Quantifying the potential for wind and phoresy to drive off-plant movement of crapemyrtle bark scale, Acanthococcus lagerstroemiae (Kuwana) (Hemiptera: Eriococcidae): Implications for spread in urban landscapes

1. After non-native herbivores arrive in a novel habitat and establish, they must then disperse to new host plants to continue invading. While flight-capable insect herbivores can expand their non-native ranges by flying to new hosts, insects that lack wings, such as female scale insects, are often aided in their spread by wind and phoretic dispersal vectors such as birds and mammals. 2. After being detected in 2004 in a Texas nursery, crapemyrtle bark scale (CMBS), Acanthococcus lagerstroemiae (Kuwana) (Hemiptera: Eriococcidae), has established in 15 different states across the Southeast and Middle Atlantic regions of the USA. 3. We quantified the wind speeds leading to scale insect dislodgement and, in the laboratory and field, evaluated the willingness of CMBS nymphs to crawl onto vector proxies (bird feathers and mammal fur). 4. Our findings demonstrate that CMBS nymphs can become dislodged from ecologically relevant substrates at low wind speeds (similar to 7 m/s) and provided evidence that CMBS nymphs willingly, though not preferentially, crawl onto vector proxies, a final step before dispersing phoretically. Our results indicate that trees with high-density infestations of CMBS are frequent sources of propagules, given that (i) wind speeds that shake small tree branches are likely to dislodge active crawlers and (ii) active crawlers appear willing to walk onto birds and mammals. 5. The apparent ease of between-tree spread by CMBS via multiple modes of dispersal presents challenges for preventing the spread of this insect, but field confirmation of these mechanisms is needed to understand their relative importance.