THE ABILITY OF COTTON LEAFWORM, Spodoptera FIELD STRAIN TO DEVELOP RESISTANCE TOWARD SOME CONVENTIONAL AND BIOINSECTICIDES

This study aim to investigate the development of resistance in cotton leaf worm (CLW) to spinosad and abamectin comparing with the development of resistance toward cypermethrin and methomyl as conventional insecticides. All tested strains were selected by dipping the whole 4 th instar larvae. Spinosad was only the compound which tested by feeding method in addition to the dipping technique. The spinosad feeding resistant strain (SFRS) was built up by supplying the 4 th instar larve of S. littoralis (Boisduval) with castor leaves treated with spinosad for 23 successive generations continuously in the laboratory. Using dipping technique, spinosad, abamectin, cypermethrin and methomyl resistant strains ( SDRS, ADRS, CDRS and MDRS; respectively ) were selected by exposing the 4 th instar larvae for 25 generations to each of spinosad and abamectin; and for 32 generations to each of cypermethrin and methomyl. Results indicated that the ability of the field strain of CLW to develop resistance toward spinosad by the two tested techniques were very high. Resistance ratio (RR) values for SDRS and SFRS were 108 and 87 fold, respectively. The ability of building up resistance toward abamectin and methomyl were almost like each other and not as high as spinosad ( only 19 fold ). Developing resistance toward cypermethrin was quiet higher than abamectin and methomyl ( 31 fold ) but still not as high as spinosad. The present study was to compare the speed of building up resistance among cypermethrin, spinosad and abamectin. The cypermethrin dipping resistant strain (CDRS) was built by exposing the 4th instar larvae of parent field strain to cypermethrin for 32 successive generations in the laboratory using larval dipping method. Out of 32 generations, 18 were tested with a series of cypermethrin concentrations for measuring their relative resistance to cypermethrin. Table (4) shows the LC50 values of tested generations. Figure (12) shows the relationship between tested generations and resistance ratios. RR values increased rapidly from G1 to G6 to reach 8.6 fold, then increased to be 10.23 in G7. In G8 generation, RR value jumped to 15.6 fold. From G8 to G15, RR values were fluctuated. From G15 to G20, RR increased to 26.35-fold then increased gradually to reach 36-fold resistance to cypermethrin in G 32. The slope values of CDRS (Table 4) indicate that the earlier selected generations were generally higher than those of the rest generations except of some fluctuations. In other words, the selected generations became more heterogeneous after selecting the strain by cypermethrin for about 18 generations. These observations suggest that cypermethrin selected generations may be able to become higher resistant and more homogenous in the case of selecting it for more than 32 generations. The present results indicate that the ability of tested strain of cotton leafworm employed to develop resistance toward cypermethin by selection is quiet higher than abamectin but not as high as spinosad. The present results are in agreement with El-Sayed et al . (1985) who published that the cotton leafworm, S. littoralis selected with cypermethrin for 15 generations has reached 13.89-fold of resistance. Ishaaya and Klein (1990) collected S. littoralis larvae from Israel cotton fields that had been heavily sprayed with conventional insecticides. They found that these insects have more than 102- times resistance to cypermethrin