Cities as a glimpse of the future

This is a guest post by our Research Associate Elsa Youngsteadt about the work and meaning behind her new research published in Global Change Biology.

About a year ago, I found myself sitting ruefully in a patch of chiggery grass by the side of the road near the little town of Bahama, North Carolina, waiting for a tow truck. I had stuck the lab pickup firmly in a ditch. It was tilted at an embarrassing, sickening angle and had one wheel lodged against the mouth of a culvert. Helpful passers-by with chains and four-wheel drives kindly offered to pull me out, but really only made matters worse.

My memory is already fuzzy about the sequence of events, but somewhere in there—

Gloomy scales and the beetle that loves them. Each white or gray bump is a gloomy scale. The twice stabbed lady beetle is one of their predators. Photo: S.D. Frank

Gloomy scales and the beetle that loves them. Each white or gray bump is a gloomy scale. The twice stabbed lady beetle is one of their predators. Photo: S.D. Frank

between slipping into the ditch, the failed rescue attempts, and the final arrival of the giant tow truck—I did actually hike into the woods and get what I came for: eight slender red maple branches, clipped from trees growing in NC State’s Hill Forest.

I found my way to this particular spot, ditch and all, by following the trail of a plant biologist who had collected maple branches there more than 40 years ago during the height of the Nixon administration and the Vietnam War. In those days, the forest was cooler. The fevered dog days of summer now average about 1.4 degrees C (about 2.5 degrees F) hotter than they did then—and that should make a difference to the trees and the insects that live on them.

Specifically, it ought to make a difference to gloomy scale insects. These little sap-sucking insects seem to like it hot. My colleague Adam Dale has been studying gloomy scales in the city of Raleigh, and he’s found that street trees in the hottest parts of the city have far more scales—sometimes 200 times more–than those in the cooler parts of the city.

The scales drink tree juices, so more scales are bad for trees. A couple of degrees

Sad, bedraggled, gloomy scale infested red maple trees. Photo: SD Frank

Sad, bedraggled, gloomy scale infested red maple trees. Photo: SD Frank

warming can make the difference between a stately shade tree and a sad, bedraggled specimen with dead branches, sparse leaves, and grimy, scale-encrusted bark.

We thought that if warming gives scales such a powerful boost in the city, global warming could do the same thing for scale insects in rural forests. But we still had no direct evidence that what happens in the city represents what happens in rural areas over time.

This seemed like hard evidence to get. Unlike birds and butterflies, the drab, millimeter-long gloomy scale has not invited enthusiastic long-term monitoring. But perhaps we could scavenge scale-insect information from another source—and this is why I became extremely grateful to scores of plant biologists like the one who archived a foot-long maple twig from Hill Forest in 1971.

These historical plant specimens are stored in collections known as herbaria, where they are affixed to stiff pieces of paperboard, labeled, and stacked in mothball-scented cabinets. It turns out that many of these old twigs still have scale insects intact, stuck firmly but inconspicuously to the spots where they once lived.

An herbarium specimen used in the study. Photo: EK Youngsteadt

An herbarium specimen used in the study. Photo: EK Youngsteadt

It made perfect sense that they would be there, but it still felt outlandish when, only 12

branches into my first search in the UNC Herbarium, there was a gloomy scale—the same species that burdens our urban red maples. It was beautifully preserved, looking like it was collected last week instead of 30 years ago. Even on 100-year old branches, the scales looked perfect.

So I counted them. And kept counting them on more than 300 historical specimens from the southeastern US, then matched up their abundance with historical temperatures for the year and location where each specimen was collected.

There it was: During relatively cool historical time periods, only 17% of branches had scale

Gloomy scale covers preserved on an old herbarium specimen. Photo:EK Youngsteadt

Gloomy scale covers preserved on an old herbarium specimen. Photo:EK Youngsteadt

insects. But during relatively hot periods, 36% were infested. In other words, scale-infested branches were more than twice as common during hot periods than cool periods—exactly as we would expect if scale insects benefit from warming in rural forests as they do in the city. Furthermore, the most heavily infested twigs were ones that had grown at temperatures similar to those of modern urban Raleigh.

But the historical specimens weren’t the whole story. The past several years have been warmer than even the historically warm time periods, so to test our prediction, we needed to go back to places where those old branches were originally collected, and see if their scale infestations had actually gotten worse.

Thanks to the careful records of those past plant collectors, I was able to track down 20 of the forest sites across North Carolina where red maple branches were collected in the ‘70s, ‘80s, and ‘90s (and only put the truck in a ditch at one of them). At 16 of the 20 sites, gloomy scale populations were denser than they were on the original branches from the same locations. Overall, I found about five times more scales in 2013 than in the earlier decades.

Careful records and herbarium tags from the past helped Elsa relocate the collection sites. Photo: EK Youngsteadt

Careful records and herbarium tags from the past helped Elsa relocate the collection sites. Photo: EK Youngsteadt

This isn’t good news, but it’s also not time to panic about gloomy scales killing our forests. Although the rural scale insects clearly benefited from warming, just as they do in Raleigh, they still never got as abundant as the ones we see in town. The reasons for that difference are an open question (I have some guesses, but that’s a different story). So, although I’d put money on gloomy scales getting more common in rural North Carolina over the next several decades, I wouldn’t yet say how much more common.

But this really isn’t just about gloomy scale. It’s about cities as an advance guard of climate change. If we can look at scales’ response to urban warming and correctly predict their increased abundance due to global warming, can we do it for other organisms, too? Can we do it for functions, like pollination and biological control of pests?

I hope we can start watching urban ecosystems for problem insects and using that information to stand forewarned about future ecological changes in natural areas. The experiments we have made by paving our cities and making them heat up may have much more to tell us about how organisms will handle future warming.

This post is based on a new study:

Youngsteadt, E., Dale, A.G., Terando, A.J., Dunn, R.R. and Frank, S.D. 2014. Do cities simulate climate change? A comparison of herbivore response to urban and global warming. Global Change Biologydoi: 10.1111/gcb.12692.  PDF

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Urbanization is good for pests and bad for trees

My wife is from a neighborhood outside Baltimore called Lawyer’s Hill. This is where, in the 18th century, lawyers (and I assume doctors and other gentlemen) had country houses and could escape the summer heat. Lawyer’s Hill is only 3 miles from Baltimore but, based on their significant investment in houses and land, it must have provided significant relief. So what was (and still is) the difference between Baltimore City and Lawyers Hill? Trees.

Historic Lawyer's Hill (left) and downtown Baltimore City. Images from Google Maps.

Historic Lawyer’s Hill (left) and downtown Baltimore City. Images from Google Maps.

Trees cool the environment by shading houses, roads, and sidewalks that absorb heat. If

Damage by gloomy scales. Notice dead branches and sparse canopy. Photo: SD Frank

Typically shabby red maples with damage by gloomy scales. Notice dead branches and sparse canopy. For more (better) pictures visit Adam’s picture gallery featured in the Bulletin of the Ecological Society of America. Photo: SD Frank

you have every walked barefoot from the pool (or wherever you spend time barefoot) to your car you know that pavement is hot and that you scurry from one patch of shade to another. All the heat absorbed by pavement that does not radiate into the soles of your feet radiates into the air. Trees also cool the environment by evaporative cooling called transpiration in which they release water vapor through their leaves. Of course there are other reasons cities are hot. Air conditioners, industrial processes, and vehicles all generate heat. An unshaded bus stop is hot but even hotter when the bus is idling next to it.

All this heat can be bad for people. Heat alone poses a risk to human health as does exposure to solar radiation and pollutants that become more concentrated in hot areas. So why don’t cities plant more trees? Many of them do and try to preserve the trees they have. Unfortunately, arthropod pests are more abundant on urban trees and urban tree survival is low.

In two papers released today, Adam Dale, PhD student extraordinaire, has tackled the questions of why herbivores are more abundant on urban trees and what are the consequences for urban tree health. Adam works on gloomy scale, Melanaspis tenebricosa, an armored scale that feeds on almost every red maple within city limits (go look at the closest red maple, then get back to work).

All the gray bumps on this trunk are gloomy scales sucking nutrients from the tree. Photo: SD Frank

All the gray bumps on this trunk are gloomy scales sucking nutrients from the tree. For more (better) pictures visit Adam’s picture gallery featured in the Bulletin of the Ecological Society of America. Photo: SD Frank

In his first “Urban warming trumps natural enemy regulation of herbivorous pests” published in Ecological Applications he shows that urban warming seems to be the primary factor associated with gloomy scale abundance on urban trees. He supports this by identifying an amazing physiological mechanism: scales at warm sites can have 3 times as many babies as scales at sites just 2.5 degrees cooler!

Adam came up with a way to count gloomy scale embryos to determine that warm scales produce more babies. Photo: AG Dale

Adam came up with a way to count gloomy scale embryos to determine that warm scales produce more babies. For more pictures of scale embryos visit Adam’s picture gallery featured in the Bulletin of the Ecological Society of America. Photo: AG Dale

Adam’s next question was: So what about the trees? Do scale insects and temperature increase plant stress or reduce tree growth? This is what urban foresters need to know if they are going to make management decisions. Why manage scales if the heat kills trees anyway? Adam’s second paper “The effects of urban warming on herbivore abundance and street tree condition” in PloS One shows that both scale insects and heat are associated with poor tree condition. This means trees with scales and particularly hot trees with scales are more likely to have dead branches, sparse foliage, and generally look worse that cool trees without scales.

Urbanization is increasing and a new paper from Adam Terando and colleagues from NCSU and the USGS Southeast Climate Science Center suggests urbanization will expand more than previously thought. See a piece on The Rise of Charlanta by Rob Dunn. You notice in the image of Lawyer’s Hill that subdivision construction is underway. Each of these house will get a lollipop tree, probably a red maple or worse an ornamental plum, but the canopy will never be restored. To conserve trees and their valuable benefits for human and environmental health we need to understand even more about why pests become more abundant on urban trees and which trees should be planted to establish resilient urban forests. Its clear from Adam’s work that red maples are not a good choice for hot southern cities.

A gallery of photographs of Adam’s research was featured in the Bulletin of the Ecological Society of America.

New banker plant paper!

Sarah Jandricic, a postdoc in the lab, just published a paper titled “The effect of banker plant species on the fitness of Aphidius colemani Viereck and its aphid host (Rhopalosiphum padi L.)” in Biological Control.

Effects of grain species on wasp abundance.

Effects of grain species on wasp abundance.

The paper describes the effects of different grain species on aphid and parasitoid wasp development and fitness. The goal was to identify which gain species would make the best banker plant for greenhouse growers. Each grain, barley, wheat, rye, and oats, had different effects on aphid and parasitoid development time, fecundity, abundance, and other measures. However, wheat and barley consistently produced parasitoids with high fitness and in great abundance. Importantly, cultivar within species had little effect so whatever cultivar grows best in your region should work.

Two new cankerworm papers

Last week we had two papers published about cankerworms. Cankerworms are caterpillars that defoliate oaks and other tree species.  Cankerworms outbreaks have

Cankerworm larva. Photo: SD Frank

Cankerworm larva. Photo: SD Frank

become increasingly common in urban areas like Charlotte, Durham, and Raleigh. Damage can be extensive.  Many trees on campus were completely defoliated. In Charlotte we have seen entire neighborhoods defoliated. Trees can only tolerate two or three consecutive years of defoliation before their growth declines or they die. More about cankerworms here.

The first paper, published in Arboriculture and Urban Forestry, is “The effect of sticky bands on cankerworm abundance and defoliation in urban trees by Bobby

Bobby and Greg Bryant counting cankerworms on a sticky band. Photo: NCSU Bulletin, http://bulletin.ncsu.edu/2013/01/cankerworm/

Bobby and Greg Bryant counting cankerworms on a sticky band. Photo: NCSU Bulletin, http://bulletin.ncsu.edu/2013/01/cankerworm/

Chanthammavong who has worked in my lab for almost two years.  He conducted this research as an undergraduate during the cankerworm outbreak last year.  He wanted to test the efficacy of sticky bands, a non-insecticide management tactic, to help reduce cankerworm damage to campus trees. Sticky bands exploit an unusual aspect of cankerworm biology: the females moths are wingless. Therefore they have to climb up trees to lay eggs.  Sticky bands are wrapped around trees to capture the moths as they climb. If they never reach the canopy to lay eggs then you shouldn’t have cankerworms or damage come spring.  

Sticky bands worked fairly well and reduced cankerworm abundance in the trees but did

Maple partially defoliated by cankerworms. Photo: SD Frank

Maple partially defoliated by cankerworms. Photo: SD Frank

not reduce overall defoliation. Lots of cities including Charlotte and Durham, NC use sticky bands each year on hundreds or thousands of trees. Since the tiny caterpillars can float from tree to tree (entomologists call it ballooning) banding at a large scale probably works better than banding only a few of the trees in an area. Bobby was supported in part by an NCSU Undergraduate Research Grant. His work appeared in the Bulletin and Technician.

The second paper “Bad neighbors: urban habitats increase cankerworm damage to non-host understory plants” in Urban Ecosystems tests the hypothesis that cankerworm damage to plants is greater in urban settings than in forests.  Cankerworms often drop from host trees and land on understory plants or the ground.  We found that in urban settings where plant communities and habitat were simple (just shrubs and mulch in landscape beds) the shrubs below cankerworm infested trees were

Native dogwood damaged by cankerworms in an urban landscape. Photo: SD Frank

Native dogwood damaged by cankerworms in an urban landscape. Photo: SD Frank

defoliated more than the same species of shrubs in forests where plant communities were more  complex (shrubs, ground cover, leaf litter, etc).  There is a caveat though: cankerworms damage native plant species more than exotic, ornamental species. As cankerworm outbreaks become more frequent and severe in urban areas they will damage more than just their primary hosts but damage may be reduced by diversifying the plants and structure of urban landscapes. This work was funded in part by the Southeast Climate Science Center.

These papers address our goals of understanding the ecology of urban ecosystems, why pest damage is often more severe in urban landscapes, and developing management tools to protect urban trees.

Recent parasitoid paper: Editor’s Choice

Sara Prado, an esteemed alumni of our lab, just had her paper selected as Editor’s ChoiceScreen Shot 2014-04-15 at 10.38.00 AM in Ecological Entomology. When given the choice, parasitoids select the hosts that will increase offspring fitness. Thanks Mom! It turns out this has benefits for biological control in greenhouses because the parasitoids prefer the green peach aphid, an important greenhouse pest, over grain aphids introduced on banker plants.

Nice job Prado!

Read the whole paper here.