The use of a natural insect repellent to protect livestock in South Africa against biting flies and midges is important for the:
• Prevention of basic distress arising from excessive insect irritation
• The reduction of the possibility of disease by preventing biting insects, capable of transmitting diseases affecting livestock, from actively feeding on the host and thereby transmitting the disease.

The use of insect repellents can help to reduce both the irritation caused by insect worry and the incidence of disease transmission. The South African Bureau of Standards (SABS) was commissioned to test the efficacy of a formulation of citronella oil.

The use of citronella oil as a repellent is well-documented. Reference is made to the US Department of Agriculture (Environmental Protection Agency Website) detailing the efficacy of citronella oil as a repellent against midges and flies.

Every formulation has different properties depending on the excipients used to enhance the efficacy and duration of effect. This specific formulation was submitted to the SABS to be tested for efficacy against local strains of flies and midges.

The test
The test was conducted according to Section 4 of SANS Method 5695, adapted to comply with the following requirements:
• 50 mosquitoes were used per test cage
• 100 house flies were used per test cage
• Each cage held one guinea pig
• There were four replicates for each treatment group
• Two untreated groups were included
• The test sample was applied using a hand-trigger application
• Untreated controls were exposed to the same insect challenge two hours after the treatment commenced (NB: A second exposure would have exposed the test animals to unnecessary stress and pain)
• To ensure that the mosquitoes were in fact still active, the test officer exposed his own arm to the mosquitoes in the test cage. The mosquitoes continued to attack the arm for the full four hours of the test.

The test is designed to test the degree of repellency of the test formulation against yellow fever mosquitoes (Aedes aegypti) and the common house fly (Musca domestica) under controlled laboratory conditions. The test animals were treated with a 10% ready-to-use formulation of citronella oil applied by means of a hand trigger application bottle and spread with a cotton wool swab.

The use of insect repellents can help to reduce both the irritation caused by insect worry and the incidence of disease transmission. The test animals were exposed to the challenge at one-, two-, three- and four-hour intervals after treatment, using the exposures as stated in the preamble above. The untreated control groups were exposed to the same challenge two hours after commencement of the trial.

The results
Efficacy was measured by means of the following objective criteria:
Mosquitoes: The number of bites recorded over a 3-minute period (skin penetration); and the number of pecks over a 3-minute period (the mosquito lands but immediately flies off without penetration).
Flies: The number of landings over a 3-minute period (flies land and remain on the surface); and the number of touchdowns over a 3-minute period (flies land but immediately fly off).

Conclusions
Under the conditions of the controlled laboratory test, the citronella oil-treated group showed a positive repellent action of >80% over the untreated control group for mosquitoes, for up to four hours. Under the conditions of the controlled laboratory test, the citronella oil-treated group showed a repellent action >80% after two hours and approximately 60% after four hours. Unfortunately no exposure to an untreated control was carried out at four hours for flies.

Dr RD Sykes is a qualified veterinarian and is currently consulting for various companies on registration requirements for stock remedies and veterinary medicines.