TETRANYCHUS V. STETHORUS -

In January vigorous colonies of the gorse spider mite (Tetranychus lintearius), which feeds on gorse, were noticed on several gorse bushes. Patches of web up about 40 cm in diameter were seen and on warm days the small red adult mites, about 0.5mm long, swarmed around the periphery of the web in great numbers. The colonies continued in this manner until the first eggs were seen towards the end of March, suspended in the web or in clumps, probably of several hundred, on the gorse twigs. As these started to hatch out, small six legged larvae first appeared and later, eight-legged, pale-coloured nymphs of different instars. Some of the nymphs apparently developed into normal brick red adults but some green forms were also noticed. (The precise nature of these various forms is still being investigated; there seem very few British published papers or pictures about the gorse mite – or do you know an acarologist who knows better?).

Early in April the villain of the piece, or hero depending on your point of view, first appeared. Small black beetles, c. 1.6 mm long (Photograph) were seen more and more frequently within the web and were clearly doing more than merely passing by. They were identified as Stethorus punctillum, one of Britain’s tiniest ladybirds. The numbers of ladybirds soon built up and eggs, larvae and pupae were found in addition to the adults. As the numbers of ladybirds increased, so did the numbers of mites decrease. The ladybird eggs were laid amongst the mite eggs and the small newly hatched larvae were seen to eat mite eggs and the small, newly hatched, six-legged larvae. Larger larvae were frequently seen to seize adult mites and consume them in a minute or so. Some colonies of mites have been more or less wiped out; heavy rain on 1 April washed away the external web and now, at the end of May, all that remains are small patches of dilapidated web, usually beneath the gorse canopy, sometimes with a few moribund mites, so that unless the colony had been noted earlier in the season no one would know that it had been there at all. Stethorus punctillum was not included in the recent, long-term Cambridge Ladybird Survey because only the larger, more easily identified species of ladybird (chiefly Coccinellinae) were studied, but available records indicate that it is widespread in south and central England. Unlike many of the well-known ladybirds such as the 7-spot and 2-spot that feed on aphids, Stethorus punctillum feeds almost exclusively on mites. In this country, the beetle is chiefly associated with deciduous woodlands, orchards and hedgerows so that its occurrence in numbers on heathland appears highly noteworthy. We are currently aware of only one recent, reliable record of the beetle in Suffolk. If you know of others, please let DN know. At present progress it seems that most of the mite colonies will be wiped out. These dramatic events raise questions about predator-prey relations and the biology of the ladybird. In previous years some gorse bushes in Westleton have been almost completely invested in web, perhaps as much as 2 m² in area. Unfortunately these were not looked at closely, but on present evidence it seems unlikely that Stethorus was present. Has it has appeared or reappeared in the district, rather than not being noted?

Observations will continue and a more detailed account will be prepared later. If you have seen any mite colonies in your area, please let MK know.

Michael Kirby , The Studio, Blythburgh Road, Westleton, Saxmundham, IP17 3AS.
email: e_j_m_kirby@btinternet.com

David R. Nash, 3 Church Lane, Brantham, Suffolk, CO11 1PU
email: dr.nash@btinternet.com

© 2003   Suffolk Naturalists' Society