Larvae of the tiny moth, Marmara gulosa, burrow on the surface of the rind, causing unsightly, serpentine blisters, or mines. Although it prefers the fruit of grapefruit, Lane Late oranges, pomelos, and limes, the pest also goes to leaves and suckers.
Beth Grafton-Cardwell, University of California research entomologist at the Kearney Agricultural Center, Parlier, says, "It’s been in the SJV for a number of years, but only since 2000 has it been found on late citrus varieties and other crops, such as cotton, Red Globe grapes, stonefruit, avocado, walnuts, and peppers." It was documented in Pasadena on oranges as early as 1915.
Grafton-Cardwell, best known for her work on aphid-transmitted viruses, and other scientists are tracking the peelminer in a project supported by the California Citrus Research Board (CCRB).
The SJV outbreak is blamed on a shipment of citrus from Mexico to Tulare County in 1998. Known previously to go to grapefuit, oleander, and willow in relatively small numbers, the miner was not considered a crop threat at the time. The Mexican fruit may have carried a new sub-species of the moth.
Difficult to control
"It is very difficult to control with insecticides," she told a recent CCRB meeting in Tulare. Her pesticide trials in 2001 showed the miner, which kicks out six to eight generations each summer, resisted the arsenal of materials from organophosphates and pyrethroids through neonicotinoids. She suspects that is so for multiple reasons.
"For one thing, we had no pheromone traps to guide timing of treatments. The larvae in their mines are well protected. At the same time, the fruit is growing rapidly."
The good news, however, is that the mining insects are vulnerable to biological controls, one being the Cirrospilus coachellae wasp collected in the Coachella Valley, where the miner hit citrus hard in 1995. UC, Riverside entomologists are rearing it for release in the SJV to supplement existing parasitic wasps.
"The good thing about the wasp is it lays more than one egg per miner larvae, so you get more bang-for-the-buck. It brought miner populations in Coachella Valley down to manageable levels, and we hope it does the same here."
Researchers also found that when miner populations reach a certain level, a virus appears as another natural control.
Meanwhile, entomologists with UC, Riverside, the Kearney Ag Center, Cal Poly San Luis Obispo, and the California Department of Food and Agriculture are working on developing pheromone techniques and finding new parasites in Mexico.
"Ultimately, I expect biological control will be the only solution to control of this pest," she said. A part of the suggested approach is to use milder materials in citrus to encourage the parasites to become established.
The issue of quarantine restrictions for citrus peelminer "is heating up," she said. Still unclear is whether its larvae can survive the washing and waxing of citrus. "I doubt it, but we will have to have proof for foreign countries concerned about it."
She plans to collect infested fruit and run it through a packing line this season. For the project, she asked that citrus growers finding citrus peelminer-infested fruit contact her at KAC, telephone (559) 646-6591.
Another question is what’s ahead for table grapes -- particularly varieties exported under stringent restrictions -- which go through no washing before shipment.
Since multiple crops are concerned, Grafton-Cardwell has convened UC specialists to discuss a united effort. The pooling of information revealed a notable sidelight: much less peelminer showed up on Bt cotton.
She said citrus growers might consider encouraging use of Bt cotton near their groves, perhaps reimbursing cotton growers for the added costs of the genetically engineered varieties.
For the moment, growers can do something to manage the peelminer by controlling its several weed hosts, including morningglory, pigweed, and purslane. Another measure is removing all mined fruit three to four weeks before new fruit sets in the spring.
Citrus growers have another relatively new pest, citrus leafminer, to contend with, although it does not pose the broad economic threat of the peelminer. It has spread from Asia throughout the world, reaching Florida in 1993 and since moving westward to the Imperial Valley by 2000. Observers expect it could easily move to the SJV.
Two tiny wasp parasites, one of them the same that attacks peelminer, control it on mature trees, and no pesticide treatments are recommended.
On non-bearing citrus, however, heavy populations of it can damage new flush foliage and reduce vigor. Treatments for infested trees two to four years old are either Admire as a systemic or Agri-Mek and oil as a foliar application. The number of applications is restricted.
Organophosphates, carbamates, and pyrethroids showed no effect on citrus leafminer.
Grafton-Cardwell recommended growers become familiar with damage caused to leaves by both miner species to distinguish them.
Another CCRB researcher, Mikeal Roose, professor of genetics at UC, Riverside, reminded growers that watching the neighbors is still the best guide when selecting a citrus rootstock. "If you haven’t tested alternatives on your ground, find out what other people near you are using."
However, he also named other basic considerations in selecting from a dozen major and eight minor rootstocks. In addition to compatibility with the scion chosen, growers need to consider the intrinsic fruit quality desired. The vigorous Volkamericana, for example, is a match for mandarins where a high solid and acid content is sought, but it is not a good choice for satsumas.
"Another issue is fruit size. For navels, you want a rootstock that reduces fruit size," he said.
Soil type can’t be overlooked. Calcareous or saline soils must be considered, along with drainage.
The three major disease problems to remember are phytophthora, citrus nematode, and citrus tristesa virus.
The genetically identical Troyer and Carrizo rootstocks are most commonly used in California, and have the positives of good balance of traits and low risk. On the other hand, Roose warned, they perform poorly on calcareous soils, have only moderate yield-to-tree-size, and are somewhat susceptible to phytopthora and citrus nematode.
In his CCRB trials on phytophthora at UC, Riverside from 1995 through 2000, James Adaskaveg, plant pathologist there, learned that Naiad, a surfactant, when metered into irrigation water, controlled the disease with greater efficacy than the fungicide Ridomil.
Naiad, he explained, disrupted the zoospore colonization of the root-rot pathogen. Used primarily as a soil amendment to enhance water penetration, the blend of non-ionic and anionic elements is not registered as a pesticide and therefore, technically, cannot be used as such.
"It amounts to a regulatory glitch," he said. If Naiad were to be registered as a pesticide, its manufacturer would have to undergo the costly registration process. He said other surfactants with a similar content of non-ionic material would perform likewise.
Use of Naiad, at 20 parts per million on a weekly basis, he said, was calculated at $150 to $170 per acre, per year.
New unregistered fungicide formulations form a threshold of "a revolution" in control of one or more phytophthora life-cycle stages, in an integrated approach with surfactants and Ridomil, he added. Fenamidon and mefenoxam attack zoospores, while mefenoxam and iprovalicarb act against mycelia and sporangia.
"There are now more new fungicide formulations than in the history of agriculture, more so than even insecticides and herbicides," he said.