IT’S WAR ON GORSE

The headline from a New Zealand newspaper highlights a scheme to control gorse, which since its introduction from Britain by the early settlers has become a noxious, invasive weed. Amongst the measures to control it the gorse mite Tetranychus lintearius which feeds on gorse, weakening and stunting the bushes was introduced from Britain and Western Europe. The help of schoolchildren has been enlisted to spread the mite around; 10 year olds are shown how to rear large numbers of mites and then to inoculate gorse bushes. As well as learning about biological control they also contribute to conservation of native bush which is threatened by the alien weed.

All has not been plain sailing, however, for natural hazards such as rain and insect predators batter and besiege the mite colonies and the mites have a roller coaster existence. Similarly at Westleton the mites have big population fluctuations from year to year. During 2004-5 no colonies were seen on the Common or the Heath although they were frequent the year before. In August 2005 they reappeared and more than 20 large, healthy webs were seen and all looked set for an opportunity to watch the build up of predators in the colonies. During the middle of September, however, there were spells of intense, thundery rain (e.g. about 25 mm on 16 September) which washed away the webs. Twigs, previously covered in a strong fabric-like web making a ‘tent’ over the tips of the spines beneath, in which the mites shelter and lay large clusters of eggs (Kirby, 2005) had mostly been washed clear of web and there were only a few remnants looped around the lower branches. Where the rain had rolled up the web it trapped and killed large numbers of mites. The surviving mites responded by congregating at the tips of downward-pointing spines or flowers, forming a ‘tear drop’ which seemed to migrate downwards until it hung secured only by a few strands of silk.

Within about a week all signs of them had gone from some bushes, the clusters of mites presumably had dropped off and scurried away into the litter under the bush. Perhaps they go into some sort of dormancy to reemerge in Autumn 2006?

Reference

      Michael Kirby: Westleton

Gorse mites migrating to a ‘tear drop’ cluster
after rain damage to their web

At the lower left the fabric-type web has been rolled up by
heavy rain trapping many mites.
In the centre mites are congregating in a tight cluster
which will increase in size and migrate down to form a ‘tear drop’.
Note the two small, black ladybirds (Stethorus punctillum)