Potential Crawling Distance for an Invading Urban Tree Pest: Implications for Settling Decisions and Between-Tree Movement

Scale insects are prevalent urban landscape pests and are considered to move between neighboring trees via wind-aided and phoretic dispersal by cryptic, short-lived crawlers. Crawling likely plays a central role in these dispersal processes by shaping their ability to move onto phoretic hosts or to locate host plants and/or settling locations after being blown off a tree. Additionally, walking capabilities of crawlers drive on-tree movement, such as how far they disperse before settling. Here, we studied movement of the damaging, non-native crapemyrtle bark scale (CMBS), Acanthococcus lagerstroemiae (Hemiptera: Eriococcidae). We aimed to determine survival times for crawlers under starved conditions, evaluated crawler walking distances on various substrates, and combined survival times and walking distances from these experiments to simulate potential walking distances over the lifetime of the crawler stage and 1, 6, 12, 18, 24, 48, and 72-h intervals. We found that half of the crawlers survived approximately three days, with < 25% surviving up to seven days. On different substrates, crawlers walked five times farther on average on carboard than white posterboard over 15-min periods; crawling distances did not significantly vary during 5-min trials using colored construction paper, however. Combining these results, we estimated crawlers could move a median distance of 62 m (range: 2–211 m) if they walked for the entirety of their life as a crawler. These findings provide insight into a vulnerable life stage of scale insects and suggest that crawlers blown off of trees could walk to new hosts and easily move between adjacent trees.