Timing of pesticide application can be the difference between spider mite outbreaks or suppression on honeylocust

Pesticide use against early season pests of the urban forest has the capacity to cause spider mite outbreaks. Using thornless honeylocust (Gleditsia triacanthos var. inermis) as a model, we applied both topical and systemic insecticides to target various stages of calico scale (Eulecanium cerasorum) a key pest of this tree in the Indianapolis area. We then measured impacts of these applications on populations of honeylocust spider mite (Platytetranychus multidigituli) and its associated predatory mites. We found that early season applications of bifenthrin caused outbreaks of honeylocust spider mite, but that later season applications suppressed spider mite problems. Pyriproxyfen, a product capable of killing calico scale crawlers when applied in June, did not affect predatory mite populations, nor did they stimulate outbreaks of spider mites. Soil applications of imidacloprid and dinotefuran were not regularly associated with spider mite outbreaks.   Due to irregular irrigation and inconsistent performance against calico scale, it is difficult to determine whether the absence of outbreak was due to lack of product uptake or a property of the compound. Our findings suggest follow up applications may be necessary to reduce spider mite populations if broad spectrum pesticides cause outbreaks when used early in the season.