SAN DIEGO — The county of San Diego will make a mosquito larvicide drop Thursday on more than 30 waterways around the county.

Vector Control officials conduct mosquito larvicide drops about once a month for half the year — April or May through October.

In recent years, a trio of non-native mosquito species have moved into the county, carrying tropical diseases including yellow fever, dengue and Zika. While there have been no known occurrences of the diseases in San Diego, scientists say the new mosquitoes are quieter and harder to kill.

The drops also aim to combat the spread of the potentially fatal West Nile virus. The virus is usually carried by birds but mosquitoes can transmit it to other animals, including humans, by biting them.

About 80% of people who become infected with West Nile virus don’t show symptoms, which can include headache, fever, nausea, skin rash or swollen glands, according to Vector Control officials. The virus is deadly in rare cases and most who show symptoms suffer an illness similar to the flu.

Residents are advised to combat local mosquito populations by emptying household items like buckets and plant saucers that may hold standing water. When outside, residents are advised to wear insect repellant that contains ingredients like picaridin or oil of lemon eucalyptus.

Residents can obtain mosquitofish from the county for free to control larvae and breeding in items like swimming pools and bird baths that may contain standing water.

The county also encourages residents to report local mosquito activity, dead birds and possible breeding areas to the Vector Control Program, which can be reached at 858-694-2888. Residents can learn more about the county’s efforts to counter local mosquito activity at its Fight the Bite webpage.