Thursday, 22 July 2010

A mystery... scale insects on European manuka

Manuka (Leptospermum scoparium) is native to New Zealand and southeast Australia. It is common throughout New Zealand and is well known for being the source of manuka honey which is sought after for its healing properties, and for being a good source of firewood (particularly for smoking fish). Its trait of having numerous white flowers has also ensured that it is fairly commonly grown as a garden plant, and so has been exported around the world for this purpose.


So when a scale insect that apparently is specific to the plant is found in Italy and Corsica in 2004 and 2006, one would imagine that it came from New Zealand or Australia right?


Acanthococcus mariannae was described yesterday in a Zootaxa paper by Giuseppina Pellizzari and Jean-Fran├žois Germain. The 30 or so specimens that went into the description were all collected from manuka from Italy and France. Surprisingly though, despite the author's (reasonable) assumption that the insect was introduced to these countries on the plants, this species has not (yet?) been found in either New Zealand or Australia. Moreover, specialists familiar with scale insects in these countries had not noticed it before. While it is likely that further searching will reveal it on manuka in NZ or Australia. However there is the lingering question: if it's not, where did it come from and how has it started attacking manuka?


Scale insects aren't glamorous. They are little more than a bag of fluid that get sap pumped into them. But they are important in a properly functioning ecosystem; and when they get out of control the consequences can be severe. This can be illustrated by an example that also involves manuka: the incidence of manuka blight in New Zealand in the 1940s and 1960s.


Manuka naturally harbours large numbers of the scale insect Coelostomidia wairoensis which produces a lot of honeydew. This in turn provides a food source for the sooty mould Capnodium walteri which covers the branches of manuka forming thick, black deposits.


In the late 1930s manuka in Canterbury (South Island of New Zealand) started to grow sick and die. By the late 1940s it was reported that it was hard to find living manuka in the region. Farmers assisted in the spread by moving infected manuka around the country to control what they viewed as a weed. The culprits were found to be two species of Eriococcus scale insect that presumably had been introduced from Australia. The effect on the plant appears to be due to nutrient stress from having large numbers of scale insects sucking on it as opposed to being a disease transmitted by the insects.


In 1957 it was discovered that a fungus was killing E. orariensis, the species that damaged manuka most severely. Subsequently, the numbers of this scale insect declined dramatically with a corresponding increase in manuka numbers. Manuka has also seen a rise in popularity and is no longer viewed as being as significant a weed.


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