Earlier this week, one of my neighbors told me that her Meyer lemon tree has had a bad infestation of scale insects for several years and that all of her attempts to eradicate the pests had failed. I took a look at her tree and agreed that she had a definite scale infestation, likely Citrus Soft Brown Scale and/or Citricola Scale. What to do? She is a determined follower of organic gardening guidelines so was hoping I could supply her with a magic bullet to solve her problem but one that would not involve using anything not approved by the Organic Materials Review Institute (OMRI) or accepted as recommended practice in an Integrated Pest Management (IPM) program. I assured her that I knew of no magic bullet but that with consistent treatment, she should be able to overcome the scale problem.
Why is citrus scale a problem? It is discussed at http://ipm.ucanr.edu/PMG/r107301311.html, the University of California’s IPM site. It says this about the damage scale insects can do: “Heavy feeding by the soft brown scale reduces tree vigor, kills twigs, and reduces yields. Sooty mold grows on excreted honeydew and may affect fruit grade. The honeydew also attracts ants, which interfere with the biological control of a number of pests.” And I would add that in a small garden, the unappealing look of the scale and the sooty mold that usually accompanies it definitely damages the appearance of a tree.
So what treatment is recommended? The website describes the basic approach: “Management of brown soft scale focuses on preserving its natural enemies and controlling ants.” That mention of controlling ants is important; ants guard scale insects from attack by predators because they feed on the honeydew. And though I have no proof, I suspect that ants may actually move scale from one plant to another, restarting an infestation after the gardener thinks she has the problem licked.
So how to control ants on citrus? The only method I know that is truly effective is to block the ants’ access to the tree canopy by applying one of the sticky trap products. The gardener first makes sure that no part of the tree canopy touches anything that would allow ants access (for example a citrus twig must not touch the soil, a fence, a wall or another tree.) Then masking or paper tape is tightly wrapped around the lower trunk of the tree, and the sticky trap substance is applied on top of the tape. (Don’t allow the sticky stuff to touch the bark of your tree as it will kill the bark and the underlying tissues. And for goodness sake, don’t accidentally get it on your hands or in your hair, as I once did, as it is nearly impossible to wash out!) The sticky trap material will form a barrier to the ants so that they will no longer be able to get past it and up into the tree. (Renew it if you notice that it is losing its sticky nature or debris has become stuck in it, providing a way that ants could cross over the trap.)
How to then eradicate the scale that is on the tree? Regular chemical insecticides, or even soap sprays are not effective but horticultural oil is: it smothers the pests, so the next step is to spray your tree thoroughly with one of the mineral oil products sold specifically as OMRI-approved insecticides. Follow the directions on the bottle for plants that are evergreen, or in leafed-out or growing condition; don’t use the directions for dormant spraying since citrus trees don’t actually go dormant. As a bonus, the oil will smother other sucking insects such as aphids as well. The unsightly sooty mold will gradually disappear once the insects and their honeydew are gone.
For appearance sake and to increase the chances that you’ll adequately cover the pests with the oil, you might first rub off as much of the scale as possible, and on badly infested twigs, even prune them away. Follow the first spraying with another one a few weeks later to be sure you hit not only the obvious adults but also the nearly invisible crawling stage.
Thereafter, regularly inspect the tree, keep the ant barrier active, any new growth pruned away from touching anything, and respray with oil immediately if you notice any signs of a repeat attack. And I probably don’t need to remind you that a tree receiving what it needs for maximum vigor (sun, well-draining slightly acid soil, deep irrigation during the dry season, and possibly a fertilizer containing iron chelate) will be better able to fight off insect attacks of any kind.
Garden tips are provided courtesy of horticulturist Sharon Hull of the San Lorenzo Garden Center. Contact her at 831 423-0223.
