PAPER PATHOLOGY ⁄ BIOLOGY Matan Shelomi, 1,* B.S.; Leia M. Matern, 1,* B.S.; Jenna M. Dinstell, 1 ; Daren W. Harris, 1 ; and Robert B. Kimsey, 1 Ph.D. DEET (N,N-Diethyl-meta-toluamide) Induced Delay of Blowfly Landing and Oviposition Rates on Treated Pig Carrion (Sus scrofa L.) ABSTRACT: The question of whether the insect repellent N,N-Diethyl-meta-toluamide (DEET) affected fly attraction, oviposition, and larval development was investigated; in part, to determine whether the common habit of wearing DEET as a repellent could affect the rate of human decomposition. Experiments using pig surrogates of human decedents were carried out in a rural environment. Dead piglets were sprayed with DEET, and fly behavior, colonization levels, and maggot development were compared with those in nonsprayed controls. Piglets treated with DEET experi- enced significant delays in fly visitation and oviposition and delayed appearance of each larval instar, as well as reduced total larval numbers (p < 0.01 for all variables), with subsequently reduced decomposition (p < 0.05). Such changes in fly behavior and larval population development would significantly impact the estimation of the period following the death from entomological evidence in decedents wearing DEET at the time of their death. KEYWORDS: forensic science, forensic entomology, insect repellents, N,N-Diethyl-meta-toluamide, Calliphoridae, Sarcophagidae, ovipo- sition, decomposition Humans commonly employ insect repellents to defend against biting insects. N,N-Diethyl-meta-toluamide (DEET) appears most frequently as the active ingredient in topical repellents (1). Origi- nally developed as a mosquito repellent (2), DEET successfully repels several genera of mosquitoes (3–6). DEET affects many other blood-seeking invertebrates as well, including sand flies (7), assassin bugs (8), Simuliid flies (9), Tsetse flies (10), face flies (11), ticks (12,13), and even leeches (14). That DEET also affects fruit flies suggests an invertebrate-repelling mechanism not limited to the prevention of blood feeding (15,16). DEET may also affect the blowflies seeking to lay eggs on the remains of homicide vic- tims. Indeed, the common household repellent Citronella oil pro- tects dead rats from Calliphorids (17). Thus, the effects of DEET on blowfly behavior may alter their role in decomposition and con- found the significance of insect evidence in homicide investigations. The extensive previous research on mosquito repellency might suggest how DEET would function in this new context. DEET could block odors and other factors produced by human skin that mosquitoes find attractive (18,19), or otherwise alter mosquito per- ception of these behavioral modifiers (20). The strength of these hypotheses faded following the discovery of DEET-sensitive olfac- tory receptor neurons that directly detect DEET and ultimately eli- cit avoidance behavior in mosquitoes (21). DEET also affects mosquito behavior in fundamentally different ways from toxicants that cause it to be classified as a ‘‘repellent’’ (22). If this repellency hypothesis prevails, the behavior of carrion-feeding flies, such as Calliphoridae and Sarcophagidae, should also be influenced by DEET. For example, one would expect to find reduced fly landing rates on a decedent, which had been wearing DEET at the time of death, together with corollary delays in oviposition and maggot population development. A controversial but key assumption of forensic entomology pos- its that blowflies locate and colonize an exposed body soon after death. DEET on the decedent would theoretically extend this inter- val, delaying onset of insect infestations. The duration of maggot infestation on a body is a type of minimum postmortem interval (PMI—period between homicide and finding the victim) and is used to estimate time of death or other parameters in homicide investigations (23,24). The length of the largest maggots then approximates this minimum PMI, because maggots appear to grow in a continuous manner; consequently, maggot length or instar becomes an indicator of maggot age. Temperature, however, regu- lates the rate of growth and development of immature insects. Thus, to determine this minimum PMI, one compares the lengths or instar of the largest maggots collected from a decedent to pub- lished growth rate studies carried out at temperatures prevalent at the scene prior to discovery. Failure to account for a repellent- induced delay in the laying of the first eggs would lead to an underestimation of the minimum PMI with consequences for the investigation (25). If a person should die covered with DEET, as could be expected in a hiker, camper, hunter, or soldier (26), the period prior to maggot infestation would be extended, truncating the estimates of the minimum PMI. In a more general behavioral context, the effects of DEET on blowfly behavior would improve the understanding of how DEET works (27,28) by further testing the theory of DEET repellency. In 1 Department of Entomology, University of California-Davis, 1 Shields Avenue, Davis, CA 95616. *These authors contributed equally to this work. Received 21 April 2011; and in revised form 9 July 2011; accepted 21 Aug. 2011. J Forensic Sci, November 2012, Vol. 57, No. 6 doi: 10.1111/j.1556-4029.2012.02159.x Available online at: onlinelibrary.wiley.com Ó 2012 American Academy of Forensic Sciences 1507