The appeal of natural repellents
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| credit:Philippe Faucon www.desert-tropicals.com |
Recent research has identified particular plant extracts which could help to keep insect pests at bay. Catnip (Nepeta cataria L.), a member of the mint family which drives cats crazy, has also been found to repel cockroaches. Scientists at Iowa State University have identified the compound (nepetalactone) in catnip and have demonstrated that areas treated with the extract keep cockroaches away. The compound has no obvious effect on humans and therefore could possibly be added to food packaging to reduce food storage losses and perhaps keep pests out of food shipments. Tests are also to be conducted to discover if the same compound could be used as a personal insect repellent against mosquitoes.
The Iowa team have also been working on the fruit of a thorny member of the mulberry family (Maclura pomifera), also known as 'osage orange' or 'hedgeapple', which was planted by early settlers in the central US as hedgerows to keep in livestock. The fruit is reputed to repel cockroaches, spiders, other insect pests and even mice. The fruit are sold in local shops as an insect repellent and tests have confirmed that both cockroaches and grain beetles avoid them.
The oil of another member of the mint family, peppermint (Mentha piperita), has been found to repel adult mosquitoes and also to kill the larvae. Scientists from the Malaria Research Centre and the Centre for Rural Development and Technology in Dehli tested locally extracted peppermint oil on the larvae of three mosquito species: Aedes aegypti (which carries dengue fever); Anopheles stephensi (malaria); and Culex quinquefasciatus (filariasis and West Nile virus). Films of peppermint oil in trays of water (3ml per m2) killed over 85% or more of the larvae of each species. Volunteers doused in the oil found it offered good protection against all mosquito species (~85%). It was particularly effective against Anopheles culicifacies, which is responsible for the majority (75%) of malaria transmissions in the northern plains of India. Although large quantities of oil would be required to treat mosquito-breeding sites around a village, peppermint can be easily cultivated and is cheaper and safer to use than many commercial insecticides.
Identification of other compounds that plants have evolved to deter insect attack could pave the way for further use of natural repellents against insects. Other well-known essential oils, for example lemongrass (Cymbopogon citratus), are reputed to be effective in preventing insect bites. Rosemary, can also be used to repel moths. In addition, recent surveys of desert and semi-desert plants have revealed a range of compounds (alcohols, alkaloids and terpenes) that are repellent and could possibly be used for crop storage protection. The advantage of using plant materials is that they can be continuously propagated, they are biodegradable and, in the case of plant extracts that have traditionally been used in foods (e.g. dill or fenugreek), these compounds will present no risk to human health. However, large quantities of natural materials may be required for effective control, which may not be considered practical or feasible in some situations.
