2017

Spruce Gall Aphis - the Masters of Masking in Forests

When spending time in nature, often a few things are left unnoticed by the human eye. This is especially true with regard to our smallest neighbours - insects. The most attentive forest visitors have often encountered damage caused by peculiar coniferous pests - the gall aphis.

Their presence can be spotted by specific "nurseries" or galls that resemble small cones. At the beginning of their development, the galls are green, then they are gradually decorated with a red or purple pattern, later becoming yellowish and finally yellow-brown. Their surface may be decorated with resin drops.

The size of galls usually does not exceed 3 centimetres, but it may change - galls of the eastern spruce gall aphis are shorter, less roundish, and in the beginning they are darker than galls of the late spruce gall aphis.

The gall aphis is anholocyclic, that is, it is not gendered. It means that these insects propagate with unfertilised eggs. The large number of galls in spruce forests is due to the mild winter, which has been particularly favourable for the gall aphis.

Spring is the time when the breeding season of the spruce gall aphis begins. Females - the founders of the colony, develop from larvae that have managed to hibernate. They have no wings, but they have a great appetite. By sucking juice, they accumulate strength, until each female lays a large amount of eggs. In the places where a female insect has fed and laid her eggs, galls develop. Although the young gall aphis females have wings, they do not travel far - a large part remain in their native spruce tree or move to an adjoining tree.

When larvae hatch, they slowly crawl into one of the cameras, where they grow and feed on fresh spruce juice. As the larvae develop, also the galls swell, and their cameras open in the second half of summer. The young larvae get out of their "nurseries" to turn into a couple of millimetres-long, winged females.

Soon, each female lays a few dozens of eggs on the needles of the tree, and the hatched larvae will hibernate until the new spruce buds appear next year. Then they will suck cone juice until they become founding females.

But what happens to the old, empty galls? They dry out, harden, turn black, until they start looking like old, empty spruce cones, thus, lagging far behind them size-wise. The old galls can remain in spruce-trees for several years. If shoots are damaged by gall aphis, next year usually needles fall off, sometimes branches and even the whole spruce tree wither.