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

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

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.

Parasitoids 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.

About the Author:

Emily Meineke
Emily earned her PhD in the Frank Lab in 2016. She’s interested in how human pressures (like urbanization) change the insects that live around us. Find her at emilykmeineke.com.