By ERNIE REZENTS
For The Maui News—
Following last month’s article on some of the pests that raise havoc in our gardens, I received several “what can I do to control the Hibiscus Erineum Mite” from people on the street and from a lady in Maalaea. She asked for the name of a systemic miticide to combat this devastating pest. This mite prefers to feed on the Chinese Red Hibiscus but will also damage hybrid hibiscus plants to a lesser extent. Unfortunately most of the chemicals will provide only a temporary control, and I do not know of a systemic miticide. The word systemic means that the plant absorbs the chemical so it is not washed off. When the plant pest feeds, it takes the chemical in and dies. Temik, now off the market because of misuse and its toxicity, was an outstanding systemic insecticide/miticide. There is now a substitute called Oxymil, but it controls everything but mites. Both of these chemicals are (were) not for use around the house and you need (needed) to be licensed to buy and apply them. Garden shops have many miticides to choose from including the SunSpray Ultra Fine Oil that lists the Erineum Mite as a target pest. I will discuss this product next time. You may want to replace your damaged hibiscus plants with a hibiscus that is not so sensitive, or to replace your hedges with plants with fewer or no pests. A good biological control is not available yet, and it may be a long time before one is found.
Crops with limited production frequently do not have pesticide clearance because the return of investment is slow and it may not be financially profitable for the company. For instance the popular Seashore Paspalum turfgrass is not as widely grown in the United States as is Bermudagrass. Many pesticides are cleared for use on Bermuda but not for Seashore Paspalum. Tests are conducted locally and if the grass is not sensitive to the pesticide and the pesticide is effective against the weed, insect, etc., a special use label is issued by the Federal Government for a certain length of time. It is important not to misuse pesticides, and labels must be followed, not to jeopardize product clearance for use on specific crops.
Hundreds of plants are known to have insecticidal properties, and the number of separate compounds is probably in the thousands. Botanical pesticides were in common use until the 1940’s when they were displaced by modern synthetic pesticides that seemed cheaper, easier to apply, and longer-lasting. However as health and environmental hazards of synthetic petrochemical pesticides increase, and as pests become resistant to these synthetic compounds, interest in plant-derived pesticides is increasing. Another approach to solving the plant pest problems is to develop plants with a genetic resistance to their enemies. Genes that provide this resistance are being inserted into plants in the laboratory or through plant breeding. This takes a long time to accomplish and just a few plants have been successfully manipulated.
It is important to realize that just because a pesticide is derived from a plant it does not mean that it is safe for humans and other mammals or that it cannot kill a variety of life. For example, Nicotine affects the neuromuscular functions causing insects to convulse and die. The same fate can befall people exposed to high doses. So we must be careful whenever we use any and all pesticides. Saving features of botanical pesticides are that they tend to break down into harmless compounds within hours or days in the presence of sunlight and soil microbes will easily decompose them.
Plants products that control pests.
Botanical products such as garlic, neem, nicotine, pyrethrum, and rotenone are popular. I have used nicotine, Black Leaf 40, to control pigeon lice and as an insecticide on roses, especially for control of the Chinese Rose Beetle. Black Leaf 40 is still on the store shelves but is not as popular as it used to be.
Pyrethrum is sold as a powder or a liquid and is made from the dried Chrysanthemum flower heads of the plant, Chrysanthemum cinerariaefolium. It affects the nervous system of insects and causes instant paralysis. Unfortunately some insects can detoxify themselves and survive; so an additional compound is included in the product to block the insect’s ability to break down the toxin. Smart! Other names such as Pyrethrin – refers to the active ingredient in Pyrethrum, and Pyrethroids – the synthetic compounds that resemble pyrethrins in chemical structure. Pyrethroids are synthetic, not organic, far more stable in sunlight, more toxic to insects, and last longer on the plants.
Neem’s insecticidal, fungicidal, and bactericidal properties, and its safety to mammals, have aroused interest among researchers. Neem oil from seeds of the tree Azadirachta indica, is a complex mixture of many active compounds. Its ingredients act as repellents, feeding inhibitors, growth retardants, sterilants, and direct toxins. Because of these multiple modes of action, it is unlikely that insects or pathogens will develop resistance to neem compounds. Neem has both contact and a limited systemic action in plants. Neem has very low mammalian toxicity and controls a wide range of plant pests. Certain beneficial insects and predatory mites escape the effects of neem. Neemix, a product containing Neem, is best used as a preventative, rather than a curative, spray. It can be used along with other pesticides.
Rotenone is from the root of the Lonchocarpus plant. It controls many species of insects including external parasites of animals likes fleas and ticks. It is harmless to warm blooded animals, but will kill beneficial insects and fish. It is short lived and is often included with Pyrethrum in powdered or liquid formulations.
Dipel is one of the names of a commercial product containing Bacillus thuringiensis var. kurstaki. Thuricide, Worm Attack, etc. are other names of products containing this bacillus. BTK is a bacterium that is specifically for control of all types of caterpillars and lacks toxic effects on nontarget species, including humans and the natural enemies of caterpillars. The bacillus has toxic protein crystals in its spores. It is classified as a stomach poison because when the plant material with BTK sprayed on is eaten by a caterpillar the gut of the caterpillar breaks down the crystals thus releasing the poison. A caterpillar that ingests BTK may live for several days; turn a darker color and even hang from the leaf’s surface by its rear appendages indicating that it is languishing in death. However it does not continue feeding and therefore causes no further damage to plants. BTK is sold in liquid, wettable powder, and dust formulations that have been vastly improved to correct some of its original short-comings. Because BTK is a living organism, it must be protected from high temperatures. When it is kept between 70-75oF, BTK powders remain active for 2 or 3 years. Stored in refrigerators will prolong its life even longer. BTK should not be exposed to direct sunlight because it is broken down by ultraviolet rays, and not kept in a car trunk on a hot day because the heat will adversely affect the living bacteria. Using a wetting agent, or a spreader-sticker, enhances plant coverage.
The Taiwan government is very interested in using biological means to control plant pests because farmers have used and misused chemicals for too long. My very good plant pathologist friend in Taiwan, Dr. Shan-da Liu, discovered two fungi that control two other fungi. He discovered that a solution containing the Green Mascareen Fungus controls caterpillars feeding on coconut leaves. This control is somewhat similar to the control of caterpillars by BTK. The second discovery is that Soy Bean and Azuki Bean (red bean for moon cakes) seeds coated with a material containing the fungus Trichoderma controls the soil borne fungus Rhizoctonia. Soy and Azuki beans are typically planted in rice fields after the rice has been harvested. Rhizoctonia lays waiting for the follow up crops to be planted and destroys the plants. Trichoderma has also proven to be effective in controlling Rhizoctonia on Taiwan golf course Bermudagrass.
Many natural product pesticides may last just a few hours or days, or do not control the target pest quickly. But because we are all concerned about the environment, environmentally safe products are being improved to overcome these and other limitations.
We must read the entire label and follow the instructions. I read my labels nearly every time I use a pesticide. Amounts to apply per gallon is different for different pests.
Next time I will write about Horticultural Oils as an insecticide and the beautiful flowering Double Buttercup tree. You can see this tree in bloom on the MCC campus (almost over), at the Izumi Nursery, and in my neighbor’s yard as you drive by. Spectacular!!