Insects as a normal part of forests
Forests are full of insects, and the great majority do no noticeable harm. They pollinate flowers, break down dead wood, feed birds and other animals and form part of the food web. A few native species, though, can sometimes multiply to very large numbers and kill or weaken many trees across a wide area. These population surges are called outbreaks, and the insects involved are often described as forest pests.
The label depends on human values. An outbreak that kills timber trees is a loss to an owner, while the same event may create dead standing trees that provide habitat for woodpeckers and cavity-nesting species. This article covers the two main groups behind most outbreaks: defoliating insects and bark-boring insects. Introduced insects and diseases that arrive from other regions are discussed under invasive species, and the way trees fight back is covered in the discussion of tree defenses.
Defoliators
Defoliators feed on leaves or needles. Many are the larval stages of moths, butterflies or sawflies. Examples that have been recorded in forests around the world include tent caterpillars, spongy moth caterpillars, budworms in conifer forests and various loopers and sawfly larvae.
When defoliator numbers are high, trees can lose much of their foliage in a season. A healthy broadleaf tree often grows a second set of leaves, although this costs stored energy and slows growth. Conifers are generally more vulnerable because they cannot easily replace lost needles, and several years of heavy defoliation can kill them. Trees weakened by defoliation are also more susceptible to other stresses, including drought and secondary pests.
Defoliator outbreaks in some forests have been described as cyclic, rising to high numbers every several years before collapsing, though the timing is irregular and varies between species and regions.
Bark-boring insects
Bark beetles and related wood-boring insects live in or under the bark of trees. Adults tunnel beneath the bark, lay eggs, and the larvae feed on the inner bark, where the tree transports sugars and water. When enough tunnels encircle the trunk, the tree can die, since movement of water from roots to leaves and sugars back down is disrupted. Many bark beetles also carry fungi that stain wood and further interfere with the tree.
Key features of many bark beetle species:
- Some attack mainly weakened or dying trees and rarely cause widespread problems.
- A smaller number can overcome healthy trees when their populations are high, by attacking in mass, often coordinated through chemical signals called pheromones.
- Many are specialized to particular tree species or groups, such as pines or spruces.
Other boring insects, such as wood-boring beetles and moth larvae, tunnel more deeply into wood, which can reduce timber quality even when the tree survives.
Why outbreaks happen
Outbreaks usually result from several conditions combining rather than one cause. Commonly cited factors include:
- Weather. Warm temperatures can speed insect development and increase survival, and mild winters may allow more individuals to survive in species limited by cold. A warm, dry spell can also stress trees.
- Tree stress. Trees weakened by drought stress produce fewer defensive compounds such as resin, which makes them easier for insects to overcome. Windthrow and storm damage can also supply large amounts of fresh, weakened wood that boosts beetle breeding.
- Stand structure. Large areas of similar-aged trees of the same species can offer abundant food. Dense, crowded stands compete for water and are often more vulnerable. Mixed stands may be less susceptible, because host trees are scattered and some species resist particular pests.
- Natural enemies. Birds, parasitic wasps, predatory insects and disease organisms often keep populations in check. Outbreaks may start when these controls fail or lag behind.
- Past management and disturbance. Fire suppression, past logging patterns and plantation layouts can all influence the age, density and species makeup of a forest, and in turn how easily insects spread.
How much climate warming has contributed to particular outbreaks is an active research area, and the answer differs by insect and region. Many researchers expect that changing conditions will alter outbreak patterns in some forests, but it is difficult to predict precisely where.
Effects on forest ecosystems
The consequences of an outbreak range from minor to landscape-scale change.
Tree mortality and growth loss. Even where trees survive, repeated defoliation can reduce growth for years. In severe cases, large stands die, changing the canopy and the amount of light reaching the ground.
Changes in species composition. If an insect specializes in one tree species, its decline can shift a forest toward other species, which may be more resistant or may simply be different. Young trees and understory plants can respond quickly to the extra light.
Wildlife. Dead standing trees provide food and nesting sites for many birds and other animals, and fallen logs add to deadwood habitat. Meanwhile, animals that depend on live canopy or specific tree species may decline. Insects themselves are an important food source: some birds and bats increase in number during outbreaks.
Nutrient and water cycles. Large amounts of caterpillar droppings and falling leaves can alter nutrient inputs to the soil. When many trees die, water use by the stand drops, which may change streamflow for a time.
Fire and carbon. Dead and dying trees can influence how a later fire behaves, although research on how strongly outbreaks affect fire has shown mixed and context-dependent results. Dying trees stop taking up carbon and begin to release it as they decompose, while regrowth gradually draws carbon back down. Over long periods this feeds into the global carbon cycle and affects how carbon storage in a forest is counted.
People. Outbreaks affect timber supplies, water, recreation, scenery and safety, since dead trees can fall near roads and homes.
How forests respond
Most forests recover from outbreaks. Surviving trees and seedlings grow into the gaps, and new seedlings establish. Because insect outbreaks often target one species or age class, the result can be a forest with a different mix from before rather than a completely cleared landscape. Recovery times can range from a few years in the case of defoliated broadleaf trees to decades where many mature trees have died.
Management options
Responses depend on goals, scale and insect biology, and they are usually most effective before an outbreak peaks.
- Monitoring. Traps, aerial surveys and field plots help detect rising populations early. Programs that measure forest health indicators are an important part of this.
- Silviculture. Stand thinning reduces crowding and tree stress. Favoring a mix of species and ages can reduce the chance that a single pest affects the whole stand.
- Sanitation. Removing recently infested trees, promptly handling storm-felled timber and avoiding leaving fresh logs on site during beetle flight periods can lower breeding material.
- Biological and targeted control. Some programs use natural enemies or specific biological products against defoliators. Chemical insecticides are sometimes used on high-value trees or small areas, but they are rarely practical across large forests and may affect non-target species.
- Accepting the outbreak. In remote or protected forests, managers may choose to leave outbreaks alone, treating them as part of natural disturbance.
Once an outbreak is large, options are limited, which is why prevention and early action usually matter more than late-stage control. A landowner who suspects an outbreak should seek local forestry or extension advice, since correct identification of the insect is the first step.