Which Insecticide Should I Use?
By Bob Chapman
"Insecticide" is a generic term used to denote something that will kill insects. There have been hundreds of insecticides found in nature, invented and marketed over the years. Of these insecticides only a few have been found to be safe to use by home gardeners. Many have been banned because of possible harm to humans and animals.
Types of insects typically found in our gardens
- Sucking insects. These insects stick their beak into the tissues and suck out the sap or fluids inside the leaf (or rose bud) often causing stunting, a stippled effect or even killing the entire leaf. Among the many sucking insects are aphids, scales, mealybugs, thrips, spittlebugs, cicadas, leafhoppers, psyllids, treehoppers and mites (mites are members of the spider family).
- Chewing insects. Holes in the leaves and chewed, serrated edges of leaves are typical signs of these plant destroyers. Included are caterpillars, loopers, beetles, grasshoppers, crickets, earwigs, cutworms and various other worms.
- Boring insects. These insects (usually beetles) tunnel into the plant, and lay their eggs. The hatched larvas eat the wood inside twigs, branches and trunks. The entrance to their tunnels is usually visible and sometimes the excrement from the larva is found around the hole or in little piles on the surface(s) below
- Leaf miners. The larva of certain flies, moths, sawflies or beetles feed on the inside layers of leaves, creating distinctive visible tunnels as they travel between layers of leaves.
- Leaf rollers. Certain caterpillars hide themselves from their enemies by rolling the leaf around them and then eat merrily away with less fear of them being eaten.
Do a little detective work to determine the type of insect doing the damage. Take a critical look at the plant(s) or insect(s) attacking them and try to determine what type of damage is being done (see the list above) or what insect is seen.
Once you have determined what type of insect or the species of insect is causing harm to your prized plant(s), then you can select the insecticide from the listings below to control them.
Organic insecticides
These insecticides kill insects when they eat a leaf that has been sprayed or dusted, by smothering them, by affecting the insect's nervous system or by contact with the insect. Most are derived from plants that have been found to kill insects. The list below is not meant to be complete (new plants or methods are found frequently) but will introduce the more popular or commonly found organic insecticides. Most organic insecticides are available through catalogue purchases, online or at specialty stores. The list is not in any order of popularity or usage.
- Horticultural oils. Horticultural oil (highly refined petroleum product such as Volck Oil), mineral oil, garlic oil, citrus oil and Neem oil (containing azadirachtin, from the seeds of the neem tree of India and Africa) are widely used by organic gardeners. Oils suffocate the insect and must contact and cover the insect to be effective. Some fungicidal effects are also claimed with the use of some oils. Horticultural oils are found in most stores or nurseries.
- Insecticidal soap (not a true soap, but the potassium salts of fatty acids found in animal fats and plant oils). Some use homemade detergent mixtures. Soaps, either commercial products or homemade, enter the breathing tubes of insects, causing their collapse and the death of the insect. Many stores and nurseries offer insecticidal soap.
- Sabadilla. Made from the seeds of Schoenocaulon officinale of Venezuela, containing alkaloids. Sabadilla affects the nerve cells of insects, causing paralysis and then death. It is used as dust. Moderately toxic to honeybees and mammals and can cause violent allergic reactions in susceptible individuals if ingested or absorbed through the skin or mucous membranes. It is effective on a wide range of insect larva and adults. Must make contact with the target insect. Use only in the evening to avoid the insecticide contacting honeybees.
- Boric acid. Very effective when combined with a food that attracts ants and cockroaches. Commercially made traps are available in stores or nurseries or you can make your own. Mix four teaspoons of boric acid powder in three cups of water and one cup of granulated sugar in a pan and bring it to a boil, stirring to dissolve the solids. Pour the mixture into small shallow containers, such as bottle caps or the snap-on caps and place near ant runs and away from pets or kids. The ants carry the sugary solution back to the nest. The boric acid forms crystals in their stomachs and explodes, killing the ant. Other ants eat the dead ants and they too are killed. Boric acid is not harmful in small amounts but can be poisonous if ingested in large amounts.
- Ryania. From the shrub Ryania speciosa and sold as a dust. It is touted as a broad-spectrum insecticide. Ryania is a stomach poison. Often found in mixtures of ryania, pyrethrin and rotenone.
- Rotenone. Extracted from a variety of plants and once very popular, but because of new products available and rotenone's deadly poisonous nature, is now seldom used. Rotenone was used to kill fish by native tribes, who mashed the roots of various plants of the Derris genus and cast the pulp into the water. It is a broad spectrum insecticide but breaks down quickly in sunlight.
- Pyrethrins or Pyrethrum. The Pyrethrum daisies (Chrysanthemum cinerarariifolium and C. coccinium) contain compounds that kill insects on contact. Pyrethrins are effective broad-spectrum insecticides and can safely be used in the garden, on vegetables and on fruit trees. These sprays and dusts are often mixed with sulfur or other compounds, giving them fungicidal properties or with other botanical insecticides. Products containing pyrethrins are commonly found.
- Lime. An old-time insecticide. Wear protective gear, including gloves, goggles and a respirator when applying to prevent inhalation of the dust.
- Ammonia. Household ammonia is used to control many insects, but refrain from use in hotter weather. Ammonia may injure the leaves of some plants so test before usage.
- Quassia. From the wood and bark of the Quassia amara or Bitterwood tree. The tree is not attacked by insects as the bark is one of the bitterest known. Typically, the bark and wood chips are spread over the soil as a repellant or are ground up to make sprays.
- Alcohol. The sprays containing 70% isopropyl alcohol (rubbing alcohol) are used outdoors and indoors on waxy-foliaged houseplants, controlling many sucking insects.
- Bacillus thuringiensis (commonly called "Bt"). Probably the most widely used (there are over 35 varieties of this bacteria) biological control. After Bt is sprayed on the plant, caterpillar pests (includes cabbage loopers, codling moth larvae, imported cabbage worms, spruce budworms, diamondback moths, gypsy moth larvae, tomato hornworms and others) ingest it as they feed. It does not kill the insect or larva immediately, but the insect stops feeding. It may live for several more days, but soon darkens, dies and drops to the ground. Bt is often found in stores or nurseries.
Inorganic insecticides
Scientists, aware of the desperate need to overcome the damage that insect pests do to agricultural crops and ornamentals in the home landscape, work diligently to develop synthetic insecticides. Some are developed as species-specific, that is they will control only a few of the hundreds of plant-damaging insects.
In order to develop effective inorganic or synthetic insecticides these scientists have studied insects, determining how their nervous system(s) works, methods by which they expend energy, their outer body covering (cuticle), their endocrine systems and how water is stored or is lost in the insect.
Inorganic insecticides that are available for use by homeowners include those that attack the nervous system of insects. There are two main parts of the nervous system: the nerve fibers (also called axons), which carry small electrical impulses, and the synapse, the microscopic gap between nerve fibers. Because each nerve fiber ends at a synapse, the nerve impulse must be carried across the synapse chemically. The insecticides that attack the synapse are called synaptic poisons. The insecticides that attack the nerve fiber itself are called "axonic" poisons. Some of these insecticides can affect the nervous system of humans and caution must be used when using them.
Some common inorganic insecticides for use by home gardeners
- Malathion: Malathion has been on the market for over 50 years and is probably the most widely used insecticide over the years. Malathion kills a wide spectrum of insects upon contact and when they walk across a surface that has been sprayed. Has a long residual effect.
- Imadicloprid: This synaptic insecticide is absorbed by the plant roots and carried to all parts. When any insect bites into the tissues it is killed. This systemic insecticide is effective for about six months. Cannot be used on fruits or vegetables.
- Sevin: A carbaryl insecticide, it kills dozens of insect species, adults as well as larvae. It has a long residual, resisting being washed off by rains. Sevin has a synaptic reaction in insects.
- Pyrethroids. A chemical copy of the pyrethrums (Chrysanthemum cinerariaefolium) used by organic gardeners. Pyrethroids are axonic poisons. The pyrethroids are widely used. The names of pyrethroids found in products on the market today include Allethrin, Bifenthrin, Cyfluthrin, Cypermethrin, Esfenvalerate, Permethrin, Resmethrin, Sumithrin, Tetramethrin, Tralomethrin and Transfluthrin. All of the above are slight modifications of pyrethroids and are widely used. Some are granulated or used as a dust.
- Acephate. This widely used insecticide inhibits the insect's nervous system, causing toxic levels of acetylcholine, a neurotransmitter.
- Dimethoate. A systemic organophosphate. It interferes with the activities of cholinesterase, an enzyme essential for the proper working of the nervous system of insects.
SUMMARY
So how do I select the insecticide to use? First, know what insects(s) you are trying to control. Are they sucking the sap from your plants or chewing the leaves? Are they found underground and eating the roots? Does the insect leave holes in the twigs, branches or trunk of the shrub or tree? If you don't know the insect that is damaging your plants, but you can see the damage, such as a stippled effect upon leaves or flowers or twigs being chewed upon, select a wide spectrum insecticide that will control most species of insects that cause damage. Many insecticides will state in bold or enlarged print the insect(s) the product controls such as ants, worms, grubs, aphids, etc. and how they are controlled.
If you prefer using only organic insecticides you'll need to patronize specialty stores that carry these products, order online or from catalogues sent by firms selling organics.
If you don't want to go to the bother of dealing with organic insecticides, you'll find lots of stores and nurseries that have many of the inorganic insecticides.
Be aware of dangers in handling and using insecticides.
Some insecticides can harm humans if contact is made with the concentrate. READ the fine print on the container. NOTE any cautions the user must take when using the insecticide. FOLLOW all directions carefully, using only the amounts recommended and the limitations stated on the label. DISPOSE of empty containers properly to avoid contaminating the environment.