Experts tackle readers’ garden questions.
Q: I have a 10- to 15-year-old navel orange tree that has just a few leaves that are very deformed. Is this a concern or just frost damage?
Glenn Jue, Sacramento
Bee garden writer Debbie Arrington: That’s not frost damage, but the work of the citrus leafminer, according to Sacramento’s “bug man,” Baldo Villegas.
“It’s just cosmetic,” he said. “It won’t harm the fruit.”
The citrus leafminer (Phyllocnistis citrella) is a tiny moth – its wingspan is only a quarter-inch – that first invaded California from Mexico in 2000. It’s since spread throughout the state.
These critters are most active from midsummer through fall and early winter, when citrus trees are growing new foliage. This moth gravitates to newly emerged leaflets or “flush.” Active at night, the moths rest during the day on the underside of leaves, where the females also lay their eggs.
Once the eggs hatch, the larvae start eating immediately in a meandering pattern on tender leaves, distorting the foliage. As the larvae mature, these shallow feeding trails or “mines” fill up with larvae poop creating visible frass trails. The larvae pupate inside their mines, rolling the leave edges and protecting the pupa with silk. Their whole life cycle can take only two weeks or as many as seven weeks, depending on weather.
According to University of California research, the citrus leafminer attacks oranges, mandarins, lemons, limes, grapefruit and other citrus. Usually, trees that are more than 4 years old tolerate the leaf damage with no ill effects. Leafminers may retard the growth of younger trees and can be problematic in nurseries or young orchards.
Parasitic wasps control citrus leafminers in other regions where this pest is active but have not caught up with the recent California invasion. Don’t apply fertilizers rich in nitrogen when leafminer populations are high; new foliage will be severely damaged.
UC integrated pest management guidelines recommend patience. These attacks usually are short-lived. Once the leaves mature, they harden and the leafminers can’t feed. The damaged leaves can stay on the tree; although ugly, they still provide food for the tree.
For more information on citrus leafminers, check out the UC IPM pest notes online at http://ipm.ucanr.edu/PMG/r107303211.html.
The Bee’s Debbie Arrington is a consulting rosarian and lifelong gardener. email@example.com, 916-321-1075, @debarrington.
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