The known unknowns in international border interceptions of insects

Propagule pressure (the number of individuals arriving in new areas) is one of the key drivers of establishment success of invasive species, including insects. However, border interception data, which have the potential to act as a proxy for true arrival rates of insects into a country, are seldom used in this way. In large part this is due to the limited amount of interception data in many countries, difficulties accessing such data, and when these data are available, difficulties in addressing their biases caused by variation in interception probability for different taxa due to policy changes and operational influences. The type of interception data required to fully quantify arrival probabilities are rarely available. To improve the use of interception data as a proxy for propagule pressure, we investigated the relative number of established species and intercepted species by taxonomic group and by biological characteristics, using several international datasets. We identified higher ratios of intercepted to established species for plant feeding insect groups relative to ratios for non-plant feeding groups, even in countries with broad-spectrum border inspection strategies. This could be due to higher interception probabilities for plant feeding insect groups, caused by phytosanitary preferences in inspections, or lower establishment probabilities for plant feeding insect groups. To address this variation and improve establishment predictions based on interception frequency, we developed a methodology to account for the ratio of established species to intercepted species per taxonomic grouping.