According to the Global Drought Information System (2015), drought is severe in many parts of the world and has been worsened by the El Niño event in 2015. Parts of central Europe are experiencing continued drought, southeastern Asia is particularly hard hit, as are the sub-Saharan and equatorial regions of Africa. Drought is severe throughout much of Australia, with some provinces having received no rainfall in three years. In South America, Brazil and the southern Andes remain trapped in drought.

In North America, drought has intensified in the High Plains and the Midwest, and is expanding in the Maritime Provinces in Canada. The ongoing exceptional drought in California prompted officials to issue mandatory water restrictions for the first time ever in April of 2015.

Drought, along with other aspects of climate change, invasive species of pests, pollution, and features associated with urbanization (including the replacement of green space with impervious surfaces), imperils trees in and around cities.

Here, we examine reasons why and how this happens.

Effects of drought are compounded by impervious surfaces in cities, making ash trees more vulnerable to attack by pests (including clearwing borers).

PHOTO: INTERNATIONAL SOCIETY OF ARBORICULTURE

What does drought mean with respect to trees and arthropod pests, and the insects and mites that consume them?

Several decades ago, scientists around the world noticed that outbreaks of herbivorous insects often followed prolonged periods of unusual weather, including extreme heat or cold, excessive amounts of rainfall, or prolonged drought. After observing drought- associated outbreaks of small sucking insects called psyllids on Eucalyptus in Australia, an eminent ecologist, T.C.R. White (1969), framed the plant-stress hypothesis. This hypothesis states that plants under abiotic stress, such as drought, become a better source of food for herbivorous insects owing to changes in their physiology.

Water deficits are believed to increase the nutritional quality or weaken the natural defenses of plants, thereby increasing herbivore fecundity, development, survival and increasing the likelihood of pest outbreaks (White 1969; D. F. Rhoades 1983; White 1984). In an important review of drought stress and insect outbreaks, W. J. Mattson and R. A. Haack (1987) provided several examples of insect outbreaks on a wide variety of trees following drought. Their list of eruptive pests contained buprestid beetles, including two-lined chestnut borer attacking several genera of hardwoods; longhorned beetles attacking fir; many species of bark beetles attacking fir, pine and spruce; aphids attacking hawthorn and Eucalyptus; sawflies attacking pine; and several species of caterpillars attacking spruce, pine and fir.

Drought-stressed Eucalyptus trees are more likely to succumb to attack by the eucalyptus longhorned borer.

How does drought change plants to favor insect outbreaks?

Mattson and Haack (1987) proposed several mechanisms that might enhance insect outbreaks following drought indirectly by improving plant quality or by directly affecting insect or mite pests. They noted that drought stress and elevated temperatures often accompany each other and jointly conspire to abet pest outbreaks.

Drought alters many facets of plant physiology, including the expression of genes involved with the production of proteins, and rates at which cells divide, enlarge and differentiate. Changes in these fundamental processes may make drought-stressed plants more nutritionally balanced or nutrient rich for herbivorous insects and mites (Mattson and Haack 1987). Mattson and Haack (1987) also argued that plants stressed by drought may be more attractive to pests due to chlorotic coloration (many sucking insects are attracted to yellowing plants) or the low frequency acoustic signals emanating from trees as water columns in xylem elements rupture.

Drought may also enhance emissions of volatile plant compounds, like ethanol, which serve as host – location cues for insects. Due to reductions in photosynthesis and transpirational cooling, drought-stressed plants are warmer than unstressed plants. Mattson and Haack (1987) suggested that elevated temperatures might boost insect detoxification systems, enabling them to better deal with plant defensive compounds or enhance immune function of pests, making them less susceptible to pathogens.

Higher temperatures may also create more favorable conditions for beneficial microbial symbionts to flourish in the insect’s gut, enhancing their digestion and performance. Also, bark beetles that inoculate trees with blue stain fungi may benefit from drought stress if elevated temperatures support more luxuriant growth of the fungi on which beetles feed.

Conversely, drought may inhibit germination of pathogenic fungi that attack insects, thereby lessening the lethal effect beneficial fungi have and limiting their ability to arrest pest outbreaks (Mattson and Haack 1987). Elevated foliar temperatures of drought-stressed trees may accelerate insect and mite development and facilitate their escape from natural enemies, while allowing more generations to be completed in less time (M.J. Raupp 2013, Raupp et al. 2010, 2012). Important recent studies by E.K. Meineke et al. (2013) and A.G. Dale and S.D. Frank (2014) clearly demonstrate that elevated temperatures, which often accompany drought, contribute in important ways to outbreaks of scale insects on a variety of trees in urban landscapes.

The North American Drought Monitor reported extreme and exceptional droughts in many parts of the United States this spring.

Do certain insects and mites differ in their responses to drought stress?

Controversy still swirls around this question, although patterns have begun to emerge from several comprehensive reviews of water stress and its impact on insect and mite pests.

An early synthesis by G.L. Waring and N.S. Cobb (1992) revealed somewhat conflicting results of the effects of drought on insects and mites. An analysis of experimental studies of water stress, where water was experimentally added or denied, found the general response of sucking and chewing insects to be negative; that is, water-stressed plants supported lower survivorship, growth, reproduction or population growth than well-watered trees.

In contrast, studies of drought and chronic water stress in natural habitats provided strong evidence that chewing and sucking insects benefited by feeding on drought-stressed plants. Mites and gall-forming insects were unaffected by experimental or natural drought stress (Waring and Cobb 1992).

A subsequent review of experimental drought stress by J.S. Koricheva et al. (1998) confirmed the negative effect of drought stress on both chewing and sucking insects. An interesting study conducted by L.M. Hanks and R. F. Denno (1993) in Maryland found survival of white peach scale on mulberry to be reduced on water-stressed trees in urban environments.

A study of water-stressed oaks in Lincoln, Nebraska, found aphids and lace bugs to be more abundant in urban settings than on less water-stressed oaks growing on a nearby campus (B.M. Cregg and M.E. Dix 2001). A more recent review of the literature by D.A. Herms (2002) revealed that water stress decreased the performance of leaf-feeders in some cases, increased performance in others, or had little effect on some. In summarizing work on bark beetles, Herms (2002) reported similarly variable results.

To reconcile these widely disparate findings, several authors have proposed that effects of plant stress on insect resistance may be nonlinear. In other words, increasing levels of stress do not necessarily result in ever-decreasing levels of resistance (P.L. Lorio 1986; Mattson and Haack 1987; S. Larsson 1989; Herms and Mattson 1992). Mattson and Haack (1987) and Herms and Mattson (1992) provided compelling arguments indicating that plant growth reduced by low to moderate levels of stress may result in resources directed to defense, thereby elevating resistance to pests relative to plants experiencing no or high levels of stress. This nonlinear relationship may provide an explanation for some studies finding negative effects of drought stress on insects while other studies show positive outcomes of plant stress on attacking insects.

Although not as lethal as the emerald ash borer, larvae of the banded ash clearwing (Podosesia aureocincta) cause significant damage to native ash trees in urban forests.

In his review of resistance in woody plants to insect attack, Herms (2002) called special attention to the plight of drought-stressed trees with respect to attack from wood- boring insects. He and others cite studies linking elevated plant stress to increased susceptibility to borer attack, especially attack by bark beetles and other species of cambium feeders (Larsson 1989; Herms 2002; A.F. Huberty and Denno 2004). For example, green ash trees (Fraxinus pennsylvanica) planted in a downtown urban environment experienced more severe drought stress and suffered higher levels of damage from borers than did trees planted on a park-like campus (Cregg and Dix 2001).

Similarly, flowering dogwood (Cornus florida), an understory species that does not tolerate midday water stress, was much more susceptible to colonization by the dogwood borer when planted in full sun relative to trees planted in at least partial shade (D.A. Potter and G.M. Timmons 1981). Water stress also increased the susceptibility of eucalypts to the eucalyptus longhorned borer (Phoracantha semipunctata) (Hanks et al. 1999). Our recent studies of susceptibility of green ash, (F. pennsylvanica) and Manchurian ash (F. mandshurica) to attack by wood borers generally support findings of the aforementioned studies. Roundheaded borers and bark beetles were significantly more abundant in both species of water-stressed ash trees. However, emerald ash borer (Agrilus planipennis) and clearwing borers in the genus (Podosesia) were unaffected by water stress (H. Martinson et al. 2014).

Does drought stress affect some tree types more so than others?

In the aforementioned review of drought stress, Waring and Cobb (1992) found resistance in conifers to be significantly compromised following water stress, while broadleaf trees generally demonstrated no clear pattern. In some cases, herbivores performed better, in other cases worse. Woody shrubs and vines exhibited a pattern opposite to that of trees in that water stress generally had a negative effect on insect performance. And although these patterns were observed, the processes underlying these differences are not fully known.

The literature review by Koricheva et al. (1998) similarly found the reproduction of sucking insects to be favored on water-stressed conifers but not so on deciduous trees. Also, and in contrast to Waring and Cobb (1992), Koricheva et al. (1998) found colonization of woodborers to be greater on water-stressed angiosperms than on water-stressed conifers.

The effects of water stress and drought are highly variable among taxa of plants and insects, and are affected by conditions in which plants grow. Effects of drought on plants and insects are often compounded by and confounded with other stressors, such as temperature, soil conditions, and pulsed inputs of nutrients and pesticides. This complicates knowing how much any single factor contributes to plant resistance or outbreaks of insects. While groups of insects, such as leaf chewers versus sap suckers, may differ in their responses to drought stress, a preponderance of evidence indicates that many kinds of wood-boring insects are favored by some level of water stress or drought.

Resin-based defenses of conifers may be compromised by drought, facilitating colonization by bark beetles.

The lethal potential of so many wood- boring insects – including natives such as western and mountain pine beetles and other scolytid borers, bronze birch borer and other Agrilus species – make a strong case for mitigating water stress whenever possible. The recent arrival of egregious non-native borers including Asian longhorned beetle, emerald ash borer, goldspotted oak borer, redbay ambrosia beetle, polyphagous shot hole borer, granulate ambrosia beetle, and a never-ending stream of others compound the dilemma for trees under stress in urban forests.

Several tactics can be used to ameliorate the effects of drought on trees in urban settings. Cregg and Dix (2001) recommend minimizing moisture stress by increasing or conserving soil moisture through supplemental irrigation, mulching, increasing planter-space size, and selecting trees proven to survive in urban environments that have elevated temperatures and evaporative demands. They recommend planting species tolerant to soil compaction, road salts, pollutants and atmospheric drought, which seems to be an ever-increasing feature of climate change. Mitigating climate change through reduced use of fossil fuels and ameliorating the effect of heat islands by planting more trees, shrubs, and groundcovers are recommended efforts (Raupp 2014a).

Other techniques to reduce water stress include the replacement of impervious surfaces with ones that allow water infiltration, increasing greenspaces in urban areas, avoiding soil compaction and aerating compacted soils (Raupp 2014a). Coupled with a warming world (Raupp 2014b) and a steady stream of new invasive species (Raupp 2013), I can’t help but think that widespread and lingering drought spells trouble for trees in many regions of the world.