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Once you start enjoying the fruits and vegetables in your organic garden, it's not long before you start receiving visitors. Meet the arthropods. Those freeloading vegetarians who come swinging their many limbs and whose intent it is to share the bounties of your hard labor.
Organic gardeners have a few options. Ignore them — pray for some leftovers. Physically remove them, or, murder. If you choose the last option, why not try a lethal whiff or contact with some killer herbs and spices.
Research into the effectiveness of plant essential oils as botanical pesticides continues and is being confirmed. Several products have been formulated and commercialized.
In the US, these products bypass the regulatory control of scheduled poisons or registered pesticides as they are considered food safe products.
What are they and how do they work?
Plants produce essential oils
There is a long history of our oriental friends enjoying the therapeutic benefits of essential oils, particularly in Egypt, Persia and India.
The process of producing essential oils is generally via steam distillation. The end result is a volatile oil containing 100's of compounds, some identifiable, many not.
Terpenoids play a major role in repelling insects says Canadian entomologist and toxicologist Dr Murray Isman, who has been investigating the development of pesticides for 30 years.
Contact and fumigant
His particular interest is in discovering how the compounds in essential oils affect insects and their fate in the environment.
Isman says that while some of the pure essential oils compounds are slightly toxic (to humans) i.e. carvacrol and pulegone, a proprietary mixture of essential oil constituents fed to rats at a high dose were not lethal.
"Essential oils have several modes of action against insects and mites including repellent and antifeedant deterrence, inhibition of molting and respiration, reduction in growth and reproduction, and cuticle disruption."
Many of these effects result from interference on the invertebrate octopamine pathway. Octopamine is a neurotransmitter unique to invertebrates.
The advantages of these many modes of action is that they may delay resistance development in the target pest.
"Essential oils may have minimal direct and/or indirect effects on pest enemies, although bees appear to be sensitive," says Isman.
Any plant essential oils containing eugenol or thymol, ie thyme (Thymus vulgaris) rosemary (Rosmarinus officinalis) and clove (syzygium aromaticum) are effective pesticides. They can be applied as a contact or fumigant.
The Greenhouse whitefly Trialeurodes vaporariorum, is a major pest of greenhouse vegetables, especially tomatoes, cherry tomatoes, cucumbers, and ornamentals. This insect has also developed immunity to many conventional insecticides. Several essential oils have been found to be effective against all insect stages, including eggs, nymphs and adults.
- Caraway seed
- Clove leaf
- Lemon eucalyptus
- Tea tree
A big eater of greens are the small spider mites (Tetranychus sp). Their feeding on chlorophyll in plants cells interferes with the plant's ability to grow. When bronzing occurs under leaves, you can guarantee numbers are high. So voracious are their appetites they can kill the plant.
Did you know some mites live in the gills of edible mushrooms?
the oil of oregano, (Origanum vulgare) or thyme (Thymus vulgaris) or Mint (Mentha spicata) where the Carmine spider mites, (Tetranychus cinnabarinus) are feeding. These minute acarides will go on a hunger strike, stop growing and procreating.
Isman and a colleague, studied the effect of rosemary oil (Rosmarinus officinalis) against the two spotted spider mite (Tetranychus urticae Koch) on tomato plants. The oil was effective on contact and even high doses did not harm the tomato plants. Other researchers have also found it useful as a fumigant.
The problem with any form of lethal control is the effect on pest predators. In the study above it was found that when both spider mites and predatory mites (commonly used for biological control) were sprayed with different pesticides containing rosemary oil, no mortality was found among predators, but up to 60% mortality was observed in the two spotted mites.
The researchers believe this might be due to the differences in the way rosemary oil is metabolised by predatory and phytophagous mites.
Moths and aphids
Through Isman's research with rosemary essential oil, it was illustrated that camphor was the most toxic compound to larvae of Pseudaletia unipuncta, a noctuid moth larva, followed by d-limonene and p-cymene. Against the larvae of Trichopulsia ni, (cabbage looper) α-terpineol was the most toxic followed by p-cymene and β-pinene. Rosemary oil also had good toxicity to aphids.
Many of the Eucalyptus species are potent for insect control (E. alba, E. camaldulensis, E. citriodora, E. deglupta, E. globulus, E. Rob) (See Appendix 1.) Eucalyptus globulas- Blue Gum, offers very good control of many insect pests.
The oldest tree on earth, Ginkgo biloba has been investigated as a pest deterrant with lab tests showing snails were repelled from eating lettuce leaves to which Gingko extract had been applied.