Urban warming and drought stress – good news for scale insect pests

Have you ever seen tiny, bark-colored bumps all over the trunk or branches of a tree? In a forest? What about in a parking lot or along a roadside? Maybe you haven’t looked, but if you do, the latter of the two locations is where you’ll strike gold.

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Gloomy scale-infested red maple branch. Photo: A.G. Dale

These tiny bumps are scale insects, sap-feeding pests that drain plants of their nutrient reserves. Scale insects are rarely abundant or damaging in rural or natural landscapes, but are drastically more abundant in cities, which reduces the condition and services provided by our most common urban trees.

The scale insect we are particularly interested in is the gloomy scale, Melanaspis tenebricosa. This insect is drastically more abundant on urban red maple (Acer rubrum) trees than those in natural forests. Maples are the most common urban landscape tree in the eastern U.S.

Recent research from my lab and my colleague’s lab at NC State University demonstrates that scale insect phenomenon is due to a combination of factors. Cities are warmer – called urban heat islands – because they are covered in paved surfaces like roads, parking lots, and buildings. Not only does that make them warmer, but it also reduces the amount of water that gets to tree roots. Heat + less water = drought stress.

Previous research has found that drought stress benefits plant-feeding insects by helping them reproduce more and develop more quickly. Separate research from my lab has demonstrated that urban heat directly benefits scale insect pests in a similar fashion. However, until now no study has investigated how drought and heat interact to affect insect pests feeding on established urban trees.

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Irrigation bag on red maple. Photo: A.G. Dale

We tested this interaction over two years by loading up a 300-gallon water tank and driving it all over the city of Raleigh, NC to water established red maple street trees that were infested with gloomy scale. We did this twice a week for two years, and at the end, collected dozens of adult female gloomy scales from each tree. We dissected these females from irrigated and non-irrigated trees and counted the number of embryos that were developing inside of them.

It turns out that gloomy scales produce significantly more offspring as temperatures warm (as previous evidence suggested), but more importantly, that gloomy scales on drought stressed trees produce significantly more offspring than those on watered trees. Not only do heat and drought stress matter individually, but they combine to additively benefit these urban tree pests!

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Gloomy scale egg cells and embryos. Photo: A.G. Dale

Managing scale insects typically relies on insecticides, which is difficult, expensive, and often unsuccessful. Therefore, effective cultural practices that prevent scale insect infestations and damage are the best approach. We have begun to chip away at this by developing impervious surface thresholds that can guide urban tree planting decisions. However, we can’t remove and replant tens of thousands of red maples already in our urban landscapes. This most recent study suggests that irrigating already-planted red maple trees can mitigate the negative effects of heat and drought by reducing the number of offspring these scale insects produce.

As cities continue to grow and the climate continues to warm, factors like heat and drought will become more prevalent and severe for urban trees. Therefore, addressing these issues by making more informed urban tree management decisions is critical to developing more economically and environmentally sustainable urban landscapes.

This article is published in the journal PLoS One. Find the entire article here: http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0173844

Also, a UF press release can be found here: http://news.ifas.ufl.edu/2017/03/gloom-and-doom-when-these-insects-are-on-hot-dry-red-maple-trees/

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