Journal of Insect Physiology 47 2001 303–310 www.elsevier.comlocatejinsphys
Inhibition of host-seeking response and olfactory responsiveness in Anopheles gambiae following blood feeding
Willem Takken , Joop J.A. van Loon, Wim Adam
Laboratory of Entomology, Wageningen University, P.O. Box 8031, 6700 EH Wageningen, The Netherlands Accepted 13 June 2000
Abstract
The effect of a single blood meal on the host-seeking response of Anopheles gambiae was investigated in the laboratory using a behavioural bioassay, whereas possible changes at the chemosensory level were monitored using electroantennogram recording
EAG. To avoid the possible confounding effect of body size, mosquitoes of a large size class only were used. Five-day old female mosquitoes were given a blood meal on a human arm and exposed to the emanations of a human hand in an olfactometer at 3,
24, 40, 48 and 72 h following the meal and their behaviour and EAG response to host stimuli were compared with that of unfed mosquitoes controls of corresponding age. During egg development, mosquitoes had access to glucose and an oviposition tray.
The ovarian development of blood-fed mosquitoes that responded to host odours was compared with that of blood-fed mosquitoes that had not been exposed to host odours. The EAG response of blood-fed and control mosquitoes to host odour was examined
upon stimulation with air led over incubated human sweat, hexanoic acid, indole and geranyl acetone. EAGs were recorded at times after a blood meal corresponding with those used in the behavioural experiment. There was no host-seeking response at 3 and 24
h post blood meal pbm. Seven percent of the mosquitoes responded to human emanations 40-h pbm, 27 at 48 h and 68 at 72 h following a blood meal. The average response of controls to host stimuli varied from 35 at t
= 40 h to 67. There was no
ovarian development in the unfed group of mosquitoes. Of the mosquitoes that responded to host odour 48 h pbm, 12.5 n =
5 had ovaries in Christophers’ stage IV and the remainder in stage V. Of the mosquitoes that responded 72 h pbm, 66.7 n
= 94
had ovaries in stage V and 31.2 n =
44 had recently oviposited. Maximum EAG amplitudes recorded from blood-fed and control mosquitoes were similar for mosquitoes in Christophers’ stages I–III, whereas in stage IV EAG amplitudes recorded from the blood-
fed group were significantly lower than those of the corresponding control group in response to headspace of incubated human sweat and to indole. The results show that there was a strong inhibition of host seeking in An. gambiae for a period of at least 40
h following a blood meal. Host-seeking returned to pre-blood meal levels 72-h post feeding and was associated with egg maturation. The inhibition of host-seeking behaviour was accompanied by an inhibition of olfactory sensitivity to headspace of incubated sweat
and indole just before the resumption of the host-seeking response. The implications of these findings for mosquito surveillance with host odours are discussed.
2001 Elsevier Science Ltd. All rights reserved.
Keywords: Anopheles gambiae; Host-seeking behaviour; Blood feeding; Electroantennogram; Olfaction; Hand odours
1. Introduction
The mosquito Anopheles gambiae Giles sensu stricto henceforth termed An. gambiae is considered the most
important malaria vector in Africa. It is an efficient vec- tor because of its propensity to seek blood meals from
humans and to feed and rest indoors White, 1974.
Corresponding author. Tel.: +
31 317 484652; fax: +
31 317 484821.
E-mail address: willem.takkenusers.ento.wau.nl W. Takken.
0022-191001 - see front matter
2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 2 - 1 9 1 0 0 0 0 0 1 0 7 - 4
Therefore, fundamental knowledge on host-seeking behaviour is important for the development of strategies
for malaria control. The behaviour of this mosquito is characterized by a nocturnal blood feeding pattern, with
activity peaks between midnight and 6.00 a.m. Haddow, 1954. Normally, one blood meal suffices to complete a
gonotrophic cycle, but in the wild varying proportions of mosquitoes undergo a “pre-gravid” phase, in which
two blood meals on different nights are required to com- plete the first gonotrophic cycle Gillies, 1954. Mos-
quito size may affect this behaviour Lyimo and Takken, 1993; Takken et al., 1998, large individuals taking up
304 W. Takken et al. Journal of Insect Physiology 47 2001 303–310
only one blood meal per cycle and small females feeding at least twice during the first gonotrophic cycle. It
appears that the first blood meal of small females is small and used primarily to build up a metabolic energy
reservoir Takken et al., 1998.
In Aedes aegypti L. host seeking is inhibited following a blood meal and this is not restored until eggs have been
deposited Klowden, 1987. The inhibition is caused by two separate mechanisms: first, host-seeking behaviour
is inhibited by distention of the abdomen following blood ingestion Klowden and Lea, 1978; Klowden and
Lea, 1979a. Second, later in gonotrophic development, host seeking is inhibited by a factor in the haemolymph
that reduces the sensitivity of one type of lactic-acid sensitive olfactory receptor neurons Klowden and Lea,
1979b; Davis, 1984. This inhibition starts with the release of an initiating factor by the ovaries 6–12 h pbm
Klowden et al., 1987. The behavioural inhibition due to oocyte development starts 24–30 h after a blood meal
and reaches a maximum 36–72 h post blood meal Klowden and Lea, 1979b. Thus, in Ae. aegypti one
blood meal suppresses host-seeking behaviour through- out the gonotrophic cycle. Host seeking is restored after
the deposition of the eggs Davis, 1984. In a compara- tive study on host-seeking behaviour of several aedine
and anopheline species, Klowden and Briegel 1994 found that An. gambiae did not express the inhibition of
host-seeking behaviour following a blood meal in con- trast to Ae. aegypti and Ae. albopictus Skuse. The
authors ascribe these differences to the different feeding strategies employed by these species, suggesting there is
no need for host seeking inhibition in nocturnal mosquito species such as An. gambiae.
Mosquitoes reared under sub-optimal conditions are small, emerge with a lower amount of energy and are
less effective in host seeking compared to their larger siblings Klowden et al., 1988; Renshaw et al., 1994;
Takken et al., 1998. The latter authors studied the effects of female mosquito size on the use of a blood
meal in An. gambiae. They found that small females use their first blood meal to build up internal reserves. Large
mosquitoes, on the other hand, use their first blood meal for egg development. The process of blood meal utiliz-
ation and its physiological consequences might therefore be different between Ae. aegypti and An. gambiae.
Recent field studies in Thailand and Puerto Rico, how- ever, have shown that Ae. aegypti also takes multiple
meals during one gonotrophic cycle Scott et al., 1993. The two species may therefore employ the same strategy
for blood feeding and egg production.
In the present study we investigated the influence of blood feeding on the host-seeking response of An. gam-
biae using a behavioural assay and electrophysiological recordings. The gonotrophic status of responding and
non-responding mosquitoes was studied as well. In order to overcome the possible confounding effect of the need
for small mosquitoes to ingest two blood meals during one gonotrophic cycle, mosquitoes used were of a uni-
form, large size class by rearing larvae at low densities and ad libitum feeding conditions.
2. Materials and methods
