Assessing the impact of the many variables — both abiotic and biotic — that influence population dynamics is no easy task. In the 22 February Nature, Ottar Bjørnstad of the National Center for Ecological Analysis and Synthesis in Santa Barbara, California, and co-workers show how plastic boxes stocked with meal moths and their natural enemies can be used to test theories of population dynamics.

The Indian meal moth — a pest that feeds on stored products such as flour, nuts and dried fruit — was cultured alone or either with a parasitoid wasp or a baculovirus, both natural enemies of the meal moth. Bjørnstad et al., in a study supported by mathematical modelling, demonstrate that the lag between the introduction of a parasite and its effect on the density of the host population depends on the strength of the interaction between the two populations (Nature 2001, 409:1001-1006). This has been predicted by theory but, until now, has been difficult to test.

In the absence of enemies, the meal moth population showed cycles of abundance. Older moth larvae cannabilised smaller larvae, so low numbers of adult moths reflected the lagged effect of an earlier abundance of cannibals. The virus had no apparent effect, whereas studies including wasps showed a deepening in the troughs of the cycle — the number of adult moths decreased with past wasp abundance.

Previous work by Bjørnstad's collaborators showed that, when both the virus and the wasp confront the moth together, all three species become extinct.

In an accompanying News and Views article, Michael Hochberg, of the Université de Montpellier II, France, and Arthur Weis, of the University of California, Irvine, California, remark that these techniques could also be used to show how the use of two natural enemies as biological pest controls yields outcomes that are different to the outcome of using either enemy alone.