Insecticides and Pesticides For House Plants

The best way to hold insects and other pests at bay is to keep your house plants healthy. But if problems occur, it’s important to use the right insecticide for the job. Insects and pests breed quickly; the sooner you tackle them, the more likely you are to succeed. Technically, insects have segmented bodies, six jointed legs and no backbones, while other harmful creatures, such as slugs, are called pests. Here, ‘pest’ refers to both.

Isolate new house plants for two weeks, until you’re sure they’re not carrying pests. Examine all plants regularly. Check top and undersides of leaves, and young growing points. For light attacks, or ‘infestations’ try non-chemical methods first. Pick off and squash small pests such as aphids, then wash off stragglers under running tap-water. Remove and destroy large pests such as caterpillars. Cut off and burn badly infested leaves and shoots. If pests reappear, use pesticide.

How pesticides work

Pesticides kill on contact or by poisoning the insides of pests. Some, such as gamma-HCH, combine both methods. Poison can coat a plant or, better still, be absorbed by it, so that when pests suck sap they die. This is called systemic insecticide, since it’s taken into the plant’s system.

Contact pesticides take the fastest effect, but unless the whole plant is covered, pests may escape – leaf undersides and rolled-up leaves make excellent hiding places! Some pesticides quickly lose their killing power; others remain potent, or ‘persistent’, for a long time. Systemic insecticides take longer to work, but are more thorough, and continue working longer.

Chemical contents

When buying pesticide, check the active ingredient or ingredients, and the list of pests it kills. Similar pesticides are often available under different brand names.

Some contain one active ingredient; others contain several, and can kill greenfly, red spider mites, thrips, white flies, mealy bugs and scale insects. Some pesticides also contain fungicides, killing diseases and pests in one go! The following are commonly used pesticide chemicals.

Bioresmethrin non-persistent; non-toxic to humans and most animals Derris or rotenone non-persistent: nontoxic to humans and most animals, but poisonous to fish Dimethoate systemic Fenetrothion persistent Formothion systemic HCH, gamma-HCH or Lindane persistent Malathion non-persistent Pyrethrum non-toxic to humans and most animals

Forms of pesticide

The same chemical or combination of chemicals may be available in two or three different forms. Choice depends on cost, convenience and amount needed.

Concentrated liquid pesticides are diluted in water, then applied in a sprayer fitted with a fine nozzle. They’re fairly cheap, but you must dilute them yourself, a capful

The choice may be rather bewildering, but in fact most pesticides are of a ‘general purpose’ nature – some even contain fungicides.

Being the usual unit of measurement, and they’re hard to mix in small amounts. Powders for dissolving in water are used in the same way as liquid pesticides. Pre-measured sachets are convenient, and make up 1 lot (560ml) or 1 gal (4.5l) quantities.

Aerosols are very easy to use. You don’t have to mix anything, and can use as little as you need. They are expensive, though, and must be used at the distance recommended by the manufacturer. Avoid CFC propellants.

Plant pins are like plant food sticks, but contain systemic insecticide. The pins slowly release poison into the potting mixture, without contaminating furnishings or the atmosphere. Again, they are convenient but expensive.

Dusts are applied from puffer packs. They are useful for house plants that don’t like their leaves wet, and for applying to potting compost infested with root pests. They are also useful in winter when wetness can cause fungal disease.

Safety first

1. Always follow manufacturers’ instructions, especially regarding dilution and intervals between applications.
2. Don’t use garden pesticides indoors, unless it says you can on the label.
3. If the weather is mild, take house plants outside to treat them, then return them indoors.
4. Wear rubber gloves when preparing and using pesticides, and always wash hands and face after use.
5. If using a pesticide on herbs or patio crops such as tomatoes, make sure it is suitable, and allow the recommended time to pass between treating and harvesting.
6. Keep pesticides out of the reach of children.
7. Keep pesticides clearly labelled, in their original container.

Pest control for the patio and greenhouse – Dealing with patio pests

The same guidelines about choice and use of pesticides apply, but there are extra points to consider.

• Choose ‘biological’ pesticides that kill pests but don’t hurt helpful creatures such as ladybirds, bees and lacewings. The label should say whether it’s biological, but derris, pyrethrum-, nicotine and pirimicarbbased pesticides are suitable.
• Outdoors, apply pesticide sprays and powders on a still, dry day. Rain washes away contact pesticides, which you may then have to re-apply.
• As well as the pests shown on the chart, ants, wasps, woodlice, slugs and snails can cause trouble. Methiocarb and metaldehyde pellets kill slugs and snails. Use with care as they are poisonous to birds, pets and children.

Dealing with greenhouse pests

As well as the various forms of house plant pesticides, you can use fumigants, or smoke cones. When lit, these fill the greenhouse with poisonous fumes. They’re very effective, killing pests in every nook and cranny; some kill fungal spores as well as pests.

Biological control is a way of fighting pests in the greenhouse by introducing their predators or parasites. These are supplied by specialist breeders, on leaves cut into strips, which are then hung in the greenhouse. They are usually introduced at the start of every growing season.

The parasite wasp Encarsia formosa destroys white flies; the tiny mite Phytoseiulus persimilis destroys red spider mite. Don’t use pesticides, for obvious reasons! And if you are successful, the predators themselves will die off, as their source of food is gone.

Always handle pesticides with care. Wear protective gloves and, if possible, mix and apply chemical treatments outside.