Hello Mr Sakovich. About a year ago I wrote to you concerning my problem with ‘sooty mold.’
You recommended an oil/soap mixture to kill the scale and other insects that secrete a honeydew upon which the fungus grows. That has been a big help, thank you.
My continuing problem with this black fungus on my plants stems from an infestation of these insects in my taller palms [especially the Alexandras, but also in Foxtails , Royals etc. ] which I cannot reach with any type of spraying equipment that I have found. I read in a book on palms that scale could be controlled with the soap/oil as you suggested or with a ‘systemic pesticide’ applied to the root area, in liquid or granular form. He didn’t elaborate further, can you?
What are the chemicals, how do they work, where can I find them? Any knowledge I can get on this will be much appreciated.
— L. C. C.
The active ingredient of the insecticide you are looking for is Imidacloprid; it is available to homeowners as Bayer Advanced Garden Tree & Shrub Insect Control as well as other products. You will be able to find it at most garden shops.
Since the pest is high up in the palms and unreachable by conventional methods, other approaches need to be applied. One is biological control — the use of predators and parasites against garden pests. This method generally takes time to work. The other is to use a systemic insecticide, which is more or less instant.
Ants interfere with biological control. They are generally larger than the parasites. Even some of the larger predators will be chased away by the aggressive ants. So the first step is to get rid of the ants. You may notice a trail of them marching up and down the palm trunk. Their nests are usually in the ground under the palm.
Applying a pesticide to that area may eliminate the nests. Ant baits placed under the palm are also effective but will take longer to work. The baits are consumed by the ants and taken to the nest and passed to other workers and eventually the queen. A sticky material like Tanglefoot can also be applied in a band around the trunk, physically preventing the ants from crossing.
The disadvantage is that a significant amount of debris will accumulate on the material and make it ineffective. This happens especially in windy areas. The sticky material will need to be stirred up or reapplied.
Finally, a systemic insecticide can be applied to the ground; water will take it down, and the roots will absorb it. As the pest is feeding, it will ingest the material and die. The active ingredient to look for is Imidacloprid. Foliar applications are also approved.
Note: Imidacloprid is not effective on armored scales and certain other species such as cottony cushion scale. The insect infesting your palm trees could be the coconut mealybug.
It has been noted that the mulch from aromatic pencil cedars (a number of species in the genus juniperus) repel Argentine ants. Not all types of cedar chips, however, repel ants. Pine straw has been found to actually provide an ideal nesting site for ants. So if you want, you can experiment with different types of mulches to see if any local species have a repelling effect on ants.
Definitions: Systemic insecticide – one that is absorbed into the plant, either by the roots or leaves. Pests which feed on the plant will also ingest the poison. Examples include pesticides with the active ingredient imidicloprid (Bayer Rose and Flower Care) and acephate (Orthene). Contact insecticide – kills only when the pesticide contacts or touches the target pest. Examples are horticultural oils and soaps. Applying this category of insecticide when there is no pest present will be ineffective. Stomach insecticides – are usually sprayed on the plant and when the leaf is fed upon, the poison is ingested and the pest dies. An example of a stomach poison is Bt, Bacillus thuringiensis (Dipel, Gnatrol, Javelin).
Hilo resident Nick Sakovich is a professor emeritus of the University of California. He has worked in the field of agriculture for 30 years. Email your questions to Sakovich at firstname.lastname@example.org.