A pine beetle is trapped by the sap flowing out of the hole it started to make. Courtesy photo
By ROBERT DRYJA
Plants, particularly trees, have been thought to live rather solitary lives even when growing adjacent to one another.
Relationships has been seen in functional terms. A parasitic relationship involves taking nutrients from a plant and providing nothing in return. Mistletoe is a common example of a plant parasite. Its roots grow into the branches of the host plant, absorbing nutrients from it.
Caterpillars eat leaves but provide nothing in return. A symbiotic relationship in contrast involves both species benefiting. Plants benefit from butterflies carrying pollen from flower to flower. Butterflies benefit from the nectar provided by flowers.
“Parasitic” and “symbiotic” do not suggest any awareness of being harmful or helpful to one another; only that that a mechanistic exchange is occurring.
This viewpoint begins to change when more complicated relationships among certain types of fungi and a forest of trees are considered. The trees are connected to each other’s roots through underground fungal networks. The trees share water and nutrients through the networks and use them to communicate.
They send distress signals about drought and disease. Healthy trees send needed water and nutrients to distressed trees. A tree may send a chemical message of an insect attack, and other trees then generate chemicals to repeal the insects if they are attacked.
These are called mycorrhizal networks. The fine, hair-like root tips of trees join together with microscopic fungal filaments to form the connecting links of a network. The kinds of relationships can become complex. Young trees may start growing from seeds that drop directly to the ground underneath a large mature tree. The mature tree absorbs most of the sunlight for photosynthesis.
Young trees would not survive due to the lack of sunlight but the mature tree provides nutrients through its roots to mycorrhizal fungi. The fungi in turn pass the nutrient to the young trees. Young trees send out chemical messages of their nutrition needs and parent trees responds in turn. This could be called a symbiotic chemical interchange. It also could be called a parent helping its children via a form of communication.
Chemical interchanges also can occur above ground by the release of gases into the air. These gases are called pheromones and are released by both trees and insects. The pheromone interactions of pine bark beetles with ponderosa pine trees create a remarkable story. A healthy ponderosa pine tree creates sap in its roots and carries it upwards in its trunk. A single bark beetle may burrow into the bark of the tree for both food and a place to lay its eggs.
However, it is in trouble when it reaches the sap flowing just below the bark in the trunk. The beetle is trapped by the sap, drowns, and its entry hole is plugged by sap oozing out. A mass attack of many beetles is needed to overcome the sap. The first beetles to arrive may die but if enough arrive then the tree will run out sap. Later arrivals then can safely take up residence. How do the beetles do this?
A clue is provided if a person cuts a gash in a ponderosa tree and then smells the air immediately around the cut. The pheromone that is released smells like butterscotch or vanilla. Bark beetles may identify a potential host tree by other pheromones released into the air. A mass attack begins when a ponderosa pine is injured or stressed. It releases what is supposed to be a defensive pheromone.
Instead female bark beetles are attracted. The first female beetles to arrive release their own pheromones as they bore into the tree. These beetle pheromones cause a tree to produce more of its pheromones. Together, these pheromones attract more beetles. A tree’s sap defense now becomes insufficient. Male beetles are attracted and enter the tunnels of females. The males then release their own type of pheromone. The combination of tree, female and male pheromones attract even more beetles.
A tree is weakened at this point but could recover if the beetle population stopped growing or was reduced. Remarkably, the beetles come to the tree’s rescue. After male and female beetles have mated, they change the kind of pheromones they release. The new combination of tree, female and male pheromones no longer attracts other beetles to the tree. A new generation of bark beetle larva grow to maturity under the tree bark while the tree does not have enough sap to drown them. The tree in turn may recover rather than killed outright since additional mature bark beetles have stopped being attracted.
“Parasitic” and “Symbiotic” are terms that can be used to describe this series of interactions but are missing something. The beetles could be said to be communicating to one another as conditions change over time. A tree is voicing its distress. They are not only releasing chemicals into the air.
Pine bark beetle holes with sap oozing out in the trunk of a ponderosa tree. Photo by Mark McGregor
Beetles can burrow extensively underneath the bark if a tree cannot produce enough sap to block them. Courtesy/USDA Forest Service