Environmental regulation of autogeny in Culex tarsalis (Diptera: Culicidae) from Manitoba, Canada.

Temperatures less than 21 degrees C suppressed the expression of autogeny in genetically autogenous females of Culex tarsalis Coquillett from Manitoba, even under a long day photoperiod of 16:8 (L:D). Under short day lengths (8:16), the percentage of autogenous females was suppressed at temperatures from 16 to 28 degrees C; but at 32 degrees C, maximum autogenous expression occurred. When genetically similar populations were maintained at 24 degrees C and exposed to photoperiods from 14:10 to 10:14 (L:D), autogeny was suppressed from a mean of 26% progressively to 1%. When all stages were maintained at 21 degrees C, pupae were photosensitive (i.e., the percentage of autogenous females was not affected by photoperiods at which larvae were maintained). The percentage of autogenous females in field populations followed the seasonal changes in temperature and photoperiod, demonstrating environmental regulation of facultative autogeny. Autogeny appeared to be genetically stable in Cx. tarsalis. Single families from a long standing laboratory colony (greater than 60 generations) had the same range (0-80%) in percentage of autogenous females per family as did recent field-collected families from the same geographical location.

Oviposition behavior of natural populations of Culex tarsalis and Culex restuans (Diptera: Culicidae) in artificial pools.

Natural populations of Culex restuans Theobald and Cx. tarsalis Coquillett in Manitoba, Canada, preferred to oviposit in artificial pools (ovipools) where lawn sod used as an attractant was renewed regularly during the season. The sod medium was unattractive to most Cx. restuans after three weeks in the spring, after one week in midsummer, and after two weeks in autumn; it was unattractive to most Cx. tarsalis after three weeks. Cx. restuans was the first Culex spp. to oviposit each spring, and in 1986 overwintered females of Cx. restuans began ovipositing four weeks before those of Cx. tarsalis. Both species continued to oviposit until September between 1984 and 1987 or until the mean weekly air temperatures dropped to 12 degrees C. Cx. restuans selected shaded oviposition pools, located under a tall, dense tree canopy, while Cx. tarsalis selected open-field, unshaded pools. CO2-baited light trap collections of adults near the oviposition site showed that Cx. restuans females were underrepresented in comparison with the number of egg rafts collected.

Interrupted blood feeding by Culex (Diptera:Culicidae) in relation to individual host tolerance to mosquito attack.

Field studies at Delta Marsh (1991) and Winnipeg (1993), MB, and Vero Beach (1992), FL, examined variation among individual quail hosts in terms of the blood-feeding success by mosquitoes that fed on them. Culex tarsalis Coquillett was the predominant species collected in box traps baited with 2 quail at Delta Marsh in 1991, Culex nigripalpus Theobald was the predominant species collected at Vero Beach in 1992, and both Cx. tarsalis and culex restuans Theobald were collected in approximately equal numbers at Winnipeg in 1993. In 48 of 70 samples, blood feeding was skewed significantly from an even distribution between the 2 quail in each trap. In 5 of 13 samples of Cx. tarsalis collected in 1991, incomplete feeding (proportion of blood meals < or = 1/2 full) was from 2 to 8 times more likely on one bird relative to the other. In 7 of 13 samples of Cx. nigripalpus collected in 1992, incomplete feeding was from 2 to > 10 times more likely on one bird relative to the other. Incomplete feeding was from 2 to > 10 times more likely on one bird relative to the other is 7 of 11 mixed Cx. restuans/Cx. tarsalis samples collected in 1993. The probability that a given quail would interrupt a feeding mosquito was correlated negatively with the probability of the same quail being fed on. The proportion of detectable multiple meals in a given trap night was correlated negatively with the degree to which the distribution of blood meals in each trap night was skewed away from 0.5 on each bird. Individual quail varied significantly in terms of the blood feeding success of mosquitoes that fed on them.

Field evidence for multiple host contacts during blood feeding by Culex tarsalis, Cx. restuans, and Cx. nigripalpus (Diptera: Culicidae).

Field studies were conducted in Manitoba, Canada, and Florida, United States to evaluate the frequency of multiple host contacts by Culex tarsalis Coquillett, Cx. restuans Theobald, and Cx. nigripalpus Theobald, primary and secondary vectors of several encephalitis viruses in North America. Blood-fed mosquitoes were collected from box traps, each baited with a pair of quail. One quail of each pair was injected with rubidium and the other with cesium to permit the determination of the source(s) of each blood meal. Approximately 5% of all blood-fed Cx. tarsalis, Cx. restuans, and Cx. nigripalpus ingested blood from both quail during overnight exposure. The frequency of multiple feeding by these species ranged from 0 to 18.5%, 0 to 33.3%, and 0 to 17.6%, respectively. Of the 331 mosquitoes that ingested blood from 2 quail, 57 (17.2%) were not fully blood fed. Also, 1.0-3.5% of blood-fed mosquitoes had ingested blood before entering the quail-baited traps. Any increase in the number of host contacts as a result of multiple feeding, however modest, may increase disproportionately the rate at which virus transmission occurs.

Rogue ovarioles and criteria for parity diagnosis in Culex tarsalis (Diptera: Culicidae) from Manitoba.

Rogue ovarioles were identified in laboratory-reared nulliparous and 1-parous Culex tarsalis Coquillett and in wild nulliparous C. tarsalis. Parity-diagnostic criteria were extrapolated from analysis of known nullipars and 1-pars. Diagnostic criteria were based, in part, on the number of ovarioles with no dilatation and a pedicel of greater than or equal to six adjacent cells. The high proportion of ovarioles lacking pedicels in known 1-pars and the small number of diagnostic ovarioles found in 2-pars support the findings of other researchers that ovulation destroys the pedicel and any previously formed dilatations.

Parity diagnosis and ovulation in Culiseta inornata (Diptera:Culicidae).

The hypothesis that ovulation in mosquitoes results in the formation of a single basal dilatation per ovariole, regardless of previous history of the ovariole, was supported for known 1- and 2-parous Culiseta inornata (Williston). The examination of histological sections of whole ovaries of Cs. inornata, before and after ovulation in the 1st gonotrophic cycle, indicated that the pedicel is destroyed during ovulation. Dissections revealed that 88% of 1- dilated ovarioles in 1-pars lacked a pedicel, and 90% of 2-pars had a mean of only 12.5 2-dilated ovarioles (10% had none). Criteria are proposed for the separation of nulliparous, 1-parous and 2-parous (anautogenous) Cs. inornata. Fifty percent of nullipars and 42% of 1-pars had a mean of 2.6 and 2.0 rogue ovarioles, respectively. The criteria account for rogue ovarioles in nullipars, but misdiagnose 2-pars as 1-pars when the former have few (or lack) diagnostic ovarioles. This diagnostic error can be reduced using structural differences in rogue versus diagnostic ovarioles.

Development time, oviposition activity and onset of diapause in Culex tarsalis, Culex restuans and Culiseta inornata in southern Manitoba.

Development times and survival of immatures and reproductive diapause of adult females of Culex tarsalis, Cx. restuans and Culiseta inornata were investigated from hatching to adult emergence at 15, 20 and 25 degrees C in the laboratory and at natural temperatures and photoperiods in southern Manitoba. Based on patterns of oviposition in artificial pools operated from mid-April to the end of September and development time of the immature stages, 3 generations of Cx. tarsalis, Cx. restuans and Cs. inornata were possible in 1980 and 1981. In 1981, 70% of field-reared Cx. tarsalis females emerging in mid-August were in diapause. Field-reared Cx. restuans and Cs. inornata entered reproductive diapause 2-3 weeks later than Cx. tarsalis.

Effects of temperature and photoperiod on the infection of two mosquito species by the mermithid Romanomermis culicivorax.

Successful invasion by the mermithid Romanornermis eulicivorax declined linearly from 93.6 to 1.5% in Culex tarsalis and from 73.1 to 1.6% i n Aeries dorsalis larvae exposed in the laboratory at 18, 16, 14, 12, and l 0 C. Larvae of C. tarsalis were more susceptible than those of A. dorsalis at 18 and 16 C, but this relationship was reversed at 12 C. Larval mortality during the 48-h exposure period was due primarily to nematode infection . Photoperiod had no effect on infection.

Effect of Storage at Low Temperatures on Development and Survival of Postparasitic Juveniles of Romanomermis culicivorax (Nematoda: Mermithidae).

Development of postparasites and adult females of Romanomermis culicivorax Ross and Smith, 1976 may be retarded by keeping cultures at 10 or 15 C for 15 wk. Storage at 5 C resulted in high mortality of postparasites. A storage temperature of 10 C is suitable if development of postparasites, young adults, and gravid females is to be greatly retarded. None of the females were gravid when postparasites (1-6 days old) were kept at 10 C for 15 wk. Development resumed after the cultures were returned to 26 C and no significant mortality occurred. A storage temperature of 15 C is suitable if development is to be only moderately retarded. After a 15-wk storage period, only a few weeks were required to return the cultures to full production status.

How do dilatations form in mosquito ovarioles?

Ovariolar dilatations have been used to estimate parity status in mosquitoes for over 40 years. The theoretical basis for this parity-diagnostic method, which was established for mosquitoes by Polovodova 1, has been challenged by a number of Russian researchers. The findings of this new school of researchers have elicited little response from those currently using the method outside Russia, except for a recent paper by Hoc and Charlwood2. This has stimulated Andrew Fox and Reinhart Brust to review the formation of ovariolar dilatations here, and compare what they term the Old and the New Schools. Although the basic application of Polovodova's method is unaffected by the premise of either School, a clear understanding of ovarian dynamics is needed to understand the method and its limits.