Relationships between beneficial invertebrates, field margin vegetation, and thrip damage in organic leek fields in eastern England

Abstract

Beneficial invertebrate numbers in two organic leek fields in eastern England were assessed in relation to four field margin vegetation treatments (clover/grass, regeneration, two flower mixtures) using pitfall and pan traps. Damage caused by onion thrips in one field was also estimated. Clover/grass and regeneration margins generally had more beneficial invertebrates than either flower mixture margins and invertebrate numbers in the fields were least opposite flower mixture margins. Distance from field margins had a greater effect on invertebrate numbers, with most ladybirds (Coccinellidae), hoverflies (Syrphidae), lacewings (Neuroptera) and parasitic wasps (Hymenoptera) recorded from the middle of fields. High ladybird, hoverfly and parasitic wasp (Icheumonidae) numbers were associated with lowest onion thrip damage, but there was no evidence that field margin treatments were related to damage. Sown flower mixture margins did not positively affect beneficial invertebrate numbers in the leek crop. It appears that an approach in keeping with elements already in the landscape would be just as effective as the use of alien vegetation mixtures in generating invertebrate natural enemy activity in vegetable fields of eastern England.