Aedes aegypti disregard humidity-related conditions with adequate nutrition.

Weather variations have clear associations with the epidemiology of dengue fever and populations of Aedes aegypti mosquitoes. Data on humidity associations, however, lags with respect to its effect on host-biting, nectar-seeking and survival. This experimental study on Ae. aegypti, sourced from the arid tropics, investigated the effect of low and high relative humidity and diet in relation to host-biting, temporal variations in feeding frequency, and mosquito mortality. In each environmental setting, 10 replicates, containing one male and five female mosquitoes, were challenged with different nutritional sources every six hours over 12 days. Results showed that host-biting did not diminish in low humidity and was six times higher than expected. Sucrose feeding was observed to significantly moderate host-biting and water alone was inadequate for survival. The high host-biting rates help to explain the intensity of dengue epidemics, while the ability of the mosquito to disregard adverse humidity-related conditions helps to explain how dengue epidemics in arid tropical regions can be just as devastating as those in the wet tropics.

Oviposition and olfaction responses of Aedes aegypti mosquitoes to insecticides.

Insecticide applications are not particularly effective on Aedes aegypti mosquitoes which has been attributed to their 'closet' behaviour, or ability to rest in places that remain unexposed to insecticides. Some researchers have suggested that insecticides repel mosquitoes, which would result in less exposure and increased dispersal. If repellence due to insecticides is a fact, acquiring a vector-borne disease, such as dengue, could legitimately be attributed to local vector control efforts and this would lead to restitution claims. This study thus investigated the effect of insecticide presence on mosquito behaviour indirectly via oviposition and directly via olfactory response. In all experiments, oviposition in each insecticide compared to its water and ethanol controls was not significantly different. This indicates that Ae. aegypti mosquitoes are not affected by insecticide presence and that increased dispersal is unlikely to be caused by vector control spraying.

Aedes aegypti disregard humidity-related conditions with adequate nutrition

Weather variations have clear associations with the epidemiology of dengue fever and populations of Aedes aegypti mosquitoes. Data on humidity associations, however, lags with respect to its effect on host-biting, nectar-seeking and survival. This experimental study on Ae. aegypti, sourced from the arid tropics, investigated the effect of low and high relative humidity and diet in relation to host-biting, temporal variations in feeding frequency, and mosquito mortality. In each environmental setting, 10 replicates, containing one male and five female mosquitoes, were challenged with different nutritional sources every six hours over 12 days. Results showed that host-biting did not diminish in low humidity and was six times higher than expected. Sucrose feeding was observed to significantly moderate hostbiting and water alone was inadequate for survival. The high host-biting rates help to explain the intensity of dengue epidemics, while the ability of the mosquito to disregard adverse humidity-related conditions helps to explain how dengue epidemics in arid tropical regions can be just as devastating as those in the wet tropics.

In vitro efficacy of over-the-counter botanical pediculicides against the head louse Pediculus humanus var capitis based on a stringent standard for mortality assessment.

Infestation of the head louse Pediculus humanus var capitis DeGeer (Phthiraptera: Pediculidae) is an important public health problem in Australia, with up to a third of children infested in some primary schools. Insecticide resistance and inadequate attention to the application instructions of topical pediculicides are common reasons for treatment failure. This study evaluated six popular Australian over-the-counter products against head lice, primarily comprised of different botanical extracts, and compared them with permethrin 1% (Quellada) and a non-treatment control in order to assess their in vitro efficacy. We also assessed commonly used criteria for evaluating pediculicide efficacy in vitro. All tested products failed to demonstrate high levels of efficacy with the exception of Tea Tree Gel((R)), which outperformed 1% permethrin. Permethrin had a high level of efficacy, but using stringent criteria 18% of lice were not dead at 3 h, indicating some resistance to Quellada. Commonly used less stringent criteria were shown to overestimate mortality of head lice as a result of the protective phenomenon of stasis or sham death observed in exposed lice that may recover after some time. Using two different levels of stringency resulted in different rankings of efficacy for most products, with the exception of the first ranked product, Tea Tree Gel. Rankings of efficacy also varied over time, even within the different assessment criteria. Government regulatory agencies should require standard in vitro tests using stringent mortality criteria, with an observation period of >or= 6 h, to determine the efficacy of new pediculicides, and only products that cause a minimum mortality rate (e.g. 80%) in head lice collected from the target population should be licensed for sale.

Effect of diet on biting, oviposition, and survival of Aedes aegypti (Diptera: Culicidae).

The effect of adult diet on host biting, sugar probing and water probing patterns, oviposition behavior, and survival of Aedes aegypti (L.) from Charters Towers, Australia, were tested in the laboratory. The 7 diets were as follows: (1) starvation, (2) water, (3) 10% sugar, (4) blood, (5) blood with water supplement, (6) blood with 10% sugar supplement, and (7) blood with 3% sugar supplement. Biting, probing, oviposition, and survival observations were made every 6 h (0600-0800 hours, 1200-1400 hours, 1800-2000 hours, 2400-0200 hours). Biting frequency on the blood with 3% sugar diet (0.26 feeds per mosquito per 6 h) and the blood with 10% sugar diet (0.23 feeds per mosquito per 6 h) was significantly less than on blood (0.51 feeds per mosquito per 6 h) and 10% sugar (0.40 feeds per mosquito per 6 h) alone. Biting frequency was not significantly different between blood with water (0.47 feeds per mosquito per 6 h) and blood alone (0.51 feeds per mosquito per 6 h). Biting and oviposition occurred throughout the day, peaking between 1800 and 2000 hours and between 2000 and 2400 hours, respectively. Biting frequency with a 3% sugar supplement decreased after the 1st oviposition cycle on day 6 compared with unsupplemented biting. The presence of sugar delayed or inhibited oviposition. Females with access to blood with water bit and oviposited concurrently on days 4-6, 8-9, and 11, indicating a 3-d gonotrophic cycle. Survival on blood alone was not significantly lower than survival on sugar and water supplemented diets. Behavior of the Charters Towers strain proved to be significantly influenced by diet, and biting occurred opportunistically without regard for previously observed crepuscular or diurnal rhythms. The biting frequencies observed were the highest yet recorded for this species, which indicates that the vectorial capacity of the Australian Ae. aegypti may be underestimated severely.

Adaptation of Aedes aegypti (Diptera: Culicidae) oviposition behavior in response to humidity and diet.

Effects of humidity and sugar concentration on the fecundity, temporal oviposition patterns and survival of a tropical strain of Aedes aegypti (L.) were investigated. Fecundity was significantly reduced by low humidity, but was not affected by sugar concentration. Low humidity caused a significant decrease in percentage survival after 19 days as compared to high humidity. Oviposition was inhibited by host availability for eight successive days. When access to a host was no longer provided, oviposition continued for 10 days in three to four distinct cycles without additional bloodmeals. Humidity stress and high sugar concentration caused oviposition to be delayed for one to four days, which is the typical duration of extreme low humidity periods in nature. These responses are hypothesized to protect the eggs of ovipositing females against the environmental hardships of periodic humidity stress and lack of hosts, thus enabling the perpetuation of the vector and the diseases it transmits in hot and dry seasons.

Multiple host-feeding and biting persistence of Aedes aegypti.

The effects of host activity, host defensive behaviour and biting persistence on multiple host-feeding by Aedes aegypti (L.) were investigated, in laboratory conditions, on human volunteers. In four different settings, four or five volunteer hosts displayed inactive, mildly active, mildly defensive and highly defensive behaviour. Into each of the four trials, each with 20 replicates, a single mosquito was released for a period of 10 min. In all settings, the median number of hosts sought per mosquito was constant, regardless of host activity and host defensive behaviour. Host defensive behaviour was thus not seen to exert a selective pressure on mosquitoes to abandon their hosts. No relationship was found between biting persistence and the number of hosts sought, indicating that biting persistence does not affect disease transmission. As one of the female hosts was consistently and significantly less attractive to host-seeking mosquitoes than any other volunteer in all four settings, it seems that some individuals are significantly less at risk from mosquito-borne pathogens than others.

Efficacy of carbon dioxide, 1-octen-3-ol, and lactic acid in modified Fay-Prince traps as compared to man-landing catch of Aedes aegypti.

The attractants 1-octen-3-ol and lactic acid significantly decreased catches of Aedes aegypti in Townsville, Australia, by 50% in a controlled laboratory environment and by 100% in the field when compared to carbon dioxide baited bidirectional Fay-Prince trap catches. Evaluation of an omnidirectional alteration on a bidirectional Fay-Prince trap revealed no significant improvement in catch size when compared to both the bidirectional trap and man-landing catch (MLC). Cumulative evening MLC (1730-2000 h) was twice that of the morning MLC (0600-0830 h), which has implications on the precise estimation of the man-biting rate. The MLC sampling method is shown to be a quick, simple, effective and cheap alternative to expensive traps in areas not currently experiencing arbovirus transmission.

The Mossie-Buster: a hose-driven insecticide delivery tool for the control of container-breeding mosquitoes.

A novel insecticide delivery tool, the Mossie-Buster, was recently developed to control larval populations from urban breeding sites in Townsville, Australia. This functional and user-friendly control device directly targets the main breeding sites of Aedes aegypti (L.) with a focused delivery of insecticides. The Mossie-Buster comprises a commercially available device and attachments that mix an insecticide solution into the flow of water emitted from a hose. Attached to the device is a trigger for controlled insecticide release. Preliminary laboratory and field trials demonstrated the tool to be effective in eliminating all Ae. aegypti present in various typical breeding containers in different environmental conditions for a minimum of 2 wk in exposed areas to 3 months in an unexposed area.

The gecko: an environmentally friendly biological agent for mosquito control.

Laboratory experiments with Aedes aegypti mosquitoes investigated the effects of light, mosquito density and physiological state on predation rates by the Australian gecko Gehydra dubia and the exotic Asian house gecko Hemidactylus frenatus. For both gecko species a positive correlation was demonstrated between prey density and the predation rate. Using Ae.aegypti males and unfed females as prey in a terrarium (0.054 m3), consumption rates reached 76-108/day for G.dubia and 63-109/day for H.frenatus, with significantly more female mosquitoes than males being eaten in most experiments. Comparing dark with semi-illuminated conditions no consistent contrast of predation rate was demonstrated. Gehyra dubia predation rates on various Australian mosquito species were compared in an experimental room (32 m3) for 24 h exposure with photoperiod L:D 12:12 h Five photophilic species (Aedes vigilax, Anopheles annulipes, Coquillettidia xanthogaster, Culex annulirostris, Cx sitiens) suffered 78-100% predation, compared with only 33-53% predation of four non-photophilic species: Aedes aegypti, Ae.notoscriptus, Ae.vittiger and Cx quinquefasciatus. This demonstrates the potential benefit of domestication for geckoes that learn to hunt at light. When offered a mixture of unfed, freshly blood-fed and gravid females of Ae.aegypti in an illuminated terrarium, both gecko species consumed significantly more unfed than fed or gravid female mosquitoes, presumably because the latter rested whereas the former foraged more actively. H.frenatus consumed significantly more mosquitoes of all categories than did G.dubia per 24 h: unfeds 5.1 +/- 0.1% v. 4.5 +/- 0.5%, blood-feds 4.5 +/- 0.3% v. 4.0 +/- 0.5% and gravids 4.8 +/- 0.4% v. 3.9 +/- 0.5%. Possible relevance of these results to competitive displacement of G.dubia by H.frenatus is discussed.