Ladle, M.

Welton, J. S.

NERC Open Research Archive

recii-tXe-CciVinCAInstitute of Al-c7ILFreshwater  1 ',-'-'Ecology C..)NaturalEnvironmentResearch CouncilINSTITUTE OF FRESHWATER ECOLOGYRiver Laboratory, East Stoke, Wareham, Dorset BH20 6B8LicProposed measures for control ofSimulium posticatum 1990M. Ladle & J.S. WeltonProject leader: M. LadleDate: August 1990Customer: North Dorset District CouncilIFE report no.: RL/T11053c5/2TFS project no.: T11053c5This is an unpublished report(and should not be citedwithout permission, which should be sought through theDirector of the Institute of Freshwater Ecology in thefirst instance.The Institute of Freshwater Ecology is part of theTerrestrial and Freshwater Sciences Directorate of theNatural Environment Research Council.1. INTRODUCTION1.1 BackgroundThe "Blandford fly" problem has been under surveillance by local authoritiessince the latter part of the 1960s. Research by IFE scientists has shownthat the problem is caused by the females of Simulium posticatum (formerlySimulium austeni) which often bite humans to obtain a blood meal. The larvaeof these flies are aquatic and live in the River Stour. At present, about 40kilometres of river are affected. Although other species of Simulium occurin the River Stour there are usually several weeks, in March, April and May,when the larvae of the "Blandford fly" are virtually the only ones present.The life history of the fly has also been researched and it is known thatthe eggs are laid in June-July, above the water in specific areas ofvertical, loamy river bank, shaded by trees and frequently dissected bydesiccation cracks. This is a unique egg laying strategy for this group ofinsects. The eggs undergo a series of environmentally triggered changes,including a resting stage or diapause, before hatching in late winter orearly spring.Upon hatching, the larvae of the "Blandford fly" drift downstream andattach, by a combination of silk threads and abdominal hooks, to waterplants in the more swiftly flowing reaches of the river. They feed byfiltering suspended solids out of the river water. By comparison with otherSimuliidae the larvae of the "Blandford fly" are slow feeders and may takeone or two hours to completely exchange their gut content.Larvae pupate on the water plants between late April and June, emerging asadults after a few days. The pupae are sedentary and do not feed. Afteremergence the male flies (which do not bite) form swarms near trees, housesand other convenient visual "markers". The female flies have their bloodmeal, mate and ultimately return to the river to lay their eggs.1.2 Potential control measuresIn theory the "Blandford fly" could be controlled at any stage in its lifecycle. Many potential control measures have been considered.The eggs are possibly the stage which is most susceptible to control beingimmobile, concentrated in well-defined locations and present in situ forabout eight months. The disadvantage of attacking this stage is that it isthe most numerous phase of the insect's life. There is no known means ofcontrol for the egg stages of Simulium. In the present case physical removalhas been tested by IFE but it would be extremely labour intensive and thelength of river bank to be treated is very large.Adult flies are widely dispersed and, although they are vulnerable tosprayed insecticides, it is improbable that any form of chemical controlcould be applied at this stage because of the dangers to human beings and tothe environment.The aquatic stages, being confined within the river, are susceptible to somespecialised forms of control. In the past weed cutting has been used as ameans of removing pre-emergent larvae and thus reducing the numbers ofemerging adults. However, this method is very expensive and the removal oflarge quantities of aquatic plant material and the associated fauna isextremely damaging to the environment. Most chemical controls are illegal inthis country and, in general, the addition to river waters of materialsdamaging to fish and/or fish food organisms is forbidden.1The material Bti is a biologically produced insecticide which is veryspecific in its action. Essentially it consists of a crystalline proteinwhich, when swallowed by certain insects, is activated by conditions in thegut to become a lethal toxin. The larvae of Simuliidae, including those ofthe "Blandford fly" are known to be susceptible to Bti. A formulation of Btiproduced by the Sandoz Company and trade named "TEKNAR HP-D" was used in asmall scale trial in 1989.1.3 Trial control in 1989The conclusions of this trial were:-"TEKNAR HP-D" is an effective larvicide when used against the larvae ofSimulium posticatum (the "Blandford fly") under the conditions prevailing inthe River Stour, with more than 90% reduction in numbers following the twoapplications.The efficacy of "TEKNAR HP-D" varies with river conditions and it ismuch more effecive in low discharge and clear water than in high dischargeand turbid water.Assay using mosquito larvae, indicated that the sample of "TEKNAR HP-D"used in the present trial had a slightly lower toxicity than was expected onthe basis of a standard internationally recognised Bti preparation.Water samples taken from the carrier stream during application of"TEKNAR HP-D" were assayed and had only very low levels of Bti activity.Sediment samples taken from the carrier stream following application of"TEKNAR HP-D" showed no Bti activity at any time.From (4) and (5) above it is concluded that the toxic properties of"TEKNAR HP-D" are rapidly lost in association with River Stour water andsediments.Samples of drifting invertebrates from the stream often contained bothliving and dead Simulium posticatum larvae even when no "TEKNAR HP-D" hadbeen applied.The numbers of living Simulium posticatum larvae drifting increasedsharply following application of "TEKNAR HP-D" but only at the samplingpoint 10 m downstream of the application point.The proportions of dead Simulium posticatum larvae in drift samplesincreased after applications of "TEKNAR HP-D". The increase was most evidentat sampling points 100 m and 200 m downstream of the application pointfollowing the second application. With regard to this it should be notedthat drifting larvae (whether living or dead) are derived from the treatedpopulation upstream of the sampling point and, in consequence, more areavailable are 100 m and 200 m than at 10 m.There was no increase in drifting activity of any group other thanSimulium posticatum following treatment with "TEKNAR HP-D".With the exception of S. posticatum there was no change in theproportion of dead to live organisms of any group (including chironomidlarvae) in the drift following application of "TEKNAR HP-D".2Samples of drift collected on 21 April 1989 from 10 m and 200 mdownstream of the application point and returnedto the laboratory forincubationshowed no mortality of invertebratesother than four individualsof three taxa. One hundred chironomidlarvae in the samples were still aliveon 3 May 1989 when the experimentterminated.To detect any changes in the character of the stream fauna followingtreatmentwith "TEKNAR HP-D", biotic indiceswere calculated.There were nosignificantchanges in the BMWP and ASPT values, both of which are widelyaccepted as indicators of water quality, following treatment. Control andtreatment sample sites all showed high values indicating the presence ofrich and diverse communities of bottom-living animals including a highproportionof pollution sensitivespecies.Samples of chironomid larvae taken from the river bed showed noincreases in the proportionof dead larvae following treatment with "TEKNARHP-D". The proportion of dead larvae varied considerably from site to siteand seemed to be directly associated with local differences in river bedconditions.No dead fish were observed in the streambefore or after treatmentwith"TEKNAR HP-D". In the course of sampling for invertebratesthirty-one fishof three species were caught; these included both fry and adults of thebullhead (Cottus gobio) and fry of the perch (Perca fluviatilis) and of acyprinid (Cyprinidae).There was no evidence that treatment with "TEKNARHP-D" had any adverse effect on these species even though it is likely thatsome of them feed on the larvae of Simulium posticatum at times.Dixidae were not present in the stream either before or after treatmentwith "TEKNARHP-D".1.4 ObjectivesThe ultimate objective is to reduce populations of Simulium posticatum inthe River Stour to a level at which the biting problem in the area issubstantiallylessened.This proposal takes into account the 'Guidelinesfor biological monitoring'put forward by the PesticidesRegistrationSection, 28 February 1990.1.5 PermissionsHSE has given the North Dorset DistrictCouncil permission to experimentallytreat eight sites on the River Stour in 1991 with a Bti preparation ("TEKNARHP-D") as the next phase in the control of Simulium posticatum, followingthe successfultrial in 1989. Although the treatment is to be restricted tothe area of the main river downstream of Blandford but no less than 7kmupstream of the Longham intake of BournemouthWater Co., it should be notedthat large populations of Simullum posticatum occur downstream of thissection.2. TREATMENTSITESSites 1, 2, 3 and 4 (Figure 1) will be treated initially and sites la, 2a,3a and 4a will be treated only if significantpopulations of S. posticatumare found in the preliminarysurvey.32.1 PreliminarysurveySamples of weed will be taken from various sites on the main river startingin early March 1991. The number of larvae per gram of weed and consequentlythe population density will be calculated at each sampling point. This willidentify the main areas of larvae and will be used to locate the treatmentpoints precisely.2.2 Number of monitoring sitesAlthough a maximum of eight sites may be treated in the present trial thecost and effort of monitoring all of these would be prohibitive. It issuggested that only the most upstream site and the most downstream sitetreated should be monitored. Absolute controls for experiments can only becarried out above the upstream site but additional controls would be set upabove any other sites monitored. The reasons for choosing these two sitesare that the upstream one has the only true control and the furthestdownstream should reveal any cumulative effect of the treatments.Bti ASSAYMosquito larva assays of sequential water samples will be organised by DrLehane on behalf of NDDC. If required, samples could be taken for assay atfrequent intervals from the surface of the water and from within the watercolumn over the peak period of Bti passage as determined by salt dilution(see section 4). It should be noted that the costs of this excercise arelikely to be high if a great many samples need to be examined.HYDRAULICMODELIf a hydraulic model of Bti dispersal in the river is believed to beessential, the applicant should contact the Institute of Hydrology or someother hydraulics consultant for advice. However, the cost of suchinvestigations would probably be prohibitive and since the Bti would only beapplied in areas of turbulent water where mixing occurs, we believe thatsuch detailed studies should not be necessary.The use of an inert tracer, having similar physical characteristics to thebacterial spores, to simulate the translocation of Bti in the river inrelation to the potable water intake at Longham is, in our opinion, notpracticable. The use of detritus as suggested in the committee's guidelineswould not be possible (we have 25 years experience of detritus studies inrivers).The use of a 'stream indicator' to show when water samples should be takenwas in fact done in the 1989 trial when sodium chloride was used as anindicator prior to application to fix the most suitable sampling times. Thisapproach is the most precise and practicable one available. Salt dilution isa British standard method and as far as we are aware is an excellentapproach to tracing the timing of passage of any body of water. The use ofFluorescein (as suggested by the committee) would give no advantage inphysical properties over salt and could, because of its high visibility andcarcinogenic properties , cause adverse public reaction. Even using saltdilution, the problems of tracing in a large volume of water and over longdistances will be considerable. This problem would be compounded if detailedmodelling was required for all treatment sites.4Practicalities of salt dilution need to be considered. The method canprovide a/ information on timing of translocation from treatment point towater sampling points b/ information on the discharge of the river (ifadequate data are not available from the NRA) to permit precise calculationof Bti volumes required for treatment.THE POSSIBILITY OF Bti IN WATER SUPPLYTesting for the presence of Bti in the "effluent" of the treatment plant atLongham by Bournemouth Water Co. would largely negate the requirements for 3and 4 above.APPLICATION OF BtiAs in the 1989 trial there is a possibility that any treatment could bevitiated by adverse flow or weather conditions. In view of this possibilityit would be appreciated if, as in 1989, flexibility could be given for asecond treatment if only limited success was achieved in the first instance.MONITORING THE EFFECTS OF Bti ON RIVER FAUNA7.1 Effect on Simulium posticatum larvae.Take 30 weed samples each from a control section (upstream of theapplication point) and four sections at approximately 20, 50, 100 and 1000mbelow the application point. The exact distances will depend on the depthand velocity of the river and the availability of significant numbers oflarvae before treatment. Samples will be taken on days -1, 0, +1 (weconsider that a single daily sample would be adequate and do not understandthe rationale of three samples as suggested by the committee) 1 week, 2weeks, 6 weeks and 12 weeks. Count and assess the population density fromall samples. Examine all samples from each location on each day to determinethe proportion of living and dead simuliids. Samples taken after 6 and 12weeks are unlikely to contain Simulium posticatum larvae but are likely toinclude other Simulium species which can be used to monitor any residualeffects of Bti.The alternative suggestion (of the committee) of experimentally testing theresidual toxicity of Bti on weed samples taken three months after treatmentcould be difficult. Weed beds would change extensively over the periodfollowing treatment and it might be problematical to obtain suitable samplesto test.7.2 Effect on chironomid larvaeTake thirty 10-second kick samples each from 1 control and 3 sections at 20,100 and 1000m downstream of each application site to assess proportions ofliving and dead chironomid larvae (using ten samples from each section) andabundance of Chironomidae (using thirty samples from each section). Samplesto be taken on days -1, 0, +1, +3, lweek, 2 weeks, 4 weeks, 6 weeks, 8weeks, 10 weeks and 12 weeks.Many species of riverine chironomids have extremely short generation timesof 2-4 weeks. Consequently after this time it is considered extremelyunlikely that many individuals present in the river would have been presentat the time of treatment. However, samples taken after this period mayenable detection of residual Bti activity (if any) in the epiphytes on whichmany of these insects graze.57.3 Effect on benthic faunaTake triplicate 3-minute kick samples each from 1 control section and threetreatment sections, 20, 100 and 1000m below the application site of Bti ondays -1, 0, +1, 2 weeks, 6 weeks and 12 weeks. Sort, identify and analyse atfamily level for BMWP and ASPT scores and RIVPACS community analysis toassess any changes taking place in the fauna.This analysis will be more informative than simple diversity indices.RIVPACS is the nationally used standard method for comparing observedcommunity structure with that expected to be present.7.4 Effect on DixidaeDixidae live in the surface film of the water along river margins and wouldnot be sampled by any of the other methods used. A separate pond-net searchwill be conducted for these insects at each site prior to treatment to berepeated after treatment if any are found.7.5 Drift samplesTake 1 hr drift samples from a control section and four sections at 20, 50,100 and 1000m downstream of the treatment point, at each treatment site onthree occasions during the hours of daylight on days -1, 0, +1, 4 weeks and10 weeks. Sort and count for drifting fauna and assess proportions of livingand dead Simuliidae and Chironomidae. Examine for other taxa which might bein a dead or distressed condition.Take drift samples in the hours of darkness on days -1, 0 and +1 fromcontrol and four sections at 20, 50, 100 and 1000m downstream of thetreatment point. Samples to be taken for lhr at dusk, midnight and dawnapproximately. Sort and count for drifting fauna and assess proportions ofliving and dead Simuliidae and Chironomidae. Examine for other taxa in adead or distressed condition.Drift sampling is a difficult and time consuming activity and particularproblems arise because of the spatially variable nature of driftingactivity. It is improbable that several suitable sampling points will befound in each stretch at which there will be no interference between nets.Because of this only a single drift net will be sited at each point andreliance will be placed on the differences between control and treatmentsamples. An attempt will be made to site drift nets in positions havingsimilar depths and velocities. To reduce the effort required withoutdetriment to the data it is proposed that no attempt should be made toquantify the drifting Simulium but simply to estimate the live:dead ratio insamples. Weed sampling should provide satisfactory estimates of anydisplacement of living Simuliidae which may occur.6

Proposed measures for control of Simulium posticatum 1990

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Proposed measures for control of Simulium posticatum 1990

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