Study finds urban warming increases scale insect populations and reduces tree health

A study published this past March revealed that urban warming increases the abundance of some tree pests, while decreasing tree health.

North Carolina State University researchers Adam Dale and Steven Frank tested the hypothesis that warm temperatures stimulates the reproduction of herbivorous pests such as armored scale insects (Melanaspis tenebricosa), while also increasing water stress and decreasing tree health. They studied several populations of red maple (Acer rubrum) trees in the city of Raleigh, NC.

They based their study on three objectives:

  1. Determine the effect of temperature on M. tenebricosa abundance
  2. Determine how temperature and M. tenebricosa abundance affect tree water potential and growth
  3. Determine how temperature and M. tenebricosa abundance affect tree condition.

Tree condition was measured on a scale, from “excellent” health—those with healthy foliage and full canopies—to “poor” health—those that had many dead or broken branches, exposed roots and crispy leaves.

As they measured the relationships of temperature on other factors, they discovered a positive relationship between temperatures and M. tenebricosa abundance, and a negative relationship between temperatures and scale insect populations on tree water potential and growth.

In fact, data over a 7-month time period showed that temperature and scale abundance could predict water potential.

Data also showed that as scale insect abundance increased, the likelihood of finding a tree in poor condition also increased. Although only 10% of the red maples were in poor condition, they were nearly always in a warmer area. However, 90% of the trees were in less than excellent condition. Over 63% of trees in poor condition were located in warm areas.

In general, Drs. Dale and Frank found that warming temperatures and scale abundance reduced tree condition.

Source: Dale, A.G., and Frank, S.D. (2014). The effects of urban warming on herbivore abundance and street tree condition. PLoS ONE. 9(7): e102996 doi: 10.1371/journalpone.0102996.

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