Abstract
The insect repellent N,N-diethyl-3-methylbenzamide (DEET), is a multimodal compound that acts as a spatial repellent as well as an irritant (contact repellent), thus being perceived by the insect’s olfactory and gustatory systems as an odorant and a tastant, respectively. Soon after DEET was developed, almost 6 decades ago, it was reported that it reduced mosquito feeding on blood mixed with this repellent. It is now known that the mosquito proboscis senses contact repellents with the tips (labella) of the labium, which remain in direct contact with the outer layers of the skin, while the stylets, including the feeding deterrent sensor (labrum), penetrate the skin. We designed a behavioral assay that allowed us to measure feeding deterrence without complications from contact or spatial repellency. Using the southern house mosquito, Culex quinquefasciatus, we demonstrate here that when DEET was mixed with blood and covered by Parafilm® layers, the mean number of landings and duration of contacts with surfaces covering blood mixed with DEET or blood plus solvent (dimethyl sulfoxide) did not differ significantly thus implying that DEET did not leak to the outer surface. The feeding times, however, were significantly different. When blood was mixed either with 0.1 or 1% DEET, female southern house mosquitoes spent significantly (P<0.0001) less time feeding than the time spent feeding on blood mixed only with the solvent. By contrast, significant differences in the mean times of feeding on blood containing 1% picaridin and blood plus solvent were significant at 5%, but not at 1% level. Like DEET, the contact repellent and insecticide, permethrin, caused a significant (P<0.0001) reduction in feeding time. We, therefore, concluded, that in this context, DEET, permethrin, and, to a lesser extent, picaridin, act as feeding deterrents.
Highlights
Chemicals used to reduce mosquito bites are repellents sensu stricto, ie, compounds that cause the responder to steer away from the source, but are excitorepellents or irritants, ie, chemicals eliciting increased locomotor activity after an insect makes contact with the source [1]
With a modified version of the surface landing and feeding assay [12], we were able to demonstrate that reduced feeding on blood spiked with DEET was due to a deterrent rather than contact repellency effect
DEET did not leak and, contact repellency was not at play. This is demonstrated by the fact that mosquitoes landed randomly on the various surfaces of the arena (S1 Video) and that the number and duration of the landings on the surface covering blood spiked with DEET did not differ from the similar data recorded for the side covering blood with solvent only (Fig 2)
Summary
Chemicals used to reduce mosquito bites are repellents sensu stricto, ie, compounds that cause the responder to steer away from the source, but are excitorepellents or irritants, ie, chemicals eliciting increased locomotor activity after an insect makes contact with the source [1]. From a strict mechanistic viewpoint, these 2 groups should be named noncontact and contact disengagents, respectively [2]. From a more pragmatic perspective, the end result is the same, ie, mosquitoes are kept at bay by sensing odorants in the vapor phase (spatial repellents) and/or by detecting non-volatile tastants (contact repellents) upon direct contact with these chemicals (on a skin surface, for example) [3].
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