Notes on the Ecology of the Dark Rice Field Mosquito, Psorophora Confinnis, in Coachella Valley, California (Diptera: Culicidae)1

Abstract

P. confinnis occur in a variety of temporary waters in this area in southern California, but on many ranches larvae were found only in irrigated date lanes. Here larval-pupal densities are higher in grassy areas and under pruned date fronds than in areas bare of cover, and larvae concentrate near cow droppings. Size of fourth-instar larvae tends to vary inversely with larval density. Fourth-instar larvae and pupae were once observed completing their development in muddy areas where irrigation water had disappeared from the surface. In the laboratory they can do so on moist filter paper, but third-instar larvae develop no farther. Larvae and pupae are attracted to light at night. Water persists in the date lanes for an average of 5.1 days after each semimonthly flooding; the aquatic development of confinnis can be completed within this time only at water temperatures of 26°C or higher. Hence adults are numerous only from mid-June through September. The scarcity or absence of adults on irrigated ranches lacking live-stock suggests that cattle are the necessary principal host. Females usually cannot pierce human skin until 4 hours after emergence; to survive they must feed within 24 hours. Adults are nocturnal and both sexes are attracted to lights. Trap-light captures reach a peak 2 or 3 hours after sunset. During the day adults cling to grasses, especially over moist or soggy soil, and fly only a few meters when disturbed, usually at a height of 1 meter or less. Males disappear rather abruptly but the female population declines slowly as the soil dries. Tests show moist soil with grass cover strongly preferred for oviposition, and moist soil without cover, water, and dry soil are progressively less attractive. Toxaphene, ¾ gallon in 50 gallons of water per 5 acres, kills second- and third-stage larvae, but is ineffective against older aquatic stages. Control possibly could be achieved by changing the irrigation regime so that water would persist in the lanes only 3 or 4 days after each flooding.