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The effect of the "urban heat island" on pest development

Pests are often more abundant in urban areas than in rural areas, although changes in host plants and the loss of effectiveness of natural parasites do not always explain this abundance. A study conducted in United States, showed that the increase in temperature could affect the presence of a scale, Parthenolecanium quercifex, present in Mexico, Canada, United States of America
Heat: insects are more adaptable. Urban areas are generally warmer than the surrounding countryside. The so-called "urban heat island" it’s caused by the lower presence of vegetation cover and by the extensive coverage of impermeable surfaces, which sometimes is close to 100%. The phenomenon of urban heating was noticed for the first time in 1833, but the effects of the heat on the characteristics and abundance of some animals in cities remain largely unknown.
High urban temperatures should have more pronounced effects on ectothermic beings, which would result promoted in the development. In this category we also find insects, of particular interest for their role as pollinators, disease vectors and plants pests.
Search by hypothesis. The researchers hypothesized that the effect of "heat island" might be the most important driver in the development of the phenomenon.
To prove this statement, they studied the effects of warming on the biology of Parthenolecanium quercifex, a sedentary scale insect pest.
Researchers have formulated four hypotheses:
  1. P. quercifex is more abundant in cities than in rural areas;
  2. urban heating increases the presence of P. quercifex  because it decreases the parasitoids presence and activity;
  3. urban heating increases P. quercifex because it increases its fertility;
  4. P. quercifex has an adaptive and physiological advantage when it comes from warmer zones rather than when it is collected in cold areas;
To demonstrate the first two hypotheses detection data were overlaid, both for the insect larvae of parasitoids, in the thermal maps. This made possible to compare between the hotter urban areas and cooler areas outside the city.
For the third hypothesis the researchers collected the ovisacs in areas with different temperatures and counted the number of eggs present with the use of macrophotography.
For the last statement studies have been conducted in the greenhouse by comparing the physiological differences of scale insects born from egg masses from cold greenhouses or warm greenhouses at various temperatures.
High adaptability and physiological benefits. The researchers found that, in the same generation, the deposed ovisacs were 5.5 times more abundant on trees exposed to warmer temperatures and, in the next generation, up to 7 times higher.
The percentage of parasitized eggs did not differ between warmer and cooler sites, as well as the number of eggs present in the ovisacs. So the study failed to demonstrate the validity of the second and third hypothesis. As for the fourth and final hypothesis, scale insects collected from the warm greenhouses were generally favoured (more abundant) when inserted in the new environments than those collected in the cold greenhouse.
Trees in the urban environment are often affected by water stress and lack of nutrients derived from the soil conditions which are not optimal if not very hostile. This situation worsens the overall health of the plants, which are then more susceptible to parasites and pathogens.
Scale insects, according to some studies, should be favoured by water stress because, feeding on the elaborated sap, which transport significantly worsens in water shortage. No difference was observed instead.
No reduction of the presence of natural parasites was observed, so even this factor does not explain the abundance of P. quercifex in the warmer sites.
This study also showed a high capacity of this parasite to adapt to warming, whether urban or global: it has been observed an increase in the presence of the insect with warmer temperatures.
The heating causes general stress to trees which combined with a possible increase of the parasites derived from their high adaptability, is a major threat to the health of urban systems we must deal with.

Research shows a high adaptation capacity to warmer conditions of Parthenolecanium quercifex a scale which attacks oak species. However other stresses can be the inciting factors