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.

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Year to year fluctuations The greatest number of colonies was seen in 2002–3 when more than 30 were found distributed all over the study area. These were first noticed in October 2002 as patches of web of 40 cm or more in diameter with, on fine days, swarms of brick-red adult mites. Both Ulex europaeus and U. gallii were infested, roughly in proportion to the total number of each species in the population. Activity almost totally ceased by late July when only a few mites and decrepit web were found.

The timing of first appearance of the webs and development of mites were similar in the next season (2003–4). There were fewer colonies and these were probably smaller than in the preceding year. No colonies could be found on some bushes which had big infestations in the previous year.

No colonies at all were found in 2004–5 although the places where mite colonies had been observed in 2002–3 and 2003–4 were frequently revisited and some selected bushes were examined by parting the branches and searching both the tangled, matted dead spines of previous seasons growth as well as current growth. The nature of growth of the gorse bushes, many of which were of considerable age, made it impossible to search the litter underneath the bushes.

Webs were next seen in July 2005 and by 9 September 18 webs were counted in the study area. A few days later, after a spell of intense, thundery rain most of the webs were almost completely washed away and no mites could be found. Where mites were still present they were much reduced in number.

Selected bushes In winter 2002 some bushes were selected for continuous, frequent observation. (every 5–15 days). In some cases the mites disappeared and were not seen on the bush again after May 2003 but for four bushes there was an almost continuous record of mite occupation, illustrated by one particular bush. This was situated on the periphery of a large dense clump of about eight component bushes. Most of the bushes were vigorous and late flowering, but the infested bush was very early flowering and less vigorous perhaps because of mite damage. Even when the infestations were at their height, none of the surrounding, contiguous bushes was affected, perhaps indicating some variation in attractiveness to mites.

In 2002 there were about 10 distinct sites of infestation, each site about 40–50 cm in diameter. The amount of visible web declined from June onwards and reappeared in October 2003 with, again, several for sites of infestation which flourished over the winter. Microscopic examination in spring 2004 found that all stages of mites were present.

After May 2004 when the webs disappeared no mites could be found even when examining the old spines of previous season’s growth. They were not seen again until September 2005 when ten small infestations were recorded.

These were washed away by the heavy rains and none was seen until July 2006 when observations were discontinued.

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Predators and hazards All colonies were heavily predated by a small black ladybird (Stethorus punctillum) and a predatory gall midge (Feltiella acarisuga ) (Kirby, 2005).

Adult Stethorus were found among almost every sample of mites including those taken during the winter months. From March onwards eggs, larvae and pupae were also found. Feltiella appeared later and was less frequent than the ladybird; the imago was seen only once. Except for the first season (2002–3) larvae and pupae were rarely found.

Large numbers of dead mites were occasionally found trapped in rolled-up or folded fabric web and this was attributed to rainfall. The effect of rain on structure of webs was closely observed in 2005, when new webs appeared in August and early in September. These had strong, active colonies of mites and had every sign of expanding to cover substantial areas of the bushes. During mid-September, however, they were spells of intense, thundery rain, often of considerable amount (e.g. >25 mm on 16 September) which did much damage to the webs. Twigs, previously covered with a tent of fabric web, sheltering clusters of mites and eggs, were washed clear of web. Often all that remained of the web were a few remnants looped around the lower branches. Much of this was rolled up trapping and killing large numbers of mites. The surviving mites responded by congregating at the tips of downward-pointing spines or flowers, forming a ‘tear drop’ which migrated downwards until it hung secured only by a few strands of silk. Within about a week all signs of mites had gone from all bushes (White Admiral, no. 61, 16).

Mites on potted gorse plants Seedling gorse plants, c. 30–45 cm high, were dug up and potted in garden soil. During the winter 2003–4 these were ‘seeded’ with clusters of mites lifted from infested bushes. The mites quickly moved away from the seeding site and established a colony amongst the spines.

Feeding behaviour was examined in situ using a stereo microscope mounted on a retort stand. Using natural light only, the behaviour of the mites could be watched for some time without disturbing them, unlike observations using a lamp, which provoked intense activity and dispersal. Typically, mites moved onto a fresh spine in such numbers that they completely covered the surface, ‘shoulder to shoulder’ and then assumed a feeding stance, with the gnathosoma (head) pressed to the spine. They remained feeding without jostling or movement for some time and then dispersed. Viewed after their departure, the spine was covered with a film of liquid, presumably leakage from punctured cells.

Diurnal behaviour. The activity of a mite colony was followed throughout a day using a time lapse function on a digital camera set to take a picture every five minutes. The mites were most active on sunny warm days and a typical pattern of activity was recorded on 2 March 2004, a bright sunny day; maximum 9, minimum –2Â3, grass minimum temperature –6Â7 °C. The twig selected had an area covered with fabric web making a strong, opaque tent like cover stretched over the tips of the spines. When first observed at 8:00 GMT, no mites were visible. They started to appear from beneath the fabric web at

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9:17 and by 10:28 were forming dense clusters around the base of spines. By 11:19 the spines were covered in feeding mites. At noon they started to disperse and move distally up the twig where they started to spin scaffolding web (strands of silk stretching from spine tip to spine tip or to the main stem, Plate 3). They continued spinning for about an hour by which time they had covered another section of twig in fabric web. They began to disperse at 13:06 and within 15 minutes had completely retreated beneath fabric web lower on the twig.

After about three months the numbers of mites had increased and the seedling bushes were heavily infested, and in May mites were seen to leave the colonies. Typically, a cylindrical mass of mites hung by a thread (probably formed from several individual strands of silk) which swung freely in the breeze. Mites had spun a scaffolding web on the supporting spine and some appeared to be moving to the tip of the spine and joining the dangling cylinder. Immediately at the tip of the spine was a small congregation of mites, then a length of silk with a few mites on it, then another small sphere of mites and finally a long cylinder. The total length of the dangling mass was 70 mm of which about half was the main mass (Fig. 1).

When examined under a microscope the mites were seen to be closely clustered with their legs overlapping resembling swarming bees. All visible mites on the outside of the cylinder were brick-red adults although pulling apart the mass revealed a few eggs within.

The colonies were also watched to see if there was any evidence of wind dispersal. Gatherings on the tips of exposed spines in breezy conditions were watched through a microscope but there was no indication of behaviour which might lead to wind dispersal.

Discussion Infestations of gorse by Tetranychus lintearius vary from year to year. Reports about their occurrence has been reviewed by Van Eyndhoven (1967); he also looked for webs in places where gorse was common and widespread in France and England, including sites where gorse mites had been found previously, with little success and concluded that he ‘was of the same opinion as earlier authors that this species is not Figure 1. Mites congregating on the tip of a gorse spine and spinning a thread down which they move to make a ‘cylinder’ of closely packed individuals which eventually drop off.

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at all common’. He also noted that ‘the mites occurred very locally, a few separate shrubs only being infested amid a great number of sound shrubs’.

Similarly the observations in this paper show that over a small, closely observed area the occurrence of infestations is intermittent and that in some years searches for webs and mites were unsuccessful although colonies had been common in previous years. The establishment of new infestations seems to be reappearance rather than colonisation from a distant locus as in several instances the webs appeared on previously infested bushes, often among dense thickets of bushes each of which appeared equally suitable for mite colonisation.

A number of factors have been suggested for the decline and disappearance of Tetranichidae (spider mite) colonies (Schuster & Murphy, 1991). Of these, overexploitation of the host may be important as gorse mites move systematically from the established web, feeding en masse, and leaving the spines severely damaged. The colony however remains integral and groups of mites do not move away from the main body even though there is fresh foliage on other parts of the bush or uninfested bushes in close contact.

Competition from other phytophagous organisms is not involved, as among none of the mite colonies examined were other gorse foliage feeders found and most of the natural predators attack the flowers or seeds. Predation, particularly by ladybirds (Stethorus punctillum) was an important factor. There were adult beetles present in almost every mite colony examined, irrespective of date, and in spring and summer larvae and adults were seen feeding voraciously on mite eggs, nymphs and adults. In no case, however, did predation wipe out a colony, for prior to disappearance in May or June there were still many mites present.

A final factor, listed by Schuster & Murphy (1991), rain and wind, appears to be the most potent in affecting the success of mite colonies and many instances were seen where rain had damaged the web. In some cases a number of mites had survived, but did not re-establish a web and moved away from the bush as a coherent group.

Observations showed that the way in which the mites left their web was similar for both rain damaged webs and heavily exploited (potted) plants. The colony gathered at the tip of a downward pointing spine to spin a length of threads down which they descended, eventually to fall to the ground.

Comparable behaviour was seen when sample twigs, heavily infested with mites, were left in a box. After a time the mites gathered together and left the box as a group often building a bridge of silken threads from the edge of the box to the bench about 8 cm away. These observations show that gorse mites have a coordinated behaviour, one consequence of which may be that any migration involves a large group of mites rather than individuals dispersing singly, perhaps behaviour ensuring a sufficient mass to establish a new colony.

No evidence of wind dispersal was seen on the potted plants. In the field sticky traps (CDs covered with double sided sticky film) were placed around large colonies and inspected for adherent mites but none was found, although the traps were difficult to maintain and were discontinued after about two weeks.

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Although no reports about how gorse mites move from infested bushes or colonise new sites were found, there has been much experimentation on the closely related, commercially important red spider mite (Tetranychus urticae).

It had been suggested that red spider mites might be wind dispersed using silken threads like baby spiders, but no evidence for this was found by Hussey & Parr (1963). They thought that the main way in which mites left from an infested plant was by ‘roping’, that is making a silken thread down which the mites descended, ‘leap frogging’ the mites on the rope, extending it downwards and eventually dropping off.

The disappearance of mites from infested bushes poses the question about where they go to and how they survive during the periods when none are seen in suitable places for colonisation. Many species of spider mites diapause during unfavourable conditions, e.g. T. urticae which infests annual crop plants hibernates during the winter in crevices in the soil, in stakes or in a glasshouse bench (Hussey & Parr, 1963). In contrast the gorse mites show no cessation of activity during the winter months. Stone (1986) found no response to experimental photoperiodic treatment and concluded that they had did not diapause. To resolve this would involve finding gorse mites during their periods of presumed inactivity (when they are not making webs). They have been found to reappear near to sites where they have been seen previously and the most obvious hypothesis is that they diapause in the copious debris of dead gorse spines under the bushes, but to confirm this would be a formidable project.

References Hussey N. W. & Parr W. J. (1963). Dispersal of the glasshouse red spider mite Tetranychus urticae Koch (Acarina, Tetranychidae). Entomologia Experimentalis et Applicata 8: 207–214.

Kirby, E. J. M. (2005). Gorse mites and their predators. British Wildlife 16: 314–317.

Schuster, R, & Murphy P. W. (1991). Life history evolution of spider mites. In Schuster, R, and Murphy P. W. (Ed.) The Acari: reproduction, development, and life history strategies. pp. 24–48.

Stone C. (1986). An investigation into the morphology and biology of Tetranychus lintearius Dufour (Acarina, Tetranychidae). Experimental and Applied Acarology 2: 173–186.

Van Eyndhoven G. L. (1967). Tetranychus lintearius Dufour, 1932, is a valid species (Acar.) Notulae ad Tetranychidas II. Entomologische Berichten 27: 90–100.

Dr E. J. M. Kirby The Studio Blythburgh Road Westleton Saxmundham IP17 3AS

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E. J. M. Kirby

Plate 3: Gorse mite (Tetranychus lintearius) activity: at midday mites have finished feeding and commenced web making. (p. 58).