Abstract
A simulation model was used to assess the role of several mechanisms proposed to be responsible for spider mite outbreaks on cotton that are typically observed following applications of insecticides. Simulation results were compared to an outbreak that occurred after two pyrethroid applications on cotton in a controlled experiment in the San Joaquin Valley of California. In the model, physiological effects were simulated by increasing spider mite fecundity and decreasing developmental duration, whereas loss of natural enemies was simulated by increasing spider mite age-specific survival. At the levels simulated, survival had the greatest impact on maximum spider mite density, degree days (oD) to maximum density, and cumulative spider mite-oD, whereas fecundity had the least, and developmental duration had an intermediate effect. There were substantial two-way interactions among all three life history parameters, with age-specific survival having the most influence. Survival had the greatest effect on spider mite population dynamics when in combination with short developmental duration. The influence of developmental duration on maximum spider mite density was greater than comparable percentage changes in fecundity, an effect that was more pronounced at high than at low survival. Changing fecundity, developmental duration, or age-specific survival individually did not result in a spider mite outbreak of the magnitude observed in the field. However, changing these three parameters simultaneously, resulted in a simulated maximum density of 8,000/m2, which represents a 12-fold increase over the untreated control, and closely mimicked the previously observed field outbreak. It is proposed that spider mite outbreaks on cotton following insecticide applications are not solely the result of physiological stimulation, but are rather due to several life history parameters being affected simultaneously, with natural enemy-mediated survival having the greatest individual impact. Implications of chemically-induced phenomena affecting spider mite management on cotton are discussed.
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