Crow’s Nest: alder scale
If you see white fuzzy clumps on alder twigs this fall, look closely and you will also see ants.
The white fuzzy critters are scale insects (Prociphilus tessellatus) that suck the juices from the stem of the plant. The ants “shepherd” them—protect them from predators and then collect honeydew from them: a form of mutualism. Honeydew is the aphids’ sweet excrement from the plant sap.
According to bugguide.net this species of scale uses silver maple (or occasionally red maple) as the alternate host along with alder. (We only have a few silver maples on the preserve but lots of red maple.) This website also mentions that the predacious caterpillar of the harvester butterfly (Feniseca tarquinius) feeds upon these aphids.
The scale insects do little damage to the host plant. Honeydew (from a variety of scale species) can be a problem if you happen to park your car under a tree that has a lot of scale insects: it leaves a sticky residue on the surfaces beneath, which then tends to be colonized by sooty molds. These can be one or more species of black, powdery fungi that feed upon honeydew.
Posted by Daniel Barringer on September 15, 2012.