By DOUG CALDWELL (Contact)
1:40 a.m., Friday, February 15, 2008


Anyone with citrus is familiar with these symptoms: the younger leaves become curled and distorted. Are there thin, silvery trails in the leaf tissue? Sometimes aphids will cause curling and cupping of foliage. But more often than not, it is damage caused by the citrus leafminer (Phyllocnistis citrella), the larva of a really small moth (4 mm wingspan!).

As with many of our pests, citrus leafminer is originally from southeast Asia. It was found in the Homestead area in 1993.

The key diagnostic to look for are the silvery “trails” just under the leaf surface. These turn gray with age and the crumbled leaves look pretty unsightly. Citrus leafminer is most commonly found attacking leaves of grapefruit and pummelo, orange-jasmine (Murraya paniculata), various Citrus spp., kumquat and calamondin. Here it is, Feb.8, and I’ve already seen the new set of leaves attacked on the grapefruit trees.

This leafminer seems to be present year round, especially when there is a flush of new growth. Depending on weather and temperate conditions, the development time from egg to adult (a “generation”) is about 13 to 52 days. Adults live for only a few days. In Florida, a new generation is produced about every three weeks.

People from northern areas may be familiar with other landscape leafminer pests such as birch and holly leafminers. The lifestyle of a leafminer larva may be somewhat claustrophobic, but they live the easy life. They don’t have to go far to search for their next meal. In fact they live and sleep inside their food, like living in the midst of a buffet table!

They are protected from wind and rain inside the leaf and hidden from many predators and parasites (to a certain degree) that may not readily see them.

People usually want to control the citrus leafminer to minimize the aesthetic damage caused by the distortion and graying of the infested foliage. Even I hate to look at those ugly, damaged leaves amongst the dark green, shiny foliage. But consider the age of the plant. Older citrus trees (more than 4 years old) generally tolerate leaf damage without any ill effect on tree growth or fruit yield.

Citrus leafminer is more likely to cause damage in nurseries and young plants in the landscape because the new growth is stunted. Trees are unlikely to die even though they may be under constant attack. Another insidious aspect of this damage is that the wounds made by the entry and exit of the citrus leafminer may be “portals of entry” for the citrus canker bacterium.

What To Do: Be alert to new growth on young trees that are less than 4 years old. Treatment timing is tricky. It has to be early, before the tender leaves are half grown.

Citrus trees’ branches don’t seem to break into new growth at the same time. Sporadic and not simultaneous bud expansion seems to be the citrus way of growth. This makes effective control up to the homeowner rather than a landscape maintenance company. The homeowner can check the trees on a weekly basis and spray the newly expanding foliage as it appears on each branch.

There are reports from California of a pheromone trap which can trick unsuspecting males into a sticky trap thinking that there is a female moth waiting there. If you are familiar with gypsy moth traps, you know what I mean.

A pheromone trap approach will greatly aid monitoring moth flight periods and maybe reduce damage. The pheromone is available from ISCA Tech in Riverside, Calif., and other companies, such as Suterra, may have it by now as well. ISCA Tech’s list of lures is at this Web site:

www.iscatech.com/exec/lures.htm

A parasitic wasp, Ageniaspis citricola, from Australia was released in 1994-1995 by the USDA and appears to be doing a pretty good job.

Foliar sprays of 2 percent horticultural mineral oil or a product labeled for fruit trees that contains spinosad is in a Ferti-lome product called Borer, Bagworm, Leafminer & Tent Caterpillar Spray (0.5% spinosad).

Be sure that any product you spray is labeled for fruit trees.

Also avoid spraying the flowers; the oil may make them drop prematurely and other pesticides may kill bees. The oil will also help suppress the asian citrus psyllid which vectors the citrus greening disease. These psyllids also swarm the new growth like ticks on a dog.

The mineral oil spray is a good approach for the all-too-common green scale (Coccus viridis) and the sooty mold that it produces.

For more information on these pests, Google UF Featured Creatures and search for the specific insects.

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Doug Caldwell, Ph.D., is the commercial landscape horticulture extension educator with the University of Florida Collier County Extension. E-Mail dougbug@.ufl.edu ; call (239) 353-4244 ext. 203.


Source:
http://www.naplesnews.com/news/2008/feb/15/extension-service-leafminer-unfriendly-not-deadly-/

Here in Colorado due to our cold winters, leaf miners do not exist. Thanks for the post. - Millet

The pheromone traps are good news if they are not too expensive-- has anyone priced them?

That would be "possible" I suppose, but when the greenhouse exhaust fans come on they would be gone. The exhaust fans totally exchange 100 percent of the internal greenhouse air every one minute. I once tried ladybugs, but when the fans came on they were soon gone. I guess I could screen all the vents, but never have. Because of the winters, Colorado does not have much of a bug problem. - Millet

Leaf miners won't kill a young tree but they sure can aggravate one half to death and the grower too. They are about my worst citrus pest. I'm waiting on this current blossom flush to pass so that I can spray my trees with hort oil to discourage them and get rid of some persistent scale the last spraying didn't eliminate.

.....Alan.

Is it not recommended to spray oil during the bloom? -- Even if you avoid the flowers? I know if you are using a large mechanical sprayer, you can't avoid spraying bees (except by spraying at the right time of day), but with a small hand sprayer you can be more selective.

I'm not sure how one would avoid the blossoms while still getting a good coverage of the leaves using any sort of sprayer. They're rather intimately mingled. I've read avoid spraying blossoms with oil.

.....Alan.

I am pretty selective when I spray --and I try to avoid my predatory insects like assasins, lacewings and ladybugs. I usually target just the new growth limb by limb. Once the new flush has started, the primary new growth usually takes off and leaves the flowers back at the start. If I recall, I did not see many leafminers early last yr, so I probably didn't spray during the blooom on most of my trees-- I do remember spraying the kumquat while it was flowering-- I waited til almost dark to avoid the bees (the southeastern blueberry bee does most of our pollinating now).

I don't expect to see much leaf miner activity for another few weeks now when the weather warms up for good. The blossom peak is passing so by then I expect to be able to spray. If I can stay on top of the miners this will make grasshoppers my biggest pest. I have half a mind to move all the container citrus into the henyard for the summer.

.....Alan.