How do herbivorous insects respond to drought stress in trees?

Abstract

Increased frequency and severity of drought, as a result of climate change, is expected to drive critical changes in plant–insect interactions that may elevate rates of tree mortality. The mechanisms that link water stress in plants to insect performance are not well understood. Here, we build on previous reviews and develop a framework that incorporates the severity and longevity of drought and captures the plant physiological adjustments that follow moderate and severe drought. Using this framework, we investigate in greater depth how insect performance responds to increasing drought severity for: (i) different feeding guilds; (ii) flush feeders and senescence feeders; (iii) specialist and generalist insect herbivores; and (iv) temperate versus tropical forest communities. We outline how intermittent and moderate drought can result in increases of carbon‐based and nitrogen‐based chemical defences, whereas long and severe drought events can result in decreases in plant secondary defence compounds. We predict that different herbivore feeding guilds will show different but predictable responses to drought events, with most feeding guilds being negatively affected by water stress, with the exception of wood borers and bark beetles during severe drought and sap‐sucking insects and leaf miners during moderate and intermittent drought. Time of feeding and host specificity are important considerations. Some insects, regardless of feeding guild, prefer to feed on younger tissues from leaf flush, whereas others are adapted to feed on senescing tissues of severely stressed trees. We argue that moderate water stress could benefit specialist insect herbivores, while generalists might prefer severe drought conditions. Current evidence suggests that insect outbreaks are shorter and more spatially restricted in tropical than in temperate forests. We suggest that future research on the impact of drought on insect communities should include (i) assessing how drought‐induced changes in various plant traits, such as secondary compound concentrations and leaf water potential, affect herbivores; (ii) food web implications for other insects and those that feed on them; and (iii) interactions between the effects on insects of increasing drought and other forms of environmental change including rising temperatures and CO2 levels. There is a need for larger, temperate and tropical forest‐scale drought experiments to look at herbivorous insect responses and their role in tree death.

Search for top-down and bottom-up drivers of latitudinal trends in insect herbivory in oak trees in Europe

“…Latitudinal variation in plant-herbivore interactions is therefore generally associated with large-scale variability in climatic conditions (Moreira et al, 2018) and numerous studies demonstrate an effect of temperature and precipitation on plant traits (e.g. leaf N, phenolic compounds) (Chen et al, 2013;Holopainen et al, 2018;Gely et al, 2019) and herbivory (Jamieson et al, 2015;Gely et al, 2019). However, many regions deviate from the global trend in temperature and precipitation toward higher latitudes due to their proximity to oceans or the presence of mountains (De Frenne et al, 2013), which can markedly change the relationship between latitude and plant-herbivore-predator interactions (Roslin et al, 2017;Loughnan & Williams, 2019;Moreira et al, 2019).…”

Tree canopy arthropods have idiosyncratic responses to plant ecophysiological traits in a warm temperate forest complex

“…Other species, that grow in a range of microsites, from dry to wet, might reveal intraspecific responses to dry spells, from which differential arthropod responses toward individual trees can be expected. Thus, there will likely be no uniformity in response by canopy arthropod diversity towards increased environmental and climatic changes, as has been suggested for the interactions between herbivorous insects and trees globally 15 .…”

“…Moreover, for many arthropod species, being host-recurrent, shifting from one host species to another during stressful conditions will also not be possible 48 . Conversely, other, generalist arthropods might be able to move between host species, and become more numerous 15 . Ecological impacts of drought, or shifts in climatic regimes, could therefore cause arthropod populations to undergo great changes, threatening ecological integrity.…”

“…Effect of climate change on the interaction between trees and associated arthropods also requires consideration, especially the extent to which tree responses to changing conditions might spill over to higher trophic levels 15 . Our poor understanding of these interactions is concerning given that even a 1ºC increase in global temperature may have profound effects on the interactions between trees and arthropods 16 .…”

Review: Plant Eco-Evolutionary Responses to Climate Change: Emerging Directions

“…Yet, increasing atmospheric [CO2] may induce greater production of plant defenses [177]. Few studies have investigated the direct impacts of prolonged drought stress on herbivores [178], however, drought stress reduces plant fitness in many systems [179]. Additionally, climate change may accelerate the developmental rates of insect herbivores and increase the number of life cycles a species completes in a growing season [176].…”