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

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Spring 2003 Resistant Pest Management Newsletter Vol. 12, <strong>No.2</strong>occurred in the Republic of Ireland, GreatBritain, and the Netherlands. In 1988, themetalaxyl + mancozeb formulation waschanged to give a full-rate of mancozeb. Inthe early 1990s, an anti-resistance strategybased on the one developed in theRepublic of Ireland (Dowley et al., 1995)was adopted in Northern Ireland withgrowers advised to use no more than threeapplications of phenylamides at thebeginning of the spray programme only.MATERIALS and METHODS Samples ofinfected potato foliage were obtained(mainly from seed crops) by members ofthe Department of Agriculture and RuralDevelopment (DARD) Potato InspectionService (Cooke et al., 2000). Isolates werederived by bulking together the sporangiaobtained from all foliage samples within asingle crop and maintained on detachedglasshouse-grown potato leaflets thentested, using the floating leaf disctechnique (Cooke, 1986), on 100 and 2 mgmetalaxyl litre-1. Isolates were designatedresistant if they sporulated on 100 mgmetalaxyl litre-1-treated discs andsensitive if they sporulated on untreateddiscs, but not on any metalaxyl-treateddisc. Isolates that grew on discs floating on2 mg, but not on 100 mg metalaxyl litre-1,were designated intermediate. At the endof each season, inspectors providedestimates of fungicide usage for all seedpotato crops in their areas.RESULTS and DISCUSSION In the mid-1980s,a high proportion of Northern Irelandpotato crops were sprayed withphenylamide fungicides for most of thespray programme. However, during mostof the 1990s and up to 2002, PotatoInspectors' estimated that c. 40% of seedpotato crops were phenylamide-treated(Fig. 1) and the majority of growersfollowed DARD advice and used no morethan three phenylamide applications perseason (Table 1). The proportion ofisolates containing phenylamide-resistantstrains declined in the early 1990s andappeared to have stabilised around 50%(Fig. 2) up to 1999. However, in 2000 andparticularly in 2001, there was a markedincrease in the incidence of phenylamideresistance. The reason for this was notclear, since fungicide usage had notchanged. The weather in the summermonths of 1998-2000 was unusually wet74
Spring 2003 Resistant Pest Management Newsletter Vol. 12, <strong>No.2</strong>and it is possible that this may have increased thenumber of generations of P. infestans within eachseason and favoured the buildup of resistant strains(Fig. 3). In 2002, DARD and Syngenta agreed revisedrecommendations for phenylamide usage: growerswere advised to use no more than two applications perseason and to switch to an alternative product type nolater than 15 July. Subsequently, the proportion ofisolates containing resistant strains declined from 76%in 2001 to 60% in 2002. These growerrecommendations will be continued in 2003 and theincidence of resistant strains again monitored. It isconcluded that in a region such as Northern Ireland,where fit phenylamide-sensitive and -resistant strainsof P. infestans co-exist, resistance may be managed bya strategy of limited use of phenylamides early in theseason only. During the winter period when thepathogen survives in infected tubers, more resistantthan sensitive strains tend to be lost by tuber rottingand this helps to stabilise the situation (Walker andCooke, 1990). However, in regions where aggressivephenylamide-resistant strains have been introduced bymigration rather than in situ selection, as occurredrecently in Taiwan (Deahl et al, 2002), such resistancemanagement may not be possible.REFERENCESCooke, L.R. 1981. Resistance to metalaxyl in Phytophthora infestans inNorthern Ireland. Proceedings 1981 British Crop ProtectionConference - Pests and Diseases 2:641-649.Cooke, L.R. 1986. Acylalanine resistance in Phytophthora infestans inNorthern Ireland. Proceedings of the 1986 British Crop ProtectionConference - Pests and Diseases 2:507-514.Cooke, L.R., Little, G., Wilson, D.G. and Thompson, D. 2000. Up-dateon the potato blight population in Northern Ireland - fungicideresistance and mating type. Fourth Workshop of an EuropeanNetwork for Development of an Integrated Control Strategy ofpotato late blight. Oostende, Belgium, 29 September - 2 October1999. PAV-Special Report no. 6, 35-45.Deahl, K.L., Cooke, L.R., Black, L.L. Wang, T.C., Perez, F.M., Moravec,B.C., Quinn, M. and Jones, R.W. 2002. Population changes inPhytophthora infestans in Taiwan associated with the appearance ofresistance to metalaxyl. Pest Management Science 58:951-958.Dowley, L.J., Cooke, L.R. and O'Sullivan, E. (1995). Development andmonitoring of an anti-resistance strategy for phenylamide useagainst Phytophthora infestans. In Phytophthora infestans. pp. 130-136. Eds.Dowley, L.J., Bannon, E., Cooke, L.R., Keane, T. & O'Sullivan, E. BoolePress, Dublin. 382 pp. Dowley, L.J. and O'Sullivan, E. 1981.Metalaxyl-resistant strains of Phytophthora infestans (Mont.) deBary in Ireland. Potato Research 28:417-421.Walker, A.S.L. and Cooke, L.R. 1990. The survival of Phytophthorainfestans in potato tubers - the influence of phenylamide resistance.Proceedings Brighton Crop Protection Conference - Pests andDiseases - 1990 3:1109-1114.Louise R. CookeApplied Plant Science DivisionDepartment of Agriculture and Rural DevelopmentNewforge Lane Belfast, BT9 5PXUnited KingdomResearch in Resistance ManagementActivity Spectrum of Spinosad and Indoxacarb: Rationale for an Innovative Pyrethroid Resistance ManagementStrategy in West AfricaABSTRACT To face pyrethroid resistance in the cottonbollworm Helicoverpa armigera (Hübner), endosulfan(700 g/ha) has been used in a resistance managementstrategy for four years in Côte d'Ivoire, West Africa.Actually, its recommendation is being questioned withregard to its acute toxicity and environmental issues.Earlier prospects revealed that insecticides such asspinosad (48 g/ha) and indoxacarb (25 g/ha) proved aseffective as endosulfan in controlling H. armigera. Incontrast to endosulfan, the activity spectrum of thesenon-pyrethroids insecticides appears to be restricted toa few bollworm and leaf pests. The present studypointed out the strength and weakness of these newinsecticides with respect to major insect pests andbeneficial species. On the basis of their activityspectrum and in the light of cotton crop phenology andmain pest seasonal occurence, a differential schemewas designed. Indoxacarb is more appropriate to thefruiting stage (101-115 DAE (Day After Emergence))as it appeared very effective against the cotton stainerDysdercus voelkeri (Schmidt) while showing lowerperformance against Earias spp and the mitePolyphagotarsonemus latus (Bank). In contrast,spinosad is to be used preferably at the vegetative stage(45-66 DAE) as it proved safer to coccinellids, moreeffective against Earias spp while its lowereffectiveness against D. voelkeri suggests avoiding itspositioning at a late stage of cotton. Various benefitsrelated to these new insecticides strongly advise theiruse as alternatives to pyrethroids. Still, to be moreattractive, their activity needs to be reinforced by otherinsecticides in such a way to control the wholearthropod pest complex.KEY WORDS: Cotton, Helicoverpa armigera, pyrethroidresistance management strategy, Spinosad, Indoxacarb,Côte d'Ivoire.INTRODUCTIONThe development of resistance in H. armigera:Known as very effective in controllingHelicoverpa armigera (Hübner) and most cottonbollworm pests, pyrethroids have been widely used formore than twenty years in Côte d'Ivoire. Recently,laboratory data obtained on H. armigera strains within1996-1998 pointed out significant increase in the LD5075
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