Thanks to conducive weather conditions and state restrictions on how citrus is transported within the state, California still has the upper hand in controlling huanglongbing, or HLB, the citrus greening disease that has devastated Florida’s groves.

The disease is present in residential citrus but has not infiltrated commercial groves. Spraying for the Asian citrus psyllid — the vector of HLB — has been more successful in some areas than others.

“It’s slower than we expected, compared to Texas and Florida,” said Elizabeth Grafton-Cardwell. “In the Central Valley, homeowners and growers have been able to eradicate the pest, although it’s been much more challenging in Southern California. But growers are doing an outstanding job of controlling the psyllids.”

She is the director of the Lindcove Research and Extension Center with the University of California’s Department of Agriculture and Natural Resources and an entomologist with UC-Riverside, focused on citrus pest management for the past 30 years.

The majority of California’s citrus crop is grown in the Central Valley, with about a quarter of it in Southern California.

The state requires nurseries to treat orchards for psyllids before they ship plants, a requirement that is helping control their spread. In the Central Valley, where the weather is dry and hot, psyllids have a hard time surviving since they require humid conditions.

“The dry climate makes the leaves harden up really fast on the trees,” Grafton-Cardwell explained. “And the psyllid looks for soft flushed leaves to leave its eggs on, which Southern California provides but here in San Joaquin Valley, the heat hardens the leaves.”

This is why Florida’s humid conditions are much more conducive to the psyllid.

Central Valley growers have also been spraying earlier, in winter, which reduces chances of the pest gaining ground in the warmer months. The main reason it makes gains in residential trees is because homeowners can be more reluctant to spray, although they have cooperated with the California Department of Food and Agriculture in reporting instances and having trees with HLB removed. CDFA has removed about 1,800 trees so far, she said.

The incidences of HLB in residential areas have begun to get closer to commercial groves, so she thinks it’s a matter of time before it affects commercial trees. But so far, there have been no infected trees.

“We do a lot of education with homeowners and growers. We have master gardeners going door to door to educate people,” Grafton-Cardwell said. “We estimate about 60% of Californians have citrus trees in their yard, so that means maybe 15 million trees.”

Organic growers are also spraying products frequently. Biological controls, such as tamarixia radiata have been deployed but are more effective in residential citrus in urban areas than in commercial groves. Also, as she pointed out, bio control is difficult when pest populations come down, because when parasites can’t find them they disappear from the area.