Citrus leafminer did ugly things to the fall foliage flush in many San Joaquin Valley citrus groves, and although it is a threat to new plantings and nursery stock, growers of mature trees need not be alarmed.
That's the word from Beth Grafton-Cardwell, University of California (UC) Extension entomologist at Parlier, who recently assembled growers and PCAs at the Lindcove Research and Education Center near Exeter to talk about the tiny moth.
“Don't panic about leafminer. Everybody will probably have it by next year. It thrashes new foliage, but it will not devastate the industry, and we will live through it,” she said.
She called the meeting to assist growers in distinguishing citrus leafminer, Phyllocnistis citrella, which prefers new foliage and rarely goes to fruit, from the more familiar citrus peelminer, Marmara gulosa, which goes to other tree parts but typically disfigures the rind of citrus fruit, making it unmarketable. In some instances, both may be found on the same tree.
A new UC brochure detailing the two and how to identify them will be released by early 2008. The information will also be available on the Internet.
When they hatch from eggs, larvae of both species dig shallow tunnels, or mines, into the immediate plant surface. This makes the larval stages difficult to reach with contact insecticide sprays.
Grafton-Cardwell explained that leafminers burrow into either upper or lower surfaces of leaves causing curling and distortion. Their wavy pattern of mines does not cross.
Peelminer mines, however, do cross each other. A heavy infestation of leafminers can give a grotesque appearance to a new flush of foliage.
“In the San Joaquin Valley,” she said, “we tend to see most leafminer activity in the fall citrus foliage flush when the trees can tolerate it.
“So far, we've seen that the spring flush, when fruit is forming, is not really affected by leafminer, perhaps because of mortality during the winter or parasite buildup in the fall. However, one exception can be coastal lemons that produce multiple crops. Leafminer gets started in June and can suppress yields on the coast.
“In San Joaquin Valley commercial citrus orchards where the trees are more than four years old, I would not be treating for leafminer. That would be costly, disrupt natural enemies, and cause more problems than it would solve.”
However, Grafton-Cardwell added, on new citrus orchards, leafminers can damage the young trees like thrips or aphids.
She recommended treatment during the first two years with Admire (imidacloprid), which will depress, but not eradicate, leafminers enough to enable the small trees to outgrow the problem. That practice, she said, has been successful against the pest in Israel.
Nursery citrus, too, are highly vulnerable to the leafminer, and heavy infestations can retard growth. Grafton-Cardwell and a number of her UC colleagues are working on additional solutions. Several insecticides available for mature trees are not registered for non-bearing trees.
“That's where most of our pesticide screening efforts are directed now,” she said. “For the nursery situation, we are close to getting a Special Local Need registration for Intrepid, and we are working with the IR4 Program on a full registration for it in bearing citrus.”
Grafton-Cardwell said she has set out new citrus seedlings for her part of the insecticide screening process. She said the nursery industry in Australia has been using 0.25 percent refined oil treatments at a frequency of every 10 days to prevent egg laying. She added that whether this would be satisfactory for California varieties and conditions is not known, but she plans to investigate it and other paths.
Often, she noted, it is easier to obtain new registrations for products than to seek to modify existing registrations. She said she wants to evaluate several materials effective on lepidopterous pests in other crops but not previously considered for citrus.
Beyond chemical means, she also wants to learn more about the possibilities of trapping male leafminer moths before they can mate. Perhaps that, in combination with Admire and/or other pesticides, would provide a management strategy for leafminer in nursery situations.
Synthetic pheromone trapping to monitor leafminer at the Lindcove station has been very effective, and traps there charted the buildup of moths from June to a peak in late September and a rapid decline by the end of October.
While other means are being sorted out, Grafton-Cardwell said her notion at the moment is that for the long term, biological control is the answer for commercial orchards.
Among natural enemies is a wasp, Cirrospilus coachelli, which attacks both leafminer and peelminer. The wasp is established in coastal and southern counties and has been introduced to the SJV, where, she said, she feels it may be successful because the two moth species now provide a year-round food source.
A key element in the biological approach would be avoiding broad-spectrum sprays and other practices that would disrupt the natural enemies. Other parasites could be introduced if needed.
In discussing other matters relating to leafminer, she said it overwinters as pupae in outer foliage flushes, exposed to extreme cold. “A freeze has greater impact on leafminer than peelminer, which can overwinter just about anywhere on the tree.”
Leafminer is restricted to citrus and closely related species, while peelminer finds hosts in a wide variety of plants.
The leafminer's entry into California from Mexico has been traced to 2000 and it has since become established in Southern California citrus. “We've been watching it move northward into the San Joaquin Valley. We are seeing it here and there this year, and next year we expect to see more,” Grafton-Cardwell said.
Although difficult to see with the naked eye, larvae can be detected with a hand lens on leaves. Pupae can be found hidden in their silk in the curls of mined leaves. An infestation needs to be detected early before it can move into the entire branch.
David Haviland, Kern County farm advisor, was also on hand at the Lindcove gathering and said backyard citrus in the Castaic area was heavily infested by leafminer in the fall of 2006 and it has since moved north into his county.
In Florida, citrus leafminer creates wounds allowing entry of citrus bacterial canker. That disease has not been found in California, and annual surveys and regulations are in place to prevent its introduction.