Mosquitoes play an important role in the transmission of pathogens and parasites to humans and animals all over the world. Effective management of vectors requires a clear understanding of vector biology. Novel research that focuses on the factors that influence vector capacity is paramount for elucidating the complex dynamics of vector-borne disease. The research projects presented in this dissertation involve two components of the vector capacity model: density and host preference. The first study investigated how varying CO2 concentration alters mosquito collection rates at a freshwater wetlands in southern California. Host-seeking mosquitoes were collected in CDC-style traps baited with one of six CO2 release rates ranging from 0-1495 ml/min. The species collected were Aedes vexans, Anopheles franciscanus, Anopheles hermsi, Culex erythrothorax and Culex tarsalis. A biting midge Culicoides sonorensis, was also collected. CO2 concentration influenced the collection rates of all host-seeking Diptera sampled in this study with increasing CO2 concentration resulting in increasing collection of host-seeking insects for all flow rates. In the second study, the diel activity of resting mosquitoes was assessed for 24 hr periods on four separate dates at a freshwater wetlands in southern California using resting boxes. Additionally, height of resting flight activity was evaluated using paired small resting boxes placed at ground level or suspended 6 m off the ground to collect resting mosquitoes at canopy level. Aedes vexans and Culex tarsalis were the most abundant species collected in the resting boxes. Our results indicated no significant differences between canopy and ground resting box collections for these two species. Peak collection time from resting boxes occurred 30 minutes before sunrise. The last study investigated the host feeding patterns of Cx. stigmatosoma. Our results confirmed that this species feeds mainly on passerine birds.

House flies are a widespread, pestiferous insect responsible for causing human nuisance as well as pathogen transmission to humans and animals. While adult fly behavior has been relatively well-studied, little is known about larval behavior. Manipulation of larval behavior may be useful to manage fly populations, reducing adult fly numbers without the need for chemical insecticides. In this study, we determined larval temperature preference by applying a temperature gradient to developmental media containing third instar fly larvae that were initially distributed evenly throughout the media. Following a 4 to 6-h assay period during which larvae could move freely within the media, larvae were found to aggregate within a relatively narrow temperature range similar to previously reported studies.

House flies are common pests on cattle feedlots and dairies, where they develop in and feed on animal waste. By contacting animal waste, house flies can acquire human pathogenic bacteria such as Escherichia coli and Salmonella spp., in addition to other bacteria, viruses, or parasites that may infect humans and animals. The subsequent dispersal of house flies from animal facilities to nearby agricultural fields containing food crops may lead to pre-harvest food contamination with these pathogens. We hypothesized that odors from honeydew, the sugary excreta produced by sucking insects feeding on crops, or molds and fungi growing on honeydew, may attract house flies, thereby increasing the risk of food crop contamination. House fly attraction to honeydew-contaminated plant material was evaluated using a laboratory bioassay and attraction was evident for the following plant-pest-honeydew combinations: citrus mealybug on squash fruit, pea aphid on faba bean plants, whitefly on navel orange and grapefruit leaves, and combined citrus mealybug and cottony cushion scale on mandarin orange leaves. Two fungal species, Aureobasidium pullulans and Cladosporium cladosporioides, were repeatedly isolated from field-collected honeydew samples. House flies were attracted to odors from A. pullulans cultures but not to those of C. cladosporioides. Gas chromatography-electroantennogram detection (GC-EAD) and gas chromatography-mass spectrometry (GC-MS) identified possible active compounds from pea aphid on faba bean plants, whitefly on navel orange, and whitefly on grapefruit leaves. 10 different compounds were identified from honeydew aeration samples that elicited a house fly antennal response. 4 of these compounds were identified from honeydew produced by whiteflies on navel orange and whiteflies on Marsh grapefruit plants, while one compound was identified from these two whitefly honeydews samples as well as honeydew from a laboratory colony of pea aphids. Two of these compounds were attractive to house flies in a blend and individually. These compound blends and single compounds were compared to fermented vinegar compounds previously found to be attractive to house flies. This dissertation presents the first study of house fly attraction to honeydew and specific honeydew odors. It contributes materially to our understanding of house fly responses to honeydew volatiles and demonstrates that insect honeydew is attractive to house flies, supporting our hypothesis that honeydew production by sucking insects infesting food crops may contribute to attraction of house flies to food crops.

House flies (Musca domestica L.) are common synanthropic pest associated with confined animal operations and are known carriers of many disease-causing pathogens affecting humans and animals. House fly production remains a significant problem for producers. Reducing the number of house flies dispersing away from a development site may impact both pathogen transmission and nuisance to humans and animals near fly development sites. This dissertation investigates the flight behaviors of the house fly under field conditions in southern California which may be used in future fly management programs. In chapter 1, a study was conducted to investigate the diel flight activity of house flies on two different active dairies in southern California. Collection period (time of day) was a useful predictor of house fly activity, as time is essentially a proxy for diel changes in temperature, humidity, light intensity, and even wind speed in southern California. Male flight activity peaked before females. Temperature, light intensity, and wind speed had a significant effect on activity throughout the day. In chapter 2, the flying height of house flies was examined using vertical 2.13m-tall sticky traps. House flies were evenly distributed over this trap height, except for flies captured on traps positioned in or near the edge of an alfalfa field. Temperature, relative humidity, and wind speed were linearly associated with changes in flying height. The even distribution of house flies over the traps indicate house flies are almost certainly flying above top of the traps. In chapter 3, the relationship of house fly flight direction with environmental variables was investigated, using a release-recapture study. House flies are not dispersing randomly, and the short-range flight of male and female flies was toward the southeast. Flight direction was significantly correlated with time of release, solar position, wind speed, and temperature. Dispersing flies may have been using a nearby tree line as a visual cue thus flying in this direction. Chapter 4 investigated the impact a visual target would have on the direction of flight for house flies. The mean direction of flight was altered by the target but did not result in a significant shift in flight direction towards the target. More house flies captured when the target was raised and the aspect of the target facing the release point was sunward (reflecting sunlight). Overall, house fly flight behaviors are complex, and are likely impacted by all observed environmental variables. Changes in temperature and sunlight (time of day) had a wide-ranging impact on house fly flight behavior, affecting flight activity and directionality. It is anticipated that as sampling techniques become more refined the impact of weather on house fly flight behavior will become better characterized, but continued research is necessary.

Host seeking "canyon flies" (Fannia benjamini complex) (Diptera: Muscidae) cause significant nuisance to humans and animals. To determine if a barrier trapping system using attractive traps would reduce the number of canyon flies reaching a human host within a protected area, a barrier of CO₂-baited CDC-type suction traps (without light) was evaluated during the peak fly activity season at a location known for high F. conspicua Malloch activity in southern California. To select a suitable radius for the barrier, the effect of an operating CO₂-baited trap on the fly capture rate of a nearby human was evaluated with traps placed at 10, 20, 30 and 40m away from the human collector. The number of flies captured by the human collector was very significantly reduced by traps placed 10m away, only slightly by traps 20m away, and unaffected by traps > 30m away. Because of site characteristics and the rapidly decreasing human-trap interaction between 10-20 m, a 15m radius circular trap barrier was erected, comprised of 8 CO₂ traps. This barrier reduced F. conspicua capture by the human collector by 51 %. Reducing the barrier radius to 5m resulted in a 39 % reduction. The diel activity of F. benjamini Malloch and the "trail gnat", Amiota picta Coquillett, in the Carmel Valley, California was determined by hourly sweep net collections in September. Both species had a high activity period during early morning (0830-0930) with a second high activity period in the late afternoon and early evening (1630-1830). A. picta activity almost ceased during the inter-peak period, while F. benjamini remained active, though at reduced numbers, throughout the day. Capture rates of F. benjamini was higher at shaded locations than at sun-exposed locations. Neither the specific human collector nor the clothing color of a human collector had a significant effect on the rate of capture of F. benjamini. Substantially fewer F. benjamini were captured in a CO₂ trap relative to the number captured by a human collector, even when traps were baited with the host odors ammonia and 1-octen-3-ol in addition to the CO₂. The implications of these studies for management of canyon flies are discussed.

Culex tarsalis is a major public health concern as it transmits viral pathogens causing encephalitis in humans, birds, and animals. This study aims to shed light on the genetic structure and insecticide resistance profiles of Cx. tarsalis populations, particularly in the western United States where it plays a key role in transmitting viruses like western equine encephalitis virus (WEEV) and St. Louis encephalitis virus (SLEV).Using restriction-site associated DNA sequencing (RADseq) data collected from 32 populations across 13 states and two locations in Canada, this research identified major clusters as: (i) Coachella, (ii) Pacific Northwest, Rocky Mountain, and coastal California, and (iii) Midwest and desert southwest. Gene flow among neighboring populations was found to be widespread, thought to be facilitated by movement of mosquitoes among adjacent populations. Prevalence of permethrin resistance in female Cx. tarsalis was explored from five field sites across two regions in Riverside County with individual mosquitoes categorized based on their survival time post-exposure diagnostic dose of permethrin. Significant variation in resistance levels between populations from the Coachella Valley and the Inland Valley were identified and could potentially be attributed to differences in insecticide usage regionally. Additionally, the impact of age and sex on susceptibility to permethrin for Cx. tarsalis was explored. Older mosquitoes exhibited lower survival rates while similar survival rates were seen between males and females. The presence of knockdown resistance mutations (kdr) was assessed from mosquitoes tested in previous bottle bioassays using melt curve analysis and sequencing. All mosquitoes displaying resistance contained at least one copy of the phenylalanine (F) mutation. The challenges in accurately identifying heterozygous genotypes using melt curve analysis was examined. Overall, this research provides valuable insights into the genetic structure and insecticide resistance profiles of Cx. tarsalis populations. Continued monitoring and research efforts are crucial for effective mosquito control strategies and the management of insecticide resistance. Moreover, this study underscores the need for innovative approaches to combat the development of resistance and mitigate public health risks associated with mosquito-borne diseases.