POPULATION FLUCTUATIONS IN GORSE MITES

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POPULATION FLUCTUATIONS IN GORSE MITES

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POPULATION FLUCTUATIONS IN GORSE MITES MICHAEL KIRBY Gorse is occasionally infested with gorse mites (Tetranychus lintearius). The conspicuous silvery webs sometimes spreading over most of the bush and swarming with tiny red mites make detection and recognition easy. After a visitation, the effects of the mites feeding on the gorse spines leaves them bleached and dead which reduces the vigour of growth of the bush. The occurrence of mite infestations varies and in some years there was no sign of mites where previously heavy infestations had been seen. In this paper the frequency, from year-to-year, of such infestations and the impact on the growth of the host bush are described. Biotic and weather factors which may affect the progress of a mite colony are examined. Methods Observations of mite colonies were made in Westleton parish during the period from October 2002 until July 2006. Gorse, both common (Ulex europaeus) and Western gorse (U. gallii) is widespread in the sandy areas of the parish where the Norfolk Crag is not covered by glacial drift. It forms long dense ‘hedges’ along many of the footpaths which facilitated regular inspection and sampling for microscopic examination from infested bushes. Some heavily infested bushes of both species of gorse were marked in 2002 and inspected regularly throughout the four year period (including times when no mites were seen). In addition, some small potted plants were seeded with mites to allow more detailed observation of feeding and web making. Mite life cycle Mites were active during winter 2002–3 swarming over the tips of the gorse spines and investing them with a fine, glistening web. Only adult mites, brick red in colour, were found during this period. Clusters of eggs were first seen in the spring, followed by larvae and green nymphs including the resting stages. Throughout April, May, June and July all stages were present as more breeding adults entered the colony (Kirby, 2005). From early in August the numbers of all stages of mites declined and eventually could no longer be found by careful searching of the masses of dead spines within the bush as well as the green foliage. The above cycle of the mite development stages was similar in other years, defining the annual period of mite activity from September/October when mite colonies started to spin their web until July/August when activity declined and mites moved away from the site of the colony. Web building Mite web is not sticky and mites and their predators move freely around the silken threads. At first the mites build a scaffold of web, by spinning threads as they move to the tip of a spine, anchoring it and then returning to a twig to pull be thread taut and then anchor it some distance from the spine. As more and more threads are added to the web it becomes almost opaque, strong and muslin-like and a writer in 1856 described it as looking like ‘glazed satin’; here it is referred to as fabric web.

Trans. Suffolk Nat. Soc. 42 (2006)


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POPULATION FLUCTUATIONS IN GORSE MITES by Suffolk Naturalists' Society - Issuu