Assessment of Chlorella vulgaris as a biological control agent against tortoise tick Hyalomma aegyptium (Acari: Ixodidae) in Egypt

Hyalomma aegyptium is a three-host tick species parasitizing mainly tortoises in Asia, North Africa, and the Middle East. It serves as a carrier for various pathogenic bacteria and protozoa that pose threats to humans, wildlife, and domestic animals. Ticks control using chemical acaricides has negative effects to the environment and animal and human health, residues in animal products and leading to resistant ticks. So safe, eco-friendly, and cost-effective methods must be alternatively used. The green microalga Chlorella vulgaris is rich in proteins, lipids, carbohydrates and vitamins. It is used in biofuel production, wastewater treatment, and as a biofertilizer. It is used in pharmaceutical drugs with many beneficial characteristics. Examination of collected specimens in the present study ensured that they were identified as H. aegyptium nymphs. Using the powdering method, nymphs were treated with Chlorella and observed for 18 days. The results showed that the effect began 4 days after treatment, the mortality percentage reached 80%, and delayed molting period with only 20% molted into males. Morphological observations using light and scanning electron microscopes revealed a stiffened nymph body after treatment with a highly damaged capitulum, integument, and legs. Integument semithin sections showed thin, disorganized cuticle with damaged layers and destructed epidermal cells after treatment. No signs of new cuticle formation were noticed. The effect of Chlorella was either mechanical through powder particles or physiological through its effect on organs. This study may provide valuable information to help in the development of new methods to control ticks and/or improve the existing ones, allowing the creation of methods which do not induce resistance in ticks, and that are less toxic to the environment and non-target organisms.