Species-Specific Interactions between Plant Metabolites and Insect Juvenile Hormone Receptors

Farnesol, a component of plant-derived honeybee-collected resins, shows JH-like effects in Apis mellifera workers

Farnesol, a sesquiterpene found in all eukaryotes, precursor of juvenile hormone (JH) in insects, is involved in signalling, communication, and antimicrobial defence. Farnesol is a compound of floral volatiles, suggesting its importance in pollination and foraging behaviour. Farnesol is found in the resin of Baccharis dracunculifolia, from which honeybees elaborate the most worldwide marketable propolis. Bees use propolis to seal cracks in the walls, reinforce the wax combs, and as protection against bacteria and fungi. The introduction within a honeybee hive of a compound with potential hormonal activity can be a challenge to the colony survival, mainly because the transition from within-hive to outside activities of workers is controlled by JH. Here, we tested the hypothesis that exogenous farnesol alters the pacing of developing workers. The first assays showed that low doses of the JH precursor (0.1 and 0.01 µg) accelerate pharate-adult development, with high doses being toxic. The second assay was conducted in adult workers and demonstrated bees that received 0.2 µg farnesol showed more agitated behaviour than the control bees. If farnesol was used by corpora allata (CA) cells as a precursor of JH and this hormone was responsible for the observed behavioural alterations, these glands were expected to be larger after the treatment. Our results on CA measurements after 72 h of treatment showed bees that received farnesol had glands doubled in size compared to the control bees (p < 0.05). Additionally, we expected the expression of JH synthesis, JH degradation, and JH-response genes would be upregulated in the treated bees. Our results showed that indeed, the mean transcript levels of these genes were higher in the treated bees (significant for methyl farnesoate epoxidase and juvenile hormone esterase, p < 0.05). These results suggest farnesol is used in honeybees as a precursor of JH, leading to increasing JH titres, and thus modulating the pacing of workers development. This finding has behavioural and ecological implications, since alterations in the dynamics of the physiological changes associated to aging in young honeybees may significantly impact colony balance in nature.

Disruptive Effects of Two Curcuminoids (Demethoxycurcumin and Bisdemethoxycurcumin) on the Larval Development of Drosophila melanogaster

Juvenile hormones (JHs) play a central role in insect development, reproduction, and various physiological functions. Curcuminoids generally exhibit a wide range of biological activities, such as antioxidant, anti-inflammatory, antibacterial, and insecticidal, and they exhibit insect growth inhibitory effects. However, research on insecticidal properties of curcuminoids has been limited. Moreover, to the best of our knowledge, studies on JHs of insects and curcuminoids are lacking. Therefore, this study aimed to identify the substances that act as JH disruptors (JHDs) from edible plants. Demethoxycurcumin (DMC) and bisdemethoxycurcumin (BDMC), two curcuminoids from the turmeric plant Curcuma longa L. inhibited the formation of a methoprene-tolerant (Met)-Taiman (Tai) heterodimer complex in Drosophila melanogaster, as shown through in vitro yeast two-hybrid assays. An artificial diet containing 1% (w/v) DMC or BDMC significantly reduced the number of D. melanogaster larvae in a concentration-dependent manner; larval development was disrupted, preventing the progression of larvae to pupal stages, resulting in an absence of adults. Building on the results obtained in this study on curcuminoids, researchers can use our study as a reference to develop eco-friendly pesticides.