Although San Joaquin Valley citrus pests, for the most part, are manageable, fears of the Asian citrus psyllid (ACP) and the deadly citrus greening disease it transmits continue to hover in the horizon.
In an update on the most prominent insect and disease problems in SJV citrus groves, Beth Grafton-Cardwell, UC Davis entomologist, Parlier, gave a reminder that the ACP has been documented in Imperial and San Diego counties, although the disease has not.
The aphid-sized insect is the vector of huanglongbing (HLB), a bacterial disease that first destroys foliage and fruit and eventually claims the tree. The California citrus industry has put in place a task force dedicated to excluding the ACP and HLB and is screening plants in Riverside and in the SJV. Citrus plant movement in a Southern California zone is quarantined, and nurseries are cooperating with spray programs for ACP.
“An early symptom of the disease is yellow mottling of leaves, which resembles nutrient deficiency. One way to distinguish the disease is one side is yellowed while the opposite remains green,” Grafton-Cardwell said during a citrus meeting in Tulare. Infected fruit are misshapen, green, bitter, and small, have aborted seeds, and cannot be sold.
The condition can occur in five years after infection, and in China, where HLB is established, the life expectancy of trees has dropped from about 50 years to 15 years, causing entire regions to go out of production of citrus. There is no cure, and infected trees must be removed to prevent spread of the disease.
Grafton-Cardwell, who visited China recently to examine infected groves, said although the SJV climate is less conducive to the disease than that of China or India, it can survive here.
In addition to commercial citrus, the disease finds hosts on several citrus-related ornamentals, without homeowners being aware.
ACP goes to flushes of foliage and deposits eggs on new leaves causing a characteristic curling. Young stages appear with waxy tubules, and the adults are one-eighth inch long and a brownish-gray color.
Grafton-Cardwell predicted that even though HLB is not known to be in California, it will, sooner or later, enter either by illegally imported plants, most likely in someone’s backyard, or by the insect migrating on its own from Florida or Texas, via Mexico.
For commercial growers, ACP infestations could increase the number of insecticide treatments from two or three per year, to five to seven per year. “And a lot of the materials that work well on the psyllid are broad spectrum and will throw off our IPM programs. They are seeing that now in Florida with new pests flaring up,” she said.
Anticipating that change, she is investigating selective insecticides in the SJV to learn how they could be added to the existing programs. At the same time, she is monitoring research on control of ACP in Texas and Florida.
In reviewing other major insect pests, Grafton-Cardwell cited differences in two tiny moths, citrus peelminer and citrus leafminer, to avoid confusion. Peelminer goes to the rind of citrus fruit, and leafminer attacks flushes of foliage.
“Leafminers are primarily an issue for nursery citrus and orchards younger than four years old, and peelminers are a problem mostly on pummelo, grapefruit, and TI, Fukumoto, and Atwood navel varieties.”
Detection with pheromone traps and timing of pesticide treatments are less successful with peelminer than leafminer, although for some unknown reason there is an increase in effectiveness towards the end of the season.
However, she added, the traps can be useful early in the season to calculate a biofix for timing of peelminer flights. Of a potential for seven flights in the SJV, typically the third attacks pummelos and grapefruit and the fourth and fifth go to oranges. Growers can time treatments accordingly.
Grafton recommends Micromite to control eggs plus an organophosphate or pyrethroid at a low rate to deal with adults and larvae.
“I’ve seen all the pesticides for peelminer and although they work in the laboratory, in the field trials we only get about 50 percent control,” she said.
The reasons for this are peelminer deposits eggs low in the tree canopy where it is difficult for sprays to penetrate and when the fruit is rapidly developing in size, untreated areas are left on the rind.
That’s why she recommends not treating for peelminer unless damage is greater than 5 percent. One positive note: due to either weather or natural enemies, peelminer damage was less in 2008.
In contrast, citrus leafminer is building in the SJV. Traps at the Lindcove Field Station showed an average of 400 moths per trap in 2007, but that increased to nearly 1,000 in 2008.
“We start seeing leafminer moths in mid-May, but we don’t see significant leaf damage until the fall. It is mostly a problem for nurseries and new plantings.”
Several materials are available, and she said it’s best to get long-term systemic materials into young plants.
Grafton-Cardwell said the two mining pests have several parasites. Peelminers have been moderated by theirs, and if leafminer numbers do not start to decline with present natural enemies, Citrostichus, one now used in Florida, could be imported into California.
Citricola scale continues to be a challenge — its numbers expanding with the reduced use of organophosphate insecticides and aggravated by prodigious reproductive capacity.
Each female can produce at least 1,000 viable eggs, making a treatable threshold of one-half scale per leaf. The species resists biological control because parasites can reach only second instars for a brief period.
Grafton-Cardwell said citricola is developing resistance to Lorsban, which was long used to knock down populations of it for periods of two to three years. She said as much as 40 percent of the population now has resistance.
Newer materials, however, do not provide enough protection for more than one season. As a result, many SJV growers have been rotating other materials with Lorsban and treating for California red scale one season and for citricola scale the next.
The recently registered systemic, Movento, has performed well against red scale, and Grafton-Cardwell continues to research its rates, spray volumes, and application techniques.
She learned that the 500 gallons of water per acre volume is a trade-off between the higher dilution with 1,000 gallons and the lesser coverage with 250 gallons.
Although results have been varied, her trials suggest Movento needs oil to get into the tree. She added that it is very compatible with Aphytis, a natural enemy of the scale.
She plans to experiment with different irrigation regimes this year to see if they influence how long the chemical remains distributed in the tree.