The Brief

As overnight temperatures rise, Culex mosquitoes begin to hatch. They can carry the West Nile virus, which infected more than 420 people in California last year, killing 19 of them.

Another kind of mosquito is being tested for dengue virus after two people in Los Angeles County contracted the virus last fall, the first locally acquired cases in California. Both people survived.

There’s no local dengue now, but experts say the virus is likely to crop up again and the pesticides used to kill mosquitoes that can spread it have lost some of their bite.

That’s because overuse of over-the counter herbicides can cause the insects to become resistant to the chemicals, said Tristan Hallam, director of scientific programs with the San Gabriel Valley Mosquito and Vector Control District.

Some weed killers contain active ingredients similar to pest control sprays, he said.

“Because it’s being used in two separate scenarios on a consistent basis, there are more opportunities for these adult mosquitoes to have that resistance build up within these populations,” Hallam said.

Hallum tests local mosquitoes and rotates which pesticides vector control uses when too many show resistance.

The best defense is to prevent mosquito bites, and keep the insects from getting in your home or breeding nearby. Here’s how:

Think like a mosquito

• Check and repair holes in screens to keep mosquitoes outdoors.
• Stop mosquitoes from laying eggs in or near standing water. Once a week, empty and scrub, turn over, cover, or throw out items that hold water, such as tires, buckets, planters, toys, pet bowls, flower pot saucers, rain barrels, or other containers. These are places where mosquitoes lay eggs.
• Empty and wash birdbaths and wading pools weekly.
• Clean and chlorinate swimming pools; drain any water collecting on pool covers.
• Stock garden ponds with fish that eat mosquitoes such as mosquitofish, goldfish, or koi. These feed on mosquito eggs and larvae.
• Report stagnant swimming pools or “green pools” to the Public Health Environmental Health Bureau at (888) 700-9995, or to a local vector control agency.

How can I protect myself from mosquito bites?

Adults should use insect repellents, wear long sleeves and pants, and try to stay inside at dawn and dusk, when mosquitoes are most active.

How to choose an insect repellant

The label will usually list mosquitoes as one of the insects that the product repels. The label might also list diseases mosquitoes can carry that the product will help prevent, such as West Nile.

For hours of long-lasting protection, look for insect repellents with the following active ingredients, according to the Centers for Disease Control and Prevention.

• DEET
• IR3535
• Oil of lemon eucalyptus, or OLE. (”Pure” oil of lemon eucalyptus — essential oil — is not recommended. It has not been tested by EPA for safety and effectiveness.)
• Picaridin, or KBR 3023

Some brand names of repellents include:

• DEET products: Off!, Cutter, Sawyer, Ultrathon
• IR3535 products: Skin So Soft Bug Guard Plus Expedition, SkinSmart
• OLE products: Repel, Off! Botanicals
• Picaridin products: Cutter Advanced, Skin So Soft Bug Guard Plus

To protect children and babies from mosquito bites, the California Department of Public Health recommends:

• Dressing children in clothing that covers arms and legs
• Cover a child's crib, stroller, or baby carrier with a mosquito net
• Use insect repellent according to label instructions
• Do not use repellents with oil of lemon eucalyptus or para-menthane-diol on children younger than 3