California’s varied climate and diverse ecosystems mean that tick activity is not confined to a single “season,” requiring awareness year-round. These small arachnids pose a public health concern because they can transmit various disease-causing organisms to humans. The cycle of tick activity in the state is primarily influenced by environmental factors such as moisture and temperature. Understanding the specific times and places where ticks are most active is the first step toward reducing the risk of a bite.
Understanding Tick Activity Across California’s Seasons
In California, tick activity is possible throughout the entire year due to the mild climate. The true “tick season” is driven by moisture and moderate temperatures, often resulting in two distinct peaks of activity. The most significant peak occurs during late winter and early spring, with a second period sometimes resuming in the fall as temperatures cool and moisture returns.
The life cycle of the Western Black-legged Tick, the primary species of concern, involves three host-seeking stages: larva, nymph, and adult. Adult ticks are most active during the cooler, wetter months, from late fall through early spring. The nymphal stage poses the greatest risk to humans because these ticks are tiny and difficult to spot. Nymphs are most abundant in the spring and early summer, from March to July, coinciding with increased human outdoor activity.
The Most Common Tick Species and Associated Risks
The species of greatest concern in California is the Western Black-legged Tick (Ixodes pacificus), which is the principal carrier of the bacteria that causes Lyme disease in the western United States. Only the nymphal and adult female stages of this tick can transmit the Lyme disease bacteria to people. This tick can also transmit the pathogen responsible for Anaplasmosis, another bacterial illness.
Several other hard tick species are common across California and transmit different pathogens. The Pacific Coast Tick (Dermacentor occidentalis) is widespread and can transmit the bacteria that causes Pacific Coast Tick Fever, a form of spotted fever, as well as Tularemia. The American Dog Tick (Dermacentor variabilis) is found along the coastal ranges and can transmit the bacteria for Rocky Mountain Spotted Fever and Tularemia. Lyme disease is the most reported tick-borne illness in the state.
High-Risk Habitats and Regional Distribution
Tick risk in California varies significantly because of the state’s diverse geography, which ranges from coastal zones to arid desert regions. Ticks thrive in environments that offer shade and moisture, making wooded areas, chaparral, and grasslands the highest-risk habitats. They are commonly found on low-lying vegetation, such as tall grasses, shrubs, and leaf litter, waiting to attach to a passing host.
The greatest concentration of risk is found in the northern coastal counties and along the western slope of the Sierra Nevada foothills. These regions, which include oak woodlands and mixed conifer forests, provide the ideal cool and moist conditions for the Western Black-legged Tick. While coastal and mountainous areas present the highest risk, ticks can still be found in suburban backyards adjacent to natural open spaces. The arid, southeastern desert regions present a lower risk due to the lack of sufficient moisture to support tick populations.
Essential Prevention and Post-Exposure Steps
Mitigating the risk of tick-borne illness begins with proactive prevention when spending time outdoors in high-risk environments.
Prevention Steps
- Apply an Environmental Protection Agency (EPA)-registered insect repellent containing ingredients like DEET or picaridin to exposed skin.
- Treat clothing and gear with a product containing 0.5% permethrin to repel or kill ticks on contact.
- Wear light-colored clothing to spot ticks easily, and tuck pants into socks to minimize access to the skin.
- Conduct a thorough tick check on the entire body after returning indoors, focusing on the hairline, groin, underarms, and behind the knees.
- Shower within two hours of leaving a tick habitat to wash off unattached ticks.
If a tick is found attached, prompt and correct removal is necessary to minimize the chance of disease transmission. The risk of infection increases after 24 hours of attachment. Use fine-tipped tweezers to grasp the tick as close to the skin’s surface as possible. Pull upward with slow, steady pressure; avoid twisting or jerking, which can cause the mouthparts to break off in the skin. After removal, clean the bite area and hands with soap and water. Watch for symptoms such as a bullseye rash, fever, or flu-like illness in the weeks following a bite, and seek medical attention if these signs develop.