Feeding Toxicity (20% sugar solution applied) 



Before field exposure, spheres treated with either 

 imidacloprid or thiamethoxam performed well, with 

 both materials offering 90% kill of feeding AMF at 

 the lowest dose (2%) (Figure 1 ). However, higher doses 

 of each material did not necessarily correlate with 

 greater efficiency. In fact, as the dose of thiamethoxam 

 increased, fly mortality decreased. 



Through nine weeks of field exposure, spheres 

 treated with imidacloprid retained a high level of fly- 

 killing power — offering levels of control nearly iden- 

 tical to fresh spheres. Spheres treated with 

 thiamethoxam also exhibited good (77%) to excellent 

 (100%) control at low and moderate doses, while mor- 

 tality of flies exposed to the high dose began to de- 

 cline steadily after three weeks of field exposure. 



Disappointingly, the low rates of both materials 

 faltered after twelve weeks of field exposure, as eleven 

 inches of rain fell in the interval between nine and 

 twelve weeks (Table I). However, the moderate rates 

 (4%) of imidacloprid and thiamethoxam maintained a 

 reasonable level of fly-killing activity (80 and 83% 

 control, respectively). At the high dose, imidacloprid 

 retained toxicity through twelve weeks, while mortal- 

 ity after exposure to thiamethoxam dropped markedly. 



Conclusions 



Imidacloprid and thiamethoxam stem from the 

 same chemical family (neonicotinoids), and are known 

 to have similar modes of action and spectra of activity. 

 Given this, it is not surprising to see that patterns of 

 toxicity against foraging and feeding AMF on spheres 

 were very similar for the two chemicals. It appears 

 that the major difference between the two (for use on 

 spheres) is the formulation. The flowable formulation 

 of Provado (imidacloprid) mixes easily into paint and 

 is retained nicely within the latex for slow release, even 

 at relatively high doses (up to -10% a.i.). Actara, on 

 the other hand, is in a wettable granular formulation, 

 and must be thinned in water (1:1) before introduction 

 into the paint. Because of this, much more liquid must 

 be added into the paint, leaving far less latex per sphere 

 to retain the active ingredient. This is the probable 

 cause of rapid loss of thiamethoxam activity at high 

 doses under heavy rainfall. 



It is clear from this study that pursuit of contact 

 toxicity using either of these materials is fruitless. 

 However, in the presence of feeding stimulant (su- 

 crose), low doses of either material offers good AMF 

 control through nine weeks of field exposure. Not sur- 

 prisingly, under the extreme rainfall conditions of Sep- 

 tember, efficacy of these low doses declined. We are 

 nonetheless encouraged by the performance of these 

 materials on field-exposed spheres at low and moder- 

 ate doses, and feel that either can be formulated to 

 achieve our goal: reliable, safe control of flies through- 

 out the 12-14 week AMF season. 



A ckn o wledgem en ts 



We would like to thank Joe Sincuk of the Univer- 

 sity of Massachusetts Horticultural Research Center 

 for contributing orchard space for sphere deployment. 

 We are also grateful to our technical staff for prepar- 

 ing and maintaining spheres: Jonathan Black, Russell 

 Fleury, and Susan Nixson. This work was supported 

 by State and Federal LPM funds, the Horticultural Re- 

 search Center Trust Fund, the Washington State Tree 

 Fruit Research Commission, the Massachusetts Soci- 

 ety for Promoting Agriculture, and the USDA CSREES 

 Pest Management Alternatives Program. 



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Fruit Notes, Volume 64 (Number 4), Fall, 1999 



13 



