Wednesday, January 26, 2011

The cottony scale insect

The cottony cushion scale is native to Australia. It was identified in New Zealand in 1878 but spread to every place around the world where citrus trees are cultivated. In Malta it was found in a garden at St Georges near San Giljan in 1907. Immediate action was taken to slow its spread to other parts of the Maltese islands until a system to control it biologically which was giving successful results in other countries could be used in Malta. The action taken might have slowed down its spread but did not stop this insect from reaching the central parts of Malta, including Lija, Attard and Balzan which at the time were well known for the cultivation of orange trees.
Last summer a small numbers of scale insects decided to set up home on some of my aubergine plants. I allowed them to grow so that I would be able to photograph them and follow their life cycle. This species of scale insects has an oval shape and can grow up to half a centimeter long. Mature insects attach themselves to a host plant by means of waxy secretions and remain stationary. In summer they produce a white egg sac on grooves on their back in which they deposit hundreds of red eggs. A few days ago I photographed minute nymphs coming out of the egg sac. These nymphs are the dispersal stage of the insect. They crawl from one plant to another but they are so are so light that they are often lifted up by the wind and carried to other areas to establish new populations.
The nymphs damage the host plant as they suck fluids from the veins of leaves and small twigs. As they grow they break out of their skin (exoskeleton) and move to another spot leaving behind their telltale skin and waxy secretions. When they become larger they move to larger twigs and eventually to the branches or tree trunk.
Most cottony cushion scales are hermaphrodites. They fertilise themselves and produce hermaphrodite insects. Males do exist. When a male fertilises a female both hermaphrodites and males are produced.
This insect is of interest because it was one of the first pests to be successfully controlled biologically. Between 1888 and 1889 a species of ladybird, the vedalia beetle, was imported into the United States from Australia to control this pest which was threatening California’s citrus trees. The experiment was a success and farmers in other countries started to use the beetle to control the scale insect wherever it appeared. In 1911 the vedalia beetle, which is known in Maltese as nannakola tas-salib, was imported into Malta by the Department of Agriculture to control the spread of the cottony scale insect which by that time was infesting citrus trees in most parts of Malta. (This article was published in The Times on 22.12.10)

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