Control of Aphid Vector Spread of Lily Symptomless Virus and Lily Mottle Virus by Mineral Oil/Insecticide Sprays in Lilium C.J. Asjes and G.J. Blom-Barnhoorn Bulb Research Centre P.O. Box 85 2160 AB Lisse The Netherlands Keywords: lily symptomless virus, lily mottle virus, mineral oil, deltamethrin, acephate, pymetrozine, virus spread. Abstract The inhibitory effect of mineral oil, synthetic pyrethroid insecticide, organophosphorus insecticide and a pyridine azomethine insecticide on the spread of aphid-borne non-persistently transmitted lily symptomless virus (LSV; carlavirus) and lily mottle virus (LMoV; potyvirus) in Lilium was studied. The control effect of spray mixtures of mineral oil ( Luxan oil H ) and pyrethroid insecticide deltamethrin ( Decis ) serially reported about in the past was confirmed. The organophosphorus insecticide acephate ( Ypsilon ) and the pyridine azomethine insecticide pymetrozine ( Plenum ) applied as single sprays were about half as effective as deltamethrin in the control of virus spread. The effect of acephate and pymetrozine in mixed sprays with mineral oil proved to be similar in control efficacy as sprays of mineral oil plus pyrethroid. The impact of this newly found effect and mode of action of the systemic insecticides as additive to mineral-oil sprays is discussed. INTRODUCTION The control of viruses in Lilium in the Netherlands is mainly concerned with the prevention of spread of non-persistently aphid-transmitted viruses such as lily symptomless virus (LSV; carlavirus) and lily mottle virus (LMoV; potyvirus; known up till 1990s as tulip breaking virus (TBV); Dekker et al., 1993; Derks et al., 1994). Bradley et al. (1962) reported the inhibition of spread of aphid-borne non-persistently transmitted plant viruses by oil to cover the leaf surfaces. Mineral oils have been used in lilies in the Netherlands since the mid 1960s (Asjes, 1976). The synthetic pyrethroid insecticides, which are effective against both acquisition and inoculation in virus transmission, were introduced in the early 1980s (Asjes, 1981; Gibson et al., 1982; Rice et al., 1983). The addition of synthetic pyrethroids to oil allowed the volume of oil to be halved without loss of efficacy. This practice has been used since the early 1990s (Asjes, 1989; 1991; Asjes and Blom-Barnhoorn, 1994; Asjes and Blom-Barnhoorn, 2000). Sprays are applied weekly in May-July to control rapid spread, and fortnightly in August-September to control the slower spread. They are applied at a pressure of 400 kpa in 400 L of water per hectare. In this paper data on the efficacy of mineral-oil sprays with other insecticides than those of the synthetic pyrethroids will be presented. MATERIAL AND METHODS Plant Material Virus-tested lily bulbs (9-10 cm circumference; 100/plot) of cv. Vivaldi (Asiatic hybrid) were used. Vivaldi flowered in the 1st week of July and showed senescence in late September. Plots in three randomised blocks were planted in late March. Each plot had two pairs of five rows (18 cm between rows) of virus-tested lilies separated by one row of LSV+LMoV-infected bulbs of cv. Enchantment (Asiatic hybrid). The trial beds (1 m wide) with the plots were flanked by paths (50 cm) and intermediate beds (1 m) sown with grass. Dahlias were planted (1 m) between the plots. Proc. 8th Int. Symp. on Flowerbulbs Eds. G. Littlejohn et al. Acta Hort. 570, ISHS 2002 277
Chemicals The mineral-oil brand used was Luxan oil H (93 % oil; Asjes and Blom- Barnhoorn, 2000) furtherly referred to as Luxan. The synthetic pyrethroid insecticide was deltamethrin in Decis (2.5 % a.i.; 0.0025 % (w/v); 10 g/ha = 400 ml commercial product). The organophosphorus insecticide was acephate in Ypsilon (80 % a.i.; 1 kg/ha of commercial product). The pyridine azomethine insecticide was pymetrozine in Plenum (25WP; 100 g a.i./ha; 0.4 kg/ha of commercial product). In 1998 Plenum was added to the sprays of Decis at July 7 and 21, and August 4 and 18 (treatment 4 in Table 2). In 1998 Decis was replaced by Plenum (treatment 9 in Table 2) at the same dates. In 1999 the same was done at June 29 and July 13 and 27 in the 3x-treatments (treatment 5 and 8 in Table 2) and additionally at August 10 in the 4x-treatments (treatment 9 in Table 2). Brand names will be used in Tables 1 and 2. Ypsilon was kindly supplied by Luxan BV at Elst and Plenum by Novartis Agro Benelux BV at Roosendaal. Application of Sprays Emulsions were prepared shortly before use and then applied with a knapsack sprayer (Birchmeijer Helico nozzles 1.2) at a pressure of 400 kpa and at a volume equivalent to 400 L/ha. Spraying was carried out in calm weather to minimise spray drift. The equivalent dosages were 3.1 (0.78 %) and 6.25 L/ha (1.56 %) for Luxan oil H, 400 ml/ha of Decis, 1 Kg and ½ Kg/ha of Ypsilon and 0.4 Kg/ha of Plenum. In the Tables half volumes and weights will only be mentioned separately. Sprays were applied weekly in May-July and fortnightly in August-September. Assessment of Bulb Weight and Virus Infection The bulbs harvested in October were stored at 0 C till January-February. They were then washed and dried overnight. The mean weight ratios were calculated as weight of n-treated bulbs: weight of n-control bulbs x 100. The presence of LSV and LMoV in each bulb of Vivaldi was assessed by DSA-ELISA (van Schadewijk, 1986). The rates of infection were calculated for separately infected plants for LSV, LMoV or both. The percentage of spread reduction was calculated by 100 (rate of virus infection of treated plots : rate of virus infection in untreated plots x 100). RESULTS Table 1 shows weight ratios, percentages of virus infection and reduction of virus spread by mineral oil and the insecticides deltamethrin and acephate. In 1997 and 1998 weight ratios were not significantly different from the data obtained in the untreated plots, except treatment 4 in 1998. In 1999 the infestation by Botrytis troubled weight ratios too strongly to indicate a possible yield-reductive effect of mineral-oil/insecticide sprays. The control by acephate in Ypsilon was less effective than by deltamethrin in Decis, viz., 20-33 % and 41-51 % reduction of virus spread, respectively. The level of efficacy of sprays of Luxan oil H plus deltamethrin was similar to that obtained by Luxan oil H plus acephate. Sprays of Luxan oil H (3.1 l/ha) plus acephate (1/2 kg/ha) were least effective (treatment 9). Table 2 shows data on weight ratios, percentages of virus infection and reduction of virus spread by mineral oil and the insecticides deltamethrin and pymetrozine. The control by single sprays of Plenum was about half or less effective than that obtained by Decis. The control by mixed sprays of Luxan oil H plus Decis or Plenum was similarly effective. DISCUSSION Current-season infection by LSV and LMoV of lily cv. Vivaldi did not evidently affect bulb yields while primary infection of both viruses is traceable from July onwards (Asjes et al., 2000). The potential reduction of bulb yields may also be due to spray treatments, for instance by mineral oils (Asjes, 1984). The potential infestation by 278
Botrytis near the end of the field season additionally affects bulb yields as became evident in 1999 when fungicide sprays generally sufficiently effective proved unsatisfactory. The erratic infestation by the fungus may have affected virus spread only slightly or rather not at all, as the greater part (85-90 %) of spread per season occurs in May-July (Asjes et al., 1996; Asjes, 1997; 2000). The insecticides applied in single sprays were differently effective in the prevention of virus spread. The pyrethroid deltamethrin confirmed its efficacy as single spray (Asjes, 1981; 1997). Pymetrozine and acephate were less but still effective under conditions of high virus infection potential. These data confirmed earlier results on control by systemically active compounds such as imidacloprid (Asjes et al., 1996) and aldicarb (Asjes, 1991). The mode of action of systemic compounds in spread of non-persistently transmitted viruses is only partially understandable. Pymetrozine interferes with the feeding of aphids while walking mobility is not affected (Kayser et al., 1994; Fuog et al., 1996). The aphids are not knocked down on contact but seem to die of starvation (Harrewijn and Kayser, 1997). The compound does not have a deterrent or antifeedant action. Pymetrozine does not effectively inhibit virus acquisition, but it reduces subsequent inoculation of healthy plants (Harrewijn and Piron, 1994). The effect of pyrethroid insecticide to reduce virus transmission by early flight removal from treated plants (Gibson et al., 1982; Rice et al., 1983) did not become additionally evident in mixed sprays of deltamethrin and pymetrozine in 1998 (Table 2: 4 and 5). Possibly the effect of other systemically active compounds may similarly be due to reduction of inoculation rather than acquisition. However, data in literature to substantiate this assumption were not found so far. The level of control efficacy by pymetrozine and acephate as additive to Luxan oil H was practically similar to that attained by sprays of this brand plus deltamethrin. The addition of imidacloprid to mineral-oil sprays yielded a similar result in a one-year trial (unpublished results; 1998). The newly found effect of other insecticides than synthetic pyrethroids to prevent spread of non-persistently transmitted viruses in lily may also control occasional resident aphids in some cultivars, for instance in Oriental hybrids. This is particularly worthwhile for the control of Aphis gossypii, which species became more prominent in last decades (Asjes, 1997). Pymetrozine was effective in the control of this species (Kayser et al., 1994). The build-up of possible resistance to systemically active insecticides may be controllable by alternative use of pyrethroids and these compounds. The effect of reduced dosages of acephate and pymetrozine and the long-term efficacy less frequently applied than weekly must be furtherly investigated. Literature Cited Asjes, C.J. 1976. Some developments of the virus pathology in lilies in the Netherlands. In: Lily Yearbook North American Lily Society 29:120-126. Asjes, C.J. 1981. Control of stylet-borne virus spread by aphids of tulip breaking virus in lilies and tulip, and hyacinth mosaic virus in hyacinth by pirimicarb and permethrin sprays versus mineral-oil sorays. Mededelingen Faculteit Landbouw Wetenschappen Rijksuniversiteit, Gent, 46/3:1073-1077. Asjes, C.J. 1984. Control of field spread of tulip breaking virus in Lilium cv. Enchantment by different brands of mineral oil. Crop Protection 3:111-124. Asjes, C.J. 1989. Bestrijding virusverspreiding in lelies: veel factoren spelen een rol. Bloembollencultuur 100 (8):30-31. Asjes, C.J. 1991. Control of air-borne spread of tulip breaking virus, lily symptomless virus and lily virus X in lilies by mineral oils, synthetic pyrethroids and a nematicide in the Netherlands. Netherlands Journal Plant Pathology 97:129-138. Asjes, C.J. 1997. Virus in bloembollen in kaart gebracht. Bloembollencultuur 108 (4):1-50. Asjes, C.J. 2000. Management review on the control of aphid-borne lily symptomless 279
virus and lily mottle virus in Lilium in the Netherlands. Virus Research: in press. Asjes, C.J. and Blom-Barnhoorn, G.J. 1994. Air-borne field spread of tulip breaking virus, lily symptomless virus and lily virus X in lily affected by seasonal incidence of flying aphids and control by sprays of mineral oil, vegetable oil, insecticide and pheromone in the Netherlands. Acta Hort. 377:301-310. Asjes, C.J. and Blom-Barnhoorn, G.J. 2000. Use of mineral spray oils and pyrethroid insecticides to control aphid vector spread of lily symptomless virus and lily mottle virus in Lilium (Liliales: Liliaceae). Proceedings International Conference on Sprays Oils beyond 2000, Sydney, Australia (25-29 October, 1999): in press. Asjes, C.J., Blom-Barnhoorn, G.J. and Schadewijk, A.R. van. 2000. Effect of seasonal detection of lily symptomless virus and lily mottle virus on aphid-borne virus spread in Lilium in the Netherlands. Proceedings Xth Symposium on Virus Diseases of Ornamental Plants, USA (May 2000). Acta Hort.: in press. Asjes, C.J., Blom-Barnhoorn, G.J., Piron, P.G.M., Harrewijn, P. and Oosten, A.M. van. 1996. Control review of air-borne tulip breaking virus and lily symptomless virus in Lilium in the Netherlands. Acta Hort. 432:290-297. Bradley, R.H.E., Wade, C.V. and Wood, F.A. 1962. Aphid transmission of potato virus Y inhibited by oils. Virology 18:327-328. Dekker, E.L., Derks, A.F.L.M., Asjes, C.J., Lemmers, M.E.C., Bol, J.F. and Langeveld, S.A. 1993. Characterization of potyviruses from tulip and lily which cause flowerbreaking. Journal General Virology 74:881-887. Derks, A.F.L.M., Lemmers, M.E.C. and Gemen, B.A. van. 1994. Lily mottle virus in lilies: characterization of strains and its differentiation from tulip breaking virus in tulips. Acta Hort. 377:281-288. Fuog, D., Senn, R. and Bolsinger, M., 1996. Pymetrozine: a novel aphicide with a new mode of action. In: XXe International Congress of Entomology, Firenze 1996 Papers contributed by CIBA, 14-17. Gibson, R.W., Rice, A.D. and Sawicki, R.M. 1982. Effects of pyrethroid deltamethrin on the acquisition and inoculation of viruses by Myzus persicae. Annals Applied Biology 100:49-54. Harrewijn, P. and Kayser, H. 1997. Pymetrozine, a fast-acting and selective inhibitor of aphid feeding. In-situ studies with electronic monitoring of feeding behaviour. Pesticide Science 49:130-140. Harrewijn, P. and Piron, P.G.M., 1994. Pymetrozine, a novel agent for reducing virus transmission by Myzus persicae. Brigton Crop Protection Conference Pests and Diseases 2: 923-928. Kayser, H., Kaufmann, L., Schurmann, F. and Harrewijn, P. 1994. Pymetrozine (CGA 215, 944): a novel compound for aphid and whitefly control. An overview of its mode of action. Brigton Crop Protection Conference Pests and Diseases 2:737-742. Rice, A.D., Gibson, R.W. and Stribley, M.F. 1983. Effects of deltamethrin on walking, flight and potato virus Y-transmission by pyrethroid-resistant Myzus persicae. Annals Applied Biology 102:229-236. Schadewijk, A.R. van, 1986. Detection of tulip breaking virus and lily symptomless virus in lily bulbs by means of ELISA. Acta Hort. 117:121-128. 280
Tables Table 1. Effect of mineral oil and insecticides on the spread of lily symptomless virus (LSV) and lily mottle virus (LMoV) in lily cv. Vivaldi in 1997-1999. Treatment Weight ratio *) % LSV+LMoV % spread reduction 1997 1998 1999 1997 1998 1999 1997 1998 1999 1. Untreated - 100 100 100 56.1 79.1 68.9 0 0 0 2. Decis 100 95 96 28.6 47.0 33.8 49 41 51 3. Ypsilon 100 101 92 37.6 63.1 52.1 33 20 24 4. Luxan + Decis 92 108 91 7.2 10.6 8.7 87 87 87 5. Luxan + Ypsilon 99 105 89 5.0 16.7 6.3 91 79 91 6. Luxan+Ypsilon (½ kg) - - 94 - - 9.4 - - 86 7. Luxan(3.1 l)+decis - - 99 - - 11.4 - - 83 8. Luxan(3.1 l)+ypsilon - - 81 - - 13.2 - - 81 9. Luxan(3.1 l)+ypsilon(½ kg) - - 95 - - 24.5 - - 64 *) In 1999 bulb weights were variably affected by non-uniform incidence of Botrytis elliptica leaf infection near the end of the field season. Table 2. Effect of mineral oil and insecticides on the spread of lily symptomless virus (LSV) and lily mottle virus (LMoV) in lily cv. Vivaldi in 1998-1999. Treatment Weight ratio *) % LSV+LMoV % spread reduction 1998 1999 1998 1999 1998 1999 1. Untreated 100 100 79.1 68.9 0 0 2. Decis 95 96 47.1 33.8 41 51 3. Plenum 100-64.1-19 - 4. Decis/Plenum (4x) 103-45.6-42 - 5. Decis/Plenum (3x) - 98-43.8-36 6. Luxan+Decis 108 91 10.6 8.7 87 87 7. Luxan+Plenum 109 95 12.5 9.3 84 87 8. Luxan+Decis/Plenum (3x) - 95-6.0-91 9. Luxan+Decis/Plenum (4x) 109 87 11.4 9.4 86 86 *) Bulb weights were variably affected by the non-uniform incidence of Botrytis infection near the end of the field season in 1999. 281