with Deryn Thorpe

Dealing with plant pests


The mild weather of October and November is one of the worst times for insect pests and I have been waging a battle against an incursion of scale which is trying to take over my entire garden!

They have attacked plants including citrus, roses, liriope, cycads, mondo grass, wattle, and gardenias and last week I discovered some white scale on my tomatoes!

These sap sucking insects spend most of their life as immobile adults and can be soft and fluffy or have a hard coating and appear as either white oval bumps (each with a tiny black dot) on stems or 3mm-5mm pink and brown raised domes on leaves and stalks.

They are immobile for most of their life and in most gardens they are moved from plant to plant by ants which like the sweet honey dew secreted by the insects. When the weather is damp the honey dew is colonised by a black fungus known as sooty mould. This looks unsightly and stops the plant from photosynthesising.

The scales can cause the death of stems if the infestation is heavy.

Hard scale species lay their eggs beneath their scale coating. In late spring they hatch into crawlers and move to a new site.

The life cycle of the scale insect is part of God’s intricate pattern. Mature female scale insects live under the waxy covers their whole life and are headless, legless and wingless. However the males have wings and once mature emerge from beneath the cover and fly off looking for females to mate with through their waxy cover. Some females can produce young without a mate.

On plants with minor infestations I’ve been scraping them off with a gloved hand.

While I like the idea of waiting for their natural enemies like beetles, lacewings, spiders and tiny parasitic wasps to control them, I can’t wait any longer and I’ve been treating them with horticultural oil. This blocks their breathing holes and suffocates the pests. I keep a spray bottle by my front door so it is easy to give the plants a squirt whenever I see the pests.

Another safe alternative to the low toxic oil is horticultural soap which dehydrates the scale. Use these products in the morning or late afternoon, making sure that the plants are not suffering from moisture stress. Oils should not be used when the temperature is above 32 degrees (35 degrees for citrus) and soaps when the temperature exceeds 30 degrees or the plants can burn.

You can also make a grease band around the bottom of plants to stop the ants spreading the pest. ?

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