Ladle, M.

Welton, J. S.

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An experimental treatment of Simulium posticatum with Bti at selected sites on the River Stour, Dorset, 1993

0Institute ofFreshwaterEcologyAn experimental treatment ofSimulium posticatum with Btiat selected sites on the RiverStour, Dorset, 1992M. Ladle PhDJ.S. Welton PhD CBiol MIBioljr7 17,01LNatural Environment Research CouncilINSTITUTEOF FRESHWATERECOLOGYRiver Laboratory,East Stoke, Wareham,DorsetBH20 6BBAn experimentaltreatmentof Simuliurn posticaturn with Btiat selected sites on the River Stour, Dorset, 1992M. Ladle & J.S. WeltonProject leader:Report date:Report to:IFE report ref:TFS project no:M. LadleAugust 1992North Dorset DistrictCouncilRL/T11053c5/4T11053c5This is an unpublishedreport and should not be cited without permission, which should besought through the Directorof the Institute of FreshwaterEcology in the first instance.The Institute of Freshwater Ecology is part of the Terrestrial and Freshwater SciencesDirectorateof the Natural EnvironmentResearchCouncil.CONCLUSIONSIn 1992 (as in 1989 and 1991) TEKNAR HP-D (Bti) was found to be an effectivesimuliicide when used against the larvae of Simuliwn posticatum under the conditionsprevailing in the River Stour.Laboratory tests using larvae of Sitnulium posticaturn showed that the approvedconcentration (0.8 ppm for ten minutes) of Bri (used in the preceding year's trials) wasstill effective but that a lesser concentration (0.4 ppm for ten minutes) substantiallyreduced mortality (77%) showing that the permitted dose is approaching the lowesteffective concentration.Following preliminary surveys, six sites on the main river were selected for treatmentand the furthest upstream (Blandford) and the penultimate downstream (Wimborne)of these were used for monitoring the effects of treatment.Current meter measurements were used to calculate discharges and to permit suitabledilutions of Bti to be achieved. NRA discharge records for Hammoon and Throopconfumed that measurements were in the correct range.The first Bti application coincided with a flood which resulted in increased dischargeand turbidity. This reduced the efficacy of the treatment. Mortality was 33% atBlandford and 59% at Wimborne.A second application several weeks later when conditions were less adverse producedmortalities of 94.9% and 77.1% at Blandford and Wimborne respectively.The overall mortalities produced by two applications of Bti were 96% and 93% atBlandford and Wimborne respectively.INTRODUCTIONIn 1992, following the successful trials in 1989 and 1991, HSE gave the North DorsetDistrict Council permission to experimentally treat the same region of the River Stour as in1991 with a Rd preparation (TEKNAR HP-D). This was the latest phase in the attemptedcontrol of the biting pest Simuliwn posticatum. The treatment was to be restricted to the areaof the main river downstream of Blandford but no less than 7 km upstream of the Longhamintake of Bournemouth Water Co. (Fig. 1). It should be noted that large populations ofSimuliwn posticatum occur in the river downstream of this obligatory cut off point.The conduct of the present treatment took into account the "Guidelines for BiologicalMonitoring" put forward by the Pesticides Registration Section, 28 February 1990.TESTS OF Bti (TEKNAR HP-D) TOXICITYIt was required by the HSE that the Teknar HP-D (the identical batch used in 1991 and storedat 5°C in the intervening period) be tested to establish that it was still effective against thelarvae of S. posticaturn.12.1 MethodsTwo experimentswere carriedout, in vitro, in whichlarvae of Sitnuliurnspp. were subjectedto TEKNARHP-D. Twenty3-5mmlarvae of S.posticatumcollected from Blandfordon theRiver Stour were placed in each of ten 100ml plastic beakers containingRiver Stour waterfiltered througha 0.064 mm sieve. Watercurrentswere inducedand the water aeratedby theintroduction of air, under pressure, through a hypodermicneedle. Larvae were allowed tosettle and attain feeding positions for 18 hr 40 min in the first experiment and for 21 hr inthe second experimentprior to treatmentcommencing.TEKNAR HP-D was diluted to give final concentrationsof 0.8 ppm and 0.4 ppm. Theappropriateconcentrationwasaddedto five of the tenreplicates,the remainingfive replicatesbeing used as controls. After a ten minute exposure period both treatment and controlsampleswerewashedthoroughlyin filteredriverwaterover a period of fifteen minutesbeforebeing returned to containers of clean, filtered river water prior to counting. Beakers wereexamined twenty four hours after treatment and the numbers of dead (unresponsive tomechanicalstimulation)and living larvae in each beaker counted.2.2 ResultsAll larvae in control sampleswere alive after twentyfour hours but of the one hundredlarvaetreated with 0.8 ppm of Bti for ten minutesonly, only two were alive after twenty four hours(Table 1).Table 1 Number of dead larvae in each control and treatment replicate following theaddition of 0.8 ppm of Bti for 10 min to the treatmentreplicates.Replicate No. in eachexperimentTreatmentNumberdeadControlNumber dead1 20 20 02 20 19 03 20 20 04 20 20 020 19 0Total 100 98 0• The second experiment was less conclusivein its outcome than the first in the fact that arelatively high proportion of control group larvae (32%) were dead at the conclusion of thetwenty four hour period. In treatmentsamplestherewas, however,77% mortality(Table2).2Table 2 Number of dead larvae in each control and treatment replicate following theaddition of 0.4 ppm of Bti for 10 mMto the treatment replicates.Replicate No. in eachexperimentTreatmentNumberdeadControlNumber dead1 20 12 62 20 14 73 20 14 94 20 18 15. 20 19 9Total 100 77 323. PRELIMINARY SURVEYS3.1 MethodsWalkoversurveyswere conductedin March 1992to determine larval presence and the stateof the simuliidpopulations. Samplesof weedweretakenfrom various sites on the mainriverin March 1992. The relative abundanceof larvae on the weed, which gives an indication ofthepopulationdensity,wasdetermined. Thesesurveysidentifiedthe main areas of infestationand were ultimatelyused to locate preciselythe treatmentpoints.	3.2 ResultsOn the 6 March 1992thereweremanylargelarvaeof speciesother than Simulium posticatwnat all sites examined between Blandford and Wimbome. On the same date samples fromBlandfordcarrier stream,CharltonMarshalland Spetisburycontainedvery few, small larvaeof Simulium posticatum, while those from Blandford(below the weir) and from Wimborne(Julians bridge)containedmany small (first, secondand third instar) S. posticatwn.A secondinspectionon 26 March 1992revealedvery few simuliidlarvae other than those ofS. posticatum. Those other specieswhich werepresent were in the final instar and about topupate. By this date the larvae of the target species (S. posticatum) covered a wide range ofsizes, but no first instars were recorded. Larvae were abundant at Blandford (carrier streamand below the weir) and at Wimbome (Juliansbridge) but were less numerous at CharltonMarshall and Spetisbury.A pre-treatment examination at Blandford and Wimborne on 31 March gave the followingresults (Table 3).Table 3 Density of S. posticatum (mean numbers g"' weed ± 95% CL) present atBlandford and Wimbome on 31 March 1992 immediately preceding treatment.Site Number ofsamplesNumber oflarvaeConfidencelimitsBlandford 30 44.4 ±8.1Wimborne 30 237.6 ±60.4Almost all the larvae on this occasion were instars 2-4 of S. posticatum. The results of thissurvey were used as pre-treatment samples in relation to the treatment of the river withTEKNAR HP-D.	3.3 Treatment SitesAs in 1991 sites T1, T2, T3 and T4 (Fig. 2) were selected for treatment as these were themain areas of infestation determined by an inspection of the entire river between Blandfordand Wimborne. Other sites treated were at Corfe Mullen (T3A) and at Canford School (T5)as significant populations of S. posticatum were found in the preliminary surveys. Only sitesT1 (Blandford) and T4 (Wimborne) were selected for monitoring, these being the most easilyaccessible and comparable with the studies in 1991.	4. DISCHARGE AND VELOCITIESDischarge values were necessary for calculation of Bti dilution factors.4.1 MethodsThe Wessex region of the National Rivers Authority were unable to provided discharge valuesat the prescribed sampling/application points as there are only two continuous gauging stationson the Stour, one at Hammoon a considerable distance upstream of Blandford and a secondat Throop many kilometres downstream of Wimborne. The NRA were, however, extremelyhelpful and supplied maps and graphs which established that approximate interpolationbetween gauging stations is reasonable despite the three to four fold difference in dischargeat any given time (see 1991 report).It was felt that more precise information was required and flow measurements were thereforeundertaken. For precise flow estimation, measurements of width and depth were made atlevel cross sections clear of weed and major obstructions. These were combined with velocitydeterminations at a number of points on each cross section to give discharge values.44.2 ResultsThe measureddischarge at Blandfordand Wimbomeon 31 March 1992 was as followsBlandfordcarrier 1.5m3Blandfordmain river 4.6 in3sWimborne 6.3 in3s'lTable 4. NRA gauged flows (cumecs)prior to treatment on 1 April 1992Date Hammoon Throop29.3.92 1.79 6.9731.3.92 7.07 8.15S. Bti APPLICATION5.1 Methods and quantitiesThe quantitiesof TEKNARHP-Drequiredin litres to achieveconcentrationsof 0.8 ppm overten minutes was calculatedfrom the manufacturer'sformula.Volume (litres) = 0.48*How (cumec)and was as follows -- Blandfordcarrier 0.72 1,Blandfordmain river 2.2 1- CharltonMarshall 3.0 1, Spetisbury 3.0 1,- Corfe Mullen 3.0 1, Wimbome 3.0 1, Canford 3.0 1.A total of 17.92litres of suspension.The TEKNAR HP-D was carried to the sites as measured doses in closed containers andmixed in 20 1 knapsack sprayerswith sieved river water. The material was sprayed, by aqualified operative,who traversedthe river approximatelyten times during the applicationperiod. The jet of the sprayingequipmentwas totally submergedbeneath the water surfaceto avoid spray drift or loss. The sites were treated sequentially starting at Blandford, thefurthest upstreamat 0930 hr and ultimatelytreating the Canford site at.1200 hr.6. MONITORING THE EFFECTS OF Bti ON SIMULIUM POST1CATUM6.1 MethodsOn pre-treatmentday (31 March 1992), samples of larvae were taken at Blandford andWhnborne. Thirty weed sampleswere taken at each site 50 m downstream of the proposedapplicationpoint. The numberof larvae on each piece of weed was recorded along with theweight of the weed.5Sampling was repeated on 3 April 1992, two days after treatment at which time it wasexpected that all dead larvae would have been washed off the weed. Numbers of larvae andweights of weed were again recorded.6.2 ResultsThere was a 6-fold difference in density of S. posticaMm (numbers per gram of weed) on pre-treatment day between Blandford and Wimbome (Table 5).Table 5 Density of S. posticatum larvae (mean number g"' weed ± 95% CL) atBlandford and Wimbome before and after treatment with EdDate 31.3.92 3.4.92 % mortalityBlandford 44.4±8.4 29.8±6.5 33Wimbome 237.6±60.4 98.3±20.8 59Conditions were not good for the application of Bti. The water was turbid and the dischargehad increased following heavy rain. This can be seen in the change in discharge recorded bythe NRA (Table 4) where the value at Hammoon in the upper catchment of the river changedfrom 1.79 to 7.07 m3 sl just prior to application. The discharge at Throop was only slightlyhigher indicating that the flood was at this time in the upper catchment.A decision was taken not to delay the treatment as it was not known how long conditionswould remain unfavourable. This proved to be the coma decision as the water remainedturbid for three weeks.As the mortalities both at Blandford and Wimborne were low in comparison to the knownefficacy of Bti, a second treatment was planned to take place once conditions becamefavourable. Samples were taken on 21 April to determine pre-treatment densities of larvaeat the two sites. Examination of the larvae showed that many were in their final instar (pupalgill filaments evident) and indeed, some had pupated. However, this second applicationproceeded on 23 April so as to reduce the number of larvae as much as possible.The NRA gauged discharges for Hammoon and Throop on 20 April were 2.5 and 8.3 m3 s-1respectively. There was no perceptible change in discharge between this date and the day oftreatment. These discharges were interpolated to determine the quantity of Bti to be added.Two litres were added at Blandford (main river +carrier) and Charlton marshal] and 3 1wereadded at Spetisbury, Corfe Mullen and Wimborne. No treatment was possible at Canford dueto water sports at the school.6The densities found at Blandford and Wimborneimmediatelybefore the second treatmentwere very similar to the densities remaining after the first treatment indicating a stablepopulationlevel in the weeks betweenthe treatments(Table 6).Table 6 Density of S. posticatumlarvae (mean numbers g4 weed ± 95% CL) atBlandfordand Wimbomebefore and after the second treatment with Bti.Date 21.4.92 27.4.92 % mortalityBlandford 39.2±8.9 2.0±1.0 94.9Wimborne 71.96±22.2 16.5±5.6 77.1The mortality was high at both Blandfordand Wimborne (Table 6). The densities on 21April (pre-2nd treatment) at Blandford and Wimbome were 88% and 30% of the initialdensitiesrespectivelygiving overallmortalitiesfrom the two treatments of 96% and 93% atBlandfordand Wimbomerespectively.7Fig. 1 Map of the River Stour showingtreatmentareaUNTREATED AREAINCLUDING SECTIONS CONTAINING S.posticatumTREATMENT AREAREGION WHERE NO TREATMENTWAS PERMITTEDSTURMINNEWBOURNEMOUTH10kmFig.2PositionoftreatmentsitesontheRiverStourBLANDFORDFORUMLangtonLongBlandfordCharltonMarshallT1 T3AT2T3TREATMENTSITEST4 T5TarrantCrawfordSpetisburyShapwickPamphillWIMBORNEMINSTERSturminsterMarhallT5 Canford1kmCorfeMullen

An experimental treatment of Simulium posticatum with Bti at selected sites on the River Stour, Dorset, 1992

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An experimental treatment of Simulium posticatum with Bti at selected sites on the River Stour, Dorset, 1992

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