Abstract
The two-spotted spider mite, Tetranychus urticae Koch, feeds on over 1140 plant species, including economically important crops, such as corn, strawberries, hops, and grapes, causing economic losses of up to $4500 per hectare. Worldwide, T. urticae has gained notoriety for developing resistance to multiple synthetic and natural acaricidal chemistries. Resistance development is facilitated by its exceptionally fast development rate and high fecundity. To date, T. urticae has been documented as having developed resistance to 96 acaricides with a variety of modes of action. Due to mounting evidence that synthetic acaricides are a danger to human and environmental health and because of the high cost of developing and registering new acaricides, demand for biodegradable botanical acaricides—especially essential oils and insecticidal soaps—has risen dramatically. In this study, I analyzed the effects of eight organic oil or soap-based acaricides (soybean oil, rosemary oil, peppermint oil, neem oil, and mineral oil) on survival of T. urticae populations. I also compared the mortality of control mites that were not resistant to any chemistries with 11 strains of mites that had been selected for high levels of resistance to different conventional acaricides, to monitor potential cross-resistance to the oil/soap-based acaricides. Most strains exhibited no cross-resistance, although there were several cases of weak (RR ≤ 3.5) but statistically significant resistance of specific strains to specific products. Pure peppermint oil was consistently the most efficacious material tested at 100% mortality on almost all of the resistant strains; soybean oil exhibited the lowest efficacy at 17% mortality. The outcome of this study and its implications on the integrated management of T. urticae are discussed below.