Insecticides

Below are listed insecticides sorted by whether they are organic or inorganic. For simplicity we have included ‘molluscicides’ on this list (ie slug pellets). Under each group we have listed some of the insecticides available and their possible uses, however you should reference the appropriate local regulatory controls to ensure that the use listed below is permitted in your area. Insecticides can affect the pests in different ways and we have indicated which action each chemical takes:

Pest-contact insecticides

These make external contact with the pest and will take effect immediately or within a few days. Any pests arriving after the insecticide has been sprayed will not be affected, so timing is critical.

Plant-contact insecticides

These coat the plants with the insecticide which is then ingested by the pest. They are often used against insects which chew through plants, such as caterpillars. The plant parts (particularly leaves) must be well drenched with the insecticide and applications repeated as rain can reduce or remove insecticide residue, making it ineffective.

Systemic insecticides

These chemicals are drawn into the plant so that sap sucking insects (and some caterpillars) ingest them. Since they work within the plant they offer protection against newly arriving pests, not just the ones which were present when the insecticide was applied.

Organic insecticides

These are derived from plant or animal products, however this does not necessarily mean that they are permitted for ‘organic’ gardening or agriculture.

Pyrethrum/pyrethrins

This insecticide, extracted from flower heads of Chrysanthemum cinerariaefolium, has been used for more than a century and is generally available in spray or dust forms. It controls aphids, whiteflies, leafhoppers, thrips, some beetles, capsids, ants and small caterpillars through an insect-contact action.

It breaks down quickly so edible crops can be harvested the day after use. It is toxic to bees, amphibians and fish.

Rotenone/Derris

This is derived from the ground roots of Derris and Lonchocarpus plants and is used, as a spray or dust, to control aphids, thrips, small caterpillars, sawflies, raspberry/flea beetles and two spotted spider mites.

It breaks down quickly so edible crops can be harvested the day after use. It is toxic to bees, amphibians and fish.

Insecticidal soaps/fatty acids

These used to be known as ’soft soap’ sprays and some gardeners still produce them by dissolving soap in water. They work with an insect-contact action and are effective against aphids, whiteflies, thrips, mealy bugs, scale insects, leafhoppers and red spider mites.

Plant oils/plant extracts

Refined vegetable oils (eg rape seed or sunflower oil) and other extracts which are sprayed on and block the breathing holes of small insects and mites (insect-contact action). They are effective against aphids (including overwintering eggs), whiteflies, thrips, scale insects and two spotted spider mites. Bees and ladybirds are not harmed by these. Should not be used on begonias or fuchsias.

Sulphur

This is used as a general purpose glasshouse fumigant which should only be used in empty glasshouses since plants in leaf can be killed by it. Sulphur is an irritant to humans (specifically of the eyes, nose and throats) and will corrode iron, so extra care should be taken when using it.

Urea/foliar lattice

This controls pests such as whiteflies, aphids, mealybugs and two spotted spider mites by blocking their breathing pores or sticking them to the leaf surface.

Spinosad

This is used against ants, applied directly to the ant nest as granuales or mixed with water.

Synthetic (inorganic) insecticides

These are chemicals which do not occur naturally.

Synthetic pyrethrums

There are various types of synthetic pyrethrum including: bifenthrin*, bioallenthrin, cypermethrin*, deltamethrin*, phenothrin, lambda cyhalothrin*, permethrin*, resmethrin and tetramethrin. Different types have different actions, but they are all insect- or plant-contact insecticides. Those indicated with a * persist for up to three weeks, the others become inactive after a few days.

These replicate the effect of organic pyrethrum and, like pyrethrum, don’t persist so are useful for crops. They are good for using on aphids, whiteflies, leafhoppers, thrips, beetles, ants, crawling insects, wasp nests and small caterpillars. Specific chemicals also have the following additional uses:

• Bifenthrin – two spotted spider mites and codling moths.
• Deltamethrin – codling moths, plum moth, tortrix moths, weevils, apple/pear suckers, capsids, scale insects and mealybugs.
• Lambda Cyhalothrin – capsids, pea moths, weevils, sawflies, leaf curling midges and carrot flies.
• Permethrin – woodlice.

Imidacloprid

This is available in drenches, sprays ready-mixed into composts. It works with both a contact and systemic action and is effective against aphids, whiteflies, thrips, two spotted spider mites, mealybugs, scales, lily beetle, vine weevils and sciarid flies. It can also be used on lawns to treat chafer grubs and leatherjackets.

Abamectin

This contact action insecticide is used to control pests including two spotted spider mites and thrips on plants growing indoors. When used as a combination insecticide with thiamethoxam it also controls fungus gnat larvae, sciarid larvae, whiteflies, mealybugs, leaf beetles and aphids.

Acetamiprid

This is generally used as a compost drench for its systemic and contact action against vine weevil larvae. However, it is also effective against aphids, whiteflies, scale insects, mealybugs, thrips, two spotted spider mites, lily beetles and caterpillars.

Thiacloprid

A similar chemical to imidacloprid, this works against vine weevils, aphids, whiteflies, aphids, woolly aphids, scales, mealybugs, leafhoppers, thrips, capsids, small caterpillars, sawfly larvae and beetles. It has a systemic action.

Thiaxethoxam

This is another systemic insecticide controlling aphids, scale insects, whiteflies, thrips, mealybugs, leaf beetles, fungus gnat larvae, sciarid larvae and, in combination with abamectin, two spotted spider mites.

Metaledehyde

Historically, this has been the number one chemical for use on slug pellets. While still widely available it has a couple of major drawbacks: it does not always kill slugs outright and it is poisonous to other animals including birds, cats and dogs. It should be kept off the foliage of plants.

Aluminium sulphate/Ferric phosphate

These are both effective controls for slugs and snails which are relatively harmless to other wildlife (including cats and dogs) when used correctly.