Biological control continued

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Weeds in our Area (Part One Hundred and Three)

By Bob and Ena McIntyre – Garden Route


Acacia cyclops and A. longifolia
Our unpopular Acacia friends from Australia have enjoyed a fair amount of attention in the Bio-Control field. Of the 10 species that are either Category one or two, control agents have been released for eight with varying success.
One of these successes was reviewed in Article 64 in June 2006. The-once off release in 2002 of the Gall Midge near Stellenbosch has seen the Gall midge’s spread through-out the areas in South Africa that are infested with Acacia cyclops. The gall midge lays its eggs in the flowers of Acacia cyclops (Rooikrans) preventing the flower from setting seed. Very few plants with seed-bearing seed pods have been seen in the past four years. As of now it appears that this agent has virtually stopped the production of seed from this species. But for a collapse of the established population of the gall midge for some unforeseen reason, the threat of rooikrans to our coastal areas has been addressed. New plants would come from any remaining viable seedbeds and in all likelihood these will be prevented from setting seed.
Another galler that has come to the aid of our natural environment is the Gall wasp (Trichilogaster acaiaelongifoliae) which was released in 1982 (see photo). It uses Acacia longofolia (Long-leaved wattle) as a host in its production cycle. The wasp has now been established throughout the range of the weed through natural dispersal coupled with manual re-distribution. The female lays its eggs during spring and summer in immature flower buds and vegetative buds. The adult wasp lives for only a few days but during that time lays approximately 400 eggs that remain dormant until after the next winter, hatching in early spring. The larvae produce a secretion which mimics the plant’s hormones. These induce abnormal growth of the flower buds which become distorted and swollen into the characteristic galls. A ‘gall’ can contain up to 15 hollow chambers each containing a single larva (see photo) that feeds on the surrounding tissues and develops to adulthood.
Research has shown that the gall wasp has curtailed the seed production by up to 95%. The galling also suppresses the vegetative growth of the plant and can cause branch die-back. However in moist areas where the plants are not subject to water-stress, plants are able to compensate for the galling and still produce high seed numbers.
To address this weak area a seed-eating weevil (Melanterius ventralis) was released in 1985. The adult weevils feed on the green developing seed and deposit their eggs into nearly ripe pods. The developing grubs feed on the seed and can destroy two or three adjacent seeds. Once fully developed the larvae chew their way out of the pod and drop to the ground where they burrow into the soil to pupate and complete their cycle. It is estimated that the weevil has reduced the production of viable seed by up to 85%.
Read more about the wonders of working with nature and using the invader’s own natural enemies in the quest to protect our indigenous bio-diversity next time.
References: Environmental Management Vol 2 No 6 September/October 2007

Photos courtesy PPRI Stellenbosch (Fiona Impson)

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