Timing the cultivation of rotational set-aside for grass weed control to benefit chick-food insects.

Abstract

Graminivorous sawflies are important beneficial farmland insects, occurring at densities too low to cause crop damage but providing an essential source of food for the chicks of many birds of arable habitats. These sawflies overwinter as pupae in the soil and emerge as adults in the spring. Cultivation of the soil during the overwintering period is believed to disturb the insects and cause high mortality; cereal fields that are not ploughed after harvesting such as undersown spring barley fields have been found to provide key overwintering sites. Rotational set-aside following cereals would similarly provide a refuge for overwintering sawflies provided that any cultivation of the soil for weed control takes place after adult emergence is complete. The phenology of sawfly emergence has been investigated and peak emergence found to be in early May. Experiments conducted at the same time showed that two of the main grass weeds of set-aside land, black-grass (Alopecurus myosuroides) and barren brome (Bromus sterilis) did not set their first viable seeds until late May or early June. Management of set-aside land by cultivation after the third week of May will therefore successfully permit emergence of overwintering sawflies without prejudicing grass weed control.