The pest defence

Saturday 14 April 2012

Here's some good news: greenfly and their hundreds of sap-sucking relatives aren't as big a headache for gardeners as they once were.

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Dave Allan

They've stopped making the RHS's list of top 10 problems and I hardly ever get cries for help from readers. I'd be surprised if the total aphid population has dropped much but suspect there are more predators in gardens controlling greenfly and blackfly numbers. When you don't use pesticides, you let lacewing and hoverfly populations grow. Tiny parasitoid wasps, spiders and ladybirds adeptly hoover up the pests, without you even noticing.

Like most insects, aphids have a complex life cycle and, while this varies from species to species, the pattern is similar. Lettuce root aphids over-winter as eggs on poplar trees, moving to the stalks of new leaves in spring. They form tiny, pouch-like galls where they start breeding. In summer, the winged adults transfer to lettuce roots and continue breeding. Lettuce leaves will turn yellow and, during a dry spell, the whole plant might die. At summer's end, newly hatched females return to poplars, mate and lay eggs in the bark, ready for the following year. Then there are wingless aphids which may stay in the soil, even after the lettuce has been lifted. They produce winged adults which attack a new crop of lettuces the following summer, just like their relatives.As is often the case, you only see the damage wrought by the root aphid when it's too late. The currant blister aphid behaves similarly. It lays its eggs on the underside of currant leaves, then the eggs hatch in spring and form galls. By the time you see the contorted, swollen leaves, the aphids have flown off to another host plant. They return in autumn and lay eggs for the next season.Like blister aphids, most aphids specialise in one plant type. Since gardens feature different plants in a small area, no single pest species builds up. It's another story for farmers, though. Large concentrations of cereals are magnets for specialist feeders, and growers need to take steps to control them. The traditional method for farmers is to use pesticides, but new techniques are being developed and these may gradually become available to gardeners.Scientists are starting to genetically modify plants, a hugely controversial move. The organic movement has turned its back on GM, a stance I believe is profoundly wrong. GM is only a new technique for plant breeders. Gene transfer occasionally occurs naturally and should, in my opinion, be seen as the latest weapon in the researcher's armoury. Every living organism shares genes with others, so transferring one gene to another organism is no big deal.Some large-scale manufacturers have given GM a bad name by developing seeds like Roundup-ready maize, which lets farmers spray Roundup throughout a field, killing everything except the maize. But scientists at the Rothamsted Research Centre in Hertfordshire are experimenting with wheat plants. A gene from a peppermint plant has been transferred to the wheat, introducing a pheromone which exudes a peppermint smell that deters aphids. In my book, it's better to grow GM plants than douse a field with pesticides.GM plants aren't yet available to gardeners, but you can use alternative strategies if you're driven to despair. Some expensive biological controls are an option but are best used in greenhouses. One of them, a tiny insect, aphidius, lays an egg in a greenfly; after hatching, the young aphidius feeds on its host. Ladybird and lacewing larvae are available too, but, again, can only be used in greenhouses because they would disperse too widely outdoors.With some aphids, you could be your own natural predator by squashing your victims on a leaf or a stem, or you could, as a last resort, use insecticidal soap. Aphids absorb oxygen through pores in their skins; the soap blocks the pores, thereby killing the pest. To avoid killing beneficial insects as well, apply the soap in the evening.There's no doubt aphids are more damaging to stressed plants, so keep soil moist, mulch round shrubs and spray leaves with liquid seaweed. Good hygiene is also important. Grey brassica aphids over-winter in decaying leaves close to plants, so go round the cabbages and sprouts to remove yellowing and fallen leaves. nVisit www.askorganic.co.uk. Email your gardening queries to da@askorganic.co.uk

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