Wednesday, 22 January 2014

Citrus Leaf Miner

Phyllocnistic citrella

Distorted growth on citrus tree
Distorted growth as in this photo, is often caused by citrus leafminer. Citrus leafminer is a small moth pest and the damage to the leaves of citrus is caused by the larvae as they mine immature foliage. Twisted and curled leaves are generally the first symptoms noticed. Severe infestations - an average of two or more mines per leaf - can retard the growth and yield of nursery and newly planted trees, however their effect on mature trees is far less serious. Infestations generally occur in late summer and autumn and are often related to low natural enemy activity. They rarely occur in spring because the production of new growth is prolific and synchronised and quickly becomes immune to attack. Leafminer is native to eastern and southern Asia and is now widely distributed where citrus is grown throughout the world.

The moth was first recorded in Australia in and around Darwin in 1912. It was probably present several years earlier and was apparently eliminated in 1922 after a five year campaign to eradicate citrus canker. During the campaign all citrus trees north of the nineteenth parallel in the territory were destroyed. Since then rigorous quarantine measures have been in force to prevent establishment of the disease in Australia. In 1940 citrus leafminer was again found in Darwin and in 1965 it was recorded at Cairns in Qld. Between 1965 and 1985 it spread slowly southwards along the east coast of the continent to the NSW south coast. In 1988 it was still only found in the Northern Territory and in the coastal districts of NSW and Qld and was affecting less than 20% of the Australian citrus industry. In 1989 it was recorded for the first time in orchards and home gardens from Dubbo north. Between then and 1992 it spread south west into the Murrumbidgee Irrigation Areas, Hillston and the Murray River districts of NSW and Victoria. By January 1993 it reached the NSW/Victorian Sunraysia and South Australian Riverland districts. By April 1995 it had spread across the continent to south-western Western Australia. So from the first moth recorded around 1912 it has been spread throughout the continent of Australia, amazing how it can be spread by we humans possibly taking a few citrus to family and friends!

The adults are night flying moths with a wingspan of a tiny 5mm. The females lay their eggs on new young growth in the warmer months of the year. When the larvae hatch they are flat and yellow and are around 3mm long. The larvae burrows into the leaf, leaving silvery squiggly paths on the foliage.

Once grown, the larvae will curl the edges of the leaf together to help form the cocoon. Inside the cocoon, it will pupate into its final form - the small moth mentioned above.

Under optimum conditions the whole life cycle takes from two to three weeks. The affects can stunt growth and reduce yield. It can be especially damaging to the tender new growth of the plant.

Control: Because infestations are restricted to flush growth, particularly in late summer and autumn, their severity can be reduced by fertilising in winter to promote flush growth in spring when the pest is either absent or relatively scarce and by limiting flush growth in late summer and autumn by not fertilising and irrigating during summer and autumn in excess of the amount needed for normal growth. This is a bit of a problem for our Coffs' gardens as we have wet summers! Pruning of late flush growth can also be used to limit and remove the unsightly infestations.

Natural enemies include small parasitic wasps and predators such as lacewings. These predators are generally associated with heavy infestations.

Spraying with an approved citrus spray at the recommended doses and frequency to both the upper and lower surfaces of susceptible leaves should commence in January.


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