The Weekend Australian - Magazine
Keep your powder dry
Pesticides need only be your last resort
The real skill in pest control is knowing when to take action – and when not to. Only a few of the insects in our gardens are pests; most are doing great work pollinating flowers, eating other insects, feeding wildlife and recycling dead plants and animals. We should tolerate minor damage (that caterpillar chewing leaves can turn into a butterfly, after all) and only intervene when plants are at serious risk.
Correct identification of a pest or disease is vital. To this end, Denis
Crawford’s Garden Pests, Diseases and Good Bugs (ABC Books) should be in every gardener’s library. Crawford rightly takes the view that pest control strategies should proceed in this order: prevention, observation, hands-on control methods, organic or biological controls, and pesticides as a last resort. For instance, if you simply ignore the aphids covering your rose buds, pretty soon predatory ladybirds, parasitic wasps and hoverflies will move in to feast, solving the problem for you. Even soft controls such as horticultural soap and oil sprays will kill good bugs along with the bad.
We’ve come a long way from the bad old days of spraying toxic chemicals thoughtlessly on our plants. Organophosphates such as chlorpyrifos and dimethoate (Rogor) have been banned for home garden use and malathion is under review. These persistent insecticides are also toxic to mammals, birds and fish. Today’s pesticides are safer but can still cause damage if used incorrectly. “It’s the quick-fix
mentality of grabbing something to spray that has seen a lot of pests become resistant to older insecticides,” says Crawford.
The exciting developments in pest control are in “biological” products – those derived from living organisms such as plants, animals, microorganisms and fungi. One of the best known is Bt (Bacillus thuringiensis), sold as Dipel for home gardeners. It’s a soil-borne bacterium that infects the stomachs of caterpillars, causing them to stop feeding and die. It doesn’t affect any other insects, or birds and mammals that eat those grubs. Another Bacillus species called Bti has been used safely worldwide for decades against mosquito larvae in water.
Buying predatory insects is a common pest control method for commercial farms and glasshouses. For home gardeners, the most useful to buy are green lacewings, which eat a range of pest insects, and Persimilis, a predator of two-spotted mite. But if you grow a diverse range of flowering plants you’ll attract these beneficial insects and birds anyway to take care of pests. Cryptolaemus beetles, a type of ladybird, are also sold to eat mealybugs and some scales. For people with lots of infested indoor pot plants, they’re a good alternative to spraying.
Biological fungicides are also being developed. Trichoderma is a genus of soil fungi that forms beneficial relationships with plant roots. One of the earliest biofungicides (T. harzianum) colonises roots, improves plant health and kills root pathogens that cause root rot, wilting and damping off. There are no registered Trichoderma products for home gardeners but you can encourage them to prosper in your soil by regularly adding organic matter.