Conroy et al. 



JESO Volume 139, 2008 



Introduction 



In 2006, Ontario produced about 14 million potted and 21 million cut 

 chrysanthemums {Dendranthema grandiflora Tzvelev), and 3 million potted and 30 million 

 cut gerberas (Gerbera jamesonii H. Bolus ex J. D. Hook) (Statistics Canada 2006). The 

 American serpentine leafminer (Liriomyza trifolii (Burgess)) attacks both crops causing 

 aesthetic damage to leaves when larvae feed on leaf mesophyll producing serpentine mines. 

 It was reported to be a major pest in Ontario floriculture greenhouses in the late 1970s 

 (Broadbent 1982) but has been kept under control by insecticides. Floriculture producers 

 often rely on heavy application of chemicals to control pests because consumers demand a 

 high quality product free from aesthetic damage (Jones et al. 1986; Parrella 1987; Gullino 

 and Wardlow 1990). There are only a few insecticides registered for American serpentine 

 leafminer control in Canada (Chaput 2000). Abamectin and cyromazine are registered 

 systemic insecticides acting on leafminer larvae. Permethrin can be used against leafminer 

 adults but resistance to permethrin may be present in the population (Keil and Parrella 

 1990; Murphy 2004). Unfortunately, the short generation time and high reproductive 

 rate of this leafminer combined with excessive insecticide use have caused it to develop 

 resistance to many insecticides (Jones et al. 1986; Parrella 1987; Keil and Parrella 1990). 

 In 2004, growers in Ontario reported difficulty in controlling it with registered insecticides 

 (G. D. Murphy, pers. comm., 2005), suggesting that it was becoming resistant to both 

 cyromazine and abamectin. In a survey of Ontario chrysanthemum and gerbera growers 

 Conroy et al. (2007) found that 22% of respondents had observed failure to effectively 

 control the leafminer with insecticides. Reduced risk insecticides, such as spinosad and 

 chlorantraniliprole, need to be tested for control of this leafminer and compatibility with 

 integrated pest management programs. 



The objectives of this study were to determine if the American serpentine leafminer 

 has developed resistance to currently registered insecticides in Ontario since control failures 

 have been observed and to evaluate the effectiveness of reduced risk insecticides with 

 potential for inclusion in management programs. 



Materials and Methods 



American Serpentine Leafminer Culture 



Two American serpentine leafminer colonies were established. A reference 

 strain was obtained from Dr. Scott Ferguson (Syngenta Crop Protection, Florida). This 

 strain had been reared in the laboratory for over 20 years and had never been exposed to 

 insecticides currently registered for leafminer control. The second strain was collected 

 from chrysanthemum and gerbera greenhouses in the Niagara region of Ontario in 2006- 

 2007. Resistance may have developed as a result of the history of insecticide use in affected 

 greenhouses, or more likely given the synchronicity of problems in a number of greenhouses, 

 it was imported along with plant material from the United States where resistance to 

 currently registered insecticides has been documented (Ferguson 2004; G. D. Murphy, pers. 

 comm., 2005). Both strains were kept in plexi-glass cages in environmental chambers at 

 26 ± 2°C, 50-70% RH, and 18:6 L:D. The strains were kept in separate buildings following 



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