Short Communication A bioassay for natural insect repellents Eddie Hang Chio, En-Cheng Yang ⁎ Department of Entomology, National Taiwan University, Taipei, Taiwan abstract article info Article history: Received 5 July 2008 Revised 19 August 2008 Accepted 21 August 2008 Keywords: Insect repellent bioassay Asian tiger mosquito Neem oil Djulis Natural products, ED 50 A practical bioassay for natural product insect repellents was developed and validated. The Asian tiger mosquito (Aedes albopictus) was used in this study. Djulis (Chenopodium spp.), an indigenous plant from Taiwan, provided better insect repellency than neem tree (Azadirachta indica) oil, a well-known insect repellent. The % repellency of each test material was converted to 50% effective dosage (ED 50 ) by probit analysis for better comparison between test materials. The ED 50 for insect repellency in descending order was djulis leave extract (0.532%), neem oil (0.579%); djulis seed extract (0.930%) and sea lily (Crinoid spp.) (1.022%). © Korean Society of Applied Entomology, Taiwan Entomological Society and Malaysian Plant Protection Society, 2008 Published by Elsevier B.V. All rights reserved. Introduction Mosquitoes and biting midges cannot only be an outdoor nuisance but also can threaten our health by transmitting diseases (Chen et al., 1979, 1982; Epstein et al., 1998; Chuang et al., 2000; Lee and Hou, 1997). Under field conditions, these pests are difficult to control without harming the environment using conventional means. Insect repellents, therefore, can play a significant role in protecting us from mosquitoes and biting midges (Braverman et al., 1999). The most common insect repellent is DEET (N, N-Diethyl-m-toluamide)(Katz et al., 2008). However, recent publications suggest that DEET poses some concerns in reproductive and developmental toxicity in animals (Wright et al., 1992). Other disadvantages of DEET include toxic reactions and damage to plastic and synthetic fabrics (Gillij et al., 2008). Insect repellents from natural sources are believed to be safer and are, therefore, being broadly investigated. Djulis, a plant from Taiwan, has been brought to our attention. According Taiwanese aboriginals, djulis is an excellent repellent against biting midges and mosquitoes. However, this claim has never been verified by any scientific publication, to the best of our knowledge. Djulis is believed to be in the genus Chenopodium (Fam. Chenopodiaceae), but its identity has not yet been fully validated (personal communication with Prof. P.J. Tsai at NPUST, 2008). We planned to evaluate djulis' insect repellency in our laboratory by following a standard protocol. To our surprise, there is no standard laboratory protocol for determining repellency. Some authors used filter paper (Liu and Ho, 1999), some used pig skin (Kasichayanula et al., 2005, Waka et al., 2004), and others used laboratory animals and human bodies (DePaula et al., 2003). These methods each have their own strengths and weaknesses (Katz et al., 2008). In addition, repellency is reported in several different ways, including percent control, repellent index, effective dosage or effective duration (Katz et al., 2008). Without a standardized protocol and reporting system, it is impossible to compare the repellency of the same product from one laboratory to another. With these challenges in mind, we developed a practical and user-friendly bioassay protocol and we established an objective unit of measurement for insect repellents. We then compared the insect repellency of djulis and the well-known natural insect repellent, neem oil (Xie et al., 1995). The success of this bioassay convinces us that other natural products can be assessed as potential insect repellents using this approach. Materials and methods Insect and animal preparations Asian tiger mosquitoes, Aedes albopictus, were used for this bioassay. This colony of the mosquito had been maintained in our department for over 10 years using methods described by Gerber et al. (1994). All mosquitoes used in our experiments were 5–14 days old. Living mice (random stock mouse, ICR strain) were purchased from the Laboratory Animal Center of National Taiwan University (NTU). Mice were used to provide blood meals for the female mosquitoes. Journal of Asia-Pacific Entomology 11 (2008) 225–227 ⁎ Corresponding author. No. 1, Sec. 4, Roosevelt Rd., Taipei, Taiwan. Fax: +886 2 33652092. E-mail address: ecyang@ntu.edu.tw (E.-C. Yang). 1226-8615/$ – see front matter © Korean Society of Applied Entomology, Taiwan Entomological Society and Malaysian Plant Protection Society, 2008 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.aspen.2008.08.002 Contents lists available at ScienceDirect Journal of Asia-Pacific Entomology journal homepage: www.elsevier.com/locate/jape