A look into the future of large shade trees in urban heat islands

Effects of urban warming on willow oaks, scale insects, and natural enemies

Lecanium scales cover every inch of this willow oak branch. Photo: EK Meineke

Raleigh is the City of Oaks. Willow oaks, Quercus phellos, have been planted along Raleigh streets for a century and now make up much of the Raleigh tree canopy. Willow oaks are large and long-lived street trees that provide shade for and aesthetic appeal in residential and commercial areas. They, like other trees, provide other valuable ecosystem services such as atmospheric cooling and carbon sequestration. Of course, no trees evolved in urban areas, and many tree species are harmed by urban conditions, such as the urban heat island effect and drought.

A primary problem endured by many urban willow oaks throughout the South and Mid-Atlantic is infestation by scale insects. Willow oaks seem particularly susceptible to Lecanium spp. scales that become so abundant that twigs can have dozens of them per inch.

But why? In forests willow oaks are not covered in scales. What conditions in cities make trees so susceptible to scales and other pest insects? And, how do those conditions and the scale insects they incite affect tree functions and services?

Research Approach

In 2012, PhD student Emily Meineke set out to answer these questions. Over the next 4 years with a variety of novel tactics she addressed three primary questions:

Abundance of Lecanium scales on hot and cold trees across the Raleigh urban heat island.

Insects are ectotherms and need heat to develop and drive their metabolic processes. Therefore, our hypothesis was that warming would benefit scale insects, but we could not believe the extent of it. We collected twigs from trees in hotter and cooler sites throughout the Raleigh urban heat island and counted scales on each one. Scales were up to 12 times more abundant on warm trees than those just 2C cooler.

This was a good find, but cities are complicated places. Hotter trees probably had other stressors like more impervious surface cover nearby, fewer nearby trees or other vegetation, and more drought stress. Therefore, the higher scale insect abundance we observed could have been due to other factors that covaried with warming. To isolate the effects of heat, Emily grew young willow oaks in the NCSU Phytotron, so that hot and cool trees had the same soil, water, and nutrients. She collected female scales full of eggs from some hot trees and some cool trees. She attached hot and cool eggs to trees in both hot and cool phytotron chambers and let them develop and feed for 7 months.

Number of Lecanium scales surviving on willow oaks grown in hot and cold Phytotron chambers.

Emily hard at work counting scales on her Phytotron willow oaks. Photo: Becky Kirkland

Consistent with expectations, more scales from hot trees survived in the hot chamber than cool chamber. However, the surprising part was that scales from cool trees did not survive as well in the hot chamber as scales from hot trees, and scales from cold urban trees also did not survive better in the hot chamber than in the cool chamber. Scales from hot trees had some advantage by either being acclimated or adapted to hot conditions.

Details of this research can be found in:
Meineke, E.K., Dunn, R.R., Sexton, J. and Frank, S.D. (2013) Urban warming drives insect pest abundance on street trees. PLoS ONE, 8(3): e59687.

Parasitoid wasp Encyrtus fuscus emerging from oak lecanium scale in Raleigh, NC in May 2014. Photo: EK Meineke and Andrew Ernst.

Parasitoids are tiny wasps (usually) that lay eggs inside of their hosts. The parasitoid larva develops inside the host insect, then emerges from the (now dead) host as an adult. Lecanium scale insects host a community of parasitoids, and Emily found that, across Raleigh’s temperature gradient, parasitoid abundance, community composition, and parasitism rates did not change, suggesting parasitoids were not driving scale insect responses to heat.

However, Emily noticed that scales on warm trees developed earlier in the spring than scales on cold trees and that, on the warmest trees, parasitized scales produced eggs while they did so less often on the cooler trees. She pursued this observation with countless hours of dissecting scales and counting eggs and made a great discovery: hot scales did develop earlier, and by doing so, evaded parasitoids until after they had produced most of their eggs. So even though parasitism rate was the same, the consequence of parasitism for the fitness of hot and cold scales was very different, giving scale insects in hotter parts of the city an advantage over those in cooler areas.

A ghost spider has made its home on a twig infested with Lecanium scales. Photo: SD Frank

Spiders, Emily found, are among the most abundant predatory arthropods in urban oaks. Since herbivore abundance increases on hot trees, and spiders tend to be more abundant where their prey (herbivores) are more abundant, one could predict that spiders should be more abundant on hot trees. (Though spiders may not eat scale insects [we don’t know], many do eat spider mites, which Emily found increased in abundance similarly to scale insects.) However, since some spiders may not like the heat, spiders could also be less abundant on hot trees or the spider community could be made of different (heat tolerant) species. Emily worked with a star undergraduate, Anna Holmquist, to determine how warming and prey abundance affected spider communities on her sprayed and unsprayed willow oaks. The punch line is that spider abundance does not track heat-induced prey abundance. This may be another reason pest outbreaks occur on urban trees. In addition, warming almost eliminated an important spider family, Anyphaenidae, known as “ghost spiders.”

Details of this research can be found in:
Meineke, E. K., Dunn, R. R., Frank, S. D. (2014) Early pest development and loss of biological control are associated with urban warming. Biology Letters, doi: 10.1098/rsbl.2014.0586.

Meineke, E.K., Holmquist, A.J., Wimp, G.M., Frank, S.D. (2017) Changes in spider community composition are associated with urban temperature, not herbivore abundance. Journal of Urban Ecology, 3 (1): juw010. doi: 10.1093/jue/juw010.

Research Associate Elsa Youngsteadt measures willow oak photosynthesis. Photo: EK Meineke

We care about bugs (a lot), but most people care more about trees (we like trees too). People care that trees look nice, provide shade, and are healthy enough not to fall on their houses, cars, dogs, or worse. Some people (nerds) even care how much carbon trees collect through photosynthesis and sequester in their tissues. We care about this because carbon stored by trees does not contribute to the rapidly growing amount of carbon in the atmosphere (currently 406 ppm).

Through a series of laboratory and field experiments, Emily found that warming reduces tree functions, such as photosynthesis and growth, when combined with water stress. Since drought is likely to become more frequent and severe with climate change, this has implications for urban and rural trees. To assess the effects of heat and pests on an honest to goodness ecosystem service, Emily calculated carbon sequestration by the trees she studied, then used the empirical data to calculate how much carbon sequestration was lost by willow oaks in Raleigh due to urban warming. A lot. Around 23 metric tons of carbon were not sequestered when the effects of warming are included in carbon sequestration formulas.

Details of this research can be found in:
Meineke, E.K., Youngsteadt, E.K., Dunn, R.R., Frank, S.D. (2016) Urban warming reduces aboveground carbon storage. Proceedings of the Royal Society – B, 283: 20161574.



An EPA STAR Fellowship awarded to Emily Meineke also funded this research.

ECOIPM Blog: Urban Herbivore Ecology posts