Triazine Herbicide and NPK Fertilizer Exposure: Accumulation of Heavy Metals and Rare Earth Elements, Effects on Cuticle Melanization, and Immunocompetence in the Model Species Tenebrio molitor

The increasing use of agrochemicals, including fertilizers and herbicides, has led to worrying metal contamination of soils and waters and raises serious questions about the effects of their transfer to different levels of the trophic web. Accumulation and biomagnification of essential (K, Na, Mg, Zn, Ca), nonessential (Sr, Hg, Rb, Ba, Se, Cd, Cr, Pb, As), and rare earth elements (REEs) were investigated in newly emerged adults of Tenebrio molitor exposed to field-admitted concentrations of a metribuzin-based herbicide and an NPK blend fertilizer. Chemical analyses were performed using inductively coupled plasma tandem mass spectrometry (ICP-MS/MS) supported by unsupervised pattern recognition techniques. Physiological parameters such as cuticle melanization, cellular (circulating hemocytes), and humoral (phenoloxidase enzyme activity) immune responses and mass loss were tested as exposure markers in both sexes. The results showed that NPK fertilizer application is the main cause of REE accumulation in beetles over time, besides toxic elements (Sr, Hg, Cr, Rb, Ba, Ni, Al, V, U) also present in the herbicide-treated beetles. The biomagnification of Cu and Zn suggested a high potential for food web transfer in agroecosystems. Gender differences in element concentrations suggested that males and females differ in element uptake and excretion. Differences in phenotypic traits show that exposure affects metabolic pathways involving sequestration and detoxification during the transition phase from immature-to-mature beetles, triggering a redistribution of resources between sexual maturation and immune responses. Our findings highlight the importance of setting limits for metals and REEs in herbicides and fertilizers to avoid adverse effects on species that provide ecosystem services and contribute to soil health in agroecosystems.

Ingestion of Nylon 11 Polymers by the Mealworm (Tenebrio molitor) Beetle and Subsequent Enrichment of Monomer-Metabolizing Bacteria in Fecal Microbiome

BACKGROUND

Nylon 11 is a synthetic plastic widely used in commercial products such as tubing for automobiles, offshore oilfields, and medical devices. An increasing amount of nylon and other plastic wastes have been released into various environments, posing ecological threats. The biodegradation of bundled nylon polymers has been considered impossible due to their crystalline structures.

METHODS

Nylon 11 film was created and incubated with adult mealworms. The mass, as well as structures, of nylon 11 films at pre- and post-incubation with beetles were compared. The number of nylon 11 monomer degrading bacteria in feces were determined by culture-dependent approach. The t-test was utilized to examine the statistical significance.

RESULTS

We discovered that adult mealworm (Tenebrio molitor) beetle can ingest nylon 11 when stretched thin. The microscopic observation of their feces did not identify the presence of large fragments of nylon 11. The analysis of fecal bacteria revealed that while the total number of culturable bacteria did not change significantly, the number of 11-aminoundecanoic acid-metabolizing bacteria increased by 10,000-fold.

CONCLUSIONS

Our results suggest that bundled nylon 11 polymers were fragmented into smaller pieces, including monomeric units (11-aminoundecanoic acid) by adult mealworm. The monomers seem to have supported the proliferation of gut microbial communities capable of utilizing 11-aminoundecanoic acid as a carbon and nitrogen source. Our work implies the potential use of the mealworm beetle as a means to fragment nylon polymers for remediation applications.

Late-life reproduction in an insect: Terminal investment, reproductive restraint or senescence

The terminal investment, reproductive restraint or senescence theories may explain individual late-life patterns of reproduction. The terminal investment hypothesis predicts that individuals increase reproductive allocation late in life as prospects for future survival decrease. The other two hypotheses predict reduced reproduction late in life, but for different reasons. Under the Reproductive Restraint hypothesis, individuals restrain their reproductive effort to sustain future survival and gain more time for reproducing, whereas under the Senescence process, reproduction is constrained because of somatic deterioration. While these hypotheses imply that reproduction is costly, they should have contrasted implications in terms of survival after late reproduction and somatic maintenance. Testing these hypotheses requires proper consideration of the effects of age-dependent reproductive effort on post-reproduction survival and age-related somatic functions. We experimentally tested these three hypotheses in females of the mealworm beetle, Tenebrio molitor, an iteroparous and income breeder insect. We manipulated their age-specific allocation into reproduction and observed the effects of this manipulation on their late-life fecundity, post-reproduction survival and immunocompetence as a measurement of somatic protection. We found that females exhibit age-related decline in fecundity and that this reproductive senescence is accelerated by a cost of early reproduction. The cost of reproduction had no significant effect on female longevity and their ability to survive a bacterial infection, despite that some immune cells were depleted by reproduction. We found that female post-infection survival deteriorated with age, which could be partly explained by a decline in some immune parameters. Importantly, females did not increase their reproductive effort late in life at the expense of their late-life post-reproduction survival. Late-life reproduction in T. molitor females is senescing and not consistent with a terminal investment strategy. Rather, our results suggest that females allocate resources according to a priority scheme favouring longevity at the expense of reproduction, which is in line with the reproductive restraint hypothesis. Such a priority scheme also shows that a relatively short-lived insect can evolve life-history strategies hitherto known only in long-lived animals. This puts in perspective the role of longevity in the evolution of life-history strategies.