Betula pubescens is also known as the mountain or downy birch. It can be susceptible to insect invasion and there are entire stands that have been defoliated due to pest herbivores. Wood ants live in the woods and make nests in the forest humus. They are predatory and harvest most species of insect as well as take care of aphid populations. The aphids are "herded" so they are near the ants. This is because the ants eat the honeydew that aphids secrete. Therefore, ant presence in a birch tree is an indication that there is a steady supply of invertebrates and the pests ants eat.
Betula Pubescens Characteristics
The white birch or Betula pubescens can grow to 65 feet and creates a deciduous canopy in New England and European forests. It can tolerate clay soil but needs sun to thrive. It is home to numerous woodland animals including insects. The sap is sweet and attractive to moths, ants, aphids and other creatures.
The wood ant does not eat wood, but it does harvest some for nest building. Primarily they live in the woods under leaf debris piles and are seen climbing vertically up and down the trunks of trees. They hunt in the trees and tend their aphid herds. They milk the aphids to get the honeydew, which is a sweet, rich food for the ants.
Wood Ants and Tree Predators.
There are numerous studies, among them the 2004 Dearne Woods Study by the University of Sheffield, comparing the effect of wood ant presence on the invertebrates that damage trees. When the ants are in the vicinity, the incidence of other insects decreases and the trees are healthier. Conversely, aphids suck the sap from the tree and do significant damage when they are housed in a birch. They can even kill it and the symbiotic relationship between ant and aphid would be the culprit.
Aphids siphon out plant juices and can do a great deal of damage to plants. Faded foliage, twisted leaves and even branch die back, are common effects of aphid feeding. However, they usually do no lasting damage except in cases where there is a literal plague and plants are already stressed. This has occurred in drought-stricken areas where plant life is already unwell.
Ants eat plant-damaging bugs, but they also encourage and take care of aphids, who do plant damage. A 2004 Finnish Zoological Board, study confirms that the closer an ant nest is to a tree, the less likely there will be a variety of tree pests. The farther a nest is, the more invertebrates were collected. Aphid incidence was about equal whether the ants were taking care of the aphids or not. Conclusively, ants are beneficial to birch trees overall.