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News Article

Climate change imposes city trees to insect pests


Warming and drought increase scale size and reproduction on maple trees

A new study has found that increased temperature and drought improve scale insect size and number of offspring on urban maple trees, making the insects more likely to succeed as pests. These results should be taken into account when preparing cities for climate change.

The new study published in PLoS ONE last month investigated whether the combined effects of urban warming and drought stress increased the fecundity and population growth of the armoured scale insect, gloomy scale Melanaspis tenebricosa, on Acer rubrum street trees in Raleigh, North Carolina.

“We created a grid of 2x2 km squares, divided the city into equal quadrants, and randomly selected three grid squares per quadrant,” the authors from the University of Florida and North Carolina State University explain. “From each selected grid square, we picked a pair of A. rubrum from the hottest site and a pair from the coldest site resulting in 48 trees at 24 randomly selected sites.” Of each tree pair, one was given 300 litres of water per week, while the other did not receive any additional water. They also measured canopy temperature, xylem water potential, and adult female M. tenebricosa abundance and population growth on each tree, as well as adult female body size and embryo production.

On average, urban trees have heavier herbivorous pest loads than their rural counterparts. Different theories have been proposed to explain this. For example, urban areas may harbour fewer natural enemies. Also, drought stress increases sugar and nitrogen concentrations in plants, which can increase fecundity, development rate, and abundance in their herbivores. Last, urban trees may suffer from higher levels of water and heat stress becoming poorly defended and of better quality for pests. The last hypothesis largely follows from the facts that urban areas are covered with impervious surfaces that do not allow water to penetrate through the ground and into the roots, and that cities are generally warmer than the surrounding areas increasing the trees’ need for water.

A. rubrum is native throughout eastern North America and comprises over 18% of the trees in Raleigh and about 15% of trees in the region’s natural forests. M. tenebricosa is also native to the region and a common pest of Acer trees. Heavily infested trees lose branches, prematurely drop leaves, and often die.

The results showed that drought stress exacerbated the effect of warming. The pest produced over 17% more embryos on the warmest unwatered trees than the warmest watered trees, and over 65% more than on the coolest watered trees. In other words, M. tenebricosa on the warmest unwatered trees produced substantially more successful offspring than those on the coolest watered trees. The female body size also increased with tree canopy temperature and the individuals on unwatered trees were 3% larger than those on watered trees. The study did not find a difference in abundance between watering treatments. The population growth ratios were also not significantly predicted by tree canopy temperature and did not differ between watering treatments.

“As cities expand and the climate changes, warmer temperatures and drought conditions may become more widespread. These changes have direct physiological benefits for M. tenebricosa, and potentially other pests, that may increase their fitness and abundance in urban and natural forests,” the authors explain. They continue, “Understanding how multiple climatic factors like temperature and drought act together to affect insect pests and their host trees is essential for managing forests and ecosystem services under climate change.”

Subscribers to CABI’s Forest Science Database can access more information on insect pests on urban trees and related topics. For example, the search string “urban forestry” AND “insect pests” returns 227 results.

 

Reference:

Dale AG, Frank SD (2017) Warming and drought combine to increase pest insect fitness on urban trees. PLoS ONE 12(3): e0173844. DOI: 10.1371/journal.pone.0173844