CYCLES IN INSECT POPULATIONS - DELAYED DENSITY-DEPENDENCE OR EXOGENOUS DRIVING VARIABLES

Delayed density dependence, and the cycles in insect populations that it can generate, are often investigated using time-series analysis. Re cently, several authors have raised concerns about the validity of usi ng time-series analysis to detect density dependence. One particular c oncern is the suggestion that exogenous driving variables, such as cyc lic weather patterns, can lead to the spurious detection of density de pendence in natural populations. Using non-biological data (the electr icity bills of one of the authors), we show how easy it is to be misle d by the results of time-series analysis. We then present 16 years' da ta on the gall-forming sawfly, Euura lasiolepis (Hymenoptera: Tenthred inidae), and show that cycles in weather, specifically winter precipit ation, lead to the spurious detection of density dependence in time-se ries analysis. We conclude that time-series analysis cannot stand alon e as a method for inferring the action of density dependence, and urge further investigation of the effects of apparent cycles in abiotic fo rces on insect populations.