phylloxera eggs

Oak Phylloxera

Most of our oaks are the 'pubescent' oak, which has hairy leaves in spring and which fall in autumn. This year many leaves were densely spotted with yellow. It was almost impossible with my ageing eyes to see what was the cause. But with a binocular microscope the reason was clear. On the lower side of the leaves, each spot had a tiny sap sucking orange insect at its centre. These minute creatures are closely related to aphids. But they are very much smaller and lack the two rear cornicles or horns on the abdomen. My photo shows two of them with their probosci stuck into the lower surface of the leaf. At this magnification you can almost make out the individual cells of the leaf. Each of the small green bobbles grouped in blocks between the tiny veins consists of two 'guard' cells surrounding the stomata or breathing pores of the leaf. This shows just how small the insects are. Each orange bug is covered with many nodules and whilst they suck at one end they lay eggs at the other, which are deposited in a charming ring. The true aphids would at this stage be giving birth to young aphids, and not producing eggs. In both cases the young are produced without any sexual encounter. These creatures are of the group known as Phylloxera. A relative is the infamous bug which caused the near collapse of the French vineyards in the 19th century. That particular bug lives chiefly on the roots of the vine and sucks the plant to death. In that case the phylloxera bug was an accidental import from the USA. The life cycle of the oak phylloxera is somewhat complicated and not fully understood. My photo was taken in August. By September most of the orange forms were replaced with blackish winged adults and eating them were a number of lacewing larvae. By October there was no sign of the pest but the yellow spots remained. It seems that the bugs had probably flown from our oak leaves which are on the point of dropping, to one of the local evergreen holm oaks. If they succeed in that journey they will give rise to another generation which will be wingless sexual males and females. These can mate and the females lay eggs in crevices near the buds which will last through the winter. In other localities it will overwinter on the cork oak and the kermes oak, both evergreen. One researcher has claimed that are twenty one stages to the life history of this insect.

yellow spots

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