Conroy et al. 



JESO Volume 139, 2008 



strain LC S() by the susceptible strain LC 5Q . 



Results and Discussion 



Cyromazine, abamectin and spinosad resistance have been documented in Florida 

 (Ferguson 2004) but only anecdotal evidence has been reported from Ontario since 2004 

 (G. D. Murphy, pers. comm., 2005). Defining "resistance", particularly at low levels can be 

 arbitrary. Comparison of fiducial limits, as suggested by Robertson et al. (2007) indicates, 

 that as the limits of the LD 5() of the insecticide susceptible and Ontario leafminer strains did 

 not overlap, the latter was resistant to abamectin (17.5x) and cyromazine (10.2x) and also 

 showed low levels of resistance to spinosad (2.8x) and chlorantraniliprole (3. Ox) (Table 

 1 ) - these low levels of "resistance" also could be due to natural variation in the Ontario 

 population (Robertson et al. 2007) or to enhanced susceptibility of the reference strain 

 reared for many years under controlled laboratory conditions (fifrench-Constant and Rousch 

 1990). 



Resistance also can be defined as failure of an insecticide to control an insect pest 

 in the field (Ball 1981). The survey of the Ontario chrysanthemum and gerbera growers 

 discovered that 22% of respondents observed failure to control American serpentine 

 leafminer with registered insecticides (Conroy et al. 2007). Comparing the LC 4S to the 

 recommended field rates (Table 2) shows that the amount of insecticide needed to kill 95% 

 of the Ontario strain would be much higher (4-5x) than that recommended for cyromazine. 

 The recommended rate for abamectin was close to the LC t) . suggesting that resistance in 

 Ontario greenhouses may not be present. However, since the population was not gathered 

 from a single source it is possible that resistance varies between greenhouses and could be 

 higher in some, causing control failures. A low level of insecticide resistance was observed 

 with spinosad (Table 1 ). It is registered in the United States for leafminer control and thus it 

 is not surprising that the Ontario strain, possibly imported from the United States on infested 

 propagation material would show decreased susceptibility to this insecticide. However, the 

 recommended application rate would appear to be adequate to provide effective leafminer 

 control (Table 2). 



Chlorantraniliprole is a new insecticide with a unique mode of action. Comparing 

 the application rate to the LC 9S suggests that the amount needed to provide 95% control 

 also is lower than the suggested application rate (Table 2). Nevertheless, the low level of 

 insecticide resistance shown by the Ontario strain to this product suggests that it may have 

 the potential to develop a higher level of resistance (Table 1 ). If chlorantraniliprole or 

 spinosad is registered for American serpentine leafminer control, it should be in the context 

 of an integrated pest management program minimizing use in order to delay resistance 

 development. 



Conclusions 



These results stress the importance of developing a multifaceted integrated pest 

 management program for American serpentine leafminer control in Ontario. Insecticides 

 alone will not control it without rapid development of insecticide resistance. An IPM 



44 



