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The impact of nutritional variables on the development of host-seeking and biting behaviours after emergence by female Culex nigripalpus mosquitoes were studied using air-flow olfactometer and close-range biting assays, respectively. Unfed females failed to develop resting stage ovarian follicles. When offered a bird host in the absence of competing stimuli, sugar-fed mosquitoes were significantly more responsive in both host-seeking and biting than unfed controls. In a choice olfactometer assay using nectar odours (honey scented with artificial apple-blossom oil) versus host odours (a bird), unfed females preferred honey over bird odours except when honey odour was weak. After sucrose feeding, females switched from honey to bird preference. This change in behaviour was accompanied by significant accumulation of lipid and by follicular growth to the resting stage. Elevation of host responsiveness after sugar feeding was reversible; starvation ultimately resulted in females preferring honey over bird odours. When the larval diet was restricted by crowding, the wing-length and total lipid of resultant adult females were reduced. Although differences were subtle, unfed bird-responding females tended to have longer wings and more lipid than their honey-responding counterparts.

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The full length of Culex quiquefasciatus early trypsin has been cloned and sequenced and a three-dimensional (3D) model of the enzyme was built showing that the enzyme has the canonical trypsin's active pocket containing H78, D123, S129, and D128. The biosynthesis of juvenile hormone (JH) III by the corpora allata (CA) in female Cx. quiquefasciatus is sugar-dependent. Females that were maintained on water after emergence synthesize very little JH III, JH III bisepoxide, and methyl farnesoate (MF) (3.8, 1.1, and 0.8 fmol/4 hr/CA, respectively). One hour after sugar feeding, the synthesis of JH III and JH III bisepoxide reached a maximum (11.3 and 5.9 fmol/4 hr/CA, respectively) whereas MF biosynthesis reached a maximum at 24 hr (5.2 fmol/4 hr/CA). The early trypsin is transcribed with a short intron (51 nt) is spliced when JH III biosynthesis is high in sugar fed and at 1 hr after the blood meal (22 and 15  fmol/4 hr/CA, respectively). We investigated the transcriptional and posttranscriptional regulation of the early trypsin gene showing that JH III concentrations influence splicing. In the absence JH III the unspliced transcript is linked by a phosphoamide bond at the 5'-end to RNA ribonuleoprotein (RNP). The biosynthesis of the early trypsin was followed in ligated abdomens (without CA) of newly emerged females that fed blood by enema. Our results show that the early trypsin biosynthesis depends on sugar and blood feeding, whereas the late trypsin biosynthesis does not depend on sugar feeding, or JH III biosynthesis. Downregulating the early trypsin transcript does not affect the late trypsin.

When newly emerged females of the mosquito Culex nigripalpus Theobald (Diptera: Culicidae) take a sugar meal, they develop both a propensity to seek a host and resting-stage ovarian follicles. As follicle growth is indicative of Juvenile Hormone (JH) activity, we searched for possible roles of JH in the behavioural shift from nectar to blood-host odour preference by Cx. nigripalpus after emergence, using an olfactometer-choice assay. Topically applied or injected methoprene (a JH analogue) resulted in a behavioural shift and follicular growth in unfed females that increased with increasing dosages. Topical methoprene 500 ng and 4 microg resulted in a switch to bird-odour preference. JH III injection resulted in a lesser shift from honey responses to bird responses. Methoprene application caused no detectable changes in glycogen, total sugars or total lipid when assayed 24h after treatment. Additionally, as male accessory glands (MAG) have been shown to synthesize JH, we implanted intact paired MAGs from either conspecific or Aedes aegypti (L.) donors, or injected 1/3 gland pair equivalents of conspecific MAG homogenate into unfed newly emerged females. All MAG treatments failed to induce behavioural or ovarian modifications. Ovariectomy had no effect on the sugar-induced shift from nectar to host preference in the olfactometer. Thus JH, but not MAG, mimicked the effects of a sugar meal by causing both follicular growth and the shift to preference for a host.