Aedes triseriatus, the treehole mosquito, is the primary vector of La Crosse virus (LAC) cause of the most common form of encephalitis in the midwest. Both control and monitoring of this mosquito are exceptionally difficult. Continued genetic and ecological research on vector competence are proposed, with the hope than an "Achilles heel" can be exploited. Research includes a sibling species, Ae. hendersoni (non-vector of LAC), Ae. brelandi, Ae. zoosophus and Ae. atropalpus. (1) Ability to transmit LAC differs greatly in different populations (10-90%). Oral, transovarial and venereal transmission will be analyzed genetically; virus-refactory strains with genetic markers will be developed. Studies on pathology (EM, FA), salivation rate, biting behavior and fecundity will be used to assess the cost of viral infection to the mosquito. Effect of environmental stress on transmission rate will be determined. (2) Studies on genetic structure involve further isolation of biochemical and morphological mutants, as well as improvement of linkage maps. Physiological traits (barriers to LAC infection, inability to diapause in the egg. resistance to drying) will be located on the linkage map. Electrophoretic analysis of clinical variation and sibling species differences will be compared via isozyme allele frequency. Effect of breeding system will be determined in inbred lines (F10 + of brother-sister mating). (3) Ecological studies will concentrate on population dynamics and productivity of mosquitoes that breed in discarded tires. Strains with rare isozyme alleles will be used to trace field dispersal. Since virtually nothing is known of field bionomics of Ae. hendersoni, we will study this species in the forest canopy via high towers. Tire productivity and adult dispersal of Ae. atropalpus will be followed. Finally, we will try to develop improved capture methods for adults. (5) Biological control laboratory experiments and field trials will be continued with predatory mosquitoes, Toxorhynchites sp. and Anopheles barberi. Control potential of a gregarine protozoan parasite of the larval midgut, Ascogrenarina, will be determined. Larval competition experiments will be undertaken with a female-sterile mutant of Aedes aegypti; this mutant absorbs the available food, causing Ae. triseriatus to starve. (5) Our Stock Center will continue to maintain 40 species and 180 strains and to furnish them to other laboratories. Finally Ae. hendersoni will be colonized (without forced-mating) for the first time.