In recent days you may have noticed that there are suddenly a lot of aphids in the orchard. Aphid populations typically enjoy the period after nut drop. The weather cools down, and aphid populations spike. One last hurrah before winter. However you might have noted something odd about where the aphids are within the trees right now. Not only are they crawling all over the leaves, but they are all over the wood, particularly the older wood. Normally aphids are found on leaves and husks and shoots, the green plant tissue where they can access and feed on the phloem, not on the bark of the scaffold branches. So what is going on here?
It turns out we have reached an interesting point in the life cycle of the two aphid species that are important on hazelnuts, filbert aphid (Myzocallis coryli) and hazelnut aphid (Corylobium avellanae). This is the one and only sexual stage. Both of these aphids overwinter in the egg stage. In the spring the eggs hatch and the tiny aphid nymphs begin feeding on the buds as they swell. But in the fall, the vegetative tissues that aphids feed from all growing season such as leaves and husks, fall from the tree and they compost on the orchard floor. Thus, these green tissues are not suitable for egg laying and the aphids have adapted to laying their eggs in the rough bark of the 2+ year old wood. The eggs are green at first but then ripen to a shiny black color.
For most of the growing season, adult aphids reproduce asexually by giving birth to live young. This sexual stage that produces the eggs is unique. For the sexual females to produce the eggs, they must first mate with the winged sexual males. You may see some of this going now on if you inspect leaves.