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Why worm castings may be the ideal insect control for your fruit trees

March 8, 2025

I received the following email from Doug Brooks, who gardens in Claremont.

“For years, I have used some type of yearly insect control product to help protect my fruit trees. The last few years I have used the Bioadvanced Fruit, Citrus & Vegetable Insect Control product. I recently found out that California has now banned many of these products. What am I to use as a replacement? I’ve scoured the internet to no avail.”

I would recommend that you try worm castings, a euphemism for worm excrement, to prevent insect predations on your plants. There is a large body of evidence that, properly applied in dry or liquid form, worm castings deter not only insect pests but pathogenic fungi and nematodes as well. Note: Nematodes are thread-like microscopic organisms, some species of which create root galls that prevent water uptake and cause wilting of vegetable crops such as tomato, potato, cucumber, carrot, onion, and many others. There are also beneficial nematodes, however, that prey on soil-dwelling larvae of insect pests.

Worm castings owe their pest control capacity to chitinase, an enzyme that breaks down chitin, an integral constituent of insect exoskeletons, fungus cell walls, and nematode eggshells. Chitin is a fibrous material that is the second most abundant polymer — after cellulose — on earth. Like cellulose, it is not easily degraded and decomposes slowly.

As a worm digests its food, 48 different bacteria assist in the process and they remain in the excreted castings. Many of these bacteria produce chitinase that enters plants when castings are applied to the soil or sprayed in liquid form onto the plant itself. Although plants contain chitinase for defense against insect pests and pathogenic fungi — whose survival depends on the chitin in their exoskeletons or cell walls — the level of chitinase in plants is typically insufficient to fend off the pests that they attract. However, when chitinase-producing bacteria enter their systems, plants’ chitinase is supplemented and becomes more aggressive in fending off pest intruders.

One of the most popular biological pest control organisms is Bt, an abbreviation for a bacteria known as Bacillus thuringiensis, which is found in more than 130 pest control products. Bt, depending on the strain, may attack the larval stage of beetles, flies, butterflies and moths, or mosquitoes. Bt produces a toxin that enters the gut of larvae that consume it. The way the toxin gains entry is by activation of Bt’s chitinase which degrades the chitin lining a larva’s digestive tract. Bt must be applied no less than once a week to be effective, as it breaks down in sunlight. Timing is also important as application should come after eggs have hatched but before larvae are large enough to pupate. It is not a contact insecticide but should be sprayed on plant surfaces where larvae are active.

It appears that most, if not all, plants and animals contain chitinase, even if it’s in a dormant state. Our own chitinase seems to be activated when we inhale dust mites or mold spores, for example. There is even thought to be a connection between an asthma attack — which may be prompted by certain chitin-covered bacteria or viruses — and the activation of human chitinase.

Insects’ aversion to worm castings is demonstrated when they are spread over ant nests, causing the ants to remove themselves from the nests. If you have an issue with ants climbing up the trunks of your fruit trees, for example, you may want to spread worm castings under the trees to keep the ants away.

Fungi may also be abated with chitinase. In one study, chitinase extracted from a yam was sprayed on powdery mildew that had infected strawberry foliage and fruit. The mildew disappeared for two weeks, indicating that the chitinase concoction worked and that it biodegraded quickly, opening up the possibility of its use in place of synthetic fungicides.

It must be emphasized that the key to effectiveness with organic pesticides is repeat application. In the case of worm castings, if you choose to use a liquid product, spray it on the foliage twice a month during the growing season and once a month in winter. Interestingly, in perusing the labels of a number of liquid worm casting products, I did not see any indication that its benefits include insect pest or fungus disease deterrence. That appears to be a well-kept secret for now.

Gardeners and farmers have traditionally used worm castings for fertilization alone, although I would not be surprised if such fertilized plants are less likely to be infested by insect pests or infected with fungus diseases. At Apricot Lane Farms in Moorpark, a 40-foot-long vermicomposting (vermi=worm) bin is utilized in providing mineral nutrition to more than 200 varieties of fruits and vegetables grown on site. The composting bin contains some 250,000 red wiggler worms that feast on the manure of the animals and poultry that live on the farm, and decompose plant detritus, coffee grounds, and table scraps as well. The compost is brewed into a tea for application to the plants. Worm castings build long-term soil fertility due to the immense variety of bacteria and fungi found in the castings. This biodiversity means that the many different minerals and nutrients plants require will be made available by the decomposing attributes of one variety of these microorganisms or another.

“The Biggest Little Farm,” a prize-winning film about Apricots Farms’ genesis and progress is widely available for online viewing. You can make arrangements to take a tour of the farm at apricotlanefarms.com.

Chitosan, derived from the chitin found in lobsters, crabs, and other marine crustaceans, also has a number of horticultural applications. It is used to prevent bacterial, fungal, and viral infections and to mitigate nematode infestation. It is also used as a carrier for slow-release fertilizers and as a soil conditioner. Finally, It has been found to alleviate drought stress and to promote growth. These beneficial effects result following seed or soil treatment with chitosan or by spraying chitosan on the plants themselves.

Does anyone have a vermicomposting tale to tell? If so, please send it along to [email protected]. Your questions and comments, as well as gardening challenges and successes are always welcome.

California native of the week: Island morning glory (Calystegia macrostegia) is a fascinating, lush evergreen vine, growing up to 25 feet tall, but usually seen shorter than that where garden-worthy subspecies or varieties are concerned. Flowers are pale pink and reach two inches across. Rare for a native, its blooms are on display over a long period, from February until July. Some advise planting it next to a shrub since it tends to be leafless at its base and its tendrils will find their way through the shrub’s foliage, with its blooms poking through. However, you can plant it successfully in a container if you are content to see it spill out and trail down as opposed to climbing up vertically. It can grow in any soil and has been utilized for covering fences or blanketing the ground. It can handle full sun on the coast, but will need some sun protection inland. It grows rapidly and can adapt to any soil type. Give it an occasional good soak in hot weather and prune it back in the winter for best performance when spring arrives. Bees are attracted to its flowers. It’s also known as Anacapa Pink due to its Anacapa Island origin.

Orange County Register

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Why worm castings may be the ideal insect control for your fruit trees

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