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2017 | Katie Liu and Zoe Glentworth

Palmerston North Girls' High School

Katie Liu and Zoe Glentworth have been awarded Team Gold CREST for their project "The Effects of Synergy on the Insect Repelling Properties of Macropiper excelsum, Corymbia citriodora and Achillea millefolium"


Mentors: Lynn Kirkland (Herb Farm) and Andrew Carroll (Plant and Food Research)

Supporters: Ginnie Harcombe, Amanda Loveridge, Bill Kirkland

Supervising teacher: Dr. Heather Meikle

Gold CREST Medals: Professor Barry Scott, FRSNZ, Vice President/Academy Chair – Biological and Life Sciences, attended the Gold CREST celebration and presented Katie and Zoe with their Gold CREST medals.


Their executive summary states:

The aim of this investigation was to develop an effective insect repellent spray from the New Zealand native plant Kawakawa (Macropiper excelsum), and the non-native plants Lemon
Eucalyptus (Corymbia citriodora) and Yarrow (Achillea millefolium), through investigating the synergistic effects of the three plant extracts. Commercial insect repellents contain chemicals that while effective against insects, may also be harmful to humans. Therefore there is a need to investigate the insect repellency effects of different plant extracts, so the information can be utilised for the development of plant-based insect repellents in the future.

The main objectives of this study included determining the optimum concentration of the plant extracts for maximum insect repellency, and investigating whether synergistic effects
are present in combinations of Kawakawa, Lemon Eucalyptus and Yarrow. Extracts were tested for their effectiveness against the common housefly (Musca domestica). The housefly
was used to represent biting insects, as the common housefly is in the Muscidae family, which is closely related to the Culicidae family, which contains the mosquito. The World Health Organization (WHO) published a Laboratory Repellent Bioassay Method for testing mosquito repellents in 2007. With this method, human participants have a control area of
skin, treated with the repellent. The WHO method has been altered to meet the needs of this study.

Rongoā is the traditional Māori process of using New Zealand native plants for their healing and medicinal properties. Māori knowledge of traditional plants with insect repelling
properties is a valuable resource for developing plant-based repellents, and this knowledge has been utilised for this study. Kawakawa contains active ingredients including myristicin
and elemicin, which have insect repelling properties. The active ingredients, including the defense protein juvadecene, are produced when the Kawakawa leaves are bruised, or fed
on nocturnally by the native New Zealand Looper Caterpillar (Cleora scriptaria). Lemon Eucalyptus contains the active ingredient P-Menthane-3,8-diol (PMD), and Yarrow contains the active ingredients Thujone, Azulene, Camphor and Cineole, which repel insects.

The first trial in this study, with 7 human participants, tested both high formulations (1% Lemon Eucalyptus essential oil and 3% Kawakawa extract and 3% Yarrow extract) and low
formulations (0.5% and 1% respectively), to determine the optimum volume to volume (V/V) ratio per plant extract for insect repellency. Results showed that there was no statistically significant difference (p< 0.001) between the single plant extracts' effectiveness at both high and low formulations. There was also no statistical difference in the insect repellency between the different treatments (p<0.001) Kawakawa vs Lemon Eucalyptus vs Yarrow. All formulations used in this trial repelled insects.

The second trial, with 14 human participants, investigated whether there was synergy when two or more plant extracts were used in the formulation. This was to determine if
combinations of plant extracts were more effective at repelling the housefly (Musca domestica) than those treatments with a single plant extract. Four combinations of the three
plant extracts were tested: 1. Kawakawa, Yarrow and Lemon Eucalyptus; 2. Kawakawa and Yarrow; 3. Kawakawa and Lemon Eucalyptus and 4. Yarrow and Lemon Eucalyptus. Thee combinations were compared with two controls: 1. sodium chloride solution and 2. bare human forearm. It was discovered that all of these treatments were significantly more effective at repelling the common housefly, in comparison to the controls (p<0.001).

In conclusion, the final formulations were developed in a spray form, which our survey results showed was the preferred mode of insect repellent delivery. All ten formulations
were equally effective insect repellents. There was no statistically significant difference between either the single species plant extract treatments and the combined plant extract
formulations (p<0.001), nor the different formulations of the single species repellents. Each formulation was effective at repelling houseflies regardless of plant type(s) or concentration. There was no statistically significant (p<0.001) synergy observed between the plant combinations trialled in this investigation. Future studies could focus on determining the lowest concentration at which these plant materials would be effective insect repellents.