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Pests and diseases: Aphids




  • Look for

    Colonies of aphids can be found in clusters on soft new growth of a wide variety of plants. Buds, tips of stems and the undersides of leaves are all good places to look. Aphids come in many different colours from common greenfly and blackfly to yellows, browns and whites. Many species are specific to one group of plants but they all need to be dealt with in a similar way.

    Plants affected

    • From time to time, aphids may attack almost any garden plant.

    About

    • Adult aphids are rarely more than 3mm long and are elliptical in shape.
    • Large colonies can cover areas on the youngest sections of stems, and the undersides of leaves and sometimes on flower-buds.
    • During the warmer months aphids give birth to as many as five live young a day, so large colonies can develop very quickly.
    • When the colonies become over-populated, they move to different locations by producing winged aphids.
    • During the cooler months, aphids mate and produce eggs which over-winter.
    • Aphids feed on plant sap and excrete plant sugars as honeydew.
    • Honeydew often covers the leaves of a plant and then becomes infested with black sooty moulds.

    Treatment

    Chemical

    Lambda-cyhalothrin and deltamethrin are both contact insecticides which are effective only when sprayed directly onto the aphids. A suitable systemic insecticide is thiacloprid which is taken in through the leaves and is taken up by the feeding aphids. Naturally occurring insecticides such as pyrethrum and fatty acids can also be used.

    Organic

    • Encourage natural predators such as ladybirds and lacewings and insectivorous birds.
    • Have patience – when aphid populations peak, predatory insects will soon move in. Often this is when people lose their nerve and turn to sprays and, in doing so, kill all the natural predators too.
    • Don’t over-feed plants. Too much fertilizer will lead to lots of soft, sappy growth which will encourage aphid attacks.
    • Consider planting a sacrificial crop near to vegetables. For example, nasturtiums which attract black-fly away from brassicas.
    • Infestations in greenhouses or conservatories can be effectively treated with a biological control such as ladybird larvae.

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