470 JounNer, oF THE Arr.rrRtcer.t Moseurro CoNrnor, AssocrATIoN VoL.4,No.4 AN EVALUATION OF GAMBUSIA AFF/N/S AND BACILLUS THURINGIENSIS VAR. ISRAELENS/S AS MOSQUITO CONTROL AGENTS IN CALIFORNIA WILD RICE FIELbS VICKI L, KRAMER,I RICHARD GARCIA, eNo ARTHUR E. coLwELL3 ABSTRACT. The mosquito potential of the mosquitofish and Bacillus thuringiensis var. (Bti) .control "fields israelensis were evaluatedin experimental wild rice fieldi in Lake County, California. w;;; assignedone of six treatme.nrs:^co1_t1o], 1.1 kg/ha Q. afflnis,3.4kg/ha c. iyitnis,'Gambusin bil ""ty io tyt" Vectobac" granules), 1.1 kglha G..affinis,plusBti and e.+'ue/na C. aiiinis plus'bd. iffiii!, "7 both releaserates, significantly reduced the mosquito population at densitiesexceeding fOO nsf/-i"tio* trap. Treatments with Bti significantly reducedlarval populations; however,the populition. i" itt" ftet,lt without fish reboundedto pretreatment levels within two weeks. In fields stodked*iitt C. "ffi"i";;e treated with Btr, populations remained low after Bti,treatment. Nontarget populations "f #h;;pft; were significantly lower in fields stocked with G. affinis than in fields witloot fish ott on" o. oro." sampling dates. INTRODUCTION lbs/acre), and no significant differences were found among the mosquito populations in fields Wild rice (Zizania palustris Linn.) is grown with or without fish (Kramer et al. 1987).The in Lake County, California from May through authors postulated several reasonsfor this lack October, providing ca. 300 hectares of breeding of control, such as the short (90 day) growing habitat for Cul,ex tarsalis Coquillett, Anopheles season,the wild rice plant structure, and the freeborni Aitken and. An. Mc- franciscanus omnivorous feeding nature of G. affinis. They Cracken.nWild rice acreageis increasingin Cal- concludedthat higher releaserates of G. affinis ifornia and totaled more than 6,400hectares in may be necessaryfor significant mosquito con- 1986(M. D. Andres,unpublished data). trol in wild rice fields. The mosquitofrsh, Gambusia affinis (Baftd This study therefore evaluated G. affinis at a and Girard), has been shown by several re- higher releaserate of3.4 kglha (3 lbs/acre). This searchers (Hoy and Reed 1970, Craven and rate is 15 times greater than the usual release Steelman 1968)to be an effective mosquito con- rate (0.2 lbs/acre) in California white rice fields trol agent in white rice (Oryza satiua Linn.) (Combs 1986) and, although feasible for Lake fields. Consequently, mosquito abatement dis- County, is unrealistic for many mosquito abate- tricts (MADs) in the Central Valley of Califor- ment districts. Thus G. affiniswas also evaluated nia make seasonalreleases ofthe fish into their at a morepractical rate of 1.1kg/ha (1 lb/acre). rice fields. The use of G. affinis instead of chem- Although significant mosquito control was not icals can reducemosquito control costs in white achieved at 1.7 kg/ha during the 1986 season, rice fields substantially (Lichtenberg and Getz the 1.1 kg/ha rate was selectedfor 1987because 1985).Wild and white rice plants, although cul- many variables in a rice field systemcan change tivated in a similar manner, have several differ- from one seasonto the next, potentially affect- ences, such as plant height and structure and ing the control capabilities of G. affinis. Llso, length of the growing season,that could affect the Lake County experimental wild rice fields the mosquito control effectivenessof G. affinis were first year fields in 1986 and this may have (Kramer et al. 1987). affected mosquito production (Collins and Gambusiaaffinis was evaluatedin experimen- Washino 1980). The impact of G. affinis on tal wild rice fields in Lake County in 1986 at aquatic insect and zooplankton populations was releaserates of 0.6 and I.7 kg/ha (0.5 and 1.5 also evaluatedin this study. B acillus thuringiensis Berliner v ar. israe le nsis de Barjac (Bti) has been used effectively to control mosquito larvae in a wide range of hab- 'Division of BiologicalControl, Universitv of Cal- itats (Lacey and Undeen 1986),including white ifornia, Berkeley,CA g4?20.Current address(reprint rice fields, but had not previously been tested in requests):Contra CostaMosquito Abatement Disirict, wild rice. Studies (ibid) have shown that Bti is 1330Concord Ave., Concord. CAg452O. a highly selectivecontrol agent and that natural 2Division of Biological Control, Universitv of Cal- enemies of mosquito larvae are conserved.To ifo-rnia.Berkel ey. C A 94720. 'Lake evaluatewhether natural enemies,including in- County Mosquito AbatementDistrict. 410 Esplanade,Lakeport, CA 9b458. troduced fish, can maintain mosquito popula- a Tompkins, D. 198?. Lake County Agricultural tions at a low level following an initial reduction Crop Report. Depart. Food and Agriculiure,lakeport. by Bti, tests were conductedusing the pathogen 5 pp. alone and in combination with G. affinis. Ducn\,rarn1988 Gtuaush AFFrNrsAND B"r rlt Wrlo Rtcn Ftnr,os MATERIALS AND METHODS sample G. affinis and other aquatic organisms. The trap stations, one per each side of the field, The eighteen 0.1 hectare (0.25 acre) Lake were randomly selectedon each trapping date. County rice plots used to evaluate G. affinis in On the final monitoring date (September8), four 1986(Kramer et al. 1987)were used for the 1987 traps were also set in the interior of each field study. The fields were flooded and seededon (a total of eight traps per field). All organisms June 9 using a seedbroadcaster attached to an were counted at the study site and returned to all-terrain vehicle. The wild rice plants matured their trapping location. On August 11, ten in about 90 days, and the fields were drained for trapped fish from each field (120 total) were harvest on September16. frozen for later gut content analysis. Maximum/minimum thermometers were The immature mosquito populations were placed in two of the plots, and the water tem- monitored weekly by taking 40 dips around the peratures were recorded weekly throughout the perimeter and 20 dips through the interior of season.Plant height and water depth were also each field. The perimeter density was sampled monitored weekly. A water sample from the by randomly selecting eight (two per field side) center of each field was collected on September ofthe flaggedstations eachweek. Five dips (400 13 and analyzedfor nitrate and phosphatecon- ml each) were taken at each station in a semi- tent, alkalinity, hardness,conductivity and pH. circularpattern, within 2 metersfrom the levee. There were three replicatesof each of the The interior sampleswere taken along a transect followingsix treatments:control, 1.1kg/ha (low beginning at a randomly selectedstation on one fish rate) G. affinis,3.4kg/ha (high fish rate) G. side of the field and ending at a randomly se- affinis, Bti only,1.1 kglha G. affinis plus Bri and lected station on the oppositelevee. A single dip 3.4 kg/ha G. affinis plus Bfi. All Bti treatments at 2 m along the transect. were 6 ke/ha of granular Vectobac'" (200 ITU/ was taken intervals placed in mg). Prior to flooding,treatments were assigned AII dip samples were concentrated, with rice water, and brought to to the fields using a randomizedblock design containers field (Fig.1). the Iaboratory to be immediately counted and were Gambusiaaffinis were seined from the Lake identified. Insects other than mosquitoes New Jersey light County MAD rearing ponds, weighed and re- also identified and recorded.A (Mulhern to leasedinto the 12 treatment plots on June 23, trap 1942) was operated adjacent mosquito two weeks post-flooding. Fish survival is be- the wild rice fields to monitor adult populations sample Iieved to be optimized by this two week post- in the area. The light trap flooding release schedule (Farley and Younce was collectedand countedweekly. populations were sampled 1977a).Fish releasedincluded adults and fry of The zooplankton both sexes. using a 202 micron net to concentratethe inte- Monitoring stations were flagged at 2 meter rior transect samples taken on August 3 and per intervals around the perimeter of each field. September 13. Thus 8 liters of water field (20 were filtered each date. After the Each flag was assigneda number so that stations dips) on could be randomly selectedas monitoring sites mosquito Iarvae and other aquatic organisms for the fish, mosquito and other aquatic insect were identified, the zooplankton were stained populations. with rose bengal to aid identification; they were preserved,with Every two weeks,four minnow traps (3.2mm then 5% formalin solution.Two mesh) per field were set overnight (ca. 24 hr.) to 5 ml subsampleswere drawn from eachconcen- trated 200 ml plot sample,and the zooplankton were identified and counted at 30x magnifica- tion. The Bti was applied at 6 ke/ha with a back- pack blower to the nine Bti fields on August 20 and again to the three Bti only fields on Septem- ber 8. Treatments were made when mosquito population densities were relatively high and near peak numbers, as based on expectedpop- ulation trends. Treated and control fields were sampled1, 3 and 5 dayspost-treatment after the first application, and 2 dayspost-treatment after the secondapplication. co.tro. r.r ffi ffi .or"^o^rcur,^ ffi r.. ror*^o^".ur,^ One-wayanalysis of varianceand Tukey'stest W;:"tl**" m*'xo/a^q"!', (for pairwiseconrparisons, P : 0.05)were used F.,J;:' to detect differences in the mosquito, aquatic Fig. 1. Experimental designof wild rice plots (water insect and zooplankton populations among the flow indicated by arrows), Lake County, CA, 1982. treatments. The mosquito population data were /1q JouRNer, oF THEArr,rnRrceN Mosqumo CoNrRor, Assocurror.r Vol.4, No.4 analyzedin two groups:1) Control, l.L kg/ha G. abundant prey population, a larger proportion affinis and 3.4 kg/ha G.