Laboratory studies on the olfactory behaviour of Anopheles quadriannulatus

The host preference of Anopheles quadriannulatus Theobald (Diptera: Culicidae), the zoophilic member of the malaria mosquito complex Anopheles gambiae Giles, was investigated in a dual‐choice olfactometer. Naïve female mosquitoes were exposed to CO₂, acetone, 1‐octen‐3‐ol, and skin emanations from cows and humans in various combinations. Their behavioural responses were recorded when they had entered one of either upwind traps from where the odours were being released. The mosquitoes did not respond to CO₂ when released at human or cattle equivalent concentrations. Too few mosquitoes responded to acetone to allow for a statistical analysis. The combination of CO₂+ 1‐octen‐3‐ol was repellent. Cow odour alone was slightly attractive, but there was a synergistic attractive effect of cow odour + CO₂. Surprisingly, the mosquitoes were attracted to human odour, and in a choice situation human odour was selected above cow odour + CO₂. Anthropophilic An. gambiae Giles s.s. was repelled by cow odour + CO₂ in contrast to An. quadriannulatus. In a choice situation, both mosquito species selected human odour above cow odour + CO₂. The implications of these results are discussed in the light of recent behavioural data from the field.     

Structure of host-odour plumes influences catch of Anopheles gambiae s.s. and Aedes aegypti in a dual-choice olfactometer

The effects of structure, concentration and composition of host‐odour plumes on catch of female Anopheles gambiae Giles sensu stricto and Aedes aegypti (L.) (Diptera: Culicidae) were investigated in a dual‐choice olfactometer. We demonstrate that the fine‐scale structure of host‐odour plumes modulates capture of An. gambiae and Ae. aegypti. In both species homogeneous skin‐odour plumes result in trap entry, whereas homogeneous CO₂ plumes reduce trap catch. Reduced trap catch also result from combining skin odour with a homogeneous CO₂ plume. Trap capture rates in homogeneous CO₂ plumes were concentration‐dependent and differed between the two species. Electric nets placed in front of the trap entrances intercepted mosquitoes before they could enter the traps. This showed that An. gambiae flew along CO₂ plumes, but did not enter the traps. Survivorship analysis of the trap‐entry times of Ae. aegypti indicated interactions between the time until capture and treatment. The assay's duration therefore can alter the distribution in a dual‐choice olfactometer.     

Behavioural responses of Ixodes ricinus nymphs to carbon dioxide and rodent odour

Many haematophagous ectoparasites use carbon dioxide (CO₂) and host odour to detect and locate their hosts. The tick Ixodes ricinus (Linnaeus) (Ixodida: Ixodidae) walks only small distances and quests in vegetation until it encounters a host. The differential effects of CO₂ and host odour on the host‐finding behaviour of I. ricinus have, however, never been clarified and hence represent the subject of this study. The effects of CO₂ and odour from bank voles on the activation and attraction of I. ricinus nymphs were analysed in a Y‐tube olfactometer. Carbon dioxide evoked a response in the absence and presence of host odour, but did not attract nymphs. Host odour, however, did not evoke a response but did attract nymphs in the absence and presence of CO₂. The current results show that CO₂ is an activator, but not an attractant, and that host odour is an attractant, but not an activator, of I. ricinus nymphs, and provide ecological insights into the host‐finding behaviour of I. ricinus.     

Flower and fruit volatiles assist host‐plant location in the Tomato fruit fly Neoceratitis cyanescens

Mature females of the tomato fruit fly Neoceratitis cyanescens can detect host fruit at a short distance using only visual stimuli, but little is known about the role of airborne volatile cues in the host searching strategy. A series of experiments is conducted in a laboratory wind tunnel, in which the behavioural responses of individual flies to volatiles from Solanaceae host plants (including tomato Lycopersicum esculentum Mill., bug weed Solanum mauritianum Scop. and Turkey berry Solanum torvum Sw.) are observed, according to some environmental (air speed) and physiological (age and mating status of females, time of day) factors. Mature females respond primarily to specific olfactory cues from blends of flowers or host fruit, preferentially unripe fruit for bug weed, as opposed to ripe fruit for Turkey berry or tomato. Males are also highly attracted by the odour of unripe fruit of bug weed. Wind plays a key role, as shown by the proportion of flies that reach the upwind section of the tunnel in the presence of both fruit odour and air flow (66.7%) and in the absence of either fruit odour (13.3%) or wind (36.7%). In response to fruit volatiles carried by wind, flies embark in a ‘plume tracking’ or ‘aim and shoot' flight, consistent with odour‐conditioned anemotaxis. Females respond to host fruit odour regardless of their age, egg load or mating status, and also more consistently in the afternoon, which is their preferential time of day for egg‐laying. Searching behaviour and response to host volatiles in N. cyanescens are discussed in the light of host‐finding and an adaptive strategy.     

Variation in host preferences of malaria mosquitoes is mediated by skin bacterial volatiles

The host preferences of the anthropophilic mosquito species in the Anopheles gambiae complex (Diptera: Culicidae) are mediated by skin bacterial volatiles. However, it is not known whether these mosquitoes respond differentially to skin bacterial volatiles from non‐human host species. In this study, the responses of two malaria mosquito species in the An. gambiae complex, Anopheles gambiae s.s. (hereafter, An. gambiae) and Anopheles arabiensis, with different host preferences, to volatiles released from skin bacteria were tested. Skin bacteria collected from human, cow and chicken skin significantly increased trap catches; traps containing bacteria collected from human skin caught the highest proportions of An. gambiae and An. arabiensis. Traps with bacteria of human origin caught a significantly higher proportion of An. gambiae than of An. arabiensis, whereas bacterial volatiles from the chicken attracted significantly higher numbers of An. arabiensis than of An. gambiae. Additionally, An. gambiae showed a specialized response to volatiles from four specific bacteria, whereas An. arabiensis responded equally to all species of bacteria tested. Skin bacterial volatiles may therefore play important roles in guiding mosquitoes with different host preferences. The identification of these bacterial volatiles can contribute to the development of an odour blend that attracts mosquitoes with different host preferences.     

Odour‐mediated orientation behaviour of the lily beetle Lilioceris lilii

The lily beetle Lilioceris lilii (Scopoli) (Coleoptera: Chrysomelidae) feeds on Lilium, Fritillaria and Cardiocrinum plants and is a serious pest in gardens and amenity plantings in parts of Northern Europe and North America. In the present study, the odour‐mediated behaviour of L. lilii is investigated by behavioural bioassays using a linear‐track olfactometer. The behavioural responses of L. lilii to hosts and conspecifics are, at least in part, odour‐mediated and the responses differ with respect to the physiological (reproductive) state of the adult beetle (i.e. pre‐ or post‐diapause). Significantly more diapaused female L. lilii move into air streams containing the odour of intact host plants than into clean air, and move into air streams containing odour of host plants and beetles combined in preference to odour from manually‐damaged host plants. Diapaused females also move into air streams containing odours from intact plants over those from larval‐infested plants. Pre‐diapause males move into the air streams of intact plants rather than L. lilii‐infested plants. Pre‐diapause females show no significant response in any experiment. The data indicate that the odour‐mediated responses of L. lilii are consistent with those known for other chrysomelids that produce a male aggregation pheromone to which reproductive individuals of both sexes respond.     

Questing activity in bed bug populations: male and female responses to host signals

A large‐arena bioassay is used to examine sex differences in spatiotemporal patterns of bed bug Cimex lectularius L. behavioural responses to either a human host or CO₂ gas. After release in the centre of the arena, 90% of newly‐fed bed bugs move to hiding places in the corners within 24 h. They require 3 days to settle down completely in the arena, with generally low activity levels and the absence of responses to human stimuli for 5 days. After 8–9 days, persistent responses can be recorded. Sex differences are observed, in which females are more active during establishment, respond faster after feeding, expose themselves more than males during the daytime, and respond more strongly to the host signal. The number of bed bugs that rest in harbourages is found to vary significantly according to light setting and sex. Both sexes stay inside harbourages more in daylight compared with night, and males hide more than females during the daytime but not during the night. The spatial distribution of the bed bugs is also found to change with the presence of CO₂, and peak aggregation around the odour source is observed after 24 min. Both male and female bed bugs move from hiding places or the border of the arena toward the centre where CO₂ is released. Peak responses are always highest during the night. Bed bug behaviour and behaviour‐regulating features are discussed in the context of control methods.     

Application of a reverse dot blot DNA–DNA hydridization method to quantify host‐feeding tendencies of two sibling species in the Anopheles gambiae complex

A DNA–DNA hybridization method, reverse dot blot analysis (RDBA), was used to identify Anopheles gambiae s.s. and Anopheles arabiensis (Diptera: Culicidae) hosts. Of 299 blood‐fed and semi‐gravid An. gambiae s.l. collected from Kisian, Kenya, 244 individuals were identifiable to species; of these, 69.5% were An. arabiensis and 29.5% were An. gambiae s.s. Host identifications with RDBA were comparable with those of conventional polymerase chain reaction (PCR) followed by direct sequencing of amplicons of the vertebrate mitochondrial cytochrome b gene. Of the 174 amplicon‐producing samples used to compare these two methods, 147 were identifiable by direct sequencing and 139 of these were identifiable by RDBA. Anopheles arabiensis bloodmeals were mostly (94.6%) bovine in origin, whereas An. gambiae s.s. fed upon humans more than 91.8% of the time. Tests by RDBA detected that two of 112 An. arabiensis contained blood from more than one host species, whereas PCR and direct sequencing did not. Recent use of insecticide‐treated bednets in Kisian is likely to have caused the shift in the dominant vector species from An. gambiae s.s. to An. arabiensis. Reverse dot blot analysis provides an opportunity to study changes in host‐feeding by members of the An. gambiae complex in response to the broadening distribution of vector control measures targeting host‐selection behaviours.     

Close encounters: contributions of carbon dioxide and human skin odour to finding and landing on a host in Aedes aegypti

In a wind‐tunnel study, the upwind flight and source location of female Aedes aegypti to plumes of carbon dioxide (CO₂) gas and odour from human feet is tested. Both odour sources are presented singly and in combination. Flight upwind along the plumes is evident for both CO₂ and odour from human feet when the odours are presented alone. Similarly, both odour sources are located by more than 70% of mosquitoes in less than 3 min. When both CO₂ and odour from human feet are presented simultaneously in two different choice tests (with plumes superimposed or with plumes separated), there is no evidence that females orientate along the plume of CO₂ and only a few mosquitoes locate its source. Rather, the foot odour plume is navigated and the source of foot odour is located by over 80% of female Ae. aegypti. When a female is presented a plume of CO₂ within a broad plume of human foot odour of relatively low concentration, the source of CO₂ is not located; instead, flight is upwind in the diffuse plume of foot odour. Although upwind flight by Ae. aegypti at long range is presumably induced by CO₂ and the threshold of response to skin odours is lowered, our findings suggest that, once females have arrived near a prospective human host, upwind orientation and landing are largely governed by the suite of human odours, whereas orientation is no longer influenced by CO₂.     

Modifications of pyrethroid effects associated with kdr mutation in Anopheles gambiae

Effects of knockdown resistance (kdr) were investigated in three pyrethroid‐resistant (RR) strains of the Afrotropical mosquito Anopheles gambiae Giles (Diptera: Culicidae): Kou from Burkina Faso, Tola and Yao from Côte d'Ivoire; compared with a standard susceptible (SS) strain from Kisumu, Kenya. The kdr factor was incompletely recessive, conferring 43‐fold resistance ratio at LD₅₀ level and 29‐fold at LD₉₅ level, as determined by topical application tests with Kou strain. When adult mosquitoes were exposed to 0.25% permethrin‐impregnated papers, the 50% and 95% knockdown times (KdT) were 23 and 42 min for SS females, compared with 40 and 62 min for RS (F₁ Kou × Kisumu) females. On 1% permethrin the KdT₅₀ and KdT₉₅ were 11 and 21 min for SS compared with 18 and 33 min for RS females. Following 1 h exposure to permethrin (0.25% or 1%), no significant knockdown of Kou RR females occurred within 24 h. Permethrin irritancy to An. gambiae was assessed by comparing ‘time to first take‐off’ (TO) for females. The standard TO₅₀ and TO₉₅ values for Kisumu SS on untreated paper were 58 and 1044 s, respectively, vs. 3.7 and 16.5 s on 1% permethrin. For Kou RR females the comparable values were 27.3 s for TO₅₀ and 294 s for TO₉₅, with intermediate RS values of 10.1 s for TO₅₀ and 71.9 s for TO₉₅. Thus, TO values for RS were 2.7–4.4 times more than for SS, and those for RR were 7–18 times longer than for SS. Experiments with pyrethroid‐impregnated nets were designed to induce hungry female mosquitoes to pass through holes cut in the netting. Laboratory ‘tunnel tests’ used a bait guinea‐pig to attract mosquitoes through circular holes (5 × 1 cm) in a net screen. With untreated netting, 75–83% of laboratory‐reared females passed through the holes overnight, 63–69% blood‐fed successfully and 9–17% died, with no significant differences between SS and RR genotypes. When the netting was treated with permethrin 250 mg ai/m² the proportions that passed through the holes overnight were only 10% of SS vs. 40–46% of RR (Tola & Kou); mortality rates were 100% of SS compared with 59–82% of RR; bloodmeals were obtained by 9% of Kou RR and 17% of Tola RR, but none of the Kisumu SS females. When the net was treated with deltamethrin 25 mg ai/m² the proportions of An. gambiae that went through the holes and blood‐fed successfully were 3.9% of Kisumu SS and 3.5% of Yaokoffikro field population (94% R). Mortality rates were 97% of Kisumu SS vs. 47% of Yaokoffikro R. Evidently this deltamethrin treatment was sufficient to kill nearly all SS and half of the Yaokoffikro R An. gambiae population despite its high kdr frequency. Experimental huts at Yaokoffikro were used for overnight evaluation of bednets against An. gambiae females. The huts were sealed to prevent egress of mosquitoes released at 20.00 hours and collected at 05.00 hours. Each net was perforated with 225 square holes (2 × 2 cm). A man slept under the net as bait. With untreated nets, only 4–6% of mosquitoes died overnight and bloodmeals were taken by 17% of SS vs. 29% of Yaokoffikro R (P < 0.05). Nets treated with permethrin 500 mg/m² caused mortality rates of 95% Kisumu SS and 45% Yao R (P < 0.001) and blood‐feeding rates were reduced to 1.3% of SS vs. 8.1% of Yao R (P < 0.05). Nets treated with deltamethrin 25 mg/m² caused mortality rates of 91% Kisumu SS and 54% Yao R (P < 0.001) and reduced blood‐feeding rates to zero for SS vs. 2.5% for Yao R (P > 0.05). Pyrethroid‐impregnated bednets in experimental huts and ‘tunnel tests’ gave equivalent results, showing that nets impregnated with permethrin or deltamethrin provided good levels of protection against kdr homozygous strains of An. gambiae (Kou and Tola), and against the field population at Yaokoffikro with 94% kdr frequency. The explanation seems to be that (a) high proportions of kdr females are killed by prolonged contact with pyrethroids through diminished sensitivity to the usual irritant and repellent effects, and (b) relatively few kdr females take advantage of this prolonged contact to ingest a bloodmeal.     

Use of rDNA-PCR to investigate the ecological distribution of Anopheles bwambae in relation to other members of the An.gambiae complex of mosquitoes in Bwamba County, Uganda

Abstract Environmental relationships were investigated among three species of the Anopheles gambiae complex of mosquitoes associated with the geothermal springs located in Bwamba County, Uganda. The degree of ecological isolation between An.gambiae and An. bwambae, a sibling species known only from the geothermal springs environment, was assessed on the basis of adult distribution and abundance as well as differences in larval habitats. Field data were gathered during June 1995 without knowing which of the species were being collected. Specimens identified subsequently by rDNA-PCR were used to interpret the ecological data. Ten of twenty aquatic sites sampled were found positive for immature stages of the An.gambiae complex. Larvae of An.bwambae were associated with ‘springwater’ habitats having much higher conductivity, much greater concentrations of dissolved solids and slightly higher temperature and pH than ‘normal’ fresh water sites inhabited by larvae of An.gambiae. Larval habitats of both species were unshaded: An.bwambae occurred among dense sedge (Cyperus laevigatus) whereas those of An.gambiae were almost devoid of vegetation. One mixed sample showed that larvae of both species occur together in peripheral aquatic sites with intermediate physical and ecological characteristics. In water preference tests, free-flying females were reluctant to lay eggs on bowls of water in cages; gravid females (with one wing amputated) placed on the surface of water in a cup laid eggs on seasoned rainwater (12/51 An.bwambae; 2/3 An.gambiae) as well as spring-water (39/51 An.bwambae; 1/3 An.gambiae). All three An.gambiae oviposited on the first water option, whereas 86% of An. bwambae witheld oviposition until being moved to the other type of water after 5–6 h, and 82% (36/44) of these laid eggs on geothermal water in preference to rainwater. Larval and adult collections showed that An.gambiae occurs sympatrically with An.bwambae throughout its range in the humid foothill environment of the geothermal springs, whereas the distribution of An.arabiensis overlaps only slightly with An.bwambae towards the savanna environment north of the springs.     

Odor-mediated flight behavior ofAnopheles gambiae gilesSensu Stricto andAn. stephensi liston in response to CO2, acetone, and 1-octen-3-ol (Diptera: Culicidae)

The flight behavior of Anopheles gambiae s.s. Giles and An. stephensi Patton exposed to different odor cues was studied in a wind tunnel. Odors consisted of CO₂, CO₂ + acetone (at two concentrations), and CO₂ + 1-octen-3-ol. Mosquitoes were released singly and their behavior was recorded on video. Parameters studied included flight velocity, percentage of time spent flying, percentage of time spent in plume, and number of turns toward the plume. Large differences in behavior toward the odors tested were observed. An. gambiaedid not respond well to CO ₂,whereas An. stephansiwas positively affected by this compound. In contrast, An. gambiaeresponded significantly to CO ₂ + acetone (at a low concentration), but the behavior of An. stephensiwas completely suppressed by this combination of odor stimuli. CO ₂ + a high concentration of acetone or CO ₂ + 1-octen-3-ol did not cause significant effects in An. gambiaecompared to no odor, while these treatments elicited strong behavioral responses in An. stephensi.The latter species responded particularly well to CO ₂ + 1-octen-3-ol. The results suggest that the observed differences may be inherent to the known differences in host preferences, where An. gambiaeis highly anthropophilic and An. stephensimore zoophilic. This would explain why the latter species responds well to CO ₂ and even better to CO ₂ + 1-octen-3-ol, a compound readily emitted by bovine ruminants.     

Mosquito responses to carbon dioxide in B West African Sudan savanna village

Abstract. Mosquito responses to carbon dioxide were investigated in Noungou village, 30 km northeast of Ouagadougou in the Sudan savanna belt of Burkina Faso, West Africa. Species of primary interest were the main malaria vectors Anopheles gambiae S.S. and An.arabiensis, sibling species belonging to the An.gambiae complex. Data forAn.finestus, An.pharoensis, Culex quinquefasciatus and Mansonia uniformis were also analysed. Carbon dioxide was used at concentrations of 0.04-0.6% (cf. 0.03% ambient concentration) for attracting mosquitoes to odour-baited entry traps (OBETs). The ‘attractiveness’ of whole human odour was also compared with CO₂, emitted at a rate equivalent to that released by the human bait. In a direct choice test with two OBETs placed side-by-side, the number of An.gambiae s. I. entering the trap with human odour was double the number trapped with CO₂, alone (at the human equivalent rate), but there was no significant difference between OBETs for the other species of mosquitoes. When OBETs were positioned 20 m apart, again CO₂, alone attracted half as many An.gambiae s.l. and only 40% Anlfunestus, 65% Ma.uniformis but twice as many An.pharoensis compared to the number trapped with human odour. The dose-response for all mosquito species was essentially similar: a linear increase in catch with increasing dose on a log-log scale. The slopes of the dose-response curves were not significantly different between species, although there were significant differences in the relative numbers caught. If the dose-response data are considered in relation to a standard human bait collection (HBC), however, the behaviour of each species was quite different. At one extreme, even the highest dose of CO₂, did not catch more An.gambiae s.1. than one HBC. At the other extreme, the three highest doses of CO₂, caught significantly more Ma.unifonnis than did one HBC. An.pharoensis and Cx quinquefasciatus showed a threshold response to CO₂, responding only at doses above that normally released by one man. An.funestus did not respond to CO₂, alone at any dose in sufficient numbers to assess the dose response. Within the An.gambiae complex, An.arabiensis 'chose' the CO₂,-baited trap with a higher probability than An.gambiae S.S. Also An.arabiensis, the less anthropophilic of the two species, was more abundant in CO₂,-baited OBETs than in human bait collections.     

Host feeding experience affects host plant odour preference of the polyphagous leafminer Liriomyza bryoniae

Herbivorous insects use highly specific volatiles or blends of volatiles characteristic to particular plant species to locate their host plants. Thus, data on olfactory preferences can be valuable in developing integrated pest management tools that deal with manipulation of pest insect behaviour. We examined host plant odour preferences of the tomato leafminer, Liriomyza bryoniae (Kaltenbach) (Diptera: Agromyzidae), which is an economically important agricultural pest widespread throughout Europe. The odour preferences of leafminers were tested in dependence of feeding experiences. We ranked host plant odours by their appeal to L. bryoniae based on two‐choice tests using a Y‐tube olfactometer with five host plants: tomato, Solanum lycopersicum Mill.; bittersweet, Solanum dulcamara L.; downy ground‐cherry, Physalis pubescens L. (all Solanaceae); white goosefoot, Chenopodium album L. (Chenopodiaceae); and dead nettle, Lamium album L. (Lamiaceae). The results imply that ranking of host plant odours by their attractiveness to L. bryoniae is complicated due to the influence of larval and adult feeding experiences. Without any feeding experience as an adult, L. bryoniae males showed a preference for the airflow with host plant odour vs. pure air, whereas females did not display a preference. Further tests revealed that adult feeding experience can alter the odour choice of L. bryoniae females. After feeding experience, females showed a preference for host plant odour vs. pure air. Feeding experience in the larval stage influenced the choice by adults of both sexes: for males as well as females reared on bittersweet the odour of that plant was the most attractive. Thus, host feeding experience both in larval and/or adult stage of polyphagous tomato leafminer L. bryoniae influences host plant odour preference by adults.     

Insecticide resistance monitoring of field‐collected Anopheles gambiae s.l. populations from Jinja,eastern Uganda,identifies high levels of pyrethroid resistance

Insecticide resistance in the malaria vector Anopheles gambiae s.l. (Diptera: Culicidae) threatens insecticide‐based control efforts, necessitating regular monitoring. We assessed resistance in field‐collected An. gambiae s.l. from Jinja, Uganda using World Health Organization (WHO) biosassays. Only An. gambiae s.s. and An. arabiensis (?70%) were present. Female An. gambiae exhibited extremely high pyrethroid resistance (permethrin LT₅₀ > 2 h; deltamethrin LT₅₀ > 5 h). Female An. arabiensis were resistant to permethrin and exhibited reduced susceptibility to deltamethrin. However, while An. gambiae were DDT resistant, An. arabiensis were fully susceptible. Both species were fully susceptible to bendiocarb and fenitrothion. Kdr 1014S has increased rapidly in the Jinja population of An. gambiae s.s. and now approaches fixation (?95%), consistent with insecticide‐mediated selection, but is currently at a low frequency in An. arabiensis (0.07%). Kdr 1014F was also at a low frequency in An. gambiae. These frequencies preclude adequately‐powered tests for an association with phenotypic resistance. PBO synergist bioassays resulted in near complete recovery of pyrethroid susceptibility suggesting involvement of CYP450s in resistance. A small number (0.22%) of An. gambiae s.s. ×An. arabiensis hybrids were found, suggesting the possibility of introgression of resistance alleles between species. The high levels of pyrethroid resistance encountered in Jinja threaten to reduce the efficacy of vector control programmes which rely on pyrethroid‐impregnated bednets or indoor spraying of pyrethroids.     

New repellent effective against African malaria mosquito Anopheles gambiae: implications for vector control

Anopheles gambiae Giles sensu stricto (Diptera: Culicidae) is a vector for Plasmodium, the causative agent of malaria. Current control strategies to reduce the impact of malaria focus on reducing the frequency of mosquito attacks on humans, thereby decreasing Plasmodium transmission. A need for new repellents effective against Anopheles mosquitoes has arisen because of changes in vector behaviour as a result of control strategies and concern over the health impacts of current repellents. The response of A. gambiae to potential repellents was investigated through an electroantennogram screen and the most promising of these candidates (1‐allyloxy‐4‐propoxybenzene, 3c {3,6}) chosen for behavioural testing. An assay to evaluate the blood‐host seeking behaviour of A. gambiae towards a simulated host protected with this repellent was then performed. The compound 3c {3,6} was shown to be an effective repellent, causing mosquitoes to reduce their contact with a simulated blood‐host and probe less at the host odour. Thus, 3c {3,6} may be an effective repellent for the control of A. gambiae.     

Oviposition preference in the oligophagous tomato fruit fly, Neoceratitis cyanescens

We assessed the role of visual and olfactory cues on oviposition preference in the oligophagous tomato fruit fly, Neoceratitis cyanescens (Bezzi) (Diptera: Tephritidae). In a field survey, we evaluated the stage of susceptibility of field‐grown tomatoes by monitoring N. cyanescens infestations from fruit‐setting up to harvest, in relation to post‐flowering time, size, and visual properties of fruit. In two‐choice laboratory experiments, we tested the degree to which females use visual and olfactory cues to select their host plant for oviposition. In addition, we investigated the ability of flies to avoid fruit already infested by conspecific eggs or larvae, and the influence of natal host fruit on oviposition preference. Neoceratitis cyanescens females preferentially lay their eggs in small yellow‐green unripe fruit (2–3.5 cm diameter, 10–21 days post‐flowering). Damage to fruit was significantly affected by brightness and size properties. In laboratory experiments, females chose to lay their eggs in bright orange rather than yellow domes. On the sole basis of olfactory stimuli, females showed a significant preference for unripe vs. ripe host fruit, for unripe fruit vs. flowers or leaves, and for host vs. non‐host fruit (or control). However, colour interacted with odour as females dispatched their eggs equally between the yellow dome and the bright orange dome when unripe fruit of tomato was placed under the yellow dome vs. ripe fruit under the bright orange dome. When offered real ripe and unripe tomatoes, females preferred unripe tomatoes. Females significantly chose to lay eggs in non‐infested fruit when they were given the choice between these or fruit infested with larvae. In contrast, recent stings containing eggs did not deter females from laying eggs. Rather, they could have an attractive effect when deposited within <1 h. Regardless of their natal host plant, tomato or bugweed, N. cyanescens females laid significantly more eggs in a dome containing bugweed fruit. However, 15% of females originating from tomato laid eggs exclusively in the dome with tomato, against 3% of females originating from bugweed.     

The impact of uniform and mixed species blood meals on the fitness of the mosquito vector Anopheles gambiae s.s: does a specialist pay for diversifying its host species diet?

We investigated the fitness consequences of specialization in an organism whose host choice has an immense impact on human health: the African malaria vector Anopheles gambiae s.s. We tested whether this mosquito’s specialism on humans can be attributed to the relative fitness benefits of specialist vs. generalist feeding strategies by contrasting their fecundity and survival on human‐only and mixed host diets consisting of blood meals from humans and animals. When given only one blood meal, An. gambiae s.s. survived significantly longer on human and bovine blood, than on canine or avian blood. However, when blood fed repeatedly, there was no evidence that the fitness of An. gambiae s.s. fed a human‐only diet was greater than those fed generalist diets. This suggests that the adoption of generalist host feeding strategies in An. gambiae s.s. is not constrained by intraspecific variation in the resource quality of blood from other available host species.     

Histochemical Studies of the Non‐Host Resistance and the Role of Actin Cytoskeleton in Pepper Against Colletotrichum orbiculare

The penetration process and defence reactions (hypersensitive response, oxidative burst and cell wall fortification) of Colletotrichum orbiculare were studied histochemically on pepper cultivar ‘A11’ (non‐host) and susceptible cucumber cultivar ‘Changchun Thorn’ (host). The results indicate that C. orbiculare could hardly penetrate the non‐host pepper leaves. It was papillae rather than hypersensitive response and H₂O₂ that played an important role in resisting the colonization and development of C. orbiculare on the non‐host pepper. The depolymerization of the actin microfilament weakened the papilla deposition of pepper and allowed successful penetration of the non‐adapted C. orbiculare, suggesting that the actin cytoskeleton of pepper is significant in preventing the invasion of the non‐host pathogen C. orbiculare.     

Effect of CO2 concentration on mosquito collection rate using odor‐baited suction traps

Carbon dioxide (CO₂) has been used for decades to enhance capture of host‐seeking mosquitoes when released in association with traps commonly used by mosquito and vector control agencies. However, there is little published work evaluating the effect of altering CO₂ release rates relative to the number of mosquitoes captured in these traps. This study investigated how varying CO₂ concentration altered the mosquito collection rate at a freshwater wetlands in southern California. Host‐seeking mosquitoes were captured in CDC‐style traps baited with one of six CO₂ release rates ranging from 0–1,495 ml/min from gas cylinders. Species captured were Aedes vexans, Anopheles franciscanus, An. hermsi, Culex erythrothorax, and Cx. tarsalis. A biting midge, Culicoides sonorensis, was also captured. For all species, increasing CO₂ release rates resulted in increasing numbers of individual females captured, with the relative magnitude of this increase associated to some extent with known feeding preferences of these species. We found that variation in CO₂ release rate can significantly alter mosquito capture rates, potentially leading to imprecise estimates of vector activity if the relationship of CO₂ release rate to mosquito capture rate is not considered.