Niche use patterns among some Stenodemini (Heteroptera: Miridae) of limestone grassland, and an investigation of the possibility of interspecific competition between Notostira elongata Geoffroy and Megaloceraea recticornis Geoffroy.

It was suggested in a previous paper that mortality patterns in two species of Stenodemini (M. recticornis and N. elongata) could be explained by interspecific competition (Gibson 1976a)Such competition would have been generated by seasonal changes in the foodplants used by the two species resulting in both species using virtually the same range of plants at one time of year.As with a number of phytophagous insects (McNeill and Southwood 1978) many changes in foodplant use could be explained by differential seasonal changes in nitrogen content of the plant part (leaves) that the bugs were using in different plant species. Each bug had an ideal food nitrogen range which it used whenever possible. Lack of fit to this mechanism could often be explained by defensive chemistry and physical properties of some grass species.Although the total bug population was taking under 1% of the area's primary production, a potential refuge foodplant for one bug species was very heavily eaten where it was rare, and behavioural interference between bug species suggested that space to feed in was a limiting resource.Laboratory and semi-field culture experiments showed that the presence of one bug species adversely affected the growth and/or survival of the other, although the outcome of competition was different to that in the field, probably due to the different relative and absolute densities used.Although one cannot be completely certain without field manipulation experiments, it is extremely likely that interspecific competition was occurring and could be stabilised by the losing species (N. elongata) having a 'refuge generation'.Competition between stenodemine species on the study area could usually be avoided by separation along a number of resource axes e.g. foodplant species, plant part, nitrogen level of food or emergence timing. In the present circumstance, a particular combination of bug and plant species in the area has forced one species pair into overlap. It should be possible to predict the guild structure and competition relations in other areas from this.