Application of a non-selective acaricide aggravates outbreaks of Tetranychus urticaeon apple by suppressing its predator, Typhlodromus pyri, and its competitor,Panonychus ulmi

Abstract: Outbreaks of Tetranychus urticae in apple orchards in Nova Scotia, Canada began inthe late 1990's. Preliminary observations in commercial orchards indicated there could be adverseeffects of pyridaben use in the year following application. To explore this issue in more depth wereanalyzed data from 10 commercial apple orchards surveyed and monitored mites in a researchorchard treated with pyridaben and several recently registered acaricides. Commercial orchardstreated with pyridaben in 2000 had higher mite day accumulations for T. urticae in 2001 andrequired more acaricide applications than did orchards treated with the favourably selectiveacaricides, abamectin and clofentezine. In the scab-resistant research orchard acaricides wereapplied in mid-July 2008. Bifenazate and acequinocyl were favourably selective because theywere more toxic to T. urticae, the more troublesome pest, than to P. ulmi and T. pyri.Spirodiclofen was neutrally selective because it strongly suppressed all three mites. Pyridabenwas unfavourably selective because it had prolonged activity against P. ulmi and T. pyri but onlysuppressed T. urticae for a short interval. Even though application of pyridaben, and to a lesserextent spirodiclofen, led to an unfavourable balance of T. urticae versus P. ulmi and T. pyri by theend of summer in 2008, effective biological control was quickly established in all treatments in2009. The rapid establishment of biological control in 2009 was likely due to the absence offungicide applications. Conversely, use of fungicides in commercial orchards, particularlyEBDC’s, likely impeded recovery of T. pyri in 2001 enabling adverse effects of pyridaben tocarry over from one year to the next. We conclude that pyridaben can be detrimental not onlybecause it suppresses T. pyri, but also because it shifts the tetranychid complex away from P.ulmi, a prey that is more easily regulated by T. pyri, to T. urticae, a prey that is not so easilycontrolled. In contrast, favourably selective acaricides such as clofentezine, bifenazate andacequinocyl, which are more toxic to T. urticae than to P. ulmi, and only slightly toxic to T. pyri,produce a better outcome where T. pyri and P. ulmi are in balance and T. urticae is at lownumbers.