For lepidopterans subject to population cycles, two mechanisms have been proposed to explain the mechanism behind the pattern. Extrinsic factors, such as sunspots or weather, were popular explanations when these patterns were first discovered. However there is little evidence that any such factors are as strictly periodic in their effects to produce the cyclical patterns we observe. Workers have come to accept the idea that negative feedback mechanisms involved with predators or parasitoids are probably responsible for most of the cyclical dynamics observed in lepidopterans. Although several lepidopteran systems exhibit relatively strong cyclical dynamics, the strongest exposition of the problem and the solution can be found in the larch budmoth.
The LBM system shows very regular population cycles in Europe. Since there are many long-term data sets for this organism, the population dynamics have received a great deal of scrutiny. Cycles generally have a period of 9 years, over large geographic areas. Based on time-series analysis, second-order or higher-order processes are likely to be causing the cycles, with declines in plant quality over many years of herbivory possibly being the mechanism. The decline in plant quality is hypothesized to occur as a result of LBM activity; this is sometimes termed induced susceptibility in plant tissues. Models employing declines in plant quality over several seasons as an exogenous factor and herbivore population growth dependent on plant material accurately mimic the observed dynamics in the system. Empirical evidence, however, suggests that other factors may be the main drivers of the cycles. During one LBM outbreak there was little decline in plant quality and the density of the larvae were quite low, relative to earlier outbreaks. However, the decline portion of the cycle still occurred, leading researchers to question the mechanism ofinduced susceptibility.
An alternative explanation for the production of cycles is the activity of specialized parasitoids. Many parasitoids prey upon LBM populations, although only two groups cause appreciable amounts of mortality. Simple models employing parasitoids and a time lag fit the observed dynamics significantly better than the plant quality models. There are, however, very few data regarding the exact proportion of budmoth larvae that are attacked successfully by parasitoids. A combined model that includes induced susceptibility in both plants and parasitoids actually explains the greatest amount of variation in the empirical data, suggesting that the mechanism may include the interaction between these two factors.
Owing to the lack of large-scale manipulations in this system, it is difficult to say conclusively what the true drivers of the LBM cycles are. It seems likely that many processes contribute to the effects that we see, with some redundancy in the factors. For example, population crashes still occur when plant quality stays relatively stable, so parasitoid activity may 'take over' when plant quality does not decline.
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