Laboratory evaluation of lactic acid on attraction of Culex spp. (Diptera: Culicidae)
The role of lactic acid was evaluated for attraction of Culex nigripalpus, Culex quinquefasciatus, Culex tarsalis, and Aedes aegypti in the laboratory using a dual-port olfactometer. When lactic acid was combined with chicken odor, attraction was increased for Cx. quinquefasciatus compared to chicken odor alone but not for Cx. nigripalpus, Cx. tarsalis, and Ae. aegypti. Lactic acid combined with hand odor did not change attraction of Cx. tarsalis and Ae. aegypti but decreased attraction of Cx. nigripalpus and Cx. quinquefasciatus. The addition of lactic acid to CO2 increased attraction of Ae. aegypti and Cx. quinquefasciatus but reduced attraction of Cx. nigripalpus and Cx. tarsalis. Use of commercial lactic acid baits with CO2 resulted in a similar trend except for Cx. nigripalpus which showed no difference. A blend of lactic acid, acetone, and dimethyl disulfide was attractive to Ae. aegypti (63.4%) but elicited low responses by all Culex spp. (1.3–26.8%). Addition of the blend to CO2 increased attraction of Ae. aegypti and Cx. quinquefasciatus but reduced attraction of Cx. nigripalpus and Cx. tarsalis. The mixture of compounds plus CO2 was as attractive as a hand for Cx. quinquefasciatus, Cx. tarsalis, and Ae. aegypti.
Host-finding by mosquitoes is a critical component of survival for most species and much research has focused on the cues used for host location. Of the many cues involved, host odor is generally considered the most important. Although mosquitoes feed on a broad range of hosts, humans have received the most attention. Considerable research has been conducted on odors produced by humans that elicit attraction in mosquitoes, with emphasis on compounds from sweat and skin (Smith et al. 1970, Eiras and Jepson 1991, Geier et al. 1996, Braks et al. 1999, 2001, Bernier et al. 1999, 2000, Constanini et al. 2001, Qiu et al. 2004, Okumu et al. 2010). Responses to these odors appear to be enhanced by CO2, which is a universally present emission from vertebrates. Carbon dioxide is responsible for increased flight activity, attraction in some species, and sensitization of mosquitoes to host odors (Gillies 1980, Clements 1999, Dekker et al. 2005). The attractant activity of human sweat was initially associated with lactic acid (Acree et al. 1968). Smith et al. (1970) reported the enhanced attraction of mosquitoes to sweat in conjunction with CO2 and concluded that other components of human odor may be involved as well. Subsequently, the role of lactic acid as a component of host attraction for Aedes aegypti (L.) and Anopheles gambiae Giles has been examined in detail (Acree et al. 1968, Schreck et al. 1990, Steib et al. 2001, Braks et al. 2001, Dekker et al. 2002, Bernier et al. 2003) and appears to be responsible for synergism with other compounds (Smallegange et al. 2005).
Other human-associated odors important in host location include acetone for attraction of Ae. aegypti, An. gambiae, Anopheles stephensi Liston (Takken et al. 1997, Bernier et al. 2003), dimethyl disulfide and dichloromethane for Ae. aegypti (Bernier et al. 2003), ammonia for Ae. aegypti (Geier et al. 1999) and An. gambiae (Braks et al. 2001), and other aliphatic carboxylic acids for An. gambiae (Constantini et al. 2001, Smallegange et al. 2009, Smallegange et al. 2010) and Culex quinquefasciatus Say (Puri et al. 2006). Electrophysiological and olfactometer responses to several carboxylic acids, alcohols, and aldehydes associated with human skin have been reported recently for Culex quinquefasciatus Say (Puri et al. 2006). While these compounds are most associated with humans in these studies, they are generally not unique to humans and may play a role in attraction to other hosts.
Previous studies associated high levels of lactic acid on human skin with strong attraction of anthropophilic species such as An. gambiae (Acree et al. 1968, Smith et al. 1970, Dekker et al. 2002) and Ae. aegypti (Acree et al. 1968, Smith et al. 1970). In contrast, birds, which are fed on readily by many Culex spp., may have relatively low levels of lactic acid in their emanations (Dekker et al. 2005). Lactic acid has been used to experimentally manipulate the attraction of anthropophilic species and, with the addition of lactic acid to host odor, results in increased attraction by An. gambiae (Dekker et al. 2002) and Ae. aegypti (Steib et al. 2001). In addition to its role as a host attractant, lactic acid also appears to contribute to host specificity for tsetse. The application of lactic acid onto an ox reduced attraction and feeding of the zoophilic species, Glossina morsitans morsitans Westw. and Glossina pallidipes Aust. (Vale 1979). Relatively little is known about the role of lactic acid and other human-associated compounds on attraction of Culex mosquitoes. The objective of this study was to determine if lactic acid and other human-associated chemicals influence attraction of several Culex species that differ in their attraction to humans and to compare their responses to Aedes aegypti.
MATERIALS AND METHODS
All species for this study were reared in the laboratory using methods described by Gerberg et al. (1994). Species in this study included: Culex quinquefasciatus (Gainesville, FL, established 1995), Culex nigripalpus Theobald (Vero Beach, FL, established 1999), Culex tarsalis Coquillett (Coachella Valley, CA, established 2001), and Ae. aegypti (Orlando, FL, established 1952). All of these Culex spp. feed readily on chickens yet differ in their attraction to humans (Allan et al. 2006a). Adults were maintained in screen cages with 5% sugar solutions provided continuously. Cages were held at 27–29° C and 70–85% RH under a photoperiod of 14:10 (L:D) with the onset of darkness at 20:00 for Ae. aegypti and at 11:00 for Culex spp. For bioassays, unfed 7- to 14-day-old female mosquitoes were used.
Mosquito responses were evaluated in a triple-cage dual-port olfactometer (Posey et al. 1998) which consisted of a large clear acrylic chamber leading to two circular ports upwind of the chamber. Three chambers with corresponding test ports were stacked on each other with only one chamber used for assays at a time. External air was charcoal-filtered, humidified and warmed (27 + 1° C, 60 + 2% RH) and flowed through the ports at 28 + 1 cm/s. Air flow through the ports was initiated by opening a door and at the end of a test, the door was closed, trapping mosquitoes in the ports where they could be counted. During a test, mosquitoes in the test chamber could follow an upwind air current to the treatment test port, to the control test port or remain in the chamber. Each chamber was loaded with 50–70 females collected from stock cages into release chambers using a draw box (Posey and Schreck 1981) that selectively collected active and responsive females. Mosquitoes were allowed to acclimate for ∼1 h before testing. Responses were calculated as the percentage of total mosquitoes tested that were trapped in the treatment port or the control port. Treatments and controls were randomly assigned to the left or right ports. All materials placed in the treatment or control ports for testing were handled with gloves to avoid contamination with skin compounds. Each day, mosquitoes from each stock cage used were tested for responsiveness using a hand or CO2 (5 ml/min) in preliminary olfactometer assays. If responses were below a pre-set criterion, assays were not conducted. A test consisted of placing treatment and control materials in the respective ports, opening the door to allow air flow over the materials into the test chamber, and closing the door at the end of the test. Assays consisted of 6 to 12 replicates and tests lasted for 10 min. Assays with Ae. aegypti were conducted under high light conditions (2,220–2,400 lux) between 10:00 and 15:00. Assays with Culex were conducted under low light conditions (100–150 lux) within 4 h after the onset of scotophase.
Tests were initially conducted comparing mosquito responses to a chicken (Gallus gallus domesticus L.) or human hand alone or in conjunction with L-lactic acid. For these tests, a treatment (chicken, hand, lactic acid, or combination of lactic acid and chicken or lactic acid and hand) was presented in one port and the other port was a blank control. To determine if lactic acid would negatively affect attraction to a chicken, a study was conducted using lactic acid, a chicken, or lactic acid in combination with a chicken as treatments. An unrestrained chicken was placed in an acrylic box (30.5 ×; 17.8 ×; 15.2 cm) with an average flow of 5 cm/s of air filtered, humidified, and warmed (Allan et al. 2006a). The chicken was allowed to settle (∼5 min) before the test was initiated. Chickens were handled with gloves to reduce contamination with skin oils. Attraction to a human hand was evaluated by placing a hand through the iris diaphragm of the olfactometer into an olfactometer port. Hands were not washed within an hour of the test and contact with laboratory chemicals avoided. Care was taken so that hands did not contact the interior sides of the olfactometer to avoid contamination with skin compounds. Due to inherent differences in mosquito attraction to people (Steib et al. 2001, Qiu et al. 2006), only one individual was involved in the hand assays. Treatments using the combination of a hand or a chicken with lactic acid were tested as above except that 1 g of lactic acid α(technical grade, 98% purity, Sigma-Aldrich Chemical, St. Louis, MO) was placed in a watch glass (5 cm diameter) downwind of the hand or chicken in the treatment port.
The effect of lactic acid on attraction to Culex spp. was further examined by examining responses to CO2 alone, lactic acid alone, and lactic acid in combination with CO2 to determine if responses to CO2 (5 ml/min) increased in the presence of lactic acid. For these tests, evaluations were made using technical grade lactic acid or commercial formulations of slow-release lactic acid. These products included Lurex™ (American Biophysics Corporation, North Frenchtown, RI) that releases ∼0.23 g/day from a gel matrix (K. McKenzie, personal communication) and Insectagator™ (ICA, Trinova, Forest Park, GA) that consists of 5% lactic acid (w/w) in a powder. Treatments consisted of 1 g of lactic acid, one Lurex lure, or 1 gm Insectagator powder. Emission of lactic acid from the Insectagator formulation was 9.36 ± 0.04 mg/10 min trial compared to an estimated 1.59 µg/10 min from the Lurex bait and 1.38 ± 0.45 mg for technical grade lactic acid.
Attraction of these Culex spp. and Ae. aegypti females was evaluated using a lactic acid-containing mixture of three human-derived compounds previously shown to be effective in eliciting strong attraction of Ae. aegypti in the absence of CO2 (Bernier et al. 2001, Silva et al. 2005). The mixture consisted of 480 ml acetone, 0.96 g lactic acid, and 10 ml of dimethyl disulfide, with 500 μl of the mixture applied to a watch glass immediately prior to each test. Mosquito responses were examined to the mixture alone, to CO2, and to a combination of CO2 and the mixture. Additionally, a human hand was added to the comparison to determine if these attractants were as attractive to Culex as a hand.
Attraction to the individual volatile compounds present in the lactic acid mixture was compared among the three Culex species. These included: acetone (98%), dimethyl disulfide (> 99.5%) (Sigma-Aldrich Chemical, St. Louis, MO) and lactic acid. Individual test compounds were placed in the treatment port of the olfactometer in vial caps (9 mm i.d. × 9 mm height) and compared against untreated vial caps in the control port. Compounds differed in volatility and volumes of treatments consisted of lactic acid (1 g), dimethyl disulfide (200 µl), and acetone (1 ml) so that compounds would be present for the entire assay duration. Emission rates of compounds based on reduced weights of three samples placed in the air flow of the olfactometer over a 10 min period were determined for dimethyl disulfide (273.15 ± 5.55 mg), acetone (0.65 ± 0.01 mg), and lactic acid (1.38 ± 0.45 mg).
Responses between treatments and controls and between treatments were compared in the olfactometer by a paired t-test (P < 0.05). Before analysis, data were arcsine transformed and pre-transformation means are presented in tables.
Addition of lactic acid to the air stream containing chicken odors did not significantly decrease attraction of any of the Culex spp. to the chicken (Table 1). Responses to the chicken and to the chicken + lactic acid treatments did not differ significantly for Ae. aegypti, (t= 1.00; df = 22; P= 0.16), Cx. nigripalpus (t= 0.80; df = 22; P= 0.43), or Cx. tarsalis (t= 1.43; df = 22; P= 0.08). Responses of Cx. quinquefasciatus showed a significant increase with the addition of lactic acid to the chicken odor (t= 2.83; df = 22; P= 0.01). Addition of lactic acid to the air stream containing a hand in the olfactometer also did not affect responses of Ae. aegypti (t= 0.94; df = 22; P= 0.18) or Cx. tarsalis (t= 0.43; df = 22; P= 0.33) (Table 1). Responses of both Cx. nigripalpus (t= 0.43; df = 22; P= 0.33) and Cx. quinquefasciatus (t= 2.19; df = 22; P= 0.05) to a hand were significantly lower when lactic acid was added to the air stream.
Table 1. Effect of lactic acid on attraction to a chicken or hand by Culex and Ae. aegypti.
| Chicken alone||85.3 (1.4)||85.4 (2.4)||77.8 (2.5)||96.1 (2.4)|
|Chicken with lactic acid||87.0 (3.0)||93.3 (1.2)*||72.7 (0.6)||91.5 (2.8)|
| Hand alone||38.7 (2.1)||55.0 (1.8)||8.6 (1.7)||95.2 (1.8)|
|Hand with lactic acid||21.1 (1.2)*||31.7 (3.4)*||7.2 (1.3)||92.6 (1.8)|
The addition of lactic acid, either technical grade or as a slow-release commercial formulation, affected responses to CO2 (Table 2). Responses to lactic acid alone were low for all species, with greatest responses by Ae. aegypti (19.3%) and lower responses by all three Culex spp. (0.3 – 6.7%). Responses to CO2 alone ranged from 19.1% (Ae. aegypti) to 50.9% (Cx. tarsalis). When lactic acid was combined with CO2, responses significantly greater than to CO2 alone were observed by both Ae. aegypti (t= 8.76; df = 22; P < 0.0001) and Cx. quinquefasciatus (t= 1.74; df = 22; P= 0.05). In contrast, responses to the combination of lactic acid and CO2 were significantly lower than to CO2 alone for Cx. nigripalpus (t= 2.75; df = 22; P= 0.012) and Cx. tarsalis (t= 2.38; df = 22; P= 0.022).
Table 2. Effect of technical grade and commercial lactic acid treatments on attraction to CO2 (5 ml/min) of Culex and Ae. aegypti.
| CO2||28.3 (1.9)||25.7 (3.5)||50.9 (4.8)||19.1 (2.5)|
|Technical grade lactic acid|
| Alone||0.3 (0.2)||6.7 (0.7)||1.7 (0.6)||19.3 (2.9)|
| With CO2||18.9 (2.5)*||34.7 (2.0)*||30.0 (3.1)*||73.2 (5.2)*|
|Commercial lactic acid product|
| Lurex alone||0.7 (0.4)||6.5 (2.3)||0.6 (0.6)||2.7 (1.2)|
| Lurex + CO2||21.6 (4.2)||45.4 (4.8)*||14.6 (0.8)*||53.2 (13.7)*|
| Insectagator alone||4.8 (1.9)||4.1 (0.8)||0.0 (0.0)||4.6 (1.2)|
| Insectagator + CO2||34.1 (3.3)||57.6 (5.8)*||32.6 (2.8)*||57.6 (11.5)*|
Commercial formulations of slow–release lactic acid (Lurex, Insectagator) alone attracted relatively few mosquitoes in the olfactometer (0.0 – 6.5%) (Table 2). Compared to CO2 alone, the addition of the Lurex lure resulted in significant increases in attraction of Ae. aegypti (t=− 2.19; df = 22; P= 0.03) and Cx. quinquefasciatus (t=– 2.81; df = 22; P= 0.005) but a significant decrease in attraction of Cx. tarsalis (t= 4.89; df = 22; P= 0.002). Addition of the Lurex lure did not change attraction of Cx. nigripalpus (t= 1.42; df = 22; P= 0.08). Compared to CO2 alone, the addition of the Insectagator lure resulted in increased attraction of Ae. aegypti (t=−3.69; df = 22; P= 0.008) and Cx. quinquefasciatus (t=−4.20; df = 22; P < 0.0001) but no difference in attraction of Cx. nigripalpus (t=−1.32; df = 22; P= 0.11). Response of Cx. tarsalis (t= 9.96; df = 22; P < 0.0001) decreased significantly with the addition of the lure.
Response to the lactic acid-containing mixture of compounds was high in Ae. aegypti, moderate in Cx. quinquefasciatus and Cx. nigripalpus, and low in Cx. tarsalis (Table 3). Attraction to CO2 was increased significantly in the presence of the lactic acid mixture for Ae. aegypti (∼four-fold) and Cx. quinquefasciatus (∼two-fold). For Cx. nigripalpus and Cx. tarsalis, however, attraction was decreased about 40%. Addition of CO2 to the lactic acid mixture increased attraction above that to the lactic acid mixture for Ae. aegypti and Cx. quinquefasciatus, decreased attraction of Cx. nigripalpus, with no change for attraction of Cx. nigripalpus.
Table 3. Effect of a lactic acid mixture attractive to Ae. aegypti on attraction of Culex to CO2.
|CO2||28.1 (2.2)||35.1 (4.4)||55.2 (4.2)||17.1 (2.4)|
|Lactic acid mix||17.9 (2.3)||26.8 (4.9)||1.3 (1.3)||63.4 (4.4)|
|Lactic acid mix + CO2||15.7 (1.2)||64.6 (2.9)||15.6 (4.8)||87.9 (1.5)|
|Hand||30.5 (4.3)||55.2 (5.7)||5.2 (1.5)||91.3 (3.2)|
| ||Values of t for comparisons|
|CO2 vs LA mix + CO2||3.58**||−3.90**||3.26**||23.33**|
|LA mix vs LA mix + CO2||−0.72ns1||7.80**||−2.38*||5.35**|
|Hand vs LA mix + CO2||−2.77*||1.10ns||1.6ns||−2.70ns|
|LA alone2 vs LA mix||−7.04**||−5.60**||0.50ns||7.20**|
The lactic acid mixture + CO2 was as attractive as a hand for Ae. aegypti and Cx. quinquefasciatus, less than to the hand for Cx. nigripalpus, and greater than to a hand for Cx. tarsalis (Table 3). Attraction to the lactic acid mixture was greater than to lactic acid alone (Table 3) for Cx. nigripalpus, Cx. quinquefasciatus, and Ae. aegypti but not for Cx. tarsalis.
Responses of Ae. aegypti differed from those of the three Culex spp. for the individual human-associated chemicals present in the lactic acid mixture (Table 4). Aedes aegypti was the only species that was more attracted to acetone and lactic acid than the corresponding untreated controls (P < 0.05). Dimethyl disulfide elicited significant attraction in both Ae. aegypti and Cx. quinquefasciatus (P < 0.05).
Table 4. Comparison of attraction of three species of Culex and Ae. aegypti to odors associated with humans.
|Acetone||1.3 (0.3)||1.3 (0.4)||0.6 (0.2)||28.8 (4.3)*|
|Dimethyl disulfide||1.3 (0.7)||24.8 (3.6)*||2.0 (2.0)||9.5 (1.7)*|
|Lactic acid||1.6 (0.3)||4.9 (1.3)||1.3 (0.8)||18.2 (2.9)*|
Lactic acid, in conjunction with other compounds such as host odors or CO2, influenced attraction in the three Culex species tested. In Cx. quinquefasciatus, lactic acid played a role similar to that played in Ae. aegypti, however, responses by Cx. quinquefasciatus were lower (6.7%) than those by Ae. aegypti (19.3%). For species such as Cx. nigripalpus and Cx. tarsalis, lactic acid was not an attractant (0.3–1.7%) and significantly reduced attraction to CO2 and to a hand. Responses to the chicken, however, were not decreased with the addition of lactic acid for Cx. nigripalpus and Cx. tarsalis, possibly due to the high levels of attractants emitted from the chicken. Previously, Allan et al. (2006a) compared the same three Culex species and Ae. aegypti to a human hand and to a chicken. All species responded similarly to the chicken, whereas attraction to the hand was very strong for Ae. aegypti, moderately low for Cx. quinquefasciatus, and very low for Cx. nigripalpus and Cx. tarsalis. In our study, the response of Cx. tarsalis was not reduced with addition of lactic acid to a hand, however, the overall response to a hand was very low (<10%) and the lack of attraction to human odors is in agreement with previous research on this species (McIver 1968, Allan et al. 2006a). Culex tarsalis, however, does appear to be strongly attracted by CO2 (McIver 1968, Allan et al. 2006b) and the addition of lactic acid, either alone or as in the mixture of compounds to an air stream containing CO2, reduced attraction in our study. Culex nigripalpus is not considered to respond well to humans as hosts under field (Provost 1969) or laboratory conditions (Allan et al. 2006a). In our study, the addition of lactic acid to the air stream containing a hand or CO2 lowered attraction responses of Cx. nigripalpus. The reduced responses by both Cx. nigripalpus and Cx. tarsalis in the presence of lactic acid indicates that this compound may be involved in the mediation of host selection for these species with lactic acid acting as a deterrent.
Lactic acid alone is generally not considered a good attractant of mosquitoes and it is the combination of this compound with CO2 that results in a synergized attraction in Ae. aegypti (Acree et al. 1968, Smith et al. 1970, Geier et al. 1999) and An. gambiae (Dekker et al. 2002). In landing assays, lactic acid elicited a moderate response in Cx. nigripalpus but not in Cx. quinquefasciatus (Allan et al. 2006a). In the same olfactometer study, only responses of Cx. quinquefasciatus were significantly greater than to controls. In a recent study, Dekker et al. (2005) documented an instant sensitization in Ae. aegypti in the presence of CO2. In the present study, the addition of lactic acid to CO2 enhanced attraction of Cx. quinquefasciatus and Ae. aegypti compared to CO2 alone. This same trend was observed with the commercial slow-release formulations of lactic acid and the lactic acid mixture. The mixture of lactic acid, acetone, and dimethyl disulfide previously reported effective for attraction of Ae. aegypti in the absence of CO2 (Bernier et al. 2001) elicited greater attraction in Cx. quinquefasciatus than to lactic acid. This was possibly due to attraction to other mixture components such as dimethyl disulfide, which was also attractive to this species in the olfactometer. The lactic acid mixture did not elicit attraction in Cx. nigripalpus nor did any of the individual compounds tested.
Commercial formulations of lactic acid appeared to release lactic acid with mosquito responses similar to those to technical grade lactic acid. Responses of mosquitoes to lures in conjunction with CO2 followed a similar pattern as the study with lactic acid, with increased responses by Ae. aegypti and Cx. quinquefasciatus and decreased responses by Cx. tarsalis. These results underscore the fact that attractants available commercially are not universally attractive to all mosquito species, and consideration needs to be made of the target species and their propensity for attraction to the compound(s) released from the lures.
Previous field trials have provided some indication that lactic acid may negatively impact trap collections of Culex. Kline et al. (1990) tested various attractants under field conditions and reported that the addition of lactic acid to CO2-baited CDC traps increased Cx. nigripalpus collections nearly three-fold. Addition of lactic acid to CO2-baited traps, however, appeared to reduce collections of other Culex spp. Hoel et al. (2007) reported that traps baited with lactic acid and CO2 collected fewer Cx. nigripalpus than traps baited with CO2 or octenol and CO2. Stryker and Young (1970) increased collections of only Ae. aegypti when adding lactic acid to traps. The impact of adding lactic acid as an attractant to traps that target Culex quinquefasciatus or Culex tarsalis remains to be examined.
Several studies support a role for lactic acid in host preference. Dekker et al. (2002) measured lactic acid levels from the skin of a numerous hosts including humans and cattle and suggested that the lower levels of lactic acid on bovine skin (9.4 µg/ml) compared to human skin (151 µg/ml) were related to the lower attraction of An. gambiae to bovines compared to humans. Levels of lactic acid detected from skin were proportional to the preference of the anthropophilic species, An. gambiae, for a host species. The host with the least lactic acid was the chicken (<1.0 µg/ml) which was not fed upon by An. gambiae. The preferred host, humans, had the highest levels of lactic acid of all species examined and the less preferred host, cattle, had moderately low levels of lactic acid. By adding lactic acid to cattle skin, attraction of An. gambiae was increased. Geier et al. (1996) removed lactate from human samples and found that none of the other components which contribute to the attractiveness of human skin for Ae. aegypti were effective without the addition of lactic acid. Steib et al. (2001) also reported that increasing levels of lactic acid on human skin or animal odor samples increased attractiveness to Ae. aegypti. Similarly, Vale (1979) reduced collections and feeding of tsetse flies on their preferred host, cattle, by adding lactic acid to the skin.
The three Culex species in this study belong to two of the nine host-feeding categories of Tempelis (1975) based on blood meal analysis. One category includes Cx. quinquefasciatus, which feeds readily on birds and mammals, in proportions possibly based on host abundance. There appear to be geographic differences regarding Cx. quinquefasciatus feeding on humans (Tempelis 1975), with the population from Florida feeding mainly on birds. Another category included both Cx. nigripalpus and Cx. tarsalis, species which switch seasonally from almost exclusively feeding on birds in the spring to mammal-feeding in the fall. Although neither species are considered anthropophilic (Tempelis et al. 1965, Tempelis 1975, Day 2005), they are effective arboviral vectors (Day 2005, Vitek et al. 2008). In our study, Cx. quinquefasciatus responded similarly to the human-feeding Ae. aegypti (Tempelis 1975) in their positive response to lactic acid, which is present in high levels with humans and low levels with birds (chickens). In contrast, Cx. nigripalpus and Cx. tarsalis responded negatively to lactic acid. This information is important in the development of effective surveillance tools for these species. Additional research conducted on the identification and evaluation of other host-associated chemicals may provide the basis for effective attractants for Culex.
We thank Erin Vrzal for technical assistance with this study and Corrine Bopp and Haze Brown for providing mosquitoes. Use of animals in this research was reviewed and approved (projects D207 and D469) by the University of Florida Institutional Animal Care and Use Committee, Gainesville FL. The use of trade, firm, or corporation names in this publication are for the information and convenience of the reader. Such use does not constitute an official endorsement or approval by the United States Department of Agriculture or the Agricultural Research Service of any product or service to the exclusion of others that may be suitable.