Artificial ovitrap preferences and biting activity of Culex salinarius (Diptera: Culicidae) on the upper Texas Gulf coast

Abstract

Results of monitoring 8 differing artificial ovitraps set at 7 different sites in Chambers County, TX, from March 1990 to December 1991 indicate that adult female Cx. salinarius prefer to oviposit in black ovitraps containing grass infusion as opposed to white ovitraps with and without infusions or black ovitraps without grass infusion. Adults of this species occurring in a salt marsh environment preferred to oviposit more often in ovitraps containing brackish water, while adults occurring in upland fresh water environments oviposited more often in ovitraps containing fresh water. The reason for these differences in ovitrap selection are not known at the present time. As a result of different water preferences of this mosquito species in each of the previously mentioned environments, adult females were trapped and dissected for half a year and examined for parity rates on the suspicion that adults were migrating from the brackish marsh to upland fresh water areas. Nulliparous females are indicative of young, newly-emerged mosquitoes which are more likely to have migrated from distant areas. Parous females indicate that the mosquito was collected a short distance from the trap site, as gravid mosquitoes are most likely to oviposit at the closest available breeding site. Statistical analyses of the parity data collected on Cx. salinarius specimens from Chambers County indicated that migration from marsh sites to upland fresh water sites was not the reason for the differences in water preferences with respect to this species. Because adult female Cx. salinarius appear to prefer to oviposit in ovitraps baited with a plant infusion, a study was conducted to determine which of 5 commonly occurring marsh plant species in the Gulf Coast region of Texas might serve as the best infusion material. Spartina patens (marshhay cordgrass) and Distichlis spicata (saltgrass) baited ovitraps collected the highest mean numbers of egg rafts over the 18-month study. Using Duncan's multiple range test for variability, the collection means for traps containing infusions of these 2 plant species were shown to be significantly higher than egg raft collection means obtained from ovitraps baited with infusions of Scirpus spp. (bulrush), Typha spp. (cattail) and Eleocharis spp. (spikerush). However, cattail and spikerush may stand to serve as a more practical choice for infusion material due to their abundance in both brackish marsh and upland fresh water environments. In regard to the biting activity of Cx. salinarius females in the Upper Coastal region of Texas, cloud cover and temperature were significant (p < 0.05) environmental factors influencing the biting activity of this species. On warm nights (95-700 F), peak biting activity occurred 37.2 minutes after official sunset in light of 0.12- 0.01 footcandles (fc). On cool nights (75-50 0 F), peak biting activity occurred slightly sooner, 33.2 minutes after official sunset and sometimes in light as bright of 0.59 fc. On warm nights, biting was usually initiated in light of approximately 0.10 fc; on cool nights, it was initiated earlier in brighter light (as high as 191.00 fc). It is suspected that cooler temperatures stimulated biting activity of adult females in brighter light earlier in the evening when temperatures were still relatively high, thus taking advantage of these warmer temperatures.