About a month ago a neighbor noted how poor my lime tree looked. She said her lemon tree didn’t have those “curly leaves” because she sprays it. A couple days later she brought over a blue bottle of the stuff she uses: Bayer Advanced — Fruit, Citrus and Vegetable Insect Control.
She was right. My lime tree did look ragged, its foliage sparse and damaged from having leafminers the first two years of its life. Citrus leafminers are the insects whose larvae eat zig-zag tracks, or mines, in leaves. The leaves become curled and unsightly after the leafminers exit and turn into tiny moths. The moths then lay eggs which hatch and eat mines through leaves again, and so on.
I’m leery of spraying though. The only time I’ve ever done it was five years ago when I sprayed neem oil on a lemon tree because it was infested with aphids. The tree remained infested with aphids — until I took other measures to control the ants which were protecting the aphids from predators. I’m just not convinced of the effectiveness of many sprays, and I’m always questioning whether whatever short-term benefits they do have might be outweighed by broader, long-term harm.
But my lime tree looked so bad. It had yet to produce a single piece of fruit, and I began to think that spraying might be the only way to control these leafminers and keep the tree alive.
What’s in this blue bottle? Immediately, the active ingredient jumped out at me: imidacloprid. This chemical is the most widely used insecticide in the world. It has also had its use severely restricted by the European Commission starting in December 2013 due to its effect on honey bees.
Bayer CropScience, the developer and primary manufacturer of imidacloprid, both fought this decision by the European Commission and acknowledged the toxicity of the chemical to honey bees on the blue bottle I held in my hands. From the label: “This product is highly toxic to bees . . .” To reduce the risk of harm to bees, the label therefore recommends to “not apply until after trees have flowered or when bees are actively foraging.”
What happens is imidacloprid is taken into a tree’s system so that when a leafminer larva eats into a leaf it consumes some of the chemical and dies. But flowers are also part of the tree’s system, and when bees drink nectar from the tree’s flowers they also consume imidacloprid. They may not die, but there is sub-lethal harm all the same.
My lime tree wasn’t blooming at the time — this was February — but I knew that it was about to have its first bloom of the year. Then I read on the University of California’s Integrated Pest Management webpage for imidacloprid that the insecticide “can move into nectar, so don’t apply prior to bloom.” That was specifically referring to applying an imidacloprid product on the soil, however, not on the leaves. So I’m OK to spray now?
The U.C. I.P.M. page continued, “Foliar application is not recommended because of negative impacts on natural enemies and bees.”
I was scared. It seemed that if you used imidacloprid in any way, you were taking quite a risk. In fact, you might be making a trade off: kill the leafminers at the expense of likely harm to bees and other insects that are beneficial.
I gave the blue bottle back to my neighbor, unused.
More important than the health of my single lime tree was the health of the bees in my yard generally. My avocado trees and apricot and peach and orange trees were all about to start blooming too, not to mention all of my vegetables, and I needed these bees to help pollinate the whole yard. It was really a trade off between killing leafminers at the expense of less food throughout the entire yard.
In late February, the lime bloomed and it flushed new growth and it dropped many of its old and leafminer-damaged leaves, and here’s how it looks now:
To my surprise and delight, the foliage is as uniform and vibrant green as I could color with crayons. There are no leafminers apparent anywhere. How could they have been so populous and damaging last year and now non-existent? And how could this have happened without my help?
The UC IPM webpage about citrus leafminers says, “the best course of action is to leave it alone and let the natural enemies of the citrus leafminer feed on and parasitize the larvae in the mines, rather than trying to control this pest with insecticides.”
Natural enemies? These include tiny wasps such as Cirrospilus and Pnigalio species. The UC IPM webpage goes on to explain that they “lay their eggs inside the mine, inside or on top of the leafminer larva. When the parasite egg hatches, the parasite larva consumes the leafminer larva.”
My deduction is that these natural enemies of the leafminers have eaten the leafminers near my lime out of existence. (See my update from January 2019 below.)
UC IPM: “In other areas of the world where the citrus leafminer invasion is long established, the experience has been similar: a high level of damage to citrus in the first year or two is followed by decreasing severity due to natural enemies parasitizing or consuming leafminers. These natural enemies, which are already present in the environment, survive by seeking out mining insects in which to lay their eggs. Eventually, the leafminer populations decline as the population of natural enemies increases.”
My lime tree didn’t need my help, and it certainly didn’t need the “help” of imidacloprid. It needed only to be left alone. What if these facts were also stated on the label of the blue bottle of Bayer Advanced?
UPDATE May 2016:
It’s now a year after deciding not to spray imidacloprid, and this spring’s flush has just finished on the lime as well as the other four citrus trees in the yard. Not a single leaf with leafminer damage on any of them.
UPDATE January 2019:
I left this post three years ago giving the possible impression that there were no longer leafminers damaging citrus trees in my yard. That is not the case. Citrus leafminers continue to be present on some of my dozen citrus trees.
Here is how leafminer activity has looked over the past few years: Some trees have leafminers in some years while others almost never have any leafminer damage. For example, here is my Valencia orange, on which I’ve only ever found a single leaf with a leafminer in it (I have no explanation for this):
Further, on other trees, the leafminers are present on some flushes of new growth (they always and only attack new growth) but are not present on others. For example, here is the Bearss lime tree discussed above, now three years later:
Look at the foliage up close. The newest growth is untouched by leafminers, but the previous flush of leaves had leafminer damage:
The leaf flush that tends to get attacked by leafminers most is the summer flush.
In the end, the conclusion remains: Don’t spray and the bees won’t be harmed and the natural enemies of the leafminer will do their work. Their work, however, is not to exterminate their food source but rather to keep it to a tolerably low level (from our perspective).
UPDATE December 2020:
On my citrus trees young and old I’ve still never bothered to spray for any reason, including for leafminers, and they continue to grow and produce well. The leafminer damage continues to occur only to some trees and only on some leaf flushes; it is only cosmetic and does not affect tree vigor or fruit production in any way as far as I can tell.
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