Native or non-native? Historical biogeography of an emergent forest pest, Matsucoccus macrocicatrices

Aim: A historically benign insect herbivore, Matsucoccus macrocicatrices, has recently been linked to dieback and mortality of eastern white pine (Pinus strobus L.). Previous reports indicated that its native range was restricted to New England, USA and southeastern Canada. Now, the insect occurs throughout an area extending from the putative native range, southward to Georgia, and westward to Wisconsin. Our goal was to evaluate whether its current distribution was due to recent introductions consistent with invasion processes. We considered two hypotheses: (a) if recent expansion into adventive regions occurred, those populations would have reduced genetic diversity due to founder effect(s); alternatively (b) if M. macrocicatrices is native and historically co-occurred with its host tree throughout the North American range, then populations would have greater overall genetic diversity and a population structure indicative of past biogeographical influences. Location: Eastern North America. Methods: We developed nine M. macrocicatrices-specific microsatellite markers de novo and genotyped 390 individuals from 22 populations sampled across the range of eastern white pine in the USA. We assessed genetic variability, relatedness, and population structure. Results: There were no signatures of founder effects. The only differences in genetic diversity occurred latitudinally, where the number of rare alleles and observed heterozygosity was highest in the southern range extent. Analyses of population structure indicated three distinct genetic clusters separated by the Great Lakes and the Blue Ridge Mountains. Main Conclusions: The seemingly sudden ecological shift from benign herbivore to significant pest led us to suspect that M. macrocicatrices was non-native. However, our findings suggest that this insect is native and has likely co-occurred with its host tree since the last glacial maximum. Our study demonstrates the importance of historical biogeographical reconstruction to inform how to approach an emergent pest.