Comparative morphometric and chemical analyses of phenotypes of two invasive ambrosia beetles (Euwallacea spp.) in the United States

Abstract

The polyphagous shot hole borer (PSHB), Euwallacea sp., was first detected in 2003 in Los Angeles County, California, USA. Recently, this invasive species has become a major pest of many hardwood trees in urban and wildland forests throughout southern California. PSHB is nearly identical in morphology and life history to the tea shot hole borer (TSHB), Euwallacea fornicatus, an invasive pest of hardwoods in Florida, USA and many other parts of the world. However, molecular studies have suggested that the taxa are different species. We conducted morphometric and chemical analyses of the phenotypes of Euwallacea sp. collected in southern California (Los Angeles County) and E. fornicatus collected in Florida (Miami-Dade County). Our analyses indicated that PSHB has 3 larval instars. The third larval instar was separated from the first 2 instars by head capsule width with 0 probability of misclassification. The body length, head width, and pronotal width of PSHB adult males were significantly less than those of females. Head width and pronotal width of female PSHB were significantly less than those of female TSHB. In contrast, body length, and ratio of body length to pronotal width of female PSHB were significantly greater than those of female TSHB. However, females of these 2 species could not be separated completely by these 4 measurements because of the overlapping ranges. Cuticular hydrocarbons detected in both species were exclusively alkanes (i.e., n-alkanes, monomethylalkanes, dimethylalkanes, and trimethylalkanes). Cuticular hydrocarbon profiles of PSHB males and females were similar, but they both differed from that of TSHB females. Cuticular hydrocarbons of PSHB were predominantly internally branched dimethylalkanes with backbones of 31 and 33 carbons, whereas cuticular hydrocarbons of TSHB females were dominated by internally branched monomethylalkanes and dimethylalkanes with backbones of 28 and 29 carbons. Multiple compounds within these classes appear to be diagnostic for PSHB and TSHB, respectively.

Pheromones of three ambrosia beetles in the Euwallacea fornicatus species complex: ratios and preferences

“…The chemical and behavioral data presented here indicates that these morphologically indistinguishable beetles, carrying different species of symbiotic Fusarium , and originating from three different invasive populations, have formed pheromone races, and further supports the amassing evidence that they have speciated into three cryptic species ( Kasson et al, 2013 ; O’Donnell et al, 2015 ; Chen et al, 2016 ; Stouthamer et al, 2017 ). According to Stouthamer et al (2017) , multiple haplotypes were found of PSHB in Vietnam and Taiwan, of KSHB in Taiwan and Okinawa, and of TSHB in Thailand and India, suggesting their possible native origins.…”

“…Each of these three beetle species carry different species of symbiotic Fusarium in their mandibular mycangia ( O’Donnell et al, 2015 ; Carrillo et al, 2016 ), and the inability of PSHB and TSHB larvae to survive when fed Fusarium from the other species suggests that isolation between species also takes place in their obligatory feeding requirements for their associated Fusarium species as well ( Freeman et al, 2013a ). Differences were also found between the cuticular hydrocarbon profiles of PSHB and TSHB which could potentially assist in species diagnostics since they are morphometrically indistinguishable ( Chen et al, 2016 ). These three cryptic species are similar in their polyphagous nature, in that they can attack and spread their Fusarium symbiont to hundreds of tree species in numerous families ( Danthanarayana, 1968 ; Eskalen et al, 2013 ).…”

North American Xyleborini north of Mexico: a review and key to genera and species (Coleoptera, Curculionidae, Scolytinae)

“…The different lineages are supported by rapidly evolving mitochondrial genes and more conserved nuclear gene regions. Although these potential different species display morphological differences, reliable morphological diagnosis has not been established (Chen et al 2016). …”

Hardwood injury and mortality associated with two shot hole borers, Euwallacea spp., in the invaded region of southern California, USA, and the native region of Southeast Asia

“…2;O'Donnell et al 2015), because of their morphological similarity (Eskalen et al 2013;Chen et al 2017). However, DNA sequences from mitochondrial cytochrome oxidase c subunit 1 (COI) and cuticular hydrocarbon profiles support the status of PSHB and TSHB as separate taxa (O'Donnell et al 2015;Chen et al 2017;Stouthamer et al 2017). Stouthamer et al (2017) reported that the native range of PSHB may encompass Northern Thailand, Vietnam, China, Taiwan, and Okinawa.…”

“…5;O'Donnell et al 2015, Gomez et al 2018b, is a second exotic ambrosia beetle that was first detected in 2013 in urban forests of San Diego County (Chen et al 2017;Stouthamer et al 2017). KSHB is similar morphologically to PSHB and TSHB but differs in cuticular hydrocarbon phenotype (Chen et al 2017) and in DNA sequence from the COI region (Stouthamer et al 2017); it also carries novel Fusarium sp. and Graphium sp.…”