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

Spring 2003 Resistant Pest Management Newsletter Vol. 12, No.2was 0.90, which indicated that resistance to spinosaddid not develop in P. xylostella after continuousselection at least up to the 7th generation. Crossresistanceto commonly used insecticides was alsostudied. The toxicity of commonly used insecticideswas also calculated against spinosad-selected P.xylostella. Comparisons of LC50 values indicated thatmonocrotophos (1.21278%) was the least toxic of allthe insecticides whereas cypermethrin (0.0276%) wasfound to be the most toxic. The relative toxicity ofcypermethrin, dichlorvos, malathion, endosulfan, andcarbaryl reveal that these insecticides were 43.94,20.80, 5.85, 4.24, and 4.17 times more toxic thanmonocrotophos.INTRODUCTION The diamondback moth (DBM),Plutella xylostella L. (Lepidoptera: Plutellidae) is oneof the major constraints in the profitable cultivation ofcole crops. The pest occurs in endemic form with highpopulation densities on early and late sowncauliflower. In case of severe infestation, the growinghearts are also damaged, affecting the production ofmarketable curds. The control of P. xylostella hasdepended primarily and extensively on the use ofinsecticides recommended for the last over forty years(Syed, 1992). However, the promiscuous use of anumber of commercial insecticides lead to thedevelopment of resistance in this pest in most countriesof Southeast Asia (Georghiou, 1981). Many factors likepronounced cultivation of early and late varieties ofcauliflower, intensive use of conventional insecticides,and prospects of the higher value of the crop duringoff-season have been outlined for its extraordinarypropensity to develop resistance to all classes ofcompounds. By and large one of the countermeasuressuggested to combat the menace of resistance is theintroduction of, and switching over to, newer,ecofriendly, and more potent insecticides.Spinosad is a new class of polyketide-derivedmacrolide effective against a broad range of pestsbelonging to orders lepidoptera, diptera, andhymenoptera (Sparks et al., 2001). It contains a mixtureof two very active principles: Spinosyn A andSpinosyn D, and is derived from a new species of soilbacterium Saccharopolyspora spinosa which acts bothas a contact and a stomach poison.Electrophysiological evidences have demonstrated thatit alters nicotinic currents in neuronal cell bodies andalso disrupts the functions of GABA receptors of smallneurons in the central nervous system (Salgado et al.,1997).Keeping in view the introduction of this chemicalin India and the potential of P. xylostella to developresistance to all classes of compounds, a need wasrealized to generate base line susceptibility data andquantify resistance to spinosad as well as crossresistancewith other chemicals with the aim to detectshifts in susceptibility following its commercial useand to formulate future strategies to manage this pest.MATERIALS and METHODSInsect Rearing: The culture of P. xylostella wasinitiated by collecting about 200 larvae from farmers'cauliflower fields and brought to the laboratory forfurther multiplication at a regularly maintainedtemperature of 24±20C. The larvae were reared oncauliflower leaves till pupation. The adult moths wereallowed to lay eggs in oven-dried glass jars havingcauliflower leaves to serve as substrate for oviposition.They were provided with 10% honey solution fortifiedwith multivitamins for feeding on a cotton swab. Theneonates were provided with fresh cauliflower leavesto feed upon. At every successive instar, the larvaewere shifted to clean jars containing fresh leaves. Thewhole stock was divided into two lots. One lot wasnamed as parental stock and the other was used forexposure to spinosad.Preparation of Insecticidal Concentration: Theproprietary products of all the insecticides were used toprepare one percent stock solution in acetone fromwhich further dilutions were prepared subsequently.Bioassay and Laboratory Selection: FAO methodNo.21 (Busvine, 1980) for topical application ofinsecticide to the larvae with slight modification wasfollowed. Instead of directly treating the larvae withoutany substrate, the larvae released on cut discs ofcauliflower leaves of the size of a petri dish (2.5 cmdiameter) were treated to simulate the application ofinsecticide in the field. To facilitate the movement ofthe larvae on both sides of the leaf disc, the leavesbearing slightly thick midribs and veins were selectedfor cutting the leaf discs. Before spraying, ten 3rdinstar larvae were released on the upper side of the leafdisc as one replication. 1mL each of the insecticidalconcentrations were sprayed on each side of the leafdisc with Potter's tower at 5lb/inch^2 pressure. Allthree replications were maintained for eachconcentration. After the treatment the petri dishes wereshifted to BOD at 24±10C with 60-70% relativehumidity. Larval mortality was recorded after every 72hours of the exposure by counting the larvae as deadwhen they did not resume activity after repeatedproddings. The survivals from the experiments,affording around 85 percent mortality, were reared tothe next generation. The progeny of the first survivinglot was termed the F1 generation and the exposures andselections were conducted up to 7 generations. Theparental strain was also maintained through withoutexposure.Statistical Analysis: Data on mortality was subjected toAbbott's formula for correction wherever required28