Fluorescent dust marking is commonly employed to identify and track small arthropods for studies of ecology, demography, and behavior. Despite its widespread use, no study to date has empirically tested the suitability of dust marking for studies of spider behavior. Here, we test the effects of fluorescent dust marking on proximity of cohabitation, sibling cannibalism, and non-cannibalistic mortality of western black widow spiderlings, Latrodectus hesperus Chamberlin & Ivie (Araneae: Theridiidae). Results indicate that dust-marked spiderlings cohabitated at closer proximities and died sooner than undusted spiderlings due to a greater incidence of cannibalism in the dust-marked group. Thus, we conclude that fluorescent dust marking significantly affected the cohabitation and cannibalistic behavior of L. hesperus spiderlings. Although few studies have reported adverse effects of dust marking on arthropods, our results should serve as a warning to future studies that normal behavior may be disrupted by the use of these fluorescent dust markers. Therefore, preliminary testing should be routine when determining the suitability of any marking technique for not only new species, but also new life stages and behaviors.