Pests on Palms

By Andreas Kortekamp, Maxburgstr. 24, D-76829 Landau i.d. Pfalz, Germany.
Chamaerops No. 52 - published online 12-03-2006

Top left: Seeds of Livistona infected with powder-post beetles. Each seed exhibits an exit hole.
Top right: Micrograph of a powderpost beetle
Centre left: Infested Caryota leaves showing brown and chlorotic areas due to feeding of spider mites
Centre right: Left remnant of a spider mite.
Bottom left and bottom right: Leaflet of Sabal palmetto with scale insects and a micrograph of such a scale insect (Diaspedidae) on the same leaflet.
Photos by Andreas Kortekamp

I guess you know the problem: If you grow palms at home on the window sill or outside in your garden, sooner or later (rather sooner!) you can find some of the typical enemies nibbling at your beloved plants: thrips, scale insects, white flies, maybe caterpillars of different insects, and spider mites. These pests may cause severe damage to palms if not treated with an appropriate pesticide. But what are appropriate pesticides? Maybe you have tested different products, chemicals, household remedies or commercial products. So did I! Some of them worked fine, others did not. Let me tell you what happened with my palms before and after treatment. Since insects and mites represent the most unpleasant pests on my palms, I will deal with these domestic animals primarily.

A long time ago I introduced a nasty insect myself with some contaminated soil. It is the sciarid fly. According to the literature its larvae mainly feed on organic matter in the soil and therefore plays an important role in decomposing plant debris. Unfortunately, the larvae also invade palm seeds and feel very happy in or on the emerging roots of seedlings. During the last year, cuttings of my oleander plants failed to root due to larval invasion of the root collars. Sticky yellow plastic sheets, placed between the plants, may help to catch the adult insects, and may thus lead to a lowered reproduction, but these sheets don’t work against larvae in the soil. If new seeds were placed into contaminated soil or if flies were able to migrate from one pot to the other, germination rates of palm seeds decreased markedly. The soil I now use for seeds was decontaminated at 120 °C for 1 hour in the oven (even though my wife doesn’t feel happy about the resulting special odour in our kitchen!). Pots with seeds should be separated from other plants until the young seedlings are out of danger. Older plants seem to be resistant against larvae of the sciarid fly, or have such an extensive root mass that the damage caused by this insect doesn’t matter.

Another insect feeding on my self-collected palm seeds (I took them from botanical gardens with permission) are powderpost beetles (Bostrychidae). They look like bark beetles but can be distinguished by their head projecting downwards. Since these beetles like sugars and starch, they are sometimes found in the meaty seeds of palms and other plants that have fallen from the tree at maturity and have been lying on the ground for some time. The presence of these beetles in infected seeds can be easily seen because they leave a penetration hole and a brown powder that represents the digested part of the infested seeds. Collecting of seeds still hanging on the trees instead of collecting them from the ground seems to be advisable. Alternatively, seeds can be treated with hot water (up to 80°C) for a few minutes (15 to 30 min) to kill the eggs or larvae adhering to the seeds and to stimulate germination. Even though this cannot be done with all seeds, I have observed no negative effect from this treatment on the germination of seeds from the following species: Brahea armata, B. edulis, Butia capitata, B. yatay, Caryota mitis, Livistona australis, L. chinensis, L. humilis, Acoelorraphe wrightii, Phoenix canariensis, Sabal minor, S. palmetto, Serenoa repens, Trachycarpus fortunei, Washingtonia filifara and W. robusta.

The major pest on my indoor palms, especially in the dry air during winter when relative humidity is low due to heating, is doubtlessly the red spider mite. It lives in colonies and spins a fine net that can be easily seen by spraying the plants with a fine mist of water. Furthermore, a characteristic mottling of the leaves indicates their feeding. Later on, leaves turn yellow at first and soon become brown and die. Urgency is now in order or you could lose the entire plant. Chamaedorea elegans, Caryota sp., Livistona chinensis, L. australis, and Syagrus romanzoffiana seem to be very susceptible. Unfortunately, if these palms grow in a greenhouse, winter garden or as ornamental indoor plants, they cannot be sprayed with pesticides during the winter period due to health risks. The palms may become damaged when they are treated outside at temperatures near to or below 0°C. In some books, you may find information suggesting that spraying indoor plants several times with pure water decreases infestation with spider mites. I did not observe this effect. It is important not to use alkaline water for spraying since white spots of lime remain on the leaves causing an unpleasant appearance. This may also result in a decreased rate of photosynthesis.

Another recommendation I’ve read is to put a plastic bag around the plant to increase humidity for two or three days. Unfortunately, my palms didn’t like that and the mites still remained alive. Therefore, I did some tests with the systemic acting compound Imidacloprid which is sold here as Confidor or Lizetan. In Lizetan the active component is combined with a fertilizer and used as sticks that have to be inserted into the pot. Since the active component is taken up very slowly into the plant, the curative effect may take some days or even weeks. You have to wait a longer period of time until all mites acquire a lethal dosage. I observed that Confidor, applied as a solution of 40 mg per litre to the soil, works much faster and is even cheaper compared to Lizetan, which is also available as a white resin or a spray (very expensive). Although there is a delay between application and effect, the active component is taken up systemically, protecting the entire plant for several weeks including the emerging leaves. Especially when applied as a spray (but also in the case of solutions added to the soil) you have to take care to avoid overdose. Wait at least two to three or even better four weeks between two treatments. I observed phytotoxic (poisonous for plants) effects when applied too frequently. Washingtonia filifera and W. robusta seem to be very sensitive.

The miticidal compound Abamectin is a natural substance that is toxic to mites and insects. It is quite safe due to a low toxicity to mammals and is thus used to control insect and mite pests of ornamental plants such as palms in the greenhouse; however, many insects and mites developed resistance. The use is therefore restricted. I tested another product (Kiron) that is effective either when applied directly to the insects or to the upper leaf side, since it can penetrate into the leaf. On my palms, Kiron did its job for several months and also inhibited colonization of the plant with the next mite generation, adhering to the leaves as eggs.

Another insect feeding on my palms is the scale insect. If palms are not attacked by spider mites, scales will fit into this niche. I tested some plant oils and soap with ethanol. These products should work by sealing off the insect’s air supply. The former should build up a more or less thin layer of oil through which air cannot diffuse, whereas the latter decreases the surface tension of water. This leads to an intrusion of water or ethanol, respectively, into the insect. But these compounds are sometimes phytotoxic to palms and have to be applied several times. After a second treatment leaves got several yellow spots and my Hyophorbe verschaffeltii, for example, lost most of its leaves. Therefore, I tested some insecticides based on pyrethroids (synthetic analogs of pyrethrum). These products worked very well. Fortunately, scale insects do not multiply as rapidly as spider mites do, so you can wait some days or even weeks until the weather outside is favourable for chemical treatments. Alternatively, you can also add the good old Confidor.

Keep in mind that insects may develop resistance if you always use the same active compound. It is better to change the product in order to avoid insecticide resistance. Scale insects on small palms or single palm leaves can also be removed by cleaning the leaves with a mixture of water and a drop of a commercial detergent and perhaps a small amount of ethanol (5 %).

Of course, our main objective is to achieve big and healthy plants. Nevertheless, we should keep the use of pesticides at a minimum in order to reduce hazards to humans, animals, and the environment. Until now, I haven’t read anything about repellents, feeding inhibitors, and other biologically active chemicals, except for an extract of the seed of the neem tree, which seems to act as a feeding inhibitor. I have not tested it so far. Interestingly, my Brahea armata palms seem to have their own biological control mechanism. They never showed any symptoms of damage or infection. This may be a consequence of the waxy layer on both sides of the leaves. Some plants are also known to contain bioactive compounds in the leaf wax that act against a wide range of organisms.

Who has had success with other ways of keeping their palms and other exotics free of pests and diseases? I hope you will let Chamaerops readers know how they worked. We all want to learn more about caring for our precious palms.


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