Effect of the Repellent Deet on the Antennal Chemoreceptors for Oviposition in Aedes aegypti (Diptera: Culicidae) A. KUTHIALA, 1 2 RAJ K. G U P T A , 1 3 AND E. E. D A V I S 4 5
KEY WORDS
Insecta, Aedes aegypti, deet, oviposition behavior
REPRODUCTIVE SUCCESS IN MOSQUITOES is de-
pendent in part on the ability of the gravid female to locate and select an appropriate oviposition site. Her choice is determined largely by volatile chemical cues that emanate from potential sites. These oviposition-site related substances are detected by chemosensory neurons located on the antennae of the female. Perry & Fay (1967) reported that gravid Aedes aegypti (L.) were attracted to water that contained methyl and ethyl esters of short-chain fatty acids such as ethyl propionate and methyl butyrate. Klowden & Blackmer (1987) demonstrated that ethyl propionate would stimulate "preoviposition" behavior (i.e., attraction to an oviposition site) by gravid females in an olfactometer. Davis (1976) described the electrophysiological responses to these fatty acid esters by chemosensory neurons associated with the antennal sensilla trichodea type 2 (A2-II). Because Mclver (1978) subsequently redescribed the sensilla as comprising both a short sharp-tipped and a short blunt-tipped sensilla trichodea in Ae. aegypti, it Opinions and assertions contained herein are the private views of the authors and are not to be construed as official or as reflecting the views of the U.S. Department of the Army. Use of trade names does not constitute an official approval of the products mentioned. 1 Letterman Army Institute of Research, Presidio San Francisco, Calif. 94129. 2 Current address: 9430 Ryerson St., Windsor, Ontario, Canada N8R 1V5. 3 Current address: Department of Entomology, Walter Reed Army Institute of Research, Washington, D.C. 20307-5100. 4 SRI International, Menlo Park, Calif. 94025. 5 To whom correspondence should be addressed.
is unclear which of these two types of A2-II sensilla contains the receptors that respond to the substances. Although several studies have sought to identify substances of larval, egg, bacterial, fungal, and plant origin from the oviposition sites of various mosquito species that attract gravid females to ovitraps, and a few studies have examined the repellent-deterrent activity of natural products and insecticides (Bentley & Day 1989), it is surprising that no studies on the effects of insect repellents such as deet (N,N-diethyl-mtoluamide) on oviposition behavior have been reported (Rutledge et al. 1978). These substances affect host-seeking and biting behavior of avid female mosquitoes and some (e.g., deet) exert their effects on behavior by rendering the female anosmic to a host and, thus, unable to detect volatile host attractants (Davis 1985, Davis et al. 1987). The question then arises as to whether repellents can similarly deter the oviposition behavior of gravid females. In the current article, we describe the effect of the insect repellent deet on oviposition attraction of gravid female Ae. aegypti and on electrophysiological responses of the short, sharptipped sensilla trichodea (Mclver 1978) that respond to the oviposition attractant, ethyl propionate, in gravid and nongravid females. Methods and Materials Insects. Aedes aegypti (UCSF strain) mosquitoes were reared in the insectary at 27°C, 80% relative humidity, and 16:8 (L:D) photoperiod.
0022-2585/92/0639-0643$02.00/0 © 1992 Entomological Society of America
Downloaded from http://jme.oxfordjournals.org/ by guest on February 19, 2016
J. Med. Entomol. 29(4): 639-643 (1992) ABSTRACT The insect repellent, N,N-diethyl-m-toluamide (deet), interferes with detection of the host attractant, lactic acid (LA), by LA-excited neurons on the antennae of female Aedes aegypti (L.) mosquitoes. In the current laboratory behavioral experiment, the attraction of gravid female mosquitoes to a source of ethyl propionate, an oviposition attractant, was reduced by a factor of two when deet was present. Similarly, when deet was presented together with ethyl propionate in electrophysiological experiments, sensitivity of the neural response to ethyl propionate was depressed in a transient, dose-dependent manner that could account for the reduction in behavioral attraction of gravid females to ethyl propionate. Electrophysiologically, the sensitivity of oviposition attractant receptors to ethyl propionate on gravid females was not different from that on nongravid host-seeking females. The similarity between the depression of sensitivity of the oviposition attractantsensitive neurons induced by deet and that observed in the host attractant-sensitive neurons suggest a common mode of action of deet on peripheral chemoreceptor neurons that mediate these behaviors.
640
Vol. 29, no. 4
JOURNAL OF MEDICAL ENTOMOLOGY
Mosquitoes flying into the upper collection cages in each 4-h interval were collected and expressed as a percentage of the number of mosquitoes in the source cage at the times sampled as before. Electrophysiological Experiments. Inseminated adult female mosquitoes from the two behavioral response categories were selected for the electrophysiological experiments: (1) gravid, oviposition site-seeking (not host-responsive); and (2) nongravid, not oviposition site-seeking (host-responsive). Behavioral responsiveness of the females was determined immediately before using the mosquito in the electrophysiological experiments. The ovaries of females that had taken a blood meal were examined to ensure that they were gravid. Our standard electrophysiological and odorgenerating techniques were used (Davis 1984a). After CO 2 anesthesia, the insect was secured to a brass holder and its antennae were held straight with double-stick cellophane tape. Extracellular action potentials (spikes) of the chemosensory neurons were detected using two uninsulated tungsten microelectrodes (
KEY WORDS
Insecta, Aedes aegypti, deet, oviposition behavior
REPRODUCTIVE SUCCESS IN MOSQUITOES is de-
pendent in part on the ability of the gravid female to locate and select an appropriate oviposition site. Her choice is determined largely by volatile chemical cues that emanate from potential sites. These oviposition-site related substances are detected by chemosensory neurons located on the antennae of the female. Perry & Fay (1967) reported that gravid Aedes aegypti (L.) were attracted to water that contained methyl and ethyl esters of short-chain fatty acids such as ethyl propionate and methyl butyrate. Klowden & Blackmer (1987) demonstrated that ethyl propionate would stimulate "preoviposition" behavior (i.e., attraction to an oviposition site) by gravid females in an olfactometer. Davis (1976) described the electrophysiological responses to these fatty acid esters by chemosensory neurons associated with the antennal sensilla trichodea type 2 (A2-II). Because Mclver (1978) subsequently redescribed the sensilla as comprising both a short sharp-tipped and a short blunt-tipped sensilla trichodea in Ae. aegypti, it Opinions and assertions contained herein are the private views of the authors and are not to be construed as official or as reflecting the views of the U.S. Department of the Army. Use of trade names does not constitute an official approval of the products mentioned. 1 Letterman Army Institute of Research, Presidio San Francisco, Calif. 94129. 2 Current address: 9430 Ryerson St., Windsor, Ontario, Canada N8R 1V5. 3 Current address: Department of Entomology, Walter Reed Army Institute of Research, Washington, D.C. 20307-5100. 4 SRI International, Menlo Park, Calif. 94025. 5 To whom correspondence should be addressed.
is unclear which of these two types of A2-II sensilla contains the receptors that respond to the substances. Although several studies have sought to identify substances of larval, egg, bacterial, fungal, and plant origin from the oviposition sites of various mosquito species that attract gravid females to ovitraps, and a few studies have examined the repellent-deterrent activity of natural products and insecticides (Bentley & Day 1989), it is surprising that no studies on the effects of insect repellents such as deet (N,N-diethyl-mtoluamide) on oviposition behavior have been reported (Rutledge et al. 1978). These substances affect host-seeking and biting behavior of avid female mosquitoes and some (e.g., deet) exert their effects on behavior by rendering the female anosmic to a host and, thus, unable to detect volatile host attractants (Davis 1985, Davis et al. 1987). The question then arises as to whether repellents can similarly deter the oviposition behavior of gravid females. In the current article, we describe the effect of the insect repellent deet on oviposition attraction of gravid female Ae. aegypti and on electrophysiological responses of the short, sharptipped sensilla trichodea (Mclver 1978) that respond to the oviposition attractant, ethyl propionate, in gravid and nongravid females. Methods and Materials Insects. Aedes aegypti (UCSF strain) mosquitoes were reared in the insectary at 27°C, 80% relative humidity, and 16:8 (L:D) photoperiod.
0022-2585/92/0639-0643$02.00/0 © 1992 Entomological Society of America
Downloaded from http://jme.oxfordjournals.org/ by guest on February 19, 2016
J. Med. Entomol. 29(4): 639-643 (1992) ABSTRACT The insect repellent, N,N-diethyl-m-toluamide (deet), interferes with detection of the host attractant, lactic acid (LA), by LA-excited neurons on the antennae of female Aedes aegypti (L.) mosquitoes. In the current laboratory behavioral experiment, the attraction of gravid female mosquitoes to a source of ethyl propionate, an oviposition attractant, was reduced by a factor of two when deet was present. Similarly, when deet was presented together with ethyl propionate in electrophysiological experiments, sensitivity of the neural response to ethyl propionate was depressed in a transient, dose-dependent manner that could account for the reduction in behavioral attraction of gravid females to ethyl propionate. Electrophysiologically, the sensitivity of oviposition attractant receptors to ethyl propionate on gravid females was not different from that on nongravid host-seeking females. The similarity between the depression of sensitivity of the oviposition attractantsensitive neurons induced by deet and that observed in the host attractant-sensitive neurons suggest a common mode of action of deet on peripheral chemoreceptor neurons that mediate these behaviors.
640
Vol. 29, no. 4
JOURNAL OF MEDICAL ENTOMOLOGY
Mosquitoes flying into the upper collection cages in each 4-h interval were collected and expressed as a percentage of the number of mosquitoes in the source cage at the times sampled as before. Electrophysiological Experiments. Inseminated adult female mosquitoes from the two behavioral response categories were selected for the electrophysiological experiments: (1) gravid, oviposition site-seeking (not host-responsive); and (2) nongravid, not oviposition site-seeking (host-responsive). Behavioral responsiveness of the females was determined immediately before using the mosquito in the electrophysiological experiments. The ovaries of females that had taken a blood meal were examined to ensure that they were gravid. Our standard electrophysiological and odorgenerating techniques were used (Davis 1984a). After CO 2 anesthesia, the insect was secured to a brass holder and its antennae were held straight with double-stick cellophane tape. Extracellular action potentials (spikes) of the chemosensory neurons were detected using two uninsulated tungsten microelectrodes (
