenic genes as main
drivers of impaired reproduction (Wang et al., 2021). How these effects
may be balanced with the effects of the same material on, say, survival,
is unclear. One other challenge is to explain whether the observed
trade-offs are adaptive or are an artifact of an animal being unable, in
general, to deal with, say, a toxic material.
Finally, the idea that plastic ingestion has detrimental effects on life
history traits and that this can lead to apparent trade-offs is not new
(Santos et al., 2021). What is new about our work is the trade-off and
outcome of ingesting compostable, which plays as much a detrimental role
as ingesting plastics. For example, animals may trade off fecundity for
development time after eating compostable products. Why would
compostable products produce similarly acute patterns as plastics? At
the proximate level, compostable products may contain similarly toxic
components to those of common plastics. It is known that added
components to plastics produce inflammatory responses, endocrine
disruption, neurotoxicity, oxidative stress, and metabolic alterations
that all together affect immunity, reproduction, and digestion in
insects (reviewed by Sánchez-Hernández, 2021). This may be the case for
compostable products too. For example, a study analyzed 43 biobased and
biodegradable products and found that 67% showed baseline toxicity,
42% led to oxidative stress, and 23% induced antiandrogenicity
(Zimmermann et al., 2021).
Conclusions
Considering the assertion that compostable products can serve as an
alternative to conventional plastics, our work presents contradicting
evidence: insects fed with compostable products may experience fitness
effects (growth, fecundity, and survival) as detrimental as those
observed in insects fed with plastics. The cause of this phenomenon is
currently unknown, and one potential explanation is the presence of
added toxic components in compostable products. Consequently, our work
serves as a cautionary note against the argument advocating for the
safety of using compostable materials.
Acknowledgements
This project was financed by a UNAM-PAPIIT grant IN204921.
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Tables
Table 1. Survival estimations obtained from the Multistate model with
developmental stage as state and treatment as covariate.