Identifying rarity in insects: the importance of host plant range

Some phytophagous insects are rare because their host plants are rare. This study uses data on the distribution of plants and insects from Great Britain to establish whether a threshold exists for the geographical range of a host plant below which its specific insects can be consistently described as rare. Data on four well-studied insect taxa (macro-moths, gelechiid micro-moths, beetles, and tephritid flies) that have been afforded conservation status as Red Data Book (RDB) or scarce (nationally Notable-A and -B) are examined. The proportion of species that are rare in these taxa declines as host plant range increases. Extrapolating these patterns of host range/phytophagous insect rarity to less studied groups (aphids, agromyzid flies and tortricid micro-moths) identifies a large number of insects that are probable candidates for conservation. Empirical evidence supports the view that many of the species selected by this process are indeed rare and in need of conservation. Identifying a host plant range below which insect herbivores are therefore rare provides an economical and objective method for producing a shortlist of phytophagous insect species rare enough to warrant conservation measures.