As spiders leave the kitchen, pests keep cooking

A spider in the family Anyphaenidae has made its home on a twig infested with scale insects.  Photo: Emily Meineke, Harvard University

I think by now most people accept that we can’t hope to preserve all extant creatures over the next 50 or 100 years. Global changes in temperature and habitat will help some species and hurt others, as Elsa Youngsteadt showed in her recent paper. Since we can’t save every creature, what is really important to protect? Increasingly, people try to understand and protect species and ecological interactions that generate ecosystem services for people, rather than diversity per se.

Former undergraduate researcher Anna Holmquist examines branches in the field. Photo: Emily Meineke, Harvard University

Urban warming makes street tree temperatures similar to what is expected under climate change, so we have studied them to predict the effects of warming – urban and global – on pest abundance and tree health. Street trees also host a surprising amount of arthropod diversity if you just look hard enough. In a new paper, our former graduate and undergraduate students, Emily Meineke and Anna Holmquist, with help from Gina Wimp at GWU, studied the effects of warming on spider communities in street tree canopies.

The team tested two predictions. Spiders like to eat and often become more abundant in places where prey is more abundant. So we predicted that, since heat increases herbivore abundance, spider abundance would follow. However, because some spiders probably benefit from warming while others do not, we predicted the composition (member species) of the spider community would be different in hot and cool trees.

The fitness of this spider probably increases with warming since it is hot and sweaty from exercise and yoga. Other spiders (not pictured, you can only work kids so hard) die in, or leave, hot places. Thus, yoga spiders will be more common on hot trees and the community composition will change. Artwork by: I.F.

Ghost spiders, like this one, are nondescript but perform important ecosystem functions. Photo: Matt Bertone, NCSU.

Spiders were by far the most abundant natural enemy group. However, as herbivore abundance increased with warming, spider abundance stayed the same. This is bad news for trees because it means that herbivores can increase unchecked. Instead, urban warming altered spider community structure due in part to a whole family of spiders, Anyphaenids — aptly named ghost spiders – virtually disappearing from the hottest trees in one year of the study. This is bad news for conserving urban biodiversity and also because ghost spiders feed on particular pests like lace bugs.

In this experiment, warming reduced biodiversity but also likely reduces biological control by predators, an important ecosystem service. Something happens in these trees to make a common ecological interaction – predators congregating to prey – stop happening. The consequence is that pests go nuts and trees suffer.

Read the full paper here:
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.

January 26th, 2017|Categories: Feature, Natural Enemies, Urban Ecology|Tags: , , , |

Who wins and loses with warming? Where you live matters.

Climate change is generally considered bad for people, earth’s biomes, and, of course, polar bears. But as the climate warms will all critters suffer? Will they all be affected the same way? No. In addition to the losers who slowly fizzle out under the oppressive heat, there will be winners who benefit from warming.

An animal’s response to climate change depends largely on two things: the amount of warming in a habitat and the physiological limits of the animal. It has been shown pretty convincingly that animals closer to the equator are more sensitive to warming than animals farther north. I know what you are thinking, “but tropical animals are hot all the time, they should be used to it.” I thought the same thing, but how it works is that since they are hot all the time, they live close to their thermal limits. So for animals in hot places, a little more heat pushes them over the edge.

Therefore the biological effects of climate change are expected to vary geographically, particularly for ectothermic animals such as insects. Elsa Youngsteadt and other folks in the lab took a road trip to test the hypothesis that insects at high latitudes, where it is cold, should generally benefit from warming whereas insects at low latitudes should have mixed responses: some should benefit, but others should be pushed over their thermal limits.

In a brilliant new paper Elsa reports her findings from this trip. The team sampled insects from street trees in the hottest and coolest parts of four cities–Raleigh, Baltimore, Queens, and Boston–taking advantage of the urban heat island effect as a natural warming experiment.

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Four cities at different latitudes were chosen to study warming effects on insect communities. Background map from the National Biomass and Carbon Dataset.

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One of the authors, Andrew Ernst, takes measurements at a typical study tree. Photo: E.K. Youngsteadt


In the lowest latitude city, Raleigh, some taxa became more abundant with warming while others declined. This suggests that, although some species benefited from warming, just as many species suffered. In the coldest and highest latitude city, Boston, most insect groups were unaffected or became more abundant, suggesting that warming was good for most species living in a frigid northern metropolis. Just as predicted! This doesn’t happen very often.

stickycard

Yellow sticky cards were used to sample insect communities in urban trees. Photo: E.K. Youngsteadt.

It seems good that not all taxa tank in Raleigh–but the fact that some benefit and others decline could be ecologically disruptive, too: Maybe a parasitoid and its host respond differently, or a predator and its prey. This sort of mismatch could lead to extinction of higher trophic levels if the prey does poorly, or herbivore outbreaks if the predator fails.

I’ll warn you upfront, this paper is dense and there are probably a lot of new concepts packed in that most people will need time to unpack. However, capturing the response of a whole community to a couple degrees of warming is novel and worth the read. Think about the responses of your favorite organisms. Not just in cities but across the globe.

Read the paper here.

December 16th, 2016|Categories: Urban Ecology|Tags: , , , |

New paper: Urban warming reduces aboveground carbon storage

This is a guest post from our former student (now postdoc at Harvard) Emily Meineke.

Through years of studying urban trees and the insects that eat them, we, the Frank lab, have discovered that warming in cities leads to more pests. We also know how: where it’s warmer, insects survive and reproduce better, and the effects of their natural enemies are diminished. In most conversations we have about this work, explaining these discoveries leads to the question: but what does this mean for the trees?

Street trees perform essential services like removing pollutants from air. Photo: EK Meineke

Street trees perform essential services like removing pollutants from air. Photo: EK Meineke

I tackled this question with the help of Elsa Youngsteadt by studying how warming and pests affect tree drought stress and functions like photosynthesis and stomatal conductance. Of course, as in my previous work, I studied the charmless but interesting oak lecanium scale on willow oaks which are among the largest and most common street trees in Southeastern cities.

Oak lecanium scales on willow oak. Photo: EK Meineke

Oak lecanium scales on willow oak. Photo: EK Meineke

Over three years we took hundreds of tedious measurements (thanks Elsa!) to figure out how fast our trees were growing and thus how much carbon they were removing from the air and storing in their tissue. This is called carbon sequestration and is a critical way trees reduce carbon pollution and global warming.

elsaphoto1-copy

Elsa measuring photosynthesis. Photo: EK Meineke

In a new paper, we show that the urban heat island effect significantly reduces street tree growth. This is because trees in warmer urban areas photosynthesize less. When these effects were scaled up to all the willow oak street trees in Raleigh, warming reduced citywide carbon sequestration by 12%. However, insect pests like scales and spider mites had minor effects on tree growth compared to warming, at least in the short term.

Oak spider mites damage leaf cells and reduce photosynthesis. Photo: EK Meineke

Oak spider mites damage leaf cells and reduce photosynthesis. Photo: EK Meineke and A Ernst

These results lead to several recommendations for urban forest management. First, because urban and global warming are becoming more intense, urban trees will store even less carbon in the future. However, managers may be able to reduce these effects by planting trees that are more tolerant of hot urban conditions. This highlights the need for research to identify what trees are appropriate to plant in hot urban environments. In general, this research makes us excited about science that will help landscape designers tailor green infrastructure for resilience to climate change and intensifying urbanization.

Our results also highlight the utility of cities as large-scale natural climate experiments, in which sessile organisms, such as trees and many insect herbivores, are confined to different thermal environments in close proximity. The range of urban warming they experience parallels the extent of global warming expected regionally, outside the city, over the next several decades. Therefore, cities can serve as experiments that allow scientists to address questions that are otherwise difficult or impossible to approach, such as the effects of warming on mature trees.

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 DOI: 10.1098/rspb.2016.1574

October 7th, 2016|Categories: Urban Ecology|Tags: , , , , |

Impervious surface cover is bad for trees. How much is too much?

gloomies

Gloomy scales on red maple. Photo: AG Dale

We have studied the effects of urban warming and other factors on tree pests and tree health for several years. The gist of it is impervious surfaces increase plant stress by warming the atmosphere and reducing water availability. Adam Dale and Elsa Youngsteadt studied the effects of impervious surface cover on red maples to determine how much is too much? In a new paper they answer this question to create an impervious surface threshold that planners and planters can use to determine if sites are suitable for red maples. Their analyses of impervious surface cover and red maple condition in Raleigh, NC indicate that red maple condition is most likely to be excellent or good if impervious surface cover is less that 32% within a 25m radius. At 33% to 66% impervious surface cover, trees were most likely to be in fair condition. Above 66% impervious surface cover, trees were mostly in poor condition.

 

Good to know but how do you measure impervious surface cover? Not many landscapers are going to pull up satelite images on their phones and bust out ArcGIS to measure the amount of impervious surface around a tree. Instead we came up with the Pace to Plant technique. With this technique anyone can acurrately measure impervious surface cover at 25 m radius just by pacing transects and counting the steps that fall on impervious surfaces.

pacetoplant

With an impervious surface threshold in hand hopefully landscape architects and other planners will not specify red maples on plans when impervious surface cover is high. Tree care professionals on the ground will also be able to assess if a planting site is suitable for red maples. Two small (even medium) steps for urban tree IPM.

May 17th, 2016|Categories: Landscape IPM, Urban Ecology|Tags: , , , |

High school intern wins state science fair

Some of NCSSM's award winners at the 2016 North Carolina Science and Engineering Fair. From NCSSM http://www.ncssm.edu/news/2016/04/04/2016-regional-science-fair-winners

Some of NCSSM’s award winners at the 2016 North Carolina Science and Engineering Fair. From NCSSM http://www.ncssm.edu/news/2016/04/04/2016-regional-science-fair-winners

Congratulations to Kimberly Andreassen for winning the Biological Science division of the North Carolina Science and Engineering Fair with her project “The effects of climate change on evergreen bagworm development and immunity.” Andrea attends the North Carolina School of Science and Mathematics. She spent last summer working in our lab, mentored by Warren Sconiers, to determine how different temperatures affect bagworm development and immune response. Now Kimberly is off to the Intel International Science and Engineering Fair in Phoenix, AZ to present her research again. Good luck Kimberly! Congratulations to Kimberly and Warren for their great project.

May 12th, 2016|Categories: Lab Happenings|Tags: |

Urban environments increase pathogen pressure on honey bees

bee2_elsa1Our lab’s latest paper, co-authored by Elsa Youngsteadt and Holden Appler, was published today in PLOS ONE. We examined pathogen pressure and immune response in managed and feral honey bee workers from hives located in urban* and non-urban environments. We found some very interesting results, and as science usually goes, we now have a lot more questions.

The urban bees we examined in the study, regardless of whether they were feral or managed, had higher levels of the fungal pathogen Nosema ceranae and Black Queen Cell Virus. We also tested the survival of urban bees in the lab and compared it to their more rural neighbors. Survival for bees in the most urban environments was three times lower than for those in the most rural environments.

Given the stress factors that urban settings present to foraging bees (such as pollution and higher temps), it’s easy to imagine that a compromised immune system in the urbanites might be the culprit. Kinda like when you get super stressed and stay up all night cramming for that big exam (or partying) and find yourself with a cold a few days later. But, our data didn’t support this idea; we didn’t observe a stronger immune response in rural bees relative to urban bees. Something else seems to be contributing to the higher pathogen pressures we saw in the urban bees.

We hypothesized that that ‘something else’ could be urban factors working in favor of the pathogens, making them more abundant or easier to transmit between honey bee workers from different hives. Higher frequency of worker visits at scarce urban food sources could increase the likelihood that bees will pick up diseases from their environment (think the public water fountain or the notorious buffet line). The fungal pathogen in question, N. ceranae, has also been shown to benefit from the warmer temps we see in cities.

This study sets the stage for so many more questions. If urban environments indeed enhance pathogen survival and change the way diseases spread through honey bee populations, is this a red flag for native bee species that share floral and other resources in these cramped urban landscapes? Is urbanization harming them too? Could pathogens jump from honey bees to native bees because they are more abundant or doing better in the city environment? April Hamblin and Margarita López-Uribe are looking into some of these questions and trying to tease out the effects urban living has on our neighborhood native bees.

Check out the paper for some more interesting findings not covered here, and stay tuned for more to come in the native bee department.

*the level of “urban-ness” for each hive was determined using the amount of impermeable surface (concrete, pavement, etc.) in the typical radius a worker bee flies from the hive, 1500 m.

November 4th, 2015|Categories: Lab Happenings, Pollinators|Tags: , , |

Gloomy scale crawlers are active and vulnerable

Adult gloomy scale. Photo: SD Frank

Adult gloomy scale. Photo: SD Frank

Gloomy scale, Melanaspis tenebricosa, is an armored scale that feeds on maples and other tree species. It becomes very abundant on red maples on streets and in landscapes and can cause branch dieback and tree death in some cases. It is not unusual to find trees with nearly 100% of their trunk covered in scale. Street trees are particularly prone to gloomy scale. Crawlers of this scale are active now and can be seen on bark and under scale covers. One of the reasons we have found this to be such a pest is that female gloomy scales produce about 3 times as many eggs when they live on relatively warm trees (like in a parking lot) than when they live on cooler trees (like in a shady yard). This amazing work is outlined in a recent paper by Adam Dale.

Control of this scale is complicated because crawlers emerge over 6-8 weeks so it is impossible to treat all the crawlers at once with horticultural oil or other contact insecticide. This is different than in other scales, such as euonymus scale, in which all crawlers are produced within a narrow window of 2 weeks or so. Adam Dale took a video of some gloomy scale crawlers so you can get an idea of how tiny and nondescript they are. This may also give you an idea of why scales are so vulnerable at this stage to the environment, predators, and insecticides like horticultural oil. Once they produce their thick waxy cover they are much less vulnerable to all these factors.

May 21st, 2015|Categories: Feature, Landscape IPM, Urban Ecology|Tags: , , , , |

Ants eat your crumbs. What’s the fuss?

This is a post by our Research Associate Elsa Youngsteadt about her new paper in Global Change Biology.

The first time we came back to an empty cage in Highbridge Park, I thought there was a problem.

This was a cage cobbled together out of a fry basket from a restaurant supply store plus a square of hardware cloth, and it was firmly tacked to the ground with landscape staples. With its snug, quarter-inch mesh, it should let most insects move freely, while keeping vertebrates out. With holes any bigger than a quarter-inch, mice could squirm through.

Exclusion cage made from a fry basket to keep food scraps safe from rats. Photo: EK Youngsteadt

Exclusion cage made from a fry basket to keep food scraps safe from rats. Photo: EK Youngsteadt

And they would want to, because the cage held a chunk of Nilla Wafer, a Ruffles Original potato chip, and a slice of Oscar Meyer turkey frank. Yum.

The purpose of the cage was to let us measure how much arthropods* alone were capable of eating in this Manhattan park, without interference from mice, rats, sparrows, and whatever other urban critters might shuffle by with the munchies.

(*Arthropods meaning “bugs,” like sowbugs, mites, millipedes, and insects—including ants.)

So I was dubious when we found an empty cage in Highbridge Park on the first day of our study. Could bugs alone have eaten our little pile of food in a day? When we did a trial run back home on the NC State campus, the food never disappeared completely like that. Had a mouse gotten in after all? Had a human passer-by toyed with our experiment?

So we tried again. We moved our setup to a new spot nearby, tucking it discreetly among the trees and undergrowth behind the ball field. We fortified the cage until it bristled with cable ties and landscape pins. And came back a day later.

This time, our arrival was timely. The cage was surrounded by a silent, busy swarm of pavement ants, trundling off the last fragments of potato chip, the last few cookie crumbs. Ants. Not mice, not tampering humans; ants.

This was a scene we would see over and over in the next several days. And maybe we shouldn’t have been surprised.

Ants feasting on hotdog, chip, and cookie in the cage. Photo: EK Youngsteadt

Ants feasting on hotdog, chip, and cookie in the cage. Photo: EK Youngsteadt

After all, ants are notorious visitors to kitchens and picnics, and they are incredibly important scavengers and hunters in natural habitats all over the world (except Antarctica).

But it seems that I’m not the only one impressed by our tiny urban neighbors. Our study of garbage-eating ants was published today in Global Change Biology, and The New York Times, The Washington Post, National Geographic, and Wired have already published their versions of the story. More news in the works from Germany to South Korea, and I’ve described our study to more than a dozen reporters this week.

This process has been delightful and a little exhausting and even puzzling. I mean, sure, I was surprised when ants cleaned up the food the first time—but everybody else is amazed, too? It’s news that ants eat crumbs?

Or is it news that some scientists were nutty enough to weigh hot dogs and potato chips and set them out in experiments around the most densely populated city in the US–in Central Park, in the middle of Broadway, in view of the Statue of Liberty?

Or is it news that ants are competing with rats for this junk we drop? That this little drama plays out over the crumbs to which we never gave a second thought?

I’ll let you decide. Here’s what we found—the bullet-point version:

  1. Bugs in street medians ate two to three times more junk food than those in parks.
    Co-author Ryanna Henderson sampling a median. Photo: EK Youngsteadt

    Co-author Ryanna Henderson sampling a median. Photo: EK Youngsteadt

    Street medians are little strips of trees and groundcover planted between lanes of traffic, and they host fewer ant species than do parks. Ecological theory predicts that more diverse ecosystems are more efficient, so we were surprised that bugs in medians ate more than those in parks. We indulged in some extrapolation and calculated that the bugs inhabiting the 150 city blocks of medians on Broadway and West Street are capable of eating more than a ton of food per year–the equivalent of 60,000 hot dogs.

  1. Pavement ants (Tetramorium species E) are big eaters. We didn’t watch our cages for 24 hours to see exactly which species collected how many crumbs—but at sites where we found pavement ants, more got eaten. Pavement ants originated in southern Europe and the Middle East, but have been living in American cities for more than a century. They love nesting near pavement, so they’re more common in medians than in parks. (But Highbridge Park had them, explaining its big, median-like appetite.)
  1. Bugs compete with vertebrates (rats!) for our food garbage. Alongside our cages, we offered a second set of uncaged foods, available not only to bugs but also to any hungry vertebrate. More food got eaten from this open treatment than from the cage. That means that both groups of animals (bugs and vertebrates) want the stuff we drop. In other words, they compete for it; what one group gets, the other group doesn’t. I would love to know how much of that uncaged food went to ants and how much to rats–and whether that depended on the kind of food, the size of the food, the time of day, the habitat… But those are goodies for another study someday.
  1. Hurricane Sandy was a disaster, but not for garbage-eating bugs. Several of our study sites were inundated with saltwater during Hurricane Sandy, seven months before our study. We thought this would matter to the wildlife. But we detected no effect of flooding on how much food got eaten. Maybe there was still an effect on some kinds of bugs—we’re still figuring that out—but certainly not on those that eat our crumbs.

 

I admit I’ve often tossed an apple core into the woods, not even thinking of it as litter. It’s “biodegradable,” so it doesn’t count, right? Now, my friends, we have met our urban biodegraders. To them it counts, and their work behind the scenes is making our garbage disappear.

Comic by Dorit Eliyahu  https://sites.google.com/site/doriteliyahu/home

Comic by Dorit Eliyahu
https://sites.google.com/site/doriteliyahu/home

December 2nd, 2014|Categories: Feature, Lab Happenings, Natural History and Scientific Adventures, Urban Ecology|Tags: , |

Do scary parasites mean my yard is healthy?

This is a post by our Research Associate Elsa Youngsteadt about some rare parasites she found.

In a lab full of people quietly staring through microscopes, a startled yelp occasionally breaks the silence. And I admit, it sometimes comes from me. Diving into the magnified world can be a lot like watching a scary movie on a big screen.

Usually the freak-out happens when I’m doing something soothingly monotonous, like counting tiny scale insects on leaves. I get in a groove and forget that I’m seeing everything 30 times bigger than it really is. Then a spider dashes crazily across the field of view: It appears enormous, threatening, bristly, unpredictable, and very, very close. This is when the yelping happens. But of course the beast is actually smaller than a pencil eraser and nowhere near my face, so I feel silly, compose myself, and get back to the scale insects.

But sometimes, the startling thing is something I’ve never seen before, something wild and creepy and totally distracting. This is what happened earlier this year when I settled in to identify some newly collected bees from my yard. These were dead bees, mind you, fresh out of a cyanide jar and expected to sit still and not be startling.

The first few toed the line. There was a shiny, bulbous, blue-green mason bee, and a plain little matte-black sweat bee. A ground-nesting Andrena still had flakes of clay on her jaws. All normal and non-threatening.

Andrena bee with strepsipterans peeking out of its abdomen. Photo: Matt Bertone

Andrena bee with strepsipterans peeking out of its abdomen. Photo: Matt Bertone

Then, the yelp. Spreading across the rear end of the next bee was a mass of very alive, squirmy, maggoty little larvae, looking not-so-very little. I knew in split second what they were, but I admit to quickly popping the bee back in the cyanide jar to finish things off before looking again.

I was witnessing the birth of a brood of strepsipterans—a group of parasites whose wacky life cycle involves a weird, blobby insect eating a live bee from the inside out, while that insect’s own babies eat her from the inside. At the time I caught my bee, the strepsipterans had just finished feeding on their mother and emerged into the world, via a hole in their mama’s head (technically, in her cephalothorax). How else?

Had the bee avoided my net and continued to fly around visiting plants, the larvae would have hopped off on the next flower. (Really, hopped: Despite the initial maggoty impression, these are leggy, nimble little insect tykes.) Each one would have waited at the flower-airport to board a fresh bee, which would carry it back to an underground nest tunnel. There, the strepsipteran would find the bee’s egg or larva and burrow in—disappearing inside within a few hours.

The strepsipteran then loses its legs and eyes and grows along with its young bee host. Eventually it fills the bee’s abdomen, stunting its organs and altering its development. Female Andrena bees, for example, develop the angular cheeks and hairy abdomens typical of males. (They also become very hard to identify.) One researcher even reported finding a bee with a single, giant eye arching over its head like a headband–perhaps a strepsipteran-induced developmental glitch.

Strepsipterans emerging from Andrena bee abdomen. Photo: Andrew Ernst

Strepsipterans emerging from Andrena bee abdomen. Photo: Matt Bertone

As adults, male strepsipterans have wings and return the outside world to fly around for a few hours before they die. Females, however, remain ensconced in their bees, with only their heads visible from the outside. (That’s the smooth, yellowish tissue you can see sticking out of the bee’s abdomen in the photos.)

The rest of her body is “a great sack full of eggs,” wrote entomologist W. Dwight Pierce in 1909. The thousands of eggs simply float around inside the mother’s body, absorbing nutrients from her blood. Eventually, they hatch and trek to the outside world via the “brood canal” that exits their mother’s head. Which is where they were when they made me shriek.

This whole stranger-than-fiction setup is more than just a titillating insect freak-show. As parasites, strepsipterans may have an important role in maintaining insect diversity. Parasites tend to regulate their hosts’ populations in ways that prevent any one species from becoming outrageously common—leaving room for more species overall. That process keeps ecosystems diverse and food webs stable.

Stepsipteran larvae emerging. Photo: Andrew Ernst

Stepsipteran larvae emerging. Photo: Andrew Ernst

I would bet that strepsipterans are among the parasites that help maintain diversity, although their role has never been measured. There are about 600 species of strepsipterans in the world, and they can also be found in planthoppers, cockroaches, grasshoppers and wasps. In the bee and wasp species that have been checked, 10 to 30% of individuals carry strepsipterans and are unlikely to reproduce.

That means strepsipterans are common enough to regulate the abundance of their hosts. In a tantalizing example, the European paper wasp (Polistes dominula) left its strepsipterans behind when it moved across the Atlantic to North America—and none of the North American species will parasitize it. So researchers have suggested this wasp’s fast reproduction and dominance over native North American wasps may be due, in part, to the loss of its parasites.

Of course I have no way of knowing where my one bee with her alarming brood of squirmy strepsipterans fits into the big picture. But I’d like to think she’s a good sign. I’d like to think that if I kept looking I would find that my messy yard is teeming with a diversity of slightly creepy parasites, keeping their hosts in line and yielding an endless supply of microscopic scary movies.

November 25th, 2014|Categories: Feature, Natural Enemies, Natural History and Scientific Adventures, Pollinators|Tags: |

Cities reduce zombie mothers: How pests escape their parasitoids in cities

Oak lecanium scale eggs within an ovisac. Photo: SD Frank

Oak lecanium scale eggs within an ovisac. Photo: SD Frank

In the Spring of 2011, I was a new Entomology graduate student with no prior experience with insects except the typical ant and bumble bee watching during my childhood. So I spent most nights early in my studies with my face in a microscope looking at scale insects. Scale insects are tree pests, literal bumps on a log that use their straw-like mouths to suck sap from trees. My adviser, Steve Frank, wanted me to figure out if scale insects were developing faster in warmer parts of the city. He wanted me to do this because scales are bad for the trees they live on. Trees we plant to line our streets and keep our city environment livable.

He also wanted me to look into this because insects that develop faster often have an advantage. We had already found that the scale insects were receiving an advantage of some kind in the hottest urban places; oak tree pests called oak lecanium scale insects – sexy name, I know– are around 12 times more abundant where it’s hot in the city of Raleigh than in nearby cooler spots. In other words, if you’re a Raleighite, there are more scale insects where you eat downtown than at your kid’s school (unless your kid goes to school downtown, I guess).

Parasioid larvae in lecanium scale ovisac. Photo: E.K. Meineke

Parasioid larvae in lecanium scale ovisac. Photo: E.K. Meineke

As the season progressed, and the scales started to develop, I noticed they looked pregnant. And not pregnant with their own eggs, a bad kind of pregnant. So I used tiny dissecting needles to see what was going on inside them and found larvae. They were translucent and barely moved. Some were bigger than others. All were tiny and fascinating. Scale insects don’t make larvae, so these larvae had to be from another type of animal. Only rarely do animals actually survive within other animals, so this was a really neat discovery. Though I didn’t know while it was happening, it was one of the sweet moments of being a new student: I got to rediscover something amazing that science has known for a long time. I discovered parasitoids again.

Modified by CombineZPParasitoids are tiny insects, often wasps, that drill holes into other insects or spiders and lay egg(s). The egg(s) hatch and develop within the host, eating it from inside. They often make zombies of their hosts, causing them to behave strangely. But in scale insects, they just hang out and steal resources by feeding on their blood.

I quickly noticed something else: the scales from hotter parts of the city that housed parasitoids kept producing lots of eggs, but the scales from cooler parts of the city produced fewer eggs when they were also housing parasitoids. It seemed like biological control by the parasitoids was failing in the hotter parts of the city.

We document this phenomenon in a new paper and show that, while scale insect development speeds up with warming, parasitoid development doesn’t. We also document that parasitoid control of scale insects fails where it’s hot in the city, likely due to the mismatch in development between the scales and their parasitoids. These developmental mismatches happen due to climate change between species that are associated with one another – predators and prey, pollinators and plants—across the globe, on land and in the sea, and this project documents that these same mismatches can happen in cities due to warming caused by sidewalks.