Overwintering populations of beetle larvae (Coleoptera) in cereal fields and their contribution to adult populations in the spring

Abstract

Although the ecology and distribution of adult beetles on farmland has been studied extensively, rather little is known of the ecology of their immature stages. Many species are important predators of crop pests and considerable interest has been shown in providing overwintering refuges from which they may colonise the crop in the spring. We present evidence that for at least one common species, and possibly a second, populations of larvae that have overwintered in the open field are major contributors to adult spring populations. During winter and spring 1998, larval and adult beetles were collected by barriered pitfall traps in cereal fields. During the winter, Carabidae larvae were most commonly caught, with the surface-active larvae of Nebria brevicollis being the most numerous. These were distributed randomly across fields with an activity-density of the same order of magnitude as that of adults caught subsequently in June. Thus a large proportion of the adults probably developed within the field, rather than invading from field boundaries. This is further supported since many of the beetles caught in the early summer were tenerals, indicating that they had recently hatched. For Pterostichus melanarius, catches of larvae in winter were much lower than subsequent adult catches, but again a high percentage of just-hatched tenerals in the field in June suggested that this species had overwintered in high numbers as larvae within the soil, confirming the findings of earlier studies. Larvae of this species are mostly active below the soil surface, which is probably why few were caught. If our suggestions about the importance of overwintering larvae in winter cereal fields are correct, then such open field populations should be taken into account when trying to enhance adult carabid numbers in summer.