Vol.12_No.2 - Pesticide Alternatives Lab - Michigan State University

Spring 2003 Resistant Pest Management Newsletter Vol. 12, No.2horticultural crops worldwide. Roses are being grownboth under open and greenhouse conditions and S.dorsalis is a major pest of this crop, especially undergreenhouse conditions. Dimethoate and acephate, apartfrom the new compounds ethofenprox andimpadacloprid, are recommended for control of thispest (Jhansi Rani and Eswara Reddy, 2001; Nair et al.,1991). Growers have repeatedly observed thatdimethoate and acephate have not given satisfactorycontrol against this pest in and around Bangalore,particularly under greenhouse conditions. Reddy et al.(1992) have documented the relative resistance inthrips, S. dorsalis, to different conventional insecticideson chilli from Andhra Pradesh in India. However,information regarding relative susceptibility of S.dorsalis with regard to open and greenhousepopulations from India is not available. Therefore, thepresent trial was taken up with the objective todetermine the relative susceptibility of two populationsof S. dorsalis on rose, from open fields andgreenhouses, to dimethoate and acephate.MATERIALS and METHODS Populations of S. dorsaliswere collected both from the field and fromgreenhouses from the Indian Institute of HorticulturalResearch (IIHR), Bangalore. Two insecticides viz.,dimethoate 30 EC (Rogor) and acephate 75 SP(Starthane) as commercial formulations were used forthe study. The assay procedure followed was modifiedfrom Reddy et al (1992). Tender rose leaves weredipped in suspensions of different concentrations foreach insecticide, air dried for 30 minutes, andintroduced into glass vials. Using a fine brush, tennymphs from homogeneous populations of S. dorsaliswere carefully transferred to each concentration ofinsecticide treated leaves (open and greenhousepopulations in two separate sets). The vials were sealedwith parafilm and minute holes were made forventilation. Leaves dipped in water alone were used ascontrol. Each concentration of insecticide treatmentand control were replicated thrice. Mortality countswere taken 24 hours after the release of the test insects.Based on the number of insects that responded to thedifferent concentrations of insecticides, a probitanalysis was carried out for arriving at LC50 values forboth open field and greenhouse populations using acomputer aided MSTATC package. The resistanceindex (RI) was computed according to the formulasuggested by FAO (1979) as, RI = LC50 of resistantstrain/LC50 of susceptible strain.RESULTS and DISCUSSION Data on the LC50 values ofthe two insecticides between two populations of thrips,S. dorsalis, revealed that LC50 values varied from0.1072 - 0.0253 % in the case of dimethoate and from0.0309 - 0.1455 % in the case of acephate (Table 1).Greenhouse populations were less susceptible whencompared to field populations in cases where bothinsecticides were tested. When the resistance index wastaken into consideration, greenhouse populations of S.dorsalis had developed 1.5 and 4.7 fold resistance todimethoate and acephate, respectively when comparedto open field populations (Table 1). The differentialresponse of thrips to the two insecticides in the study interms of LC50 values was attributed to thedevelopment of resistance by S. dorsalis in greenhousepopulations when compared to open field populations.Relatively less susceptibility of S. dorsalis populationsin greenhouses may be attributed to their frequentexposure to different insecticides (nearly at fortnightlyintervals) and the quick elimination of susceptiblepopulations/genes in comparison to open populations.However, in the case of open populations, there isrelatively more chance of passing of susceptible genesin successive generations by mating with resistantpopulations, resulting in more susceptible populationswhen compared to greenhouse populations.Earlier reports have indicated that citrus thrips,Scirtothrips citri (Moulton) developed resistance inDDT, dimethoate, acephate, bendiocarb, andformentanate (Morse and Brawner, 1986 and Immarajuet al., 1989). S. dorsalis, a dominant species of thripson roses, may have undergone selection for a numberof insecticides in the past which might have led tocross-resistance to related compounds that are widelyused. Hence, there is an urgent need to curbindiscriminate insecticide use on roses particularly ingreenhouses. The result also suggests that futurecontrol programmes for S. dorsalis on rose ingreenhouses need to incorporate a resistancemanagement strategy as a major component.ACKNOWLEDGEMENT The authors are grateful to theDirector, Indian Institute of Horticultural Research,Bangalore for providing necessary facilities and Dr.N.K.Krishna Kumar, Head of Division of Entomologyand Nematology for suggesting the study.REFERENCES:FAO, 1979. Recommended methods for the detection and measurementof resistance for adult aphids. FAO Method no.17 FAO PlantProtection Bulletin 27: 29-32.Immaraju, J.A., Morse, J.G. and Kersten, D.J. 1989. Citrus thrips(Thysanoptera:Thripidae) Pesticide resistance in Coachella and SanJoaquin valley of California. Journal of Economic Entomology 82:374-380.Jhansi Rani, B. and Eswara Reddy, S.G. 2001. Efficacy of selectedinsecticides against thrips, Scirtothrips dorsalis Hood on rose in63