ECR feature: Scale insects with Thomas D. Whitney

Thomas Whitney is currently a postdoc at Washington State University, Puyallup. He studies the ecology and evolution of insect species. His recent work in the Journal of Biogeography has sought to understand the extensive dieback in eastern white pines (Pinus strobus) and its association with a scale insect (Matsucoccus macrocicatrices). It has been unclear if this association is historical or recent, perhaps indicative of a recent host shift. Using population genetic approaches, Thomas sought to determine the likely context of this plant-insect association.

(left) Thomas meticulously removing scale insects from a branch in Wisconsin, USA. Not pictured very well are the hundreds of swarming mosquitos. (right) Thomas presenting his work.

Links: Personal webpage | Google Scholar | Research Gate

Institution: Washington State University – Puyallup

Current academic life stage: Postdoc

Research interests: I apply principles in ecology and evolution to better understand insect pests

Current study system: I currently study the little-known Douglas-fir twig weevil (Cylindrocopturus furnissi), a native beetle to the Pacific Northwest of North America. It has long been known to use Douglas-fir (Psuedotsuga menziesii) as a host, but only recently have we noticed it is developing within true firs (Abies spp.) as well. This has caused concern for the Christmas tree industry. This is a mystery, and the possibility of cryptic species, a host-shift, or something else entirely is what stimulates my curiosity about the system.

Recent paper in Journal of Biogeography: Whitney TD, Gandhi KJK, Lucardi RD. In press. Native or non-native? Historical biogeography of an emergent pest, Matsucoccus macrocicatrices. Journal of Biogeography. DOI: 10.1111/jbi.13702

Motivation for the paper: Eastern white pine (Pinus strobus) is an important and widespread tree across eastern North America. Since the early 2000s, the species has suffered from a novel phenomenon of branch dieback and mortality. Only recently was it discovered that an insect-pathogen complex is associated with the symptoms. The pathogen (Caliciopsis pinea) is assumed native, but there was no prior indication as to the past distribution of the insect, the eastern white pine bast scale. It was first described in Canada and was never known south of Massachusetts until 2006. Since then, reports of the insect in association with dieback symptoms have occurred frequently and as far south as Georgia and as far west as Wisconsin. Determining whether this insect has historically co-occurred with eastern white pine throughout the tree’s range was important to rule out or confirm the possibility of an invasive species. This information can help guide management strategies.

(A) Fruiting bodies of Caliciopsis pinea protruding from a bark canker. As part of an insect-pathogen complex, the feeding behaviors of eastern white pine bast scales are hypothesized to create ideal infection courts for this pathogen to penetrate the bark and establish in the cambium. These cankers are leading to white pine dieback symptoms. (B) A cluster of eastern white pine bast scales (Matsucoccus macrocicatrices) found on the bark surface feeding on the host tree’s vascular fluid. (C) Eastern white pine trees exhibiting dieback symptoms. Photo credit: Lori Chamberlain. (D) State, federal, and university researchers attend a field trip in the Southern Appalachian Mountains of Georgia, USA, as part of the first White Pine Health Workshop in February 2018.

Key methodologies: We conducted a population genetics study to assess the presence of structure across the range of the insect. We also looked for evidence of genetic bottlenecking, which would be consistent with a recent introduction from source populations to newly documented populations. We developed a panel of microsatellite markers using next generation sequencing to assess genetic diversity and structure. We also conducted landscape genetic analyses to determine if host tree connectivity (using data from Forest Inventory Analysis) could explain an apparent barrier to insect dispersal located in the Blue Ridge Mountains.

Unexpected challenges: These insects are tiny and very difficult to sample! As of now, there is no method to passively collect them. They must be hand sampled at the 2nd instar juvenile stage. This is the point in their development where they resemble a tiny black pearl – no eyes, no legs, only mouthparts perpetually inserted into the bark of the tree extracting sugars. These sentient sap-filled balloons are cryptically hidden in bark crevices and under lichen, and they require a delicate touch with forceps to pluck them from a branch or trunk. With such a big area of the continent to sample, training others was impractical. Instead, I did all the sampling myself, either travelling to sample in situ with a hand lens or receiving overnight shipments from my colleagues to sample under a microscope. It was tedious, but it was also a joy to explore these remote areas and collaborate with so many good folks.

Major result and contribution to the field: We found that the eastern white pine bast scale is indeed a native species to its newly documented areas. In fact, we found evidence to suggest it may have been associated with eastern white pine in refugial populations located in the Southern Appalachian Mountains during the Last Glacial Maximum. This rules out the possibility of the insect being a non-native invader exploiting naïve hosts. Why this insect has only now become associated with eastern white pine dieback and mortality remains a central question, but this work has successfully narrowed the possibilities.

What are the next steps? There are several steps to take with this system. One will be to investigate the mechanism that allows the insect’s feeding wounds to facilitate infection of the tree by fungal pathogens. Additionally, it will also benefit research efforts to develope a pheromone lure to accurately survey adult males, which will help us gain a better understanding of its density and range-wide distribution.

If you could study any organism on Earth, what would it be and why? I would study ice crawlers (Notoptera: Grylloblattodae), which are insect extremophiles! They live on alpine mountains, cannot tolerate temperatures over 10 °C, and are super rare. I think they’d be interesting to study for their unique biology, sure, but also because they seem to be an interesting system in terms of speciation and evolution. Oh, and how cool would that field work be?

Any other little gems you would like to share? I used to study wolf spiders that inhabit forest leaf litter. If you are unfamiliar, wolf spiders have an iridescent layer behind their retinas. At night, you can easily locate them with a headlamp – their eyes give off a subtle shimmer. It’s a fun thing to try. Whether in a forest, a grassland, or a desert, you’ll be surprised with how many wolf spiders are around.

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