The influence of cropping rotations and soil cultivation practice on the population ecology of carabids (Coleoptera : Carabidae) in arable land

Summary Use is made of detailed information on the lifecycles and phenology of carabid beetles in arable fields to analyse pitfall catches and seek evidence for the hypothesis that pre-adult instars in the soil are susceptible to soil tillage operations at particular times of the year. Strictly autumn-breeding species were trapped less frequently at the time of adult emergence in early summer when soil cultivation was done for crop establishment in late spring compared with uncultivated or autumn-sown fields. Analysis of data for species with distinct reproductive cohorts in autumn and spring produced evidence that the larval over-wintering cohort, but not the adult hibernating cohort, was similarly affected by soil cultivation. No evidence was found to suggest that cultivation at the time of egg or early larval instars directly influences the survival of populations and it is concluded that probably only cultivation-induced mortality of late larval and pupal instars has a significant population effect. The longer-term survival of most carabids in arable fields probably depends on either flexibility in their time of breeding or, for strictly autumn-breeding species, on a well-developed phase of post-emergence dispersal by adult beetles. It is concluded that soil cultivation directly impacts on the incidence of larval over-wintering populations and limits the range of lifecycles and phenologies that are possible in arable land. Such constraints may explain the phenomenon of long-term population robustness that is often exhibited by carabid populations in arable crops when they are subject to non-target pesticide effects.