What a difference a month makes. In early April, the Sitka at Glandwr Forest were bursting with the lime green buds of spring. By mid May there was an unwelcome colour in town, with many of the Sitka under pressure from the green spruce aphid.
Green spruce aphids (Elatobium abietinum) are tiny green insects, 1mm to 2mm in length, that feed on the sap stored in the mature needles of spruce trees. Although they take a relatively small amount of nutrients, the toxins that they secrete create a lot of damage. These toxins cause discolouration turning the needles yellow which spreads along the entire needle as the aphids continue to feed. This yellowing, chlorosis, comes from a lack of chlorophyll and eventually leads to needle drop.
This is the first year that the Sitka spruce (Picea sitchensis) at Glandwr have suffered so dramatically from aphid damage. A milder winter without significant hard frosts has meant that that more green spruce aphids survived the winter and were able to reproduce and quickly multiply this spring.
Sitka spruce is one of the most common trees grown in UK forests for timber. In Glandwr Forest it was planted as part of our carbon study, a scientific research program which looks at the impact of enhanced rock weathering and soil microbiome inoculation on carbon sequestration in trees and soil. The study looks at two types of forests, native broadleaf and conifer. The conifer portion of the study is focused on Sitka to ensure that the findings from the research will be relevant for commercial forestry.
At Glandwr Forest we have a range of tree species and are managing the grasslands for a variety of flowering plants. This should encourage all the natural predators of aphids - birds, ladybirds, hoverflies, lacewings, and tiny wasps. When the green spruce aphid population exploded this spring, these aphid predators couldn't keep up. But as predator numbers increase, hopefully the aphid numbers will drop and there won't be any long-term damage to these trees.
It depends! For birds and insects that feed on green spruce aphids, it might be a good year. For our Sitka - it's been tough. Sitka is particularly vulnerable to the green spruce aphid. The loss of needles stresses the tree and slows the tree's growth.
There is also a bigger message: our winters are changing. As the climate becomes milder, we might see more years like this, where pests survive in higher numbers and put extra pressure on our forests.
Resources:
Forest Research: Tools and Resources: Green Spruce Aphid: Elatobium Abietinum
The Carbon Community Blog: Exploring the History behind our Conifer Trees
