# HL-IR mediates cinnamaldehyde repellency behavior in parthenogenetic Haemaphysalis longicornis

## TL;DR

This study identifies a tick receptor that detects cinnamaldehyde, a plant compound, and shows how it triggers repellent behavior, offering a new approach for natural tick control.

## Contribution

The study identifies HL-IR as a key ionotropic receptor mediating cinnamaldehyde repellency in ticks and determines its critical binding site.

## Key findings

- HL-IR is expressed in Haller’s organs and mediates cinnamaldehyde repellency in Haemaphysalis longicornis.
- Microinjection of HL-IR dsRNA reduced cinnamaldehyde repellency and electrophysiological responses.
- The 218ASN site in HL-IR is critical for cinnamaldehyde binding.

## Abstract

Chemical repellents against arthropods have limitations in terms of toxicity and resistance. Natural plant compounds can be utilized as alternatives for developing environmentally friendly repellents for humans and animals. A variety of plant essential oils exhibit strong repellent effects against ticks; however, the mechanisms of action against ticks remain unknown. Here, we investigated the repellency of cinnamaldehyde, a primary compound found in cinnamon oil, and demonstrated that it affected the electrophysiological responses on Haller’s organs of parthenogenetic Haemaphysalis longicornis. Transcriptome data indicated that the cinnamaldehyde response was linked to ionotropic receptor (HL-IR) at various tick developmental stages. HL-IR was widely expressed in a variety of tissues and developmental stages of ticks according to RT-qPCR. In situ hybridization results showed that HL-IR was highly expressed on Haller’s organs of the ticks. Microinjection of HL-IR double-stranded RNA (dsRNA) showed that reduced transcript levels led to significant decreases in the tick repellency rate from cinnamaldehyde and the EAG response of Haller’s organ. Experiments using competitive fluorescence binding and mutation sites showed that 218ASN was the critical binding site for cinnamaldehyde and HL-IR. We conclude that Haller’s organ of ticks expresses HL-IR, and that this interaction mediates tick-repellent behavior by binding to cinnamaldehyde.

Tick and tick-borne diseases present a significant threat to human and animal health. To mitigate tick bites and the spread of tick-borne diseases, chemical repellents are commonly utilized in daily life. In recent years, the resistance and toxicity of chemical repellents have been brought to light, leading to a growing interest in plant-derived repellents as a new research focus. Ticks sense their environment primarily through their sense of smell because their vision and hearing are underdeveloped. Despite this, the mechanisms underlying tick olfaction remain poorly understood. The interaction between ticks and plant-derived chemicals is a fascinating area of study. Our findings revealed Haller’s organ is the key sensory organ for tick sensing of cinnamaldehyde. We reveal an ionotropic receptor in ticks that specifically detects cinnamaldehyde, triggering repellent responses through Haller’s organs, and elucidated its binding site. A comprehensive understanding of this process is imperative for effective tick prevention and disease control.